Overview of Process Plant
Piping System Design
Participant’s Guide
CONTACT INFORMATION
ASME Headquarters
1-800-THE-ASME
ASME Professional Development
1-800-THE-ASME
Eastern Regional Office
Southern Regional Office
8996 Burke Lake Road – Suite L102
Burke, VA 22015-1607
703-978-5000
800-221-5536
703-978-1157 (FAX)
1950 Stemmons Freeway – Suite 5037C
Dallas, TX 75207-3109
214-746-4900
800-445-2388
214-746-4902 (FAX)
Midwest Regional Office
Western Regional Office
17 North Elmhurst Avenue – Suite 108
Mt. Prospect, IL 60056-2406
847-392-8876
800-628-6437
847-392-8801 (FAX)
119-C Paul Drive
San Rafael, CA 94903-2022
415-499-1148
800-624-9002
415-499-1338 (FAX)
Northeast Regional Office
326 Clock Tower Commons
Route 22
Brewster, NY 10509-9241
914-279-6200
800-628-5981
914-279-7765
(FAX)
You can also find information on
these courses and all of ASME,
including ASME Professional
Development, the Vice President of
Professional Development, and other
contacts at the ASME Web site……
http://www.asme.org
International Regional Office
1-800-THE-ASME
Overview of Process Plant
Piping System Design
By:
Vincent A. Carucci
Carmagen Engineering, Inc.
Copyright © 2000 by
All Rights Reserved
TABLE OF CONTENTS
PART 1:
PARTICIPANT NOTES ..............................................................................3
PART 2:
BACKGROUND MATERIAL .................................................................................... 73
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
IX.
X.
XI.
Introduction ....................................................................................................................... 73
General ............................................................................................................................. 73
A.
What is a piping system .......................................................................................... 73
B.
Scope of ASME B31.3............................................................................................. 73
Material selection considerations...................................................................................... 75
A.
Strength................................................................................................................... 75
B.
Corrosion Resistance .............................................................................................. 77
C.
Material Fracture Toughness .................................................................................. 77
D.
Fabricability ............................................................................................................. 78
E.
Availability and Cost ................................................................................................ 78
Piping Components........................................................................................................... 79
A.
Fittings, Flanges, and Gaskets................................................................................ 79
B.
Flange Rating .......................................................................................................... 85
Sample Problem 1 - Determine Flange Rating ................................................................. 88
Solution ............................................................................................................................. 88
Valves ............................................................................................................................... 89
A.
Valve Functions....................................................................................................... 89
B.
Primary Valve Types ............................................................................................... 90
C.
Valve Selection Process ......................................................................................... 98
Exercise 1 – Determine Required Flange Rating ............................................................. 99
Design ............................................................................................................................. 100
A.
Design Conditions ................................................................................................. 100
B.
Loads and Stresses............................................................................................... 101
C.
Pressure Design of Components .......................................................................... 105
Sample Problem 2 - Determine Pipe wall thickness ....................................................... 110
Sample Problem 3 .......................................................................................................... 116
Exercise 2: Determine Required Pipe Wall Thickness .................................................. 121
System Design ................................................................................................................ 122
A.
Layout Considerations .......................................................................................... 122
B.
Pipe Supports and Restraints ............................................................................... 123
C.
Piping Flexibility..................................................................................................... 129
D.
Required Design Information for Piping Stress Analysis ...................................... 132
E.
Criteria for Allowable Equipment Nozzle Loads .................................................... 132
F.
When Should A Computer Analysis Be Used ....................................................... 134
G.
Design Considerations for Piping System Stress Analysis ................................... 134
Fabrication, Assembly, and Erection .............................................................................. 140
A.
Welding and Heat Treatment ................................................................................ 140
B.
Assembly and Erection.......................................................................................... 144
Quality Control ................................................................................................................ 151
A.
Inspection .............................................................................................................. 151
B.
Testing................................................................................................................... 154
Other Considerations ...................................................................................................... 156
A.
Nonmetallic Piping................................................................................................. 156
B.
Category M Fluid Service...................................................................................... 157
C.
High Pressure Piping............................................................................................. 158
Summary......................................................................................................................... 160
Part 1:
Participant Notes
3
OVERVIEW OF
PROCESS PLANT PIPING
SYSTEM DESIGN
By: Vincent A. Carucci
Carmagen Engineering, Inc.
1
Notes:
Piping System
Piping system: conveys fluid between
locations
Piping system includes:
• Pipe
• Fittings (e.g. elbows, reducers, branch
connections, etc.)
• Flanges, gaskets, bolting
• Valves
• Pipe supports
2
Notes:
4
ASME B31.3
• Provides requirements for:
– Design
– Materials
– Fabrication
– Erection
– Inspection
– Testing
• For process plants including
–
–
–
–
Petroleum refineries
Chemical plants
Pharmaceutical plants
Textile plants
– Paper plants
– Semiconductor
plants
– Cryogenic plants
3
Notes:
Scope of ASME B31.3
• Piping and piping components, all fluid
services:
– Raw, intermediate, and finished chemicals
– Petroleum products
– Gas, steam, air, and water
– Fluidized solids
– Refrigerants
– Cryogenic fluids
• Interconnections within packaged equipment
• Scope exclusions specified
4
Notes:
5
Strength
•
•
•
•
•
•
Yield and Tensile Strength
Creep Strength
Fatigue Strength
Alloy Content
Material Grain size
Steel Production Process
5
Notes:
Stress - Strain Diagram
B
S
A
C
E
6
Notes:
6
Corrosion Resistance
• Deterioration of metal by chemical or
electrochemical action
• Most important factor to consider
• Corrosion allowance
added thickness
• Alloying increases corrosion resistance
7
Notes:
Piping System Corrosion
General or
Uniform
Corrosion
Uniform metal loss. May be combined with erosion if
high-velocity fluids, or moving fluids containing
abrasives.
Pitting
Corrosion
Localized metal loss randomly located on material
surface. Occurs most often in stagnant areas or areas of
low-flow velocity.
Galvanic
Corrosion
Occurs when two dissimilar metals contact each other in
corrosive electrolytic environment. Anodic metal develops
deep pits or grooves as current flows from it to cathodic
metal.
Crevice Corrosion Localized corrosion similar to pitting. Occurs at places
such as gaskets, lap joints, and bolts where crevice
exists.
Concentration
Cell Corrosion
Occurs when different concentration of either a corrosive
fluid or dissolved oxygen contacts areas of same metal.
Usually associated with stagnant fluid.
Graphitic
Corrosion
Occurs in cast iron exposed to salt water or weak acids.
Reduces iron in cast iron, and leaves graphite in place.
Result is extremely soft material with no metal loss.
8
Notes:
7
Material Toughness
• Energy necessary to initiate and
propagate a crack
• Decreases as temperature decreases
• Factors affecting fracture toughness
include:
– Chemical composition or alloying elements
– Heat treatment
– Grain size
9
Notes:
Fabricability
• Ease of construction
• Material must be weldable
• Common shapes and forms include:
– Seamless pipe
– Plate welded pipe
– Wrought or forged elbows, tees, reducers,
crosses
– Forged flanges, couplings, valves
– Cast valves
10
Notes:
8
Availability and Cost
• Consider economics
• Compare acceptable options based on:
– Availability
– Relative cost
11
Notes:
Pipe Fittings
• Produce change in geometry
–
–
–
–
Modify flow direction
Bring pipes together
Alter pipe diameter
Terminate pipe
12
Notes:
9
Elbow and Return
90°
45°
180° Return
Figure 4.1
13
Notes:
Tee
Reducing Outlet Tee
Cross Tee
Figure 4.2
14
Notes:
10
Reducer
Concentric
Eccentric
Figure 4.3
15
Notes:
Welding Outlet Fitting
16
Figure 4.4
Notes:
11
Cap
Figure 4.5
17
Notes:
Lap-joint Stub End
Note square corner
R
R
Enlarged Section
of Lap
18
Figure 4.6
Notes:
12
Typical Flange Assembly
Flange
Bolting
Gasket
Figure 4.7
19
Notes:
Types of Flange
Attachment and Facing
Flange Attachment Types
Flange Facing Types
Threaded Flanges
Flat Faced
Socket-Welded Flanges
Blind Flanges
Raised Face
Slip-On Flanges
Lapped Flanges
Ring Joint
Weld Neck Flanges
20
Table 4.1
Notes:
13
Flange Facing Types
Figure 4.8
21
Notes:
Gaskets
•
•
•
•
Resilient material
Inserted between flanges
Compressed by bolts to create seal
Commonly used types
– Sheet
– Spiral wound
– Solid metal ring
22
Notes:
14
Flange Rating Class
• Based on ASME B16.5
• Acceptable pressure/temperature
combinations
• Seven classes (150, 300, 400, 600, 900,
1,500, 2,500)
• Flange strength increases with class
number
• Material and design temperature
combinations without pressure indicated
not acceptable
23
Notes:
Material Specification List
24
Table 4.2
Notes:
15
Pressure - Temperature Ratings
Material
Group No.
Classes
Temp., °F
-20 to 100
200
300
400
500
600
650
700
750
800
850
900
950
1000
1.9
1.8
150
235
220
215
200
170
140
125
110
95
80
65
50
35
20
300
620
570
555
555
555
555
555
545
515
510
485
450
320
215
400
825
765
745
740
740
740
740
725
685
675
650
600
425
290
150
290
260
230
200
170
140
125
110
95
80
65
50
35
20
300
750
750
720
695
695
605
590
570
530
510
485
450
320
215
1.10
400
1000
1000
965
885
805
785
785
710
675
650
600
425
290
190
150
290
260
230
200
170
140
125
110
95
80
65
50
35
20
300
750
750
730
705
665
605
590
570
530
510
485
450
375
260
400
1000
1000
970
940
885
805
785
755
710
675
650
600
505
345
Table 4.3
25
Notes:
Sample Problem 1
Flange Rating
New piping system to be installed at
existing plant.
Determine required flange class.
• Pipe Material:
• Design Temperature:
• Design Pressure:
1 1 Cr − 1 Mo
4
2
700°F
500 psig
26
Notes:
16
Sample Problem 1 Solution
• Determine Material Group Number (Fig. 4.2)
Group Number = 1.9
• Find allowable design pressure at
intersection of design temperature and Group
No. Check Class 150.
– Allowable pressure = 110 psig < design pressure
– Move to next higher class and repeat steps
• For Class 300, allowable pressure = 570 psig
• Required flange Class: 300
27
Notes:
Valves
• Functions
– Block flow
– Throttle flow
– Prevent flow reversal
28
Notes:
17
Full Port Gate Valve
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
Handwheel Nut
Handwheel
Stem Nut
Yoke
Yoke Bolting
Stem
Gland Flange
Gland
Gland Bolts or
Gland Eye-bolts and nuts
Gland Lug Bolts and Nuts
Stem Packing
Plug
Lantern Ring
Backseat Bushing
Bonnet
Bonnet Gasket
Bonnet Bolts and Nuts
Gate
Seat Ring
Body
One-Piece Gland (Alternate)
Valve Port
Figure 5.1
29
Notes:
Globe Valve
•
•
•
•
•
Most economic for throttling flow
Can be hand-controlled
Provides “tight” shutoff
Not suitable for scraping or rodding
Too costly for on/off block operations
30
Notes:
18
Check Valve
•
•
•
•
Prevents flow reversal
Does not completely shut off reverse flow
Available in all sizes, ratings, materials
Valve type selection determined by
– Size limitations
– Cost
– Availability
– Service
31
Notes:
Swing Check Valve
Cap
Pin
Seat
Ring
Hinge
Flow
Direction
Disc
Body
32
Figure 5.2
Notes:
19
Ball Check Valve
Figure 5.3
33
Notes:
Lift Check Valve
Seat
Ring
Piston
Flow
Direction
34
Figure 5.4
Notes:
20
Wafer Check Valve
Figure 5.5
35
Notes:
Ball Valve
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
36
Part Names
Body
Body Cap
Ball
Body Seal Gasket
Seat
Stem
Gland Flange
Stem Packing
Gland Follower
Thrust Bearing
Thrust Washer
Indicator Stop
Snap Ring
Gland Bolt
Stem Bearing
Body Stud Bolt & Nuts
Gland Cover
Gland Cover Bolts
Handle
Figure 5.6
Notes:
21
Plug Valve
Wedge
Molded-In Resilient Seal
Sealing Slip
Figure 5.7
37
Notes:
Valve Selection Process
General procedure for valve selection.
1. Identify design information including
pressure and temperature, valve function,
material, etc.
2. Identify potentially appropriate valve
types and components based on
application and function
(i.e., block, throttle, or reverse flow
prevention).
38
Notes:
22
Valve Selection Process,
cont’d
3. Determine valve application requirements
(i.e., design or service limitations).
4. Finalize valve selection. Check factors to
consider if two or more valves are
suitable.
5. Provide full technical description
specifying type, material, flange rating,
etc.
39
Notes:
Exercise 1 - Determine
Required Flange Rating
• Pipe:
1 1 Cr − 1 Mo
4
2
• Flanges:
• Design Temperature:
• Design Pressure:
A-182 Gr. F11
900°F
375 psig
40
Notes:
23
Exercise 1 - Solution
1. Identify material specification of flange
A-182 Gr, F11
2. Determine Material Group No. (Table 4.2)
Group 1.9
3. Determine class using Table 4.3 with design
temperature and Material Group No.
– The lowest Class for design pressure of 375
psig is Class 300.
– Class 300 has 450 psig maximum pressure
at 900°F
41
Notes:
Design Conditions
• General
– Normal operating conditions
– Design conditions
• Design pressure and temperature
– Identify connected equipment and associated
design conditions
– Consider contingent conditions
– Consider flow direction
– Verify conditions with process engineer
42
Notes:
24
Loading Conditions
Principal pipe load types
• Sustained loads
– Act on system all or most of time
– Consist of pressure and total weight load
• Thermal expansion loads
– Caused by thermal displacements
– Result from restrained movement
• Occasional loads
43
– Act for short portion of operating time
– Seismic and/or dynamic loading
Notes:
Stresses Produced By
Internal Pressure
Sl
Sc
P
t
44
Sl
=
Longitudinal Stress
Sc
=
Circumferential (Hoop) Stress
t
=
Wall Thickness
P
=
Internal Pressure
Figure 6.1
Notes:
25
Stress Categorization
• Primary Stresses
– Direct
– Shear
– Bending
• Secondary stresses
– Act across pipe wall thickness
– Cause local yielding and minor distortions
– Not a source of direct failure
45
Notes:
Stress Categorization, cont’d
• Peak stresses
– More localized
– Rapidly decrease within short distance of
origin
– Occur where stress concentrations and
fatigue failure might occur
– Significance equivalent to secondary stresses
– Do not cause significant distortion
46
Notes:
26
Allowable Stresses
Function of
– Material properties
– Temperature
– Safety factors
Established to avoid:
– General collapse or excessive distortion from
sustained loads
– Localized fatigue failure from thermal
expansion loads
– Collapse or distortion from occasional loads
47
Notes:
B31.3 Allowable
Stresses in Tension
Basic Allowable Stress S, ksi. At Metal Temperature, °F.
°°
Material
Spec. No/Grade
100
200
300
400
500
Carbon Steel
A 106
B
20.0
20.0
20.0
20.0
18.9
17.3
16.5
10.8
6.5
2.5
1.0
C - ½Mo
A 335
P1
18.3
18.3
17.5
16.9
16.3
15.7
15.1
13.5
12.7
4.
2.4
P11
20.0
1¼ - ½Mo
A 335
600
700
800
900
1000 1100
1200
1300
1400
1500
18.7
18.0
17.5
17.2
16.7
15.6
15.0
12.8
6.3
2.8
1.2
18Cr - 8Ni pipe
A 312
TP304 20.0
20.0
20.0
18.7
17.5
16.4
16.0
15.2
14.6
13.8
9.7
6.0
3.7
2.3
1.4
16Cr - 12Ni-2Mo
pipe
A 312
TP316 20.0
20.0
20.0
19.3
17.9
17.0
16.3
15.9
15.5
15.3
12.4
7.4
4.1
2.3
1.3
Table 6.1
48
Notes:
27
Pipe Thickness Required
For Internal Pressure
•
t=
PD
2 (SE + PY )
P = Design pressure, psig
D = Pipe outside diameter, in.
