Pipe Sizing, Wall Thickness
and Pressure Drop
Calculation
By
Rajesh Chawla
L&T-Sargent & Lundy Limited
Vadodara
20 May 2008
1
Introduction
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20 May 2008
General Definitions
Piping Development Process
Piping System Standards
Piping Sizing
Piping Wall Thickness
Piping Pressure Drop
Recommended Piping Velocity
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1
Definitions:
piping: assemblies of piping
components used…[for] fluid flows. Piping
also includes pipe supporting elements,
but does not include support
structures…or equipment…
piping system: interconnected piping
subject to the same design conditions
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3
More Definitions:
piping components: mechanical
elements suitable for joining or assembly
into pressure-tight fluid-containing piping
systems…pipe, tubing, fittings, flanges,
gaskets, bolting, valves and devices such
as expansion joints, flexible joints,
pressure hoses, traps, strainers, inline
portions of instruments and separators.
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& More Definitions:
design pressure: the pressure at the
most severe condition of internal or
external pressure and temperature
expected during service
design temperature: the temperature
at which, under the coincident pressure,
the greatest thickness or highest
component rating is required
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Piping Development Process
1. Establish applicable system standard(s)
2. Establish design conditions
3. Make overall piping material decisions
– Pressure Class
– Reliability
– Materials of construction
4. Fine tune piping material decisions
– Materials
– Determine wall thicknesses
– Valves
5. Establish preliminary piping system layout & support
configuration
6. Perform flexibility analysis
7. Finalize layout and bill of materials
8. Fabricate and install
9. Examine and test
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3
Piping System Standards
Provide a set of requirements for obtaining
a safe, reliable and economical installation
Are frequently called Codes; for example,
B31 piping system standards are called
Codes
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Why Pipe Sizing is Important
• As much as 30% of the total cost of the typical
plant goes for piping, piping elements and valves
• A significant amount of operating cost (energy) is
also used up in forcing flow through piping and its
components
• A significant amount of maintenance cost is also
for the piping and associated things
Proper sizing, optimal in some sense, is
therefore very necessary
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4
Avoid Oversizing & Undersizing
Oversizing
¾ Higher material and installation costs - extra but
unnecessary
¾ Delay in getting at outlets
¾ Increase heat loss from distributing piping
¾ Increased condensate formation
Undersizing
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¾
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Lower pressure at point of use during peak demand
Variation in temperature & pressure at outlet
Risk of steam starvation
Risk of erosion, water hammer and noise
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Inputs Required to Perform Pipe
Sizing and Pressure Drop
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Contract Specification
Design Criteria Document (DCD)
Heat Balance Document (HBD)
Water Balance Document (WBD)
Flow Scheme & P&IDs
Design Parameters for Piping Systems
Plot Plan
General Arrangement
Pipe layouts and piping isometric drawings
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Interface Responsibility
• Equipment Interfaces points and the design
parameters for the same
• Any specific design requirement from the
equipment vendor
• Budgeted pressure drop for Critical piping
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Pipe Sizing Criteria
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Velocity Consideration
– Simplest in approaches
– Recommended values of linear velocities for the
flowing medium are used
Available Pressure Drop Consideration
– More involved and most important method of pipe
sizing
– A minimum pipe size which causes a pressure drop at
the most equal to this maximum acceptable pressure
drop
– Would be uneconomical
Economic Considerations
– Linear velocity and available pressure drop constraints
are not stringent
– Economics is governed by the capital cost of the pipe
and accessories including fittings, insulations etc.
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Methods Use for Pipe Sizing
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Outside Diameter Controlled Pipe Sizing
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The pipes are categorized in schedule numbers per ASME B
36.10 and B 36.19
Schedule numbers bear a relation to the pressure rating of
the piping
Eleven Schedules ranging from the lowest at 5 through 10,
20, 30, 40, 60, 80, 100, 120, 140 to schedule No. 160
Stainless steel piping the schedules will be suffixed by “S”
Outside diameter remains constant and inside diameter
changes as the wall thickness increases
Inside Diameter Controlled Pipe Sizing
–
–
Used when required wall thickness is not available in the
schedule pipes
Inside diameter remains constant and outside diameter
changes as the wall thickness increases
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Definitions:
Calculated Minimum Wall Thickness (tm) - The minimum pipe wall
thickness required for design pressures and temperatures for various
materials plus allowances for mechanical strength and corrosion or
erosion, if any.
Actual Minimum Wall Thickness (ta) - The actual pipe wall thickness
including manufacturing tolerances and allowance for pipe bends, etc.
The actual wall thickness is always greater than the calculated minimum
wall thickness.
Nominal Wall Thickness (tn) - The commercially available pipe wall
thickness.