S = Allowable stress in tension, psi
E = Longitudinal-joint quality factor
Y = Wall thickness correction factor
•
•
t m = t + CA
t nom =
tm
0.875
49
Notes:
Spec.
No.
Class (or Type)
Description
Ej
Carbon Steel
API
5L
...
...
...
A 53
Type S
Type E
Type F
A 106
...
Seamless pipe
Electric resistance welded pipe
Electric fusion welded pipe, double butt, straight or
spiral seam
Furnace butt welded
1.00
0.85
0.95
Seamless pipe
Electric resistance welded pipe
Furnace butt welded pipe
1.00
0.85
0.60
Seamless pipe
1.00
Low and Intermediate Alloy Steel
A 333
...
...
Seamless pipe
Electric resistance welded pipe
1.00
0.85
A 335
...
Seamless pipe
A 312
...
...
...
Seamless pipe
Electric fusion welded pipe, double butt seam
Electric fusion welded pipe, single butt seam
1.00
0.85
0.80
1.00
Electric fusion welded pipe, 100% radiographed
Electric fusion welded pipe, spot radiographed
Electric fusion welded pipe, double butt seam
1.00
0.90
0.85
Stainless Steel
A 358
1, 3, 4
5
2
B 161
...
B 514
...
Welded pipe
0.80
B 675
All
Welded pipe
0.80
Nickel and Nickel Alloy
50
Seamless pipe and tube
1.00
Table 6.2
Notes:
28
Temperature, °F
950
1000
1050
1100
1150 & up
Ferritic
Steels
0.4
0.5
0.7
0.7
0.7
0.7
Austenitic
Steels
0.4
0.4
0.4
0.4
0.5
0.7
Other
Ductile
Metals
0.4
0.4
0.4
0.4
0.4
0.4
Cast iron
0.0
...
...
...
...
...
Materials
900 & lower
Table 6.3
51
Notes:
Curved and Mitered Pipe
• Curved pipe
– Elbows or bends
– Same thickness as straight pipe
• Mitered bend
– Straight pipe sections welded together
– Often used in large diameter pipe
– May require larger thickness
• Function of number of welds, conditions, size
52
Notes:
29
Sample Problem 2 Determine Pipe Wall Thickness
Design temperature: 650°F
Design pressure: 1,380 psig.
Pipe outside diameter: 14 in.
Material: ASTM A335, Gr. P11 ( 1 14 Cr − 12 Mo ),
seamless
Corrosion allowance: 0.0625 in.
53
Notes:
Sample Problem 2 - Solution
t=
PD
2(SE + PY)
t=
1,380 × 14
2[(16,200 × 1) + (1,380 × 0.4 )]
t = 0.577 in.
54
Notes:
30
Sample Problem 2 Solution, cont’d
tm = t + c = 0.577 + 0.0625 = 0.6395 in.
t nom =
0.6395
= 0.731 in.
0.875
55
Notes:
Welded Branch Connection
Db
Tb
Reinforcement
Zone Limits
Nom.
Thk.
c
tb
A3
A3
L4
Reinforcement
Zone Limits
Mill
Tol.
A4
A4
A1
Tr
Th
Dh
Nom.
Thk.
c
th
Mill
Tol.
d1
A2
A2
d2
d2
β
Pipe C
56
Figure 6.2
Notes:
31
Reinforcement Area
d1 =
Db − 2(Tb − c)
sin β
d1 = Effective length removed from run pipe, in.
Db = Branch outside diameter, in.
Tb = Minimum branch thickness, in.
c = Corrosion allowance, in.
β = Acute angle between branch and header
57
Notes:
Required Reinforcement Area
Required reinforcement area, A1:
A 1 = t h d1(2 − sin β)
Where: th = Minimum required header
thickness, in.
58
Notes:
32
Reinforcement Pad
• Provides additional reinforcement
• Usually more economical than increasing
wall thickness
• Selection variables
– Material
– Outside diameter
– Wall thickness
æ (D − Db ) ö
A 4 = çç p
Tr
è sin β
59
Notes:
Sample Problem 3
• Pipe material: Seamless, A 106/Gr. B for
branch and header, S = 16,500 psi
• Design conditions: 550 psig @ 700°F
• c = 0.0625 in.
• Mill tolerance: 12.5%
60
Notes:
33
Sample Problem 3, cont’d
• Nominal Pipe
Thicknesses:
Header: 0.562 in.
Branch: 0.375 in.
• Required Pipe
Thicknesses:
Header: 0.395 in.
Branch: 0.263 in.
• Branch connection at 90° angle
61
Notes:
Sample Problem 3 - Solution
d1 =
d1 =
Db − 2(Tb − c)
sin β
16 − 2 (0.375 × 0.875 − 0.0625 )
= 15.469 in.
sin 90°
A1 = thd1(2 − sinβ)
A1 = 0.395 × 15.469 (2 − sin90°) = 6.11in.2
62
Notes:
34
Sample Problem 3 Solution, cont’d
• Calculate excess area available in header, A2.
A 2 = (2d2−d1)(Th−th−c )
d2 = d1 = 15.469 in. < Dh = 24 in.
A2 = (2 × 15.469 - 15.469) (0.875 × 0.562 0.395 - 0.0625)
A2 = 0.53 in.2
63
Notes:
Sample Problem 3 Solution, cont’d
•
Calculate excess area available in branch,
•
A3.
A3 =
2L 4(Tb − tb−c )
sinβ
L 4 = 2.5 (0.875 × 0.375 − 0.0625 ) = 0.664 in.
A3 =
2 × 0.664 (0.875 × 0.375 − 0.263 − 0.0625 )
= 0.003 in.2
sin 90°
64
Notes:
35
Sample Problem 3 Solution, cont’d
• Calculate other excess area available, A4.
A4 = 0.
• Total Available Area:
AT = A2 + A3 + A4
AT = 0.53 + 0.003 + 0 = 0.533 in.2 available
reinforcement.
AT < A1
∴ Pad needed
65
Notes:
Sample Problem 3 Solution, cont’d
• Reinforcement pad: A106, Gr. B, 0.562 in. thick
• Recalculate Available Reinforcement
L41 = 2.5 (Th - c) = 2.5 (0.875 × 0.562 - 0.0625) =
1.073 in.
L42 = 2.5 (Tb - c) + Tr
= 2.5 (0.875 × 0.375 - 0.0625) + 0.562 (0.875) =
1.16 in
66
Notes:
36
Sample Problem 3 Solution, cont’d
Therefore, L4 = 1.073 in.
A3 =
2L 4 (Tb − t b − c)
sin β
A3 =
2 × 1.073 (0.875 × 0.375 − 0.263 − 0.0625 )
sin90 o
A 3 = 0.005 in.2 (vs. the 0.003 in.2 previously calculated )
A T = A 2 + A 3 + A 4 = 0.53 + 0.005 + 0 = 0.535 in.2
67
Notes:
Sample Problem 3 Solution, cont’d
• Calculate additional reinforcement required and
pad dimensions:
A4 = 6.11 - 0.535 = 5.575 in.2
Pad diameter, Dp is:
Tr = 0.562 (0.875) = 0.492 in.
Dp =
A 4 Db
5.575
+
=
+ 16 = 27.3
Tr sin β 0.492
Since 2d2 > Dp, pad diameter is acceptable
68
Notes:
37
Exercise 2 - Determine
Required Pipe Wall Thickness
•
•
•
•
•
•
•
Design Temperature: 260°F
Design Pressure: 150 psig
Pipe OD: 30 in.
Pipe material: A 106, Gr. B seamless
Corrosion allowance: 0.125
Mill tolerance: 12.5%
Thickness for internal pressure and
nominal thickness?
69
Notes:
Exercise 2 - Solution
• From Tables 6.1, 6.2, and 6.3 obtain values:
– S = 20,000 psi
– E = 1.0
– Y = 0.4
• Thickness calculation:
t=
PD
150 × 30
=
2(SE + PY ) 2[(20,000 × 1.0 ) + (150 × 0.04 )]
t = 0.112 in.
70
Notes:
38
Exercise 2 - Solution, cont’d
• Corrosion allowance calculation:
t m = t + CA = 0.112 + 0.125
t = 0.237 in.
• Mill tolerance calculation:
tm
0.237
=
0.875 0.875
= 0.271 in.
t nom =
t nom
71
Notes:
Layout Considerations
• Operational
– Operating and control points easily reached
• Maintenance
– Ample clearance for maintenance equipment
– Room for equipment removal
– Sufficient space for access to supports
• Safety
– Consider personnel safety
– Access to fire fighting equipment
72
Notes:
39
Pipe Supports and Restraints
• Supports
– Absorb system weight
– Reduce:
+ longitudinal pipe stress
+ pipe sag
+ end point reaction loads
• Restraints
– Control or direct thermal movement due to:
+ thermal expansion
+ imposed loads
73
Notes:
Support and Restraint
Selection Factors
•
•
•
•
•
•
Weight load
Available attachment clearance
Availability of structural steel
Direction of loads and/or movement
Design temperature
Vertical thermal movement at supports
74
Notes:
40
Rigid Supports
Shoe
Dummy Support
75
Base Adjustable
Support
Saddle
Trunnion
Figure 7.1
Notes:
Hangers
76
Figure 7.2
Notes:
41
Flexible Supports
Load and Deflection
Scale
Small Change in
Effective Lever Arm
Large Change in
Effective Lever Arm
Relatively
Constant
Load
Typical Variable-Load
Spring Support
Typical Constant-Load
Spring Support Mechanism
Figure 7.3
77
Notes:
Restraints
• Control, limit, redirect thermal movement
– Reduce thermal stress
– Reduce loads on equipment connections
• Absorb imposed loads
– Wind
– Earthquake
– Slug flow
– Water hammer
– Flow induced-vibration
78
Notes:
42
Restraints, cont’d
• Restraint Selection
– Direction of pipe movement
– Location of restraint point
– Magnitude of load
79
Notes:
Anchors and Guides
• Anchor
– Full fixation
– Permits very limited (if any) translation or
rotation
• Guide
– Permits movement along pipe axis
– Prevents lateral movement
– May permit pipe rotation
80
Notes:
43
Restraints - Anchors
Anchor
Anchor
Partial Anchor
Figure 7.4
81
Notes:
Restraints - Guides
Guide
Guide
x
Vertical Guide
82
Guide
Figure 7.5
Notes:
44
Piping Flexibility
• Inadequate flexibility
– Leaky flanges
– Fatigue failure
– Excessive maintenance
– Operations problems
– Damaged equipment
• System must accommodate thermal
movement
83
Notes:
Flexibility Analysis
• Considers layout, support, restraint
• Ensures thermal stresses and reaction
loads are within allowable limits
• Anticipates stresses due to:
84
– Elevated design temperatures
+ Increases pipe thermal stress and reaction
loads
+ Reduces material strength
– Pipe movement
– Supports and restraints
Notes:
45
Flexibility Analysis, cont’d
• Evaluates loads imposed on equipment
• Determines imposed loads on piping
system and associated structures
• Loads compared to industry standards
– Based on tables
– Calculated
85
Notes:
Design Factors
• Layout
• Component
design details
• Fluid service
• Connected
equipment type
• Operating
scenarios
• Pipe diameter,
thickness
• Design temperature
and pressure
• End-point movements
• Existing structural
steel locations
• Special design
considerations
86
Notes:
46
Equipment Nozzle Load
Standards and Parameters
Equipment Item
Parameters Used
To Determine
Acceptable Loads
Industry Standard
Centrifugal Pumps
API 610
Nozzle size
Centrifugal
Compressors
API 617, 1.85 times
Nozzle size, material
Air-Cooled Heat
Exchangers
API 661
NEMA SM-23
allowable
Nozzle size
Pressure Vessels, Shell- ASME Code Section
and-Tube Heat
VIII, WRC 107,
Exchanger Nozzles
WRC 297
Nozzle size, thickness,
reinforcement details,
vessel/exchanger diameter,
and wall thickness. Stress
analysis required.
Tank Nozzles
API 650
Nozzle size, tank diameter,
height, shell thickness, nozzle
elevation.
Steam Turbines
NEMA SM-23
Nozzle size
Table 7.1
87
Notes:
Computer Analysis
• Used to perform detailed piping stress
analysis
• Can perform numerous analyses
• Accurately completes unique and difficult
functions
88
– Time-history analyses
– Seismic and wind motion
– Support motion
– Finite element analysis
– Animation effects
Notes:
47
Computer Analysis Guidelines
Type Of Piping
General piping
Pipe Size, NPS
Maximum Differential
Flexibility Temp.
≥4
≥ 400°F
≥8
≥ 300°F
≥ 12
≥ 200°F
≥ 20
any
For rotating equipment
≥3
Any
For air-fin heat exchangers
≥4
Any
For tankage
≥ 12
Any
Table 7.2
89
Notes:
Piping Flexibility Temperature
• Analysis based on largest temperature
difference imposed by normal and
abnormal operating conditions
• Results give:
– Largest pipe stress range
– Largest reaction loads on connections,
supports, and restraints
• Extent of analysis depends on situation
90
Notes:
48
Normal Temperature
Conditions To Consider
Stable
Operation
Temperature range expected for most of time plant is
in operation. Margin above operating temperature
(i.e., use of design temperature rather than operating
temperature) allows for process flexibility.
Startup and
Shutdown
Determine if heating or cooling cycles pose flexibility
problems. For example, if tower is heated while
attached piping remains cold, piping flexibility should
be checked.
Regeneration
and Decoking
Piping
Spared
Equipment
Design for normal operation, regeneration, or
decoking, and switching from one service to the
other. An example is furnace decoking.
Requires multiple analyses to evaluate expected
temperature variations, for no flow in some of piping,
and for switching from one piece of equipment to
another. Common example is piping for two or more
pumps with one or more spares.
Table 7.3
91
Notes:
Abnormal Temperature
Conditions To Consider
Loss of Cooling
Medium Flow
Temperature changes due to loss of cooling medium
flow should be considered. Includes pipe that is
normally at ambient temperature but can be blocked
in, while subject to solar radiation.
Most on-site equipment and lines, and many off-site
lines, are freed of gas or air by using steam. For 125
psig steam, 300°F is typically used for metal
temperature. Piping connected to equipment which
Steamout for Air will be steamed out, especially piping connected to
or Gas Freeing upper parts of towers, should be checked for tower at
300°F and piping at ambient plus 50°F. This may
govern flexibility of lines connected to towers that
operate at less than 300°F or that have a smaller
temperature variation from top to bottom.
If process flow can be stopped while heat is still being
No Process Flow
applied, flexibility should be checked for maximum
While Heating
metal temperature. Such situations can occur with
Continues
steam tracing and steam jacketing.
92
Table 7.4
Notes:
49
Extent of Analysis
• Extent depends on situation
• Analyze right combination of conditions
• Not necessary to include system sections
that are irrelevant to analysis results
93
Notes:
Modifying System Design
•
•
•
•
Provide more offsets or bends
Use more expansion loops
Install expansion joints
Locate restraints to:
– Minimize thermal and friction loads
– Redirect thermal expansion
• Use spring supports to reduce large
vertical thermal loads
• Use Teflon bearing pads to reduce friction
loads
94
Notes:
50
System Design Considerations
• Pump systems
– Operating vs. spared pumps
• Heat traced piping systems
– Heat tracing
+ Reduces liquid viscosity
+ Prevents condensate accumulation
– Tracing on with process off
95
Notes:
System Design
Considerations, cont’d
• Atmospheric storage tank
– Movement at nozzles
– Tank settlement
• Friction loads at supports and restraints
– Can act as anchors or restraints
– May cause high pipe stresses or reaction loads
• Air-cooled heat exchangers
– Consider header box and bundle movement
96
Notes:
51
Tank Nozzle
SHELL
NOZZLE
BOTTOM
Figure 7.6
97
Notes:
Welding
•
•
•
•
Welding is primary way of joining pipe
Provides safety and reliability
Qualified welding procedure and welders
Butt welds used for:
– Pipe ends
– Butt-weld-type flanges or fittings to pipe ends
– Edges of formed plate
98
Notes:
52
Butt-Welded Joint Designs
Equal Thickness
(a) Standard End Preparation
of Pipe
(b) Standard End Preparation
of Butt-Welding Fittings and
Optional End Preparation of
Pipe 7/8 in. and Thinner
(c) Suggested End Preparation,
Pipe and Fittings Over 7/8 in.
Thickness
Figure 8.1
99
Notes:
Butt-Welded Joint Designs
Unequal Thickness
3/32 in. max.
(a)
(b)
(c)
(d)
100
Figure 8.2
Notes:
53
Fillet Welds
Figure 8.3
101
Notes:
Weld Preparation
• Welder and equipment must be qualified
• Internal and external surfaces must be
clean and free of paint, oil, rust, scale, etc.
• Ends must be:
– Suitably shaped for material, wall thickness,
welding process
– Smooth with no slag from oxygen or arc
cutting
102
Notes:
54
Preheating
• Minimizes detrimental effects of:
– High temperature
– Severe thermal gradients
• Benefits include:
– Dries metal and removes surface moisture
– Reduces temperature difference between
base metal and weld
– Helps maintain molten weld pool
– Helps drive off absorbed gases
103
Notes:
Postweld Heat Treatment
(PWHT)
• Primarily for stress relief
– Only reason considered in B31.3
• Averts or relieves detrimental effects
– Residual stresses
+ Shrinkage during cooldown
+ Bending or forming processes
– High temperature
– Severe thermal gradients
104
Notes:
55
Postweld Heat Treatment
(PWHT), cont’d
• Other reasons for PWHT to be specified
by user
– Process considerations
– Restore corrosion resistance of normal
grades of stainless steel
– Prevent caustic embrittlement of carbon steel
– Reduce weld hardness
105
Notes:
Storage and Handling
• Store piping on mounds or sleepers
• Stacking not too high
• Store fittings and valves in shipping crates
or on racks
• End protectors firmly attached
• Lift lined and coated pipes and fittings with
fabric or rubber covered slings and
padding
106
Notes:
56
Pipe Fitup and Tolerances
• Good fitup essential
– Sound weld
– Minimize loads
• Dimensional tolerances
• Flange tolerances
107
Notes:
Pipe Alignment
Load Sensitive Equipment
• Special care and tighter tolerances needed
• Piping should start at nozzle flange
– Initial section loosely bolted
– Gaskets used during fabrication to be replaced
• Succeeding pipe sections bolted on
• Field welds to join piping located near
machine
108
Notes:
57
Load Sensitive Equipment,
cont’d
• Spring supports locked in cold position
during installation and adjusted in locked
position later
• Final bolt tensioning follows initial
alignment of nozzle flanges
• Final nozzle alignment and component
flange boltup should be completed after
replacing any sections removed
109
Notes:
Load Sensitive Equipment,
cont’d
• More stringent limits for piping > NPS 3
• Prevent ingress of debris during
construction
110
Notes:
58
Flange Joint Assembly
• Primary factors
– Selection
– Design
– Preparation
– Inspection
– Installation
• Identify and control causes of leakage
111
Notes:
Flange Preparation,
Inspection, and Installation
•
•
•
•
•
•
Redo damaged surfaces
Clean faces
Align flanges
Lubricate threads and nuts
Place gasket properly
Use proper flange boltup procedure
112
Notes:
59
“Criss-Cross”
Bolt-tightening Sequence
Figure 8.4
113
Notes:
Causes of Flange Leakage
•
•
•
•
•
•
•
•
Uneven bolt stress
Improper flange alignment
Improper gasket centering
Dirty or damaged flange faces
Excessive loads at flange locations
Thermal shock
Improper gasket size or material
Improper flange facing
114
Notes:
60
Inspection
• Defect identification
• Weld inspection
– Technique
– Weld type
– Anticipated type of defect
– Location of weld
– Pipe material
115
Notes:
Typical Weld Imperfections
Lack of Fusion Between Weld Bead and Base Metal
a) Side Wall Lack of Fusion
b) Lack of Fusion Between
Adjacent Passes
Incomplete Filling at Root on One Side Only
c) Incomplete Penetration Due
to Internal Misalignment
Incomplete Filling at Root
d) Incomplete Penetration of
Weld Groove
External Undercut
Root Bead Fused to Both Inside
Surfaces but Center of Root Slightly
Below Inside Surface of Pipe (Not
Incomplete Penetration)
Internal Undercut
e) Concave Root Surface
(Suck-Up)
f) Undercut
g) Excess External Reinforcement
116
Figure 9.1
Notes:
61
Weld Inspection Guidelines
Type of Inspection
Visual
Radiography
Magnetic Particle
Liquid Penetrant
Ultrasonic
Situation/Weld Type
All welds.
Defect
•
Minor structural welds.
•
Cracks.
•
Slag inclusions.
•
Butt welds.
•
Gas pockets.
•
Girth welds.
•
Slag inclusions.
•
Miter groove welds.
•
Incomplete penetration.
•
Ferromagnetic
materials.
•
Cracks.
For flaws up to 6 mm
(1/4 in.) beneath the
surface.
•
Porosity.
•
•
Lack of fusion.
•
Ferrous and
nonferrous materials.
•
Cracks.
Seams.
•
Intermediate weld
passes.
•
•
Porosity.
•
Weld root pass.
•
Folds.
•
Simple and
inexpensive.
•
Inclusions.
•
Shrinkage.
Confirms high weld
quality in pressurecontaining joints.
•
Surface defects.
•
Laminations.
•
Slag inclusions in thick
plates.
•
Subsurface flaws.
Table 9.1
117
Notes:
Testing
• Pressure test system to demonstrate
integrity
• Hydrostatic test unless pneumatic
approved for special cases
• Hydrostatic test pressure
– ≥ 1½ times design pressure
118
Notes:
62
Testing, cont’d
– For design temperature > test temperature:
PT =
1. 5 P S T
S
ST/S must be ≤ 6.5
PT
P
ST
S
= Minimum hydrostatic test pressure, psig
= Internal design pressure, psig
= Allowable stress at test temperature, psi
= Allowable stress at design temperature, psi
119
Notes:
Testing, cont’d
• Pneumatic test at 1.1P
• Instrument take-off piping and sampling
piping strength tested with connected
equipment
120
Notes:
63
Nonmetallic Piping
• Thermoplastic Piping
– Can be repeatedly softened and hardened by
increasing and decreasing temperature
• Reinforced Thermosetting Resin Piping
(RTR)
– Fabricated from resin which can be treated to
become infusible or insoluble
121
Notes:
Nonmetallic Piping, cont’d
• No allowances for pressure or temperature
variations above design conditions
• Most severe coincident pressure and
temperature conditions determine design
conditions
122
Notes:
64
Nonmetallic Piping, cont’d
• Designed to prevent movement from
causing:
– Failure at supports
– Leakage at joints
– Detrimental stresses or distortions
• Stress-strain relationship inapplicable
123
Notes:
Nonmetallic Piping, cont’d
• Flexibility and support requirement same
as for piping in normal fluid service. In
addition:
– Piping must be supported, guided, anchored
to prevent damage.
– Point loads and narrow contact areas avoided
– Padding placed between piping and supports
– Valves and load transmitting equipment
supported independently to prevent excessive
loads.
124
Notes:
65
Nonmetallic Piping, cont’d
• Thermoplastics not used in flammable
service, and safeguarded in most fluid
services.
• Joined by bonding
125
Notes:
Category M Fluid Service
Category M Fluid
• Significant potential for personnel
exposure
• Single exposure to small quantity can
cause irreversible harm to breathing or
skin.
126
Notes:
66
Category M Fluid Service, cont’d
• Requirements same as for piping in
normal fluid service. In addition:
– Design, layout, and operation conducted with
minimal impact and shock loads.
– Detrimental vibration, pulsation, resonance
effects to be avoided or minimized.
– No pressure-temperature variation
allowances.
127
Notes:
Category M Fluid Service, cont’d
– Most severe coincident pressure-temperature
conditions determine design temperature and
pressure.
– All fabrication and joints visually examined.
– Sensitive leak test required in addition to
other required testing.
128
Notes:
67
Category M Fluid Service, cont’d
• Following may not be used
– Miter bends not designated as fittings,
fabricated laps, nonmetallic fabricated branch
connections.
– Nonmetallic valves and specialty components.
– Threaded nonmetallic flanges.
– Expanded, threaded, caulked joints.
129
Notes:
High Pressure Piping
• Ambient effects on design conditions
– Pressure reduction based on cooling of gas or
vapor
– Increased pressure due to heating of a static
fluid
– Moisture condensation
130
Notes:
68
High Pressure Piping,
cont’d
• Other considerations
– Dynamic effects
– Weight effects
– Thermal expansion and contraction effects
– Support, anchor, and terminal movement
131
Notes:
High Pressure Piping,
cont’d
• Testing
– Each system hydrostatically or pneumatically
leak tested
– Each weld and piping component tested
– Post installation pressure test at 110% of
design pressure if pre-installation test was
performed
• Examination
132
– Generally more extensive than normal fluid
service
Notes:
69
Summary
• Process plant piping much more than just
pipe
• ASME B31.3 covers process plant piping
• Covers design, materials, fabrication,
erection, inspection, and testing
• Course provided overview of requirements
133
Notes:
70
Part 2:
Background Material
71
OVERVIEW OF PROCESS PLANT PIPING SYSTEM DESIGN
Carmagen Engineering, Inc.
72
I.
INTRODUCTION
This course provides an overview of process plant piping system design. It
discusses requirements contained in ASME B31.3, Process Piping, plus
additional requirements and guidelines based on common industry practice. The
information contained in this course is readily applicable to on-the-job
applications, and prepares participants to take more extensive courses if
appropriate.
II.
GENERAL
A.
What is a piping system
A piping system conveys fluid from one location to another. Within
a process plant, the locations are typically one or more equipment
items (e.g., pumps, pressure vessels, heat exchangers, process
heaters, etc.), or individual process plants that are within the
boundary of a process facility.
A piping system consists of:
•
Pipe sections
•
Fittings (e.g., elbows, reducers, branch connections, etc.)
•
Flanges, gaskets, and bolting
•
Valves
•
Pipe supports and restraints
Each individual component plus the overall system must be
designed for the specified design conditions.
B.
Scope of ASME B31.3
ASME B31.3 specifies the design, materials, fabrication, erection,
inspection, and testing requirements for process plant piping
systems. Process plants include petroleum refineries; chemical,
pharmaceutical, textile, paper, semiconductor, and cryogenic
plants; and related process plants and terminals.
73
ASME B31.3 applies to piping and piping components that are used
for all fluid services, not just hydrocarbon services. These include
the following:
•
Raw, intermediate, and finished chemicals.
•
Petroleum products.
•
Gas, steam, air, and water.
•
Fluidized solids.
•
Refrigerants.
•
Cryogenic fluids.
The scope also includes piping that interconnects pieces or stages
within a packaged-equipment assembly.
The following are excluded from the scope of ASME B31.3:
•
Piping systems for internal gauge pressures at or above zero
but less than 15 psi, provided that the fluid is nonflammable,
nontoxic, and not damaging to human tissue, and its design
temperature is from -20°F through 366°F.
•
Power boilers that are designed in accordance with the ASME
Boiler and Pressure Vessel Code Section I and external boiler
piping that must conform to ASME B31.1.
•
Tubes, tube headers, crossovers, and manifolds that are
located inside a fired heater enclosure.
•
Pressure vessels, heat exchangers, pumps, compressors, and
other fluid-handling or processing equipment. This includes
both internal piping and connections for external piping.
74
III.
MATERIAL SELECTION CONSIDERATIONS
Piping system material selection considerations are discussed below.
A.
Strength
A material's strength is defined by its yield, tensile, creep, and
fatigue strengths. Alloy content, material grain size, and the steel
production process are factors that affect material strength.
1.0
Yield and Tensile Strength
A stress-strain diagram that is produced from a standard
tensile test (Figure 3.1) illustrates the yield and tensile
strengths. As the stress in a material increases, its
deformation also increases. The yield strength is the stress
that is required to produce permanent deformation in the
material (Point A in Figure 3.1).
If the stress is further increased, the permanent deformation
continues to increase until the material fails. The maximum
stress that the material attains is the tensile strength (Point B
in Figure 3.1). If a large amount of strain occurs in going
from Point A to Point C, the rupture point, the material is said
to be ductile. Steel is an example of a ductile material. If the
strain in going from Point A to Point C is small, the material
is brittle. Gray cast iron is an example of a brittle material.
B
S
A
C
E
Typical Stress-Strain Diagram for Steel
Figure 3.1
75
2.0
Creep Strength
Below about 750°F for a given stress, the strain in most
materials remains constant with time. Above this
temperature, even with constant stress, the strain in the
material will increase with time. This behavior is known as
creep. The creep strength, like the yield and tensile
strengths, varies with temperature. For a particular
temperature, the creep strength of a material is the minimum
stress that will rupture the material during a specified period
of time.
The temperature at which creep strength begins to be a
factor is a function of material chemistry. For alloy materials
(i.e., not carbon steel) creep strength becomes a
consideration at temperatures higher than 750°F.
3.0
Fatigue Strength
The term “fatigue” refers to the situation where a specimen
breaks under a load that it has previously withstood for a
length of time, or breaks during a load cycle that it has
previously withstood several times. The first type of fatigue
is called “static,” and the second type is called “cyclic.”
Examples of static fatigue are: creep fracture and stress
corrosion cracking. Static fatigue will not be discussed
further in this course.
One analogy to cyclic fatigue is the bending of a paper clip.
The initial bending beyond a certain point causes the paper
clip to yield (i.e., permanently deform) but not break. The
clip could be bent back and forth several more times and still
not break. However after a sufficient number of bending
(i.e., load) cycles, the paper clip will break under this
repetitive loading. Purely elastic deformation (i.e., without
yielding) cannot cause a cyclic fatigue failure.
The fatigue strength of a material under cyclic loading can
then be defined as the ability to withstand repetitive loading
without failure. The number of cycles to failure of a material
decreases as the stress resulting from the applied load
increases.
76
B.
Corrosion Resistance
Corrosion of materials involves deterioration of the metal by
chemical or electrochemical attack. Corrosion resistance is usually
the single most important factor that influences pipe material
selection. Table 3.1 summarizes the typical types of piping system
corrosion.
General or Uniform
Corrosion
Characterized by uniform metal loss over entire surface of material.
May be combined with erosion if material is exposed to high-velocity
fluids, or moving fluids that contain abrasive materials.
Pitting
Corrosion
Form of localized metal loss randomly located on material surface.
Occurs most often in stagnant areas or areas of low-flow velocity.
Galvanic Corrosion
Occurs when two dissimilar metals contact each other in corrosive
electrolytic environment. The anodic metal develops deep pits or
grooves as a current flows from it to the cathodic metal.
Crevice Corrosion
Localized corrosion similar to pitting. Occurs at places such as
gaskets, lap joints, and bolts, where a crevice can exist.
Concentration Cell
Corrosion
Occurs when different concentration of either corrosive fluid or
dissolved oxygen contacts areas of same metal. Usually associated
with stagnant fluid.
Graphitic Corrosion Occurs in cast iron exposed to salt water or weak acids. Reduces
iron in the cast iron and leaves the graphite in place. Result is
extremely soft material with no metal loss.
Typical Types of Piping System Corrosion
Table 3.1
For process plant piping systems in corrosive service, corrosion
protection is usually achieved by using alloys that resist corrosion.
The most common alloys used for this purpose are chromium and
nickel. Low-alloy steels with a chromium content of 1¼% to 9%
and stainless steels are used in corrosive environments.
C.
Material Fracture Toughness
One way to characterize the fracture behavior of a material is the
amount of energy necessary to initiate and propagate a crack at a
given temperature. This is the material's fracture toughness, which
77
decreases as the temperature decreases. Tough materials require
a relatively large amount of energy to initiate and propagate a
crack. The impact energy required to fracture a material sample at
a given temperature can be measured by standard Charpy V-notch
tests.
Various factors other than temperature affect the fracture
toughness of a material. These include the following:
•
Chemical composition or alloying elements.
•
Heat treatment.
•
Grain size.
The major chemical elements that affect a material's fracture
toughness are carbon, manganese, nickel, oxygen, sulfur, and
molybdenum. High carbon content, or excessive amounts of
oxygen, sulfur, or molybdenum, hurts fracture toughness. The
addition of manganese or nickel improves fracture toughness.
D.
Fabricability
A material must be available in the shapes or forms that are
required, and it typically must be weldable. In piping systems,
some common shapes and forms include the following:
E.
•
Seamless pipe.
•
Plate that is used for welded pipe.
•
Wrought or forged elbows, tees, reducers, and crosses.
•
Forged flanges, couplings, and valves.
•
Cast valves.
Availability and Cost
The last factors that affect piping material selection are availability
and cost. Where there is more than one technically acceptable
material, the final selection must consider what is readily available
and what are the relative costs of the acceptable options. For
example, the use of carbon steel with a large corrosion allowance
could be more expensive than using a low-alloy material with a
smaller corrosion allowance.
78
IV.
PIPING COMPONENTS
A.
Fittings, Flanges, and Gaskets
1.0
Pipe Fittings
Fittings are used to make some change in the geometry of a
piping system. This change could include:
•
Modifying the flow direction.
•
Bringing two or more pipes together.
•
Altering the pipe diameter.
•
Terminating a pipe.
The most common types of fittings are elbows, tees,
reducers, welding outlets, pipe caps, and lap joint stub ends.
These are illustrated in Figures 4.1 through 4.6. Fittings may
be attached to pipe by threading, socket welding, or butt
welding.
An elbow or return (Figure 4.1) changes the direction of a
pipe run. Standard elbows change the direction by either
45° or 90°. Returns change the direction by 180°.
90°
45°
180° Return
Elbow and Return
Figure 4.1
79
A tee (Figure 4.2) provides for the intersection of three
sections of pipe.
•
A straight tee has equal diameters for both the run and
branch pipe connections.
•
A reducing-outlet tee has a branch diameter which is
smaller in size than the run diameter.
•
A cross permits the intersection of four sections of pipe
and is rarely seen in process plants.
Tee
Figure 4.2
A reducer (illustrated in Figure 4.3) changes the diameter in
a straight section of pipe. The centerlines of the large and
small diameter ends coincide in a concentric reducer,
whereas they are offset in an eccentric type.
Concentric
Eccentric
Reducer
Figure 4.3
A welding outlet fitting, or integrally reinforced branch
connection (Figure 4.4) has all the reinforcement required to
strengthen the opening contained within the fitting itself.
80
Typical Integrally Reinforced Branch Connection
Figure 4.4
A pipe cap (Figure 4.5) closes off the end of a pipe section.
The wall thickness of a butt-welded pipe cap will typically be
identical to that of the adjacent pipe section.
Cap
Figure 4.5
A lap-joint stub end (Figure 4.6) is used in conjunction with
lap-joint flanges.
Note square corner
R
R
Enlarged Section
of Lap
Lap-Joint Stub End
Figure 4.6
81
2.0
Flanges
A flange connects a pipe section to a piece of equipment,
valve, or another pipe such that relatively simple
disassembly is possible. Disassembly may be required for
maintenance, inspection, or operational reasons. Figure 4.7
shows a typical flange assembly. Flanges are normally used
for pipe sizes above NPS 1½.
Flange
Bolting
Gasket
Typical Flange Assembly
Figure 4.7
A flange type is specified by stating the type of attachment
and the type of face. The type of attachment defines how
the flange is connected to a pipe section or piece of
82
equipment (e.g., welded). The type of flange face or facing
defines the geometry of the flange surface that contacts the
gasket. Table 4.1 summarizes the types of flange
attachments and faces. Figure 4.8 illustrates flange facing
types.
Flange Attachment Types
Flange Facing Types
Threaded Flanges
Flat Faced
Socket-Welded Flanges
Blind Flanges
Raised Face
Slip-On Flanges
Lapped Flanges
Ring Joint
Weld Neck Flanges
Types of Flange Attachment and Facing
Table 4.1
83
Flange Facing Types
Figure 4.8
84
3.0
Gaskets
A gasket is a resilient material that is inserted between the
flanges and seated against the portion of the flanges called
the “face” or “facing”. The gasket provides the seal between
the fluid in the pipe and the outside, and thus prevents
leakage. Bolts compress the gasket to achieve the seal and
hold the flanges together against pressure and other
loadings.
The three gasket types typically used in pipe flanges for
process plant applications are:
B.
•
Sheet.
•
Spiral wound.
•
Solid metal ring.
Flange Rating
ASME B16.5, Pipe Flanges and Flanged Fittings, provides steel
flange dimensional details for standard pipe sizes through NPS 24.
Specification of an ASME B16.5 flange involves selection of the
correct material and flange "Class." The paragraphs that follow
discuss the flange class specification process in general terms.
Flange material specifications are listed in Table 1A in ASME B16.5
(excerpted in Table 4.2). The material specifications are grouped
within Material Group Numbers. For example, if the piping is
fabricated from carbon steel, the ASTM A105 material specification
is often used. ASTM A105 material is in Material Group No. 1.1.
Refer to ASME B16.5 for additional acceptable material
specifications and corresponding Material Group Numbers.
85
ASME B16.5, Table 1A, Material Specification List (Excerpt)
Table 4.2
After the Material Group has been determined, the next step is to
select the appropriate Class. The Class is determined by using
pressure/temperature rating tables, the Material Group, design
metal temperature, and design pressure. Selecting the Class sets
all the detailed dimensions for flanges and flanged fittings. The
objective is to select the lowest Class that is appropriate for the
specified design conditions.
Table 2 of ASME B16.5 provides the information that is necessary
to select the appropriate flange Class for the specified design
conditions. ASME B16.5 has seven classes: Class 150, 300, 400,
600, 900, 1,500, and 2,500. Each Class specifies the design
pressure and temperature combinations that are acceptable for a
flange with that designation. As the number of the Class increases,
the strength of the flange increases for a given Material Group. A
higher flange Class can withstand higher pressure and temperature
combinations. Table 4.3 is an excerpt from Table 2 of ASME B16.5
and shows some of the temperature and pressure ratings for
several Material Groups. Material and design temperature
combinations that do not have a pressure indicated are not
acceptable.
Specifying the flange size, material, and class completes most of
what is necessary for selecting an ASME B16.5 flange. The flange
type, facing, bolting material, and gasket type and material must be
86
added to complete the flange selection process. Discussion of
these other factors is beyond the scope of this course.
Material Group
No.
Classes
Temp., °F
-20 to 100
200
300
400
500
600
650
700
750
800
850
900
950
1000
1.8
150
235
220
215
200
170
140
125
110
95
80
65
50
35
20
300
620
570
555
555
555
555
555
545
515
510
485
450
320
215
1.9
400
150
825
765
745
740
740
740
740
725
685
675
650
600
425
290
300
290
260
230
200
170
140
125
110
95
80
65
50
35
20
750
750
720
695
695
605
590
570
530
510
485
450
320
215
1.10
400
1000
1000
965
885
805
785
785
710
675
650
600
425
290
190
150
290
260
230
200
170
140
125
110
95
80
65
50
35
20
300
750
750
730
705
665
605
590
570
530
510
485
450
375
260
400
1000
1000
970
940
885
805
785
755
710
675
650
600
505
345
ASME B16.5, Pressure-Temperature Ratings (Excerpt)
Table 4.3
87
SAMPLE PROBLEM 1 - DETERMINE FLANGE RATING
A new piping system will be installed at an existing plant. It is necessary to
determine the ASME class that is required for the flanges. The following design
information is provided:
•
Pipe Material: 1¼ Cr – ½ Mo.
•
Design Temperature: 700°F.
•
Design Pressure: 500 psig.
SOLUTION
Determine the Material Group Number for the flanges by referring to ASME Table
1A (excerpted in Table 4.2). Find the 1¼ Cr – ½ Mo material in the Nominal
Designation Steel column. The material specification for forged flanges would be
A182 Gr. F11, and the corresponding material Group Number is 1.9.
Refer to Table 2 for Class 150 (excerpted in Table 4.3). Read the allowable
design pressure at the intersection of the 700°F design temperature and Material
Group 1.9. This is only 110 psig and is not enough for this service.
Now check Class 300 and do the same thing. The allowable pressure in this
case is 570 psig, which is acceptable.
The required flange Class is 300.
88
V.
VALVES
A.
Valve Functions
The possible valve functions must be known before being able to
select the appropriate valve type for a particular application. Fluid
flows through a pipe, and valves are used to control the flow. A
valve may be used to block flow, throttle flow, or prevent flow
reversal.
1.0
Blocking Flow
The block-flow function provides completely on or completely
off flow control of a fluid, generally without throttling or
variable control capability. It might be necessary to block
flow to take equipment out of service for maintenance while
the rest of the unit remains in operation, or to separate two
portions of a single system to accommodate various
operating scenarios.
2.0
Throttling Flow
Throttling may increase or decrease the amount of fluid
flowing in the system and can also help control pressure
within the system. It might be necessary to throttle flow to
regulate the filling rate of a pressure vessel, or to control unit
operating pressure levels.
3.0
Preventing Flow Reversal
It might be necessary to automatically prevent fluid from
reversing its direction during sudden pressure changes or
system upsets. Preventing reverse flow might be necessary
to avoid damage to a pump or a compressor, or to
automatically prevent backflow into the upstream part of the
system due to process reasons.
89
B.
Primary Valve Types
1.0
Gate Valve
Most valves in process plants function as block valves.
About 75% of all valves in process plants are gate valves.
The gate valve is an optimum engineering and economic
choice for on or off service. The gate valve is not suitable to
throttle flow because it will pass the maximum possible flow
while it is only partially open. Figure 5.1 illustrates a typical
full-port gate valve.
90
1.
Handwheel Nut
2.
Handwheel
3.
Stem Nut
4.
Yoke
5.
Yoke Bolting
6.
Stem
7.
Gland Flange
8.
Gland
9.
Gland Bolts or
Gland-Eye Bolts
and Nuts
10. Gland Lug Bolts
and Nuts
11. Stem Packing
12. Plug
13. Lantern Ring
14. Backseat Bushing
15. Bonnet
16. Bonnet Gasket
17. Bonnet Bolts and
Nuts
18. Gate
19. Seat Ring
20. Body
21. One-Piece Gland
(Alternate)
22. Valve Port
Full-Port Gate Valve
Figure 5.1
2.0
Globe Valve
The globe valve is the type most commonly used to throttle
flow in a process plant. In the smaller sizes, they are
91
typically used as hand-control valves. In larger sizes,
applications are limited primarily to bypasses at control valve
stations. They provide relatively tight shutoff in control valve
bypasses during normal operations; they serve as temporary
flow controllers when control valves must be taken out of
service.
Because all globe valve patterns involve a change in flow
direction, they are not suitable for piping systems that
require scraping or rodding. Globe valves are rarely used for
strictly on/off block valve operations because conventional
gate valves adequately serve that function at a lower cost
and a much lower pressure drop.
3.0
Check Valve
Check valves prevent flow reversal. Typical check valve
applications are in pump and compressor discharge piping
and other systems that require protection against backflow.
Valves which contain a disc or discs that swing out of the
flow passage area usually create a lower pressure drop in
the system than those which contain a ball or piston
element. These latter elements remain in the flowstream
and the port configurations frequently include an angular
change in flow direction. For all process designs, the
intended purpose of check valves is to prevent gross flow
reversal, not to effect complete leakage-free, pressure-tight
shutoff of reverse flow.
The selection of a particular check valve type generally
depends on size, cost, availability, and service. Ball and lift
check valves are usually the choice for sizes NPS 2 and
smaller, while swing check and plate check valves are used
in the larger sizes.
3.1
Swing Check Valve
The main components of a swing check valve (Figure
5.2) are the body, disc, cap, seat ring, disc hinge, and
pin. The disc is hinged at the top and closes against
a seat in the valve body opening. It swings freely in
an arc from the fully closed position to one that
provides unobstructed flow. The valve is kept open
by the flow, and disc seating is accomplished by
gravity and/or flow reversal.
92
Cap
Pin
Seat
Ring
Hinge
Flow
Direction
Disc
Body
Swing Check Valve
Figure 5.2
3.2
Ball Check Valve
The ball check valve utilizes a ball to prevent flow
reversal (Figure 5.3). The basic types are the
straight-through- and globe-type (90° change in
direction, similar to a typical globe valve body). Ball
check valves are available in sizes NPS ½ through 2
in all ratings and materials used in process plants.
Their low cost usually makes them the first choice for
valves sized NPS 2 and smaller, provided the
pressure drop is not a concern.
93
Ball Check Valve
Figure 5.3
3.3
Lift Check Valve
A lift check valve (Figure 5.4) usually depends on
gravity for operation. Under forward flow, a piston or
disc is lifted off the seat by the fluid while being
retained in the valve by guides. On reverse flow, the
piston or disc is forced against the seat to block
further flow. Some lift check valves utilize spring
loading to assure positive seating.
Lift check valves employing the disc- or piston-type
mechanism are available in sizes from NPS ½
through 2 in all ratings and materials used in process
plants. They are most commonly used in the higher
ASME B16.5 ratings (Class 300 and greater), and
where tighter shutoff is required. Valves of this type
should only be used in clean services.
94
Seat
Ring
Piston
Flow
Direction
Lift Check Valve
Figure 5.4
3.4
Wafer Check Valve
The wafer body or flangeless valve is a valve body
without flanges (Figure 5.5). Valves of this type are
placed between pipe flanges and held in place by the
compressive force between the flanges and
transmitted through the gaskets. The lug-wafer (or
single-flanged) valve is also shown in Figure 5.5.
Valves of this type are mounted between pipe flanges
and are held in place by cap screws, machine bolts,
or stud bolts which thread into the valve body.
95
Figure 5.5
3.5
Ball Valve
Ball valves (Figure 5.6) usually function as block
valves. Ball valves are well suited for conditions
where quick on/off and/or bubble-tight shut-off is
required. The pressure/temperature ratings for ball
valve soft seats above ambient temperatures are
usually lower than the ASME ratings for steel valves.
This is because of the lower physical properties of the
soft-seat materials. Soft-sealed ball valves are not
normally used for throttling service because the softseats are subject to erosion or distortion/displacement
caused by fluid flow when the valve is in the partially
open position.
96
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Part Names
Body
Body Cap
Ball
Body Seal Gasket
Seat
Stem
Gland Flange
Stem Packing
Gland Follower
Thrust Bearing
Thrust Washer
Indicator Stop
Snap Ring
Gland Bolt
Stem Bearing
Body Stud Bolt & Nuts
Gland Cover
Gland Cover Bolts
Handle
Ball Valve
Figure 5.6
3.6
Plug Valve
Plug valves (Figure 5.7) usually function as block
valves. They are well suited for conditions where
quick on/off and/or bubble-tight shutoff is required.
The soft-seal-types may have lower
temperature/pressure ratings than the ASME ratings
for steel valves because of the lesser physical
properties of the soft-seat materials. Soft-seal plug
valves are not normally used for throttling service
since the soft seals are subject to erosion or
distortion/displacement caused by fluid flow when the
valve is partially open.
97
Wedge
Molded-In Resilient Seal
Sealing Slip
Plug Valve
Figure 5.7
C.
Valve Selection Process
The steps that follow provide a general procedure for selecting
valves and valve components.
1. Identify the necessary design information. This includes design
pressure and temperature, valve function, material, etc.
2. Identify potentially appropriate valve types (i.e., ball, butterfly,
check, etc.) and components based on application and function
(i.e., block, throttle, or reverse flow prevention).
3. Determine valve application requirements (i.e., design or service
limitations).
4. Finalize valve selection. Check which factors need consideration if
two or more valves are suitable.
5. Provide a full technical description. This is done by specifying the
valve type, material, flange rating, etc.
98
Exercise 1 – Determine Required Flange Rating
For the piping system described below, determine the required flange rating (or
Class) in accordance with ASME B16.5.
Pipe:
1¼ Cr – ½ Mo
Flanges:
A - 182 Gr. F11
Design Temperature:
900°F
Design Pressure:
375 psig
99
VI.
DESIGN
A.
Design Conditions
1.0
General
Normal operating conditions are those expected to occur
during normal operation, excluding failure of any operating
device, operator error, and the occasional, short-term
variations stated in the applicable code. Startup and
controlled shutdown of plants and similar foreseeable
events are included within normal operation.
Design conditions are those which govern the design and
selection of piping components, and are based on the most
severe conditions expected to occur in service. A suitable
margin is used between the normal operating and design
conditions to account for normal operating variations.
ASME B31.3 does not specify what margins should be used
between operating and design conditions; suitable margins
are determined by the user based on his experience.
2.0
Determining Design Pressure and Temperature
The design pressure and temperature are used to calculate
the required thickness of pipe and other design details. The
design temperature is used to determine the material basic
allowable stress and other design requirements. The values
for design pressure and temperature are based on process
requirements.
Piping system design conditions generally are determined
based on the design conditions of the equipment to which
the piping is attached. Determining the piping design
conditions consists of:
1. Identifying the equipment to which the piping system is
attached.
2. Determining the design pressure and design temperature
for the equipment.
100
3. Considering contingent design conditions, such as upsets
not protected by pressure-relieving devices.
4. Considering the direction of flow between the equipment.
5. Verifying the values with the process engineer.
B.
Loads and Stresses
1.0
Classification of Loading Conditions
Pipe loads are classified into three principal types: sustained
loads, thermal expansion loads, and occasional loads.
Sustained loads are those that act on the piping system
during all or most of its operating time. Sustained loads
consist of two main categories: pressure and weight. The
pressure load (caused by the design pressure) usually refers
to internal pressure, although some piping systems may also
be designed for external pressure. Design pressure is
defined as the maximum sustained pressure that a piping
system must contain without exceeding its allowable stress
limits. Design pressure is normally the governing factor in
determining the minimum required pipe wall thickness.
As shown in Figure 6.1, internal pressure produces both
circumferential (i.e., hoop) stress and longitudinal stress in
the pipe wall.
101
Sl
Sc
P
t
Sl
=
Longitudinal Stress
Sc
=
Circumferential (Hoop) Stress
t
=
Wall Thickness
P
=
Internal Pressure
Stresses Produced By Internal Pressure
Figure 6.1
The weight refers to the total design weight load. The total
weight load includes the weight of the pipe, the fluid in the
pipe, fittings, insulation, internal lining, valves, valve
operators, flanges, supports and any other concentrated
loads. The weight loads produce a longitudinal stress in the
pipe wall.
A piping system will expand or contract due to changes in its
operating temperature. Thermal expansion loads are
created when the free expansion and contraction of the
piping is prevented at its end points by connected
equipment, or prevented at intermediate points by supports
and/or restraints that are installed. The resulting loads
cause thermal stresses in the pipe. Increasing the restraint
in a system increases the loading and results in higher
thermal expansion stresses. Another cause of pipe thermal
loads can be from the thermal expansion of equipment at
102
pipe-to-equipment nozzle attachment points, causing
displacements in the piping system.
The third type of loading comes from occasional loads.
Occasional loads act during a small percentage of the
system’s operating time. Occasional loads involve seismic
and/or dynamic loading. The degree of seismic loading that
must be considered varies with geographic location and is
defined by a seismic zone (Ref. ANSI/ASCE 7). Dynamic
loads may be caused by safety-relief valve discharges, valve
operation (both opening and closing), steam/water hammer,
surge due to pump start-up and shutdown, and wind loads.
2.0
Stress Categorization
To evaluate the stresses in a piping system, it is necessary
to distinguish among primary, secondary, and peak stresses.
3.0
•
Primary stresses are the direct, shear, or bending
stresses generated by the loading.
•
Secondary stresses are those acting across the pipe wall
thickness due to a differential radial deflection of the pipe
wall. Secondary stresses cause local yielding and minor
distortions. Secondary stresses, unlike primary stresses,
are not a source of direct failure from a single load
application.
•
Peak stresses are more localized stresses which die
away rapidly within a short distance from their origin.
Peak stresses occur in areas such as welds, fittings,
branch connections, and other piping components where
stress concentrations and possible fatigue failure might
occur. Peak stresses are considered equivalent in
significance to secondary stresses, but they do not cause
any significant distortion.
Allowable Stresses
The basic allowable stress is a function of material
properties, temperature, and safety factors. The basic
allowable stress provides an upper limit for the actual
stresses.
103
•
Allowable stresses for sustained loads are established to
prevent general collapse or excessive distortion of the
piping system.
•
Allowable stresses for thermal expansion loads are
established to prevent a localized fatigue failure.
•
Allowable stresses for occasional loads are established
to prevent wind and earthquake type loads from
collapsing or distorting the piping system.
Actual stresses are calculated for the following load cases:
•
Sustained loads
•
Occasional loads
•
Stress range due to differential thermal expansion
The piping system is designed such that the calculated
stresses are no larger than the appropriate allowable
stresses.
Table 6.1 (excerpted from ASME B31.3 Table A-1) lists
basic allowable stresses in tension versus temperature for
several materials.
104
Basic Allowable Stress S, ksi. At Metal Temperature, °F.
Material
Spec. No/Grade
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
Carbon Steel
A 106
B
20.0
20.0
20.0
20.0
18.9
17.3
16.5
10.8
6.5
2.5
1.0
C - ½Mo
A 335
P1
18.3
18.3
17.5
16.9
16.3
15.7
15.1
13.5
12.7
4.
2.4
1¼ - ½Mo
A 335
P11
20.0
18.7
18.0
17.5
17.2
16.7
15.6
15.0
12.8
6.3
2.8
18Cr - 8Ni pipe
A 312
TP304
20.0
20.0
20.0
18.7
17.5
16.4
16.0
15.2
14.6
13.8
9.7
6.0
3.7
2.3
1.4
16Cr - 12Ni-2Mo
pipe
A 312
TP316
20.0
20.0
20.0
19.3
17.9
17.0
16.3
15.9
15.5
15.3
12.4
7.4
4.1
2.3
1.3
1.2
ASME B31.3, Table A-1 (Excerpt),
Basic Allowable Stresses in Tension for Metal
Table 6.1
C.
Pressure Design of Components
1.0
General
Two different types of pressure may be imposed on a piping
system: external or internal. Most piping systems need only
be designed for internal pressure. Some piping systems
may be subject to a negative pressure or vacuum condition
during operation (e.g., process vacuum conditions, steamout, underwater lines, etc.) and must be designed for
external pressure. This section only discusses the internal
pressure design of straight sections of pipe. Refer to ASME
B31.3 for design requirements for external pressure.
2.0
Required Wall Thickness for Internal Pressure of
Straight Pipe
The required wall thickness for internal pressure is
calculated using the following equation:
t=
1500
PD
2 (SE + PY )
Where:
t
= Required thickness for internal pressure, in.
P
= Internal design pressure, psig
105
S
= Allowable stress in tension (Table 6.1), psi
E
= Longitudinal-joint quality factor (Table 6.2)
Y
= Wall thickness correction factor (Table 6.3)
The longitudinal-joint quality factor is based on:
•
Whether the pipe is seamless or has a welded
longitudinal seam
•
The pipe material and welding process (if welded pipe)
The wall thickness correction factor is based on the type of
steel and the design temperature.
106
Spec.
No.
Class (or Type)
Description
Ej
Carbon Steel
API
5L
...
...
...
Seamless pipe
Electric resistance welded pipe
Electric fusion welded pipe, double butt, straight or
spiral seam
Furnace butt welded
1.00
0.85
0.95
A 53
Type S
Type E
Type F
Seamless pipe
Electric resistance welded pipe
Furnace butt welded pipe
1.00
0.85
0.60
A 106
...
Seamless pipe
1.00
Low and Intermediate Alloy Steel
A 333
...
...
Seamless pipe
Electric resistance welded pipe
1.00
0.85
A 335
...
Seamless pipe
1.00
Stainless Steel
A 312
...
...
...
Seamless pipe
Electric fusion welded pipe, double butt seam
Electric fusion welded pipe, single butt seam
1.00
0.85
0.80
A 358
1, 3, 4
5
2
Electric fusion welded pipe, 100% radiographed
Electric fusion welded pipe, spot radiographed
Electric fusion welded pipe, double butt seam
1.00
0.90
0.85
Nickel and Nickel Alloy
B 161
...
Seamless pipe and tube
1.00
B 514
...
Welded pipe
0.80
B 675
All
Welded pipe
0.80
ASME B31.3, Table A-1B (Excerpt),
Basic Quality Factors for Longitudinal Weld Joints, Ej
Table 6.2
107
Temperature, °F
Materials
900 & lower
950
1000
1050
1100
1150 & up
Ferritic
Steels
0.4
0.5
0.7
0.7
0.7
0.7
Austenitic
Steels
0.4
0.4
0.4
0.4
0.5
0.7
Other
Ductile
Metals
0.4
0.4
0.4
0.4
0.4
0.4
Cast iron
0.0
...
...
...
...
...
ASME B31.3, Table 304.1.1 (Excerpt),
Values of Coefficient Y
Table 6.3
Two additional thickness allowances must be considered to
determine the final required pipe wall thickness: corrosion
allowance and mill tolerance.
Corrosion allowance (CA) is an additional thickness that is
added to account for wall thinning and wear that can occur in
service. The corrosion allowance is based on experience
and data for the particular pipe material and fluid service.
Thus:
tm = t + CA
Where:
tm
=
Total minimum required wall thickness, in.
Mill tolerance accounts for the difference between the actual
manufactured pipe wall thickness and the “nominal” wall
thickness specified in the relevant pipe dimensional
standard. The typical pipe mill tolerance is 12.5%. This
means that the as-supplied pipe wall thickness can be up to
12.5% thinner than the nominal thickness and still meet its
specification requirements. Use the following equation to
determine the minimum required nominal thickness to order:
108
t nom =
tm
0.875
Where:
tnom = Minimum required nominal pipe wall thickness, in.
Each pipe size has several standard nominal thicknesses
that are available. The nominal pipe thickness that is
specified for a system must be selected from those readily
available and be at least equal to tnom.
3.0
Curved and Mitered Pipe Segments
The minimum required thickness of curved pipe (elbows or
bends) is the same as that required for straight pipe
sections. A mitered bed is fabricated by welding straight
pipe sections together to produce the direction change. A
mitered bend is generally less expensive than a wrought
elbow for large pipe sizes (over ~ NPS 24). The minimum
required thickness for a miter may be greater that that of the
connected straight pipe sections, depending on the number
of miter welds, design conditions, size, etc. Refer to ASME
B31.3 for thickness calculation requirements.
109
SAMPLE PROBLEM 2 - DETERMINE PIPE WALL THICKNESS
A piping system must be modified to add a new, spare heat exchanger. You
have been assigned the responsibility to determine the required wall thickness
for the pipe from the heat exchanger to several pumps. The piping system will
have a design temperature of 650°F. The design pressure is 1,380 psig. The
pipe outside diameter is 14 in. The material is ASTM A335, Gr. P11 (1¼ Cr – ½
Mo), seamless. Corrosion allowance is 0.0625 in.
What is the minimum required thickness for this pipe?
SOLUTION
The following equation applies:
t=
PD
2 (SE + PY )
Based on the given information:
P
=
1,380 psig.
D
=
14 in.
For the A335, Gr. P 11 material:
S
= 16,150 psi. [Table A-1 of ASME B31.3 at 650°F
E
= 1.0 [Table A-1B of ASME B31.3]
Y
=
0.4 [Table 304.1.1 of ASME B31.3, since the
material is ferritic and the temperature is below
900oF.
Since all the required parameters have now been determined, the required
internal pressure thickness may be calculated as follows:
110
1,380 × 14
2 [(16,150 × 1) + (1,380 × 0.4)]
t = 0.577 in.
t=
In this case, a 0.0625 in. corrosion allowance has been specified.
Therefore:
4.0
tm
=
t + c = 0.577 + 0.0625
tm
=
0.6395 in.
tnom =
0.6395
= 0.731 in.
0.875
Branch Reinforcement Requirements
A pipe with a branch connection is weakened by the required
opening. Unless the wall thickness of the pipe is sufficiently
greater than that required to sustain the pressure, additional
reinforcement must be provided.
ASME B31.3 contain rules for determining the required
reinforcement for both welded and extruded outlet-type
branch connections. Branch connections can also be made
using forged or wrought fittings (i.e., tees, laterals, crosses,
couplings, or half-couplings), or an integrally reinforced
branch connection. Reinforcement calculations are not
required for forged or wrought type branch connections
because they have adequate inherent reinforcement and
have been designed and tested to meet ASME B31.3
requirements. This section discusses only branch
connections that are fabricated by welding a branch pipe to
the run pipe.
111
4.1
Area Removed By Branch Connection
A volume of metal is removed from a pipe wall when a
hole is cut in it for a branch connection. However, a
simplification is made when evaluating branch
reinforcement requirements.
An imaginary plane is passed through the branch and
run pipes, and the intersection is viewed in crosssection. The removed volume of pipe wall is then
looked at as an area (see Figure 6.2).
Db
Tb
Reinforcement
Zone Limits
Nom.
Thk.
c
tb
Mill
Tol.
A3
A3
L4
Reinforcement
Zone Limits
A4
A4
A1
Tr
Th
Dh
Nom.
Thk.
c
th
d1
A2
Mill
Tol.
A2
d2
d2
β
Pipe C
Welded Branch Connection
Figure 6.2
4.2
Limits of Reinforcement Zone
The reinforcing zone is the region where credit may
be taken for any reinforcement that is present. The
branch connection must have adequate reinforcement
to compensate for the weakening caused by cutting a
hole in the run pipe. This reinforcement:
112
•
Must be located reasonably close to the opening
to provide any practical benefit.
•
May be located in the branch pipe, the run pipe, or
both.
Additional material located outside of this zone is not
effective for reinforcement.
4.3
Branch Connection Reinforcement
Branch connection reinforcement located within the
reinforcement zone may come from one or more of
the following sources.
•
Excess thickness available in the branch or
header pipe.
•
Additional reinforcement added in the form of a
pad, ring, saddle, or weld metal.
If excess thicknesses in the branch and header pipes
do not provide enough reinforcement, additional metal
may be added.
4.4
Reinforcement Area
The required reinforcement area is based on the
metal area removed. This is calculated using:
d1 =
D b − 2(Tb − c )
sin β
Where:
d1
=
Effective length removed from the run pipe,
in.
Db
=
Branch outside diameter, in.
Tb
=
Minimum branch thickness, in.
c
=
Corrosion allowance, in.
β
=
Acute angle between branch and header
113
The required reinforcement area, A1, is then
calculated using :
A1 = t h d1(2 − sin β)
Where:
th
4.5
=
Minimum required header thickness, in.
Reinforcement Pad
Additional branch reinforcement is needed when the
required area exceeds the available area, and may be
provided by locally increasing the thickness of either
the header or branch pipe. However, it is usually
more economical to provide a reinforcement pad to
supply the additional reinforcement.
There are three variables to select in designing the
reinforcement pad:
•
Material
•
Outside diameter
•
Wall thickness
To calculate the area of the reinforcement pad, A4, the
following equation is used:
æ (D p − D b ) ö
A 4 = çç
Tr
è sin β
Where:
D01p = Outside diameter of the pad, in.
Db =
Outside diameter of the branch, in.
Tr =
Pad thickness, in.
β=
The acute angle between the branch and
header pipes.
114
The pad must be large enough to provide the
additional reinforcement needed and be within the
reinforcement zone. The pad material is generally
equivalent to that of the pipe.
The following Sample Problem illustrates the branch
reinforcement calculation procedure.
115
SAMPLE PROBLEM 3
A new steam turbine is being installed within a process plant. This will require a
new NPS 16 steam supply line to be connected to an existing NPS 24 distribution
header. The following design information has been determined:
•
Pipe material - Seamless, A 106/Gr. B for both the branch and header.
•
Design temperature
-
700°F
•
Design pressure
-
550 psig
•
Allowable stress
-
16,500 psi.
•
Corrosion allowance
-
0.0625 in.
•
Mill tolerance
-
12.5%
•
Nominal Pipe
Thicknesses
-
Header: 0.562 in.
Branch: 0.375 in.
•
Required Pipe
Thicknesses for Pressure
-
Header: 0.395 in.
Branch: 0.263 in.
•
The branch connection is made on top of the header at a 90° angle, and does
not penetrate a header weld.
Determine if additional reinforcement is required for this branch connection. If it
is, size the reinforcing pad, neglecting the area of any welds. Assume that the
pad material is equal to the header material, and that its thickness equals the
header thickness.
SOLUTION
See Figure 6.2 for the relevant nomenclature.
•
The required thicknesses for pressure were given.
•
Next, the value for the effective length removed from the run pipe, d1, must be
calculated. This equals the corroded inside diameter of the branch
connection after accounting for mill tolerance (i.e., the actual pipe wall
thickness may be up to 12.5% less than the nominal thickness).
d1 =
Db − 2(Tb − c )
sin β
116
d1 =
16 − 2(0.375 × 0.875 − 0.0625 )
sin 90°
d1 = 15.469 in.
•
Now the required reinforcement area, A1, may be calculated.
A1 = t h d1 (2 − sinβi
A1 = 0.395 × 15.469 (2 − sin90°)
A1 = 6.11in.2
The available reinforcement areas in the header and branch pipe are now
calculated. This is determined using any “excess” thickness available in the
header and branch that is not necessary to withstand the pressure (or other)
loads. Disregard any contribution from nozzle attachment welds since this is
minimal.
•
Calculate the excess area available in the header, A2.
A2 = (2d2 − d1)(Th − th − c )
First determine d2 which is the greater of d1, or,
(Tb−c ) + (Th−c ) +
d1
2 , but less than the header diameter, Dh
(0.875 × 0.375 − 0.0625 ) + (0.875 × 0.562 − 0.0625 ) + 15.469
2
= 8.43 in.
∴ d2 = d1 = 15.469 in., which is less than the header diameter of 24 in.
A2 = (2 x 15.469 - 15.469) (0.875 x 0.562 - 0.395 - 0.0625)
A2 = 0.53 in.2
•
Calculate the excess area available in the branch, A3.
117
A3 =
2L4(Tb − tb−c )
sin β
First determine L4.
L4= 2.5 (Th−c ) or 2.5 (Tb−c ) + Tr, whichever is smaller .
Since Tr = 0 (i.e., no reinforcing pad initially) and Th is greater than Tb, L4 is
based on the second equation.
L4 =
2.5 (0.875 x 0.375 - 0.0625)
L4 =
0.664 in.
A3 =
2 × 0.664 (0.875 × 0.375 − 0.263 − 0.0625 )
sin 90°
A3 = 0.003 in.2
•
Calculate other excess area that may be available, A4.
There is no reinforcing pad and the area contribution from the branch weld is
being disregarded. Therefore, A4 = 0.
•
Total Available Area:
The total available reinforcement area, AT, is calculated by adding the
contributions from each source.
AT =
A2 + A3 + A4
AT =
0.53 + 0.003 + 0
AT =
0.533 in.2 available reinforcement.
The available total reinforcement of 0.533 in.2 is obviously much less than the
required reinforcement area of 6.11 in.2. Therefore, a reinforcing pad is
required. The reinforcement pad will now be sized.
118
A106, Gr. B material will be used for the reinforcement pad. Its thickness is
set to be equal to the header nominal thickness of 0.562 in.
•
Recalculate Available Reinforcement:
Now that a reinforcing pad is being used, the available reinforcement in the
branch must be recalculated since the height of the reinforcement zone in the
branch pipe will change slightly.
L41 =
2.5 (Th - c)
L41 =
2.5 (0.875 × 0.562 - 0.0625)
L41 =
1.073 in.
L42 =
2.5 (Tb - c) + Tr
L42 =
2.5 (0.875 × 0.375 - 0.0625) + 0.562 (0.875)
L42 =
1.16 in.
Therefore, L4 = 1.073 in.
A3 =
2L 4 (Tb − t b − c)
sin β
A3 =
2 × 1.073 (0.875 × 0.375 − 0.263 − 0.0625)
sin 90 o
A 3 = 0.005 in.2 (vs. the 0.003 in.2 previously calculated)
A T = A 2 +A 3 + A 4
A T = 0.53 + 0.005 + 0
A T = 0.535 in.2 available reinforcement
•
Calculate additional reinforcement required and the pad dimensions:
The required reinforcement area is 6.11in.2, and the available area is 0.535
in.2. Therefore, the additional reinforcement area to be provided in the pad,
A4, is:
119
•
A4 =
6.11 - 0.535
A4 =
5.575 in.2
Determine the diameter of the pad, Dp.
Tr = 0.562 (0.875) = 0.492 in.
Db = 16 in.
Dp =
A4
Db
+
Tr sin β
5.575
+ 16
0.492
D p = 27.3 in.
Dp =
The pad diameter must be at least 27.3 in. to provide adequate
reinforcement. Since 2d2 = 30.938 in., this pad diameter is within the
reinforcement zone along the header and is acceptable.
The following approach of calculating the required pad width, Lr, may be used
as an alternative to calculating the pad diameter.
0.5 A4
Tr
0.5 × 5.575
Lr =
0.492
L r = 5.66 in.
Lr =
120
EXERCISE 2:
DETERMINE REQUIRED PIPE WALL
THICKNESS
A new project is being considered to transport 48° API crude oil in a carbon steel
pipe between two areas within a tank farm. The fluid being transported will have
a design temperature of 260°F. The system design pressure is 150 psig, the
pipe outside diameter is 30 in., and the pipe being used is A 106, Gr. B seamless
pipe. A corrosion allowance of 1/8 in. has been specified for the pipe. All piping
within the tank farm is designed in accordance with ASME B31.3. Assume there
is a 12.5% mill tolerance.
a.
What is the thickness required for internal pressure?
b.
What is the minimum required nominal wall thickness?
Use Table 6.1 along with Tables 6.2 and 6.3 for the necessary information.
121
VII.
SYSTEM DESIGN
A.
Layout Considerations
Operational, maintenance, and safety considerations influence the
layout of a piping system. These factors must be recognized when
designing the layout and spacing of piping and equipment. This
section discusses how these factors influence piping layout.
1.0
Operations Requirements
Operating and control points (e.g., valves, flanges,
instruments, sample points, drains, and vents) should be
located so that they can be used safely and easily. For
example, valves must be located so that they can be
reached.
There must be enough clearance above and below the pipe
to perform basic operations on valves and flanges.
There must also be enough lateral space to access valves,
sample points, vessel flanges, and other equipment that may
require operator attention.
2.0
Maintenance Requirements
The piping system must be laid out so that its components
can be inspected, repaired, or replaced with minimum
difficulty. There must be ample clearance for maintenance
equipment (e.g., cranes) and for vehicles (e.g., trucks).
Access must be provided so supports can be maintained.
There must be enough space to access and remove large
pieces of equipment if they require maintenance.
•
Access near rotating equipment is important because
cranes must reach the equipment when removal or
realignment is required.
•
Heat exchanger bundles must be pulled out for cleaning.
•
Large valves must be removed to repair or replace their
seats.
122
•
3.0
Rotating equipment requires frequent monitoring and
maintenance.
Safety Considerations
Piping layout must consider the safety of personnel near the
pipe. This specifically includes access for fire fighting
equipment and fire prevention. Fire fighting equipment
needs clearance to access major pieces of equipment (e.g.,
heat exchangers, vessels, and tankage). Pipeways must be
routed and designed to provide the necessary clearances.
There must be enough space beneath pipeways for people
to walk and work. Firewater piping must be routed so that it
would not be damaged by piping containing hazardous fluids
that could rupture.
B.
Pipe Supports and Restraints
A piping system needs supports and restraints because of the
various loads that are imposed upon it. Supports absorb system
weight and reduce longitudinal pipe stress, pipe sag, and end point
reaction loads. Restraints control or direct the thermal movement
of a piping system. The control of thermal movement may be
necessary either to keep pipe thermal expansion stresses within
allowable limits, or to limit the loads that are imposed on connected
equipment.
Selection of a specific type of support or restraint to use in a
particular situation depends on such factors as:
•
Load to be supported or absorbed.
•
Clearance available for attachment to pipe.
•
Availability of nearby structural steel that is already there.
•
Direction of loads to be absorbed or movement to be
restrained.
•
Design temperature.
•
Need to permit vertical thermal movement at a support.
123
1.0
Rigid Supports
Rigid supports are used in situations where weight support is
needed and no provision to permit vertical thermal
displacement is required. A rigid support always will prevent
vertical movement downward, will sometimes prevent
vertical thermal movement upward, and will permit lateral
movement and rotation. See Figure 7.1.
Shoe
Base Adjustable
Support
Saddle
Dummy Support
Trunnion
Rigid Supports
Figure 7.1
Hangers are a type of rigid support. They support pipe from
structural steel or other facilities that are located above the pipe
and carry piping weight loads in tension. Pipe hangers are typically
124
one or more structural steel rods bolted to a pipe attachment and to
the overhead member. A hanger rod is designed to move freely
both parallel and perpendicular to the pipe axis, and not restrict
thermal expansion in these directions. A hanger will prevent
movement both down and up. See Figure 7.2.
Hangers
Figure 7.2
2.0
Flexible Supports
Flexible or resilient supports allow the piping system to move
in all three directions while still supporting the required
weight load. Weight is supported by the use of a coil spring
having an appropriate stiffness to carry the applied weight
125
load. Since the spring is resilient, it permits vertical thermal
movement while still carrying the weight. This type of
support is used in situations where support must be provided
at a particular location, and vertical thermal expansion must
also be permitted.
There are two basic types of flexible supports: variable load
and constant-load-type. In the variable-load type flexible
support, the amount of vertical load exerted by the support
changes as a result of the pipe thermal movement (which
compresses or extends the spring). The amount of vertical
load exerted by a constant-load type support does not
change throughout its movement range. See Figure 7.3.
Load and Deflection
Scale
Small Change in
Effective Lever Arm
Large Change in
Effective Lever Arm
Relatively
Constant
Load
Typical Variable-Load
Spring Support
Typical Constant-Load
Spring Support Mechanism
Flexible Supports
Figure 7.3
3.0
Typical Restraints and Anchors
3.1
Restraints
Restraints have two primary purposes in a piping
system.
126
•
Restraints control, limit, or redirect the unrestricted
thermal movement of a pipe. They are used to
either reduce the thermal stress in the pipe or the
loads exerted by the pipe on equipment
connections.
•
Restraints absorb loads imposed on the pipe by
other conditions such as wind, earthquake, slug
flow, water hammer, or flow-induced vibration.
Excessive loads could result in high pipe stress or
equipment reaction loads, or cause flange
leakage.
There are several different types of restraints that
may be used. The selection of which type to use and
its specific design details depends primarily on the
direction of pipe movement that must be restrained,
the location of the restraint point, and the magnitude
of the load that must be absorbed. It is also possible
to restrain more than one direction at one location in a
piping system, or to combine a restraint with a
support.
3.2
Anchors
An anchor is a special type of restraint that stops
movement in all three directions. Anchors provide full
fixation of the pipe, permitting very limited, if any,
translation or rotation. An anchor is used in situations
where it is necessary to totally isolate one section of a
piping system from another from the standpoint of
load and deflection. A total anchor that eliminates all
translation and rotation at one location is not used as
commonly as one or more restraints that act at a
single location. A directional anchor which restrains
the line only in its axial direction is more commonly
used. Figure 7.4 provides several examples of
anchors.
127
Anchor
Anchor
Partial Anchor
Restraints/Anchors
Figure 7.4
3.3
Guides
A guide is a particular type of restraint that permits
movement along the pipe axis while preventing lateral
movement. Depending on the particular guide details
employed, pipe rotation may or may not be restricted.
Common situations where guides are used are in long
pipe runs on a pipe rack to control thermal movement
and prevent buckling, and in straight pipe runs down
the side of a tower to prevent wind-induced
movement and control thermal expansion. See
Figure 7.5.
128
Guide
Guide
x
Guide
Vertical Guide
Examples of Guides
Figure 7.5
C.
Piping Flexibility
Piping must have sufficient flexibility to accommodate thermal
expansion (or contraction) effects. Piping systems must be
designed to ensure that they do not fail because of thermal
stresses or produce excessive forces and moments at connected
equipment. If a system does not provide adequate flexibility, the
results can be leaky flanges, fatigue failure of the pipe, excessive
maintenance, operations problems, and damaged equipment.
129
A structure that is subject to a change in temperature will change in
dimensions. If these thermal movements are allowed to occur
without any restraint whatsoever, no pipe stresses or reaction loads
result. However, in real systems, stresses are developed in the
pipe and moments and forces are imposed on the connected
equipment and at supports and restraints installed in the system.
The basic problem is to determine the internal pipe stresses and
the external loads, and then decide if they are acceptable. A
thermal flexibility analysis is done to ensure that the piping system
is laid out, supported, and restrained such that the thermal stresses
in the pipe and the loads on the end points are within allowable
limits.
1.0
Rationale for Piping Flexibility and Support Design
Support and flexibility design is a combination of art and
science with multiple factors to consider and usually more
than one way to design the system. It requires knowledge of
how the operating and design conditions of a piping system
influence its overall design, and the supports and restraints
required for the system.
A piping system can be described as an irregular structural
frame in space because of its relatively slender proportions
when compared to structural steel systems. Elevated design
temperatures or various operating scenarios may cause
sufficient pipe thermal stress or reduce material strength
such that supplementary structural assistance to support the
piping system is required. It is also often necessary to limit
the pipe movement at specific locations to protect sensitive
equipment, control vibration, or to resist external forces (e.g.,
wind, earthquake, or shock loading).
Attention must also be paid to pipe support/restraint design
details to ensure that localized stresses in the pipe wall are
kept within allowable limits. In those situations, design
details that spread the applied load over a wider portion of
the pipe surface are used.
Planning for pipe supports and restraints should be done
simultaneously with establishing possible layout
configurations to achieve the most cost-effective design.
130
2.0
Approaches to Design
Due to the complexity of the piping flexibility and support
design process, there is no single procedure or design
method applicable for all situations. The following is one
way to approach the problem.
•
Examine the layout and operation of the piping system to
identify:
-
Layout geometry.
-
Pipe diameter and thickness, and locations of any
changes in these parameters.
-
Piping component design details such as branch
connection details and type of elbows used (i.e., long
radius or short radius).
-
Design temperature and pressure.
-
Fluid service, including its potential danger.
-
End-point movements.
-
Type of connected equipment (i.e., rotating or fixed).
-
Locations of existing structural steel.
-
Relevant operating scenarios.
-
Special design considerations (e.g., wind, vibrationprone services, orientation of loads).
•
Determine the potential effects of those conditions (e.g.,
thermal movements, loads, and stresses).
•
Determine the types of support or restraint required and
their approximate locations.
•
Determine if the situation warrants a detailed computer
analysis.
•
If required, identify which conditions apply for the
analysis and utilize an appropriate computer program.
•
Interpret the results of the analysis.
131
D.
Required Design Information for Piping Stress Analysis
Detailed piping stress analysis is done using a computer program
such as Caesar II, Simflex, or Triflex. Such programs have the
capability to consider any combination of pipe geometry, support,
restraint, and load conditions. However, several things must be
considered:
•
Applicable design conditions and operating scenarios for the
piping system.
•
Allowable stresses from ASME B31.3.
•
Load limitations, if any, on connected equipment.
•
Extent of analysis required to identify most severe case.
Design conditions that must be known to perform a detailed pipe
stress analysis are listed below:
•
Layout geometry of the piping system.
•
Pipe diameter and wall thickness.
•
Design temperature and pressure.
•
Fluid service, including whether it is dangerous.
•
End-point movements.
•
Type of connected equipment.
•
Structural steel located in the vicinity.
•
Special design considerations and load cases.
Another consideration is the number of cycles that the system will
undergo during its design life. This influences piping flexibility
design because the allowable flexibility stress is based on fatigue
failure. All ASME B31.3 piping systems are designed for a
minimum of 7000 cycles. Systems that will undergo more than
7000 operating cycles during their design life are designed using a
reduced allowable stress basis.
E.
Criteria for Allowable Equipment Nozzle Loads
A poorly designed piping system can cause damage to the
equipment it is connected to, whether the equipment is a rotating
132
type (e.g., pump or compressor) or stationary type (e.g., pressure
vessel or heat exchanger). Rotating equipment is the more
sensitive with respect to imposed piping loads because of the
moving parts and small clearances involved in its design.
Excessive piping loads imposed on rotating equipment can cause
damage, poor operation, and/or maintenance problems at levels
well below those that would cause pipe or equipment stress
concerns.
Loads that are imposed by the piping system on connected
equipment are determined from the results of the piping flexibility
analysis. These loads are then compared to allowable values
based on industry standards for particular types of equipment to
determine if they are acceptable. The allowable values can
sometimes be read from a table contained in the applicable industry
standard. Other times, the allowable loads or the equipment
stresses that they cause must be calculated. Equipment vendors
will sometimes have allowable load criteria that must be
considered. Table 7.1 summarizes industry standards that apply to
equipment nozzle load evaluations, and the parameters that are
used to determine the allowable loads.
Equipment Item
Industry Standard
Parameters Used
To Determine
Acceptable Loads
Centrifugal Pumps
API-610
Nozzle size
Centrifugal Compressors
API-617, 1.85 times
NEMA SM-23 allowable
Nozzle size, material
Air-Cooled Heat Exchangers
API-661
Nozzle size
Pressure Vessels, Shelland-Tube Heat Exchanger
Nozzles
ASME Code Section
VIII, WRC-107,
WRC-297
Nozzle size, thickness, reinforcement
details, vessel/exchanger diameter,
and wall thickness. Stress analysis
required.
Tank Nozzles
API-650
Nozzle size, tank diameter, height,
shell thickness, nozzle elevation.
Steam Turbines
NEMA SM-23
Nozzle size
Equipment Nozzle Load Standards and Parameters
Table 7.1
133
F.
When Should A Computer Analysis Be Used
Computer programs can perform numerous analyses with many
different combinations of design conditions and system geometries.
They can perform many functions that would be difficult for a piping
analyst to do “by hand.” Computers can also perform unique
functions that would be difficult or impossible to do by hand or other
methods with sufficient accuracy. Even though hand calculations
can be used in many situations, a computer program can often be
used to finalize and optimize the final design.
A computer analysis should also be used when there are several
operating combinations to be considered and other methods would
be inadequate or too time consuming, when greater accuracy is
required due to the nature of the system, and for complicated piping
systems. Computer programs are also very useful for analyzing the
stresses and loads at piping components such as valves, branches,
and bends. A piping system designer should remember that a
computer program only gives quantitative guidelines, to which they
must apply common sense and judgement.
The guidelines listed in Table 7.2 may be used to help determine
when a computer analysis should be performed:
Type Of Piping
Pipe Size, NPS
General piping
For rotating equipment
For air-fin heat exchangers
For tankage
≥4
≥8
≥ 12
≥ 20
≥3
≥4
≥ 12
Maximum Differential
Flexibility Temp.
≥ 400°F
≥ 300°F
≥ 200°F
any
Any
Any
Any
Computer Analysis Guidelines
Table 7.2
G.
Design Considerations for Piping System Stress Analysis
The following paragraphs discuss several design considerations in
piping system stress analysis.
134
1.0
Piping Flexibility Temperature
Flexibility analysis should be made for the largest
temperature difference that may be imposed on the pipe by
normal and abnormal operating conditions. This results in
the largest pipe stress range to be considered in fatigue
failure evaluation, and the largest reaction loads imposed on
equipment end connections, supports, and restraints.
Tables 7.3 and 7.4 provide guidelines to determine the
temperatures to consider in a flexibility analysis. Note that
more than one of these items might require consideration in
a particular system and lead to the need for multiple
computer calculations to identify the case that governs the
system design.
Stable
Operation
Startup and
Shutdown
Gives the temperature range expected for most of the time a plant is in
operation. Some margin above equipment operating temperature (i.e.,
use of the design temperature rather than operating temperature)
allows for process flexibility.
Must be examined to determine if the heating or cooling cycles pose
flexibility problems. For example, if a tower is heated while some
attached piping remains cold, the piping flexibility should be checked
for that case.
Regeneration
and Decoking
Piping
Must be designed for normal operation, regeneration, or decoking, and
switching from one service to the other. An example is the decoking of
furnaces.
Spared
Equipment
Requires multiple analyses to determine if the piping is adequate for
the expected variations of temperature, for no flow in some of the
piping, and for switching from one piece of equipment to another. A
common example is the piping for two or more pumps with one or more
spares.
Normal Temperature Conditions To Consider
Table 7.3
135
Loss of Cooling
Medium Flow
Temperature changes due to a loss of cooling medium flow
should be considered. This includes pipe that is normally at
ambient temperature but can be blocked in, while subject to
solar radiation.
Steamout for Air
or Gas Freeing
Most on-site equipment and lines, and many off-site lines, are
freed of gas or air by the use of steam. For 125 psig steam,
300°F is typically used for the metal temperature. Piping
connected to equipment which will be steamed out, especially
piping connected to the upper parts of towers, should be
checked for the tower at 300°F and the piping at ambient plus
50°F. This situation may govern the flexibility of lines connected
to towers that operate at less than 300°F or that have a smaller
temperature variation from top to bottom.
No Process Flow
While Heating
Continues
If process flow can be stopped while heat is still being applied,
the piping flexibility should be checked for the maximum metal
temperature. Such situations can occur with steam tracing and
steam jacketing.
Abnormal Temperature Conditions To Consider
Table 7.4
Metal temperatures that govern the flexibility design of a piping
system are not necessarily the ones associated with the most
severe coincident pressure and temperature which govern the wall
thickness of the pipe. Piping flexibility depends only on the
temperature. Therefore, a condition of high temperature and low
pressure may govern the piping flexibility design while the wall
thickness is based on a higher pressure but a lower temperature.
Pipe thermal movement is caused by a temperature change from
the piping installation temperature (i.e., the ambient temperature).
Piping analysis computer programs typically include a “default”
ambient temperature (commonly 70°F). Then, all thermal
movements and resulting thermal stresses are calculated based on
the difference between the specified pipe design temperature and
the default ambient temperature. A realistic ambient installation
temperature (typically lower than 70°F) must be used for the
specific plant site to accurately calculate the maximum thermal
stress range and reaction loads.
136
2.0
Extent of Analysis
The extent of a piping system analysis depends on the
situation. The overall purpose of the analysis is to provide
enough flexibility for the system. The engineer must analyze
the right combination of operating conditions to determine
where, and if, additional flexibility is needed to reduce pipe
stresses or loads at end points. The engineer must also
decide if it is desirable and acceptable to not include portions
of a large, complex system in the analysis to simplify the
modeling. For example, including an NPS 4 branch run in
the model of a NPS 24 main system may not be necessary.
Judicious installation of anchors or other restraints in a large
system could also help simplify the modeling by separating
the system into sections.
Use the following steps to develop the piping design:
3.0
•
Define line size, wall thickness, material, number of
temperature cycles, layout, maximum differential
temperature, and any alternative operating scenarios.
•
Determine conditions of end-point restraint and
movements.
•
Locate intermediate points of restraint and define any
limitations that they impose on piping movement.
•
Select a suitable analysis method and calculate the loads
and stresses.
•
Compare the results with the allowable stress range for
thermal expansion stresses, the allowable stress at
design temperature for weight-plus-pressure stresses,
and the applicable load criteria for connected equipment.
Modifying System Design
The initial piping system layout may not be satisfactory for
thermal flexibility stresses or loads on connected equipment.
The following guidelines may help the situation.
•
Provide more offsets or bends, or use more expansion
loops within the same space. These make the system
more flexible and reduce the thermal stresses.
137
4.0
•
Install expansion joints. However, this approach should
be the exception rather than the rule. Expansion joints
represent a "weak link" in a piping system. They may
affect the life of the system since they are more
susceptible to damage than pipe, and can create
maintenance and operational problems. Thus, the use of
expansion joints should only be considered as a last
resort. One situation where expansion joints must be
used is where pressure drop or other process
requirements dictate the use of relatively straight pipe
runs (e.g., fluidized solids transfer lines).
•
Strategically locate restraints to minimize thermal and
friction loads at equipment. Restraints could also be
used to direct pipe thermal expansion into a section of
the system that has more inherent flexibility to absorb it.
•
Use spring supports if large vertical thermal movements
are expected, or if thermal expansion causes pipe to lift
off fixed supports. Avoid fixed supports that result in
large thermal stresses.
•
Use Teflon bearing pads at supports for large-diameter
pipe or other large weight loads if friction loads are
excessive on equipment connections or structural
members.
System Design Considerations
Each type of piping system has particular factors that must
be considered when performing a detailed analysis. For
example:
•
Pump systems will often be installed with spared pumps.
Thus, various scenarios of operating vs. spared pump(s)
must be considered since portions of the system near the
pumps will be hot while other portions are cold.
•
Piping systems are sometimes heat traced. This might
be done either to reduce liquid viscosity to allow the
necessary flow, or to prevent condensate accumulation.
The condition with the process flow off while the heat
tracing remains on must also be considered since the
pipe metal temperature for this case may be higher than
the normal design temperature.
•
Piping systems connected to atmospheric storage tanks
must be designed considering movement that occurs at
138
the tank nozzle. When the tank is filled with liquid, the
shell will bulge outward and the nozzle will rotate down
due to this shell bulging (see Figure 7.7). Over a period
of time, the tank may also settle down into its foundation
with respect to the pipe. Because of these expected tank
movements, it is often necessary to use a flexible-type
pipe support located near the tank nozzle to ensure that
the tank nozzle is not overloaded.
SHELL
NOZZLE
BOTTOM
Tank Nozzle
Figure 7.7
•
It may be necessary to consider pipe frictional effects at
support points. If large enough, friction loads can restrict
pipe movement and cause unexpectedly high pipe
stresses or end point reaction loads. Typical situations
where it may be necessary to consider friction loads are
for long horizontal pipe runs, or where large concentrated
weight loads are supported near equipment nozzles.
•
The most common configuration for air-cooled heat
exchanger piping uses short, straight sections of pipe to
connect the manifold to the exchanger nozzles. The
manifold is located directly above or below the exchanger
header box. The heat exchanger tube bundle is allowed
to move laterally to accommodate the thermal expansion
of the pipe manifold. The flexibility analysis should
include the restraining effect of friction from movement of
the exchanger bundle, which will resist lateral movement
of the bundle.
139
VIII. FABRICATION, ASSEMBLY, AND ERECTION
Individual sections of pipe must be fabricated into convenient sections (i.e., spool
pieces). Individual spool pieces are then assembled and erected in the field.
A.
Welding and Heat Treatment
Welding is one of the primary ways of joining pipe. Welded joints
represent the ultimate in safety and reliability. All design codes call
for welding to be carried out using a qualified procedure and
welders. Included in the welding procedure are: base-metal
specification, electrode type and material, joint preparation (i.e.,
geometry), weld position (e.g., vertical, overhead, etc.), welding
process (including whether it is manual or automatic), techniques,
electrical details, preheat and interpass temperatures, and postweld heat treatment (PWHT) requirements.
1.0
Butt-Welds
Butt-welds are made between two components whose edges
are in close proximity. Butt-welded joints in piping systems
are primarily of the single-V configuration and are welded
from the pipe outside surface. The joint preparation and the
procedure that is used ensure that there is complete fusion
between the edges of the components being joined. Joint
designs shown in Figure 8.1 are typically used for ends of
equal thickness. The transition between ends of unequal
thickness may be accomplished by taper grinding the thicker
pipe to match the thinner, or by using weld metal to provide
a smooth transition as shown in Figure 8.2. Butt-welds are
always used to weld pipe ends together, to weld butt-weldtype flanges or fittings to pipe ends, or to weld the edges of
formed plate together when plate is used to manufacture
pipe.
140
(a) Standard End Preparation
of Pipe
(b) Standard End Preparation
of Butt-Welding Fittings and
Optional End Preparation of
Pipe 7/8 in. and Thinner
(c) Suggested End Preparation,
Pipe and Fittings Over 7/8 in.
Thickness
Butt-Welded Joint Designs
Equal Thickness
Figure 8.1
3/32 in. max.
(a)
(b)
(c)
(d)
Butt-Welded Joint Design
Unequal Thickness
Figure 8.2
2.0
Fillet Weld
The fillet weld generally requires no special joint preparation.
It is an angular weld bead that joins components normally
positioned at a 90° angle to each other. The size of a fillet
weld is stated as a leg length of the largest inscribed right
isosceles triangle. In piping systems, fillet welds are only
used for slip-on flanges, socket welds, and for welding
141
attachments to piping components (e.g., reinforcing pads,
supports, etc.). See Figure 8.3.
Fillet Welds
Figure 8.3
142
3.0
Welding Preparation Steps
The following outlines the overall steps that are required for
welding.
4.0
•
The individuals and equipment executing the welding
procedure must be confirmed to be qualified to produce
acceptable results.
•
Internal and external surfaces to be welded shall be
clean and free from paint, oil, rust, scale, or other
material that would be detrimental to either the weld or
base metal when heat is applied.
•
The ends of the components to be welded must be set to
the correct geometric shape suitable for the materials,
wall thickness, and welding process involved.
Preheating
Preheating is used, along with heat treatment, to minimize
the detrimental effects of high temperature and severe
thermal gradients that are inherent in welding. The following
identifies the benefits of preheating:
5.0
•
Dries the metal and removes surface moisture which
could result in weld porosity.
•
Reduces the temperature difference between the base
metal and the weld to reduce the cooling rate of the
weldment. This lowers the weld hardness and reduces
cooling/shrinkage stresses.
•
Helps maintain the weld pool molten longer to permit
maximum separation of impurities.
•
Helps drive off absorbed gases (e.g., hydrogen) which
could contribute to weld porosity.
Postweld Heat Treatment (PWHT)
PWHT averts or relieves the detrimental effects of high
temperature and severe temperature gradients that are
inherent in welding, and relieves residual stresses that are
created by bending and forming. Specific heat treatment
temperature and procedure requirements are specified in
143
ASME B31.3 based on the pipe material and wall thickness
being joined.
The following summarizes the principal reasons for PWHT:
B.
•
Stress relief is the most common reason for specifying
PWHT. This is the only consideration for the PWHT
requirements specified in ASME B31.3. Other reasons for
PWHT (e.g., due to process considerations) must be
specified by the user or contractor. Residual stresses will
remain in the pipe and result from shrinkage as the weld
and adjacent pipe metal cool down from elevated welding
temperatures. Residual stresses will also remain after
bending or forming processes. If these residual stresses
are too high, they can lead to premature failure of the
pipe.
•
After welding the normal grades of stainless steels (i.e.,
those that are not stabilized with alloy additions), the
material must be heat treated to restore its maximum
corrosion resistance.
•
PWHT is required to prevent caustic embrittlement of
welded carbon steel pipe that handles alkaline solutions.
Caustic embrittlement is a form of stress corrosion where
the residual stresses due to welding are sufficient to
cause failure.
•
PWHT is sometimes necessary to reduce weld hardness
in certain materials. Minimizing weld hardness reduces
the tendency to crack, especially in certain process
environments (e.g., wet H2S).
Assembly and Erection
Additional piping fabrication requirements must be considered.
Several of these are discussed below.
1.0
Storage and Handling
Improper handling and storage of pipe materials and welding
filler metals can cause damage and result in poor
construction quality and failures during operation.
144
2.0
•
Pipe should not be stored directly on the ground to help
prevent rainwater accumulation around the pipe, which
could result in corrosion.
•
Pipe should not be stacked so high that pipes or their
coatings may be damaged.
•
Fittings and valves should be stored in shipping crates or
on racks to provide protection until used.
•
End protectors should be firmly attached to prevent
damage to weld bevels, flange faces, threads, or socketweld ends.
•
Lined and coated pipes and fittings should be lifted with
wide fabric or rubber-covered slings and padding to
prevent damage.
Pipe Fitup and Tolerances
Good joint fitup is essential to making a sound weld and
minimizing the loads imposed on the piping system and
connected equipment. Depending on the welding process
used, a slight mismatch may be permissible.
•
Pipe fitup for welded joints shall be as required by the
welding procedure.
•
The tolerance for axial dimensions, face-to-face, centerto-face, and location of attachments should be ±1/8 in.
maximum.
•
Flattening of bends, measured as the difference between
the largest and smallest outside diameter at any crosssection, should not exceed 5% of the nominal diameter of
the pipe (3% at the ends).
•
Lateral translation of branches and connections from
centerline of run should not exceed ±1/16 in.
•
Flange bolt holes shall straddle the centerlines. Rotation
of flanges, measured as the offset between elevation of
bolt holes on opposite sides of a flange centerline, should
not exceed ±1/16 in.
•
The tilt of flanges measured at the periphery across any
diameter should not exceed 1/32 in. from the square
position. Use of a 1/64 in. tolerance is often necessary
for flanges at load-sensitive equipment.
145
3.0
Alignment of Pipe Attached to Load-Sensitive
Equipment
Special care must be taken for load-sensitive equipment,
especially rotating equipment. Specifically, in attaching pipe
to rotating equipment, the installation should avoid putting
excessive forces and moments on the machinery nozzles
which could result in misalignment.
•
Installation of piping that is connected to rotating
equipment should preferably start at the machine nozzle
flange. This will reduce the possibility of having a large
mismatch between the pipe and machine flanges if pipe
installation is begun from the opposite end of the system.
•
Bolt on succeeding pipe sections as appropriate up to the
first support. Adjust this support as required to just
contact the pipe at its bearing point. Proceed to any
other adjacent supports which should be similarly
adjusted.
•
One or more field welds are typically used to join the
piping nearest to the machine with the rest of the system.
The number and location of these field welds are
determined such that they will permit final position
adjustments to achieve acceptable flange alignment at
the machine nozzle.
•
Spring supports should be locked in their cold position
during pipe installation.
•
All spring supports will be adjusted in the locked position
just until they contact their respective support points. If
spring-support adjustment is insufficient, modifications to
associated structural members or shimming will be
required.
•
Final bolt tensioning of component flanges close to the
machinery should be done after initial alignment of nozzle
flanges.
•
Piping that requires any sections to be removed for
flushing after completing field welds should have final
nozzle alignment and component flange boltup
completed after replacing flushed sections.
•
For piping over NPS 3 connected to machinery, flange
alignment must be within more stringent limits than is
146
specified for general piping systems. More stringent
limits are required to minimize the loads that are imposed
by flange boltup.
• Precautions should be taken to prevent ingress of debris
into machine internals during construction of connecting
pipework.
4.0
Flange Joint Assembly
Flange joint assembly procedures directly affect the ability of
the flange to be leak-tight in service. In many low-pressure,
low-temperature, and/or nonflammable services, many rules
of good flanged joint design and makeup can and have been
violated with no adverse consequences. However, it is
dangerous to break these rules in critical, high-temperature
services since the results can be serious leakage problems
with consequent fires. The primary factors for successfully
making up a flanged joint and controlling leakage are the
following:
•
Proper selection and design of the flanged joint.
•
Proper preparation, inspection, and installation of the
flanged joint.
•
Identifying and controlling the causes of leakage.
Flanged joint assembly and leakage control are discussed
below.
5.0
Flange Preparation, Inspection, and Installation
The following discusses the primary steps that are required
to achieve a properly assembled flanged joint.
•
Redo Damaged Surfaces. Warped or badly corroded
flanges should be replaced or refaced. Reface flanges
with tool marks or scratches across the gasket seating
surface.
•
Clean Faces. All gasket and flange surfaces should be
clean. Remove all burrs, rust, and dirt from flange faces
with scrapers or wire brushes.
147
•
Align flanges. Flanges at rest should be within the
alignment tolerances previously discussed, with the
flanges practically mating before the bolts are installed.
Bringing the flanges into alignment should not leave any
residual stresses in the piping system. Residual stresses
could lead to flange leakage in service or overload
problems in systems that are connected to load-sensitive
equipment. This becomes more important with
increasing pipe diameter, as the residual stress increases
with increasing diameter for the same amount of
misalignment.
•
Lubricate Threads and Nuts. Lubricate the bolt threads
and the nut faces where they will contact the flange.
Lubrication helps increase the amount of bolt load that
goes into tightening the flange rather than into
overcoming friction.
•
Place Gasket Properly. The gasket must be centered on
the flange faces to achieve a reliable joint, but holding the
gasket in place can be a problem. If something must be
used to hold the gasket, a high-temperature grease may
be used sparingly in systems that operate at less than
200°F. No grease, paste, or adhesive should be used to
hold gaskets for systems operating at 200°F or more.
The high temperature causes these materials to burn off,
which could damage the gasket and cause leakage.
Thin cellophane tape may be used on the outside edges
of a gasket, but never on the seating surfaces. Tape on
the seating surfaces will deform the gasket during joint
assembly, burn out at operating temperature, and thus
provide a leakage path. Centering rings on spiral-wound
gaskets help by allowing the gasket to be supported in
the proper position by a few bolts while the other bolts
are inserted. Sheet gaskets should be cut so that their
outside diameter corresponds to the bolt position, again
to help centering.
•
Use Proper Flange Boltup Procedure. Flanges may be
made up using a wrench and hammer, an impact wrench,
a torque wrench, or a stud tensioner. The most important
aspects of a proper boltup procedure, regardless of
method, are to:
148
6.0
-
Use a "criss-cross" pattern bolt-tightening sequence,
as is used when bolting a wheel onto a car. This
approach helps to achieve a uniform bolt load around
the flange. See Figure 8.4.
-
Use at least three rounds of tightening around the
flange, increasing the applied load in each round,
with two rounds at the maximum load. This
approach helps achieve uniform bolt load around the
flange circumference.
-
For the most critical high-temperature or highpressure flanges, use a method that permits
measuring the applied load (i.e., torque wrench or
stud tensioner). In this way, there is greater
assurance that uniform bolt load is achieved. For
such applications, a maximum stud stress during
boltup of 40-50,000 psi is the normal target.
Causes of Flange Leakage
Most of the primary causes of flange leakage are directly
related to poor inspection or installation. These are
summarized below:
•
Uneven Bolt Stress. An incorrect boltup procedure or
limited working space near one side of a flange can leave
some bolts loose while others crush the gasket. This is
especially troublesome in high-temperature services,
when the heavily loaded bolts relax during operation.
•
Improper Flange Alignment. Improper flange alignment,
especially nonparallel faces, causes uneven gasket
compression, local crushing, and subsequent leakage.
•
Improper Gasket Centering. If a gasket is off-center, it
will be unevenly compressed and more prone to leakage.
•
Dirty or Damaged Flange Faces. Dirt, scale, scratches,
protrusions, or weld spatter on gasket seating surfaces
provide leakage paths or can cause uneven gasket
compression that results in leakage.
•
Excessive Loads in the Piping System at Flange
Locations. Excessive piping system forces and moments
at flanges can distort them and cause leaks. Common
causes of this are inadequate flexibility, using excessive
149
force to align flanges, and improper location of supports
or restraints.
•
Thermal Shock. Rapid temperature fluctuations can
cause flanges to deform temporarily and leak.
•
Improper Gasket Size or Material. Using the wrong
gasket size or material can cause leakage.
•
Improper Flange Facing. A rougher flange-surface finish
than specified for spiral-wound gaskets can result in
leakage.
Typical "Criss-Cross" Bolt-Tightening Sequence
Figure 8.4
150
IX.
QUALITY CONTROL
A.
Inspection
Prior to initial operation, each piping installation, including individual
components and overall workmanship, shall be examined. The
following requirements are based on ASME B31.3.
Defects must be identified before a piping system can be tested or
go into operation. Defect identification is especially important in
welded areas. A good weld starts with a proper design and is
executed using a qualified procedure and welder. However, the
quality that is achieved in a particular instance may not be
acceptable for a variety of reasons. The method of weld
examination needed to ensure that welds of acceptable quality are
achieved must be specified. Not all welds are inspected in the
same manner. Determining the proper type of weld inspection is a
function of technique, weld type, anticipated type of defect, location
of weld, and pipe material.
The following are common weld defects (illustrated in Figure 9.1):
•
Lack of fusion between adjacent weld passes.
•
Lack of fusion between weld bead and base metal.
•
Incomplete penetration due to internal misalignment.
•
Incomplete penetration of weld groove.
•
Concave root surface.
•
Undercut.
•
Excess external reinforcement.
•
Cracks.
Table 9.1 summarizes the primary weld inspection methods, where
they are typically used, and the types of defects they can locate.
151
Typical Weld Imperfections
Figure 9.1
152
Type of Inspection
Visual
Situation/Weld Type
All welds
Radiography
Magnetic Particle
Liquid Penetrant
Ultrasonic
Defect
•
Minor structural welds
•
Cracks
•
Slag inclusions
•
Butt welds
•
Gas pockets
•
Girth welds
•
Slag inclusions
•
Miter groove welds
•
Incomplete penetration
•
Ferromagnetic materials
•
Cracks
•
For flaws up to ¼ in.
beneath the surface
•
Porosity
•
Lack of fusion
•
Ferrous and nonferrous
materials
•
Cracks
•
•
Seams
Intermediate weld passes
•
•
Porosity
Weld root pass
•
•
Folds
Simple and inexpensive
•
Inclusions
•
Shrinkage
•
Surface defects
•
Laminations
•
Slag inclusions in thick plates
•
Subsurface flaws
Confirms high weld quality in
pressure-containing joints
Guidelines for Weld Inspection
Table 9.1
The following inspection guidelines also apply:
•
ASME B31.3 specifies weld examination requirements and
acceptance criteria based on fluid service category (i.e.,
Normal, Severe Cyclic Conditions, and Category D fluid
services).
•
For P-Nos. 3, 4, and 5 materials, examination shall be
performed after heat treatment. Thus, any defects caused by
heat treatment will be present.
•
For a welded branch connection, the examination of and any
necessary repairs to the pressure-containing weld shall be
completed before any reinforcing pad or saddle is added.
Thus, the reinforcement will not prevent inspection and repair.
153
B.
•
At least 5% of all fabrication shall be visually examined.
•
100% of fabrication for longitudinal welds, except in
components made in accordance with a listed specification,
shall be visually inspected.
•
Random visual examination of the assembly of threaded,
bolted, and other joints.
•
Random visual examination during the erection of piping.
•
Not less than 5% of circumferential butt- and miter-groove
welds shall be examined fully by random radiography or
random ultrasonic examination.
•
Not less than 5% of all brazed joints shall be examined, by inprocess examination.
•
Piping in severe cyclical service requires additional
examination.
Testing
The piping system must be pressure tested after it has been
completely fabricated, erected, and inspected. The pressure test
demonstrates the mechanical integrity of the system before it is
placed into operation. The following highlights several test
requirements.
•
A hydrostatic test must be used unless otherwise approved for
special situations.
•
The hydrostatic test pressure at any point in a metallic piping
system shall be as follows:
a) Not less than 1½ times the design pressure.
b) For design temperatures that are above the test
temperature, the minimum test pressure shall be calculated
as follows, except that the value of ST/S shall not exceed
6.5:
PT =
1.5 PS T
S
154
Where:
c)
PT
=
P
=
Internal design pressure, psig
ST
=
Allowable stress at test temperature,
psi
S
=
Minimum hydrostatic test pressure, psig
Allowable stress at design temperature,
psi
If the test pressure as defined above would produce a
stress in excess of the yield strength at test
temperature, the test pressure may be reduced to the
maximum pressure that will not exceed the yield
strength at test temperature.
•
Pneumatic strength tests, when approved, shall be conducted at
110% of the design pressure.
•
Instrument take-off piping and sampling system piping, up to
the first block valve, shall be strength tested with the piping or
equipment to which it is connected.
155
X.
OTHER CONSIDERATIONS
A.
Nonmetallic Piping
The following highlights several aspects of nonmetallic piping
design. Refer to ASME B31.3 for additional details.
Examples of nonmetallic piping include:
•
Thermoplastic Piping. Piping fabricated from a plastic which
is capable of being repeatedly softened by an increase of
temperature and hardened by a decrease of temperature.
•
Reinforced Thermosetting Resin Piping (RTR). Piping
fabricated from a resin capable of being changed into a
substantially infusible or insoluble product when cured at room
temperature, or by application of heat, or by chemical means.
Some differences in the design of nonmetallic piping vs. metallic
piping in normal fluid service include:
•
Allowances for variations of pressure or temperature, or both,
above design conditions are not permitted. The most severe
conditions of coincident pressure and temperature will be used
to determine design conditions.
•
Piping systems shall be designed to prevent thermal expansion
or contraction, pressure expansion, or movement of piping
supports and terminals from causing:
-
Failure of piping supports from overstrain or fatigue.
-
Leakage at joints.
-
Detrimental stresses or distortions in connected equipment.
•
The stress-strain behavior of most nonmetals differs
considerably from that of metals. Therefore, the assumptions
that stresses throughout the piping system can be predicted
from strains, or that displacement strains will produce
proportional stress because of fully elastic behavior of the piping
materials, are generally not valid.
•
In addition to the requirements of flexibility and support for
metallic piping in normal fluid service:
156
B.
-
Nonmetallic piping shall be supported, guided, and
anchored to prevent damage to the piping.
-
Point loads and narrow areas of contact between piping and
supports shall be avoided.
-
Suitable padding shall be placed between piping and
supports where piping damage may occur.
-
Valves and equipment that would transmit excessive loads
to the piping shall be independently supported.
•
Thermoplastics shall not be used in flammable fluid service
above ground and shall be safeguarded when used in most fluid
services.
•
Nonmetallic piping is joined by bonding. Bonding can be
achieved through many methods including adhesive, wrapping,
heat fusion, hot gas welding, and solvent cementing.
Category M Fluid Service
The following highlights several aspects of Category M fluid
service. Refer to ASME B31.3 for additional details.
Category M defines a fluid service in which the potential for
personnel exposure is judged to be significant, and a single
exposure to a very small quantity of the toxic fluid can cause
irreversible harm to breathing or points of bodily contact. The
following highlights several provisions, in addition to those specified
for normal fluid service, that apply to Category M Fluid Service.
•
Design, layout, and operation of piping shall be conducted to
minimize impact and shock loads.
•
Conditions which could lead to detrimental vibration, pulsation,
or resonance effects should be avoided or minimized.
•
No allowances may be made for pressure-temperature
variations. The coincident pressure-temperature conditions
requiring the greatest wall thickness or the highest component
rating will determine design temperature and pressure.
•
All fabrication, as well as all threaded, bolted, and other
mechanical joints, shall be visually examined.
•
A sensitive leak test in addition to the required leak test must
be included.
157
•
C.
The following may not be used:
-
Miter bends not designated as fittings, fabricated laps, and
nonmetallic fabricated branch connections
-
Nonmetallic valves and specialty components
-
Threaded nonmetallic flanges
-
Expanded, threaded, and caulked joints
High Pressure Piping
The following highlights several aspects of high pressure piping
design. Refer to ASME B31.3 for additional details.
Design Conditions and Criteria
Piping is generally considered to be high pressure if it has a
pressure over that allowed by Class 2500 for the specific
design temperature and Material Group. However, there are
no specific pressure limitations for the application of the
rules for high pressure piping.
In most cases, the design pressure of each component in a
high pressure piping system must be at least equal to the
pressure at the most severe condition of coincident internal
or external pressure and temperature expected during
service. The design temperature of each component in a
piping system is the temperature at which, under the
coincident pressure, the greatest thickness or highest
component rating is required.
Consideration must be given to the ambient effects on a
piping system.
•
The cooling of a gas or vapor may reduce the pressure
sufficiently to create a vacuum.
•
The heating of a static fluid in a piping component causes
a pressure increase.
•
Moisture condensation can result in atmospheric icing
when piping system design minimum temperature is less
than 32°F.
158
In any case, the design must allow the system to either
withstand or provide some type of relief from the ambient
effects.
Other effects to consider include:
2.0
•
Dynamic Effects (e.g., impact, wind, earthquake,
vibration, discharge reactions).
•
Weight Effects (e.g., live loads, dead loads).
•
Thermal Expansion and Contraction Effects.
•
Effects of Support, Anchor, and Terminal Movements.
•
Allowable stresses.
•
Wall thickness calculation requirements.
•
No allowance for pressure above the design pressure
permitted.
•
Particular fabrication details not permitted (e.g., miters).
Examination
While the examination of High Pressure Piping is very similar
to that of piping in normal fluid service, it must be more
extensive. For example, in normal fluid service, a sample
selected at random per the inspector's judgement is
sufficient to make a determination as to the acceptability of
the material. In high pressure piping, 100% of the material
and components must be examined. Also, only 5% of the
fabrication must be examined for normal fluid service,
whereas 100% of fabrication must be examined in high
pressure piping.
3.0
Testing
Prior to initial operation, each piping system shall be either
hydrostatically or pneumatically leak tested. Each weld and
each piping component (except bolting and individual
gaskets to be used during final assembly) shall be tested. If
the testing is done on the equipment prior to installation, an
additional test of the installed piping system shall be
conducted at a pressure not less than 110% of the design
pressure. If the initial testing is done on the installed piping,
then the additional test is not necessary.
159
XI.
SUMMARY
A process plant piping system includes much more than just straight sections of
pipe. It also includes fittings, flange assemblies, valves, pipe supports, and
restraints. ASME B31.3 specifies the design, materials, fabrication, erection,
inspection, and testing requirements for process plant piping systems. This
course provided an overview of process plant piping system requirements,
including items that are not explicitly included in B31.3 (e.g., valve selection and
design, flexibility analysis guidelines, equipment nozzle load requirements, etc.).
Participants can use this information on their jobs, and are prepared to take more
extensive courses if appropriate.
160