Allowable Stress - The maximum stress allowed for a material for a given
temperature.
Schedule Pipe - Tubular products manufactured to the dimensional
requirements of ASME B36.10 and ASME B36.19.
Bend Radius - The distance from the center point of the bend to the
centerline of the bent pipe. Bend radius is usually specified as a multiple
of the nominal pipe size.
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Pipe Sizing Procedure
– Assume a pipe diameter
– Determine the flow rate
– Determine the effective pipe length
– Calculate the permissible loss of head
– Determine the pipe diameter
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Equations for Pipe Wall
Thickness
WALL
THICK
tm =
ta =
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SCHEDULE
PIPE
PD
+A
2 S +2 yP
tm
×F
.875
CAMERON
EXTRUDED PIPE
P ( ID)+2 A( S + yP )
2( S + yP−P )
tm + .005 (without bend)
or
tm x F
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Thickness Calculation as
Per B 31.1
ASME B 31.1 Power Piping Code in clause 104.1.2
gives formula for minimum thickness as
tm = PDo
+A
2(SE +Py)
Where;
=
tm
P
=
=
Do
SE
=
Min. reqd. wall thickness
Internal design Pr.
Outside Dia. of Pipe
Max. Allowable Stress
From Appendix ‘A’.
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Thickness Calculation as Per
B 31.1
Y
=
Coefficient From Table 104.1.2.(A)
A =
Additional Thickness to compensate for
1) Mat. removed for threading
2) Corrosion and erosion
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B 31.1 Pipe Bends- Thickness of Pipe
Table 102.4.5 give min. recommended thickness prior to
bending as;
Radius of
Bends
≥
6D
5D
4D
3D
min. thk.
Prior to bending
1.06 tm.
1.08 tm.
1.14 tm.
1.25 tm.
31.3 Do not contain above table but gives the formula to
calculate the same.
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Thickness Of Straight Pipe Under
External Pressure
The pipe with a large ratio of diameter to wall
thickness will collapse under an external
pressure which is only a small fraction of internal
pressure which it is capable of withstanding.
To determine the wall thickness under
external pressure, the procedure outlined in the
BPV Code ASME Section VIII Div. 1 UG-28
through UG-30 shall be followed.
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Thickness Of Straight Pipe Under
External Pressure
Example:
A 6" (150 mm) NB pipe has an external Design Pressure of
400 psig at 7500 F. The material of construction of pipe is
seamless austenitic stainless steel to ASTM A 312 TP 304L.
The corrosion allowance is nil. Calculate thickness and select
proper schedule.
Refer ASME Section VIII Div.1. UG 28
Assume value of ‘t’ and determine ratios
L and
Do
Do
t
Do for 6" NB pipe
= 6.625"
Assume SCH 5 S pipe
Nominal thickness
= 0.109"
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Thickness Of Straight Pipe Under
External Pressure
Minimum thickness considering negative mill tolerance of 12.5%
t = 0.875 x 0.109 = 0.095"
Consider,
L
= 50
Do
Since L is unspecified
Do
6.625
=
=
69.7
t
0.095
From Graph (Fig. G) in ASME Section II Part D
Factor A = 0.000225
From Graph (Fig. HA-3) in ASME Section II Part D
Factor B = 2750 For the above factor A and for 7500F
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Thickness Of Straight Pipe Under
External Pressure
Allowable pressure Pa
4
B
=
3
Do/ t
4 x 2750
=
=
52.6 psig
3 x 69.7
This is less than the Design Pressure
Therefore, assume higher thickness.
Consider SCH 80 S pipe
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Thickness Of Straight Pipe Under
External Pressure
Nominal thickness
Minimum thickness
= 0.432"
= 0.875 x 0.432
= 0.378"
Do
6.625
=
= 17.5
t
0.378
Do
Factor A for the new value of
is 0.0038
t
Corresponding factor B = 5500
Allowable Pressure, Pa
4 x 5500
= 419 psig
3 x 17.5
More than Design Pressure
Hence select SCH 80S pipe
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Pressure Drop in Piping System
Two methods are commonly used:
By adding experimentally determined values of
L/D for each component in the equation
V2
ΔH = f (L / D)
2g
where;
ΔH =
f
=
L
=
D
=
V
=
g
=
Pressure drop due to friction, ft H2O
Darcy’s friction factor from Moody Diagram non - dimensional
Equivalent length of pipe segment, ft
Internal diameter of pipe, ft
Fluid velocity, ft/sec
Acceleration due to gravity, 32.2 ft/sec2
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Pressure Drop in Piping System
By introduces a resistance coefficient, K,
into the above equation;
V 2
ΔH = K
2g
where;
K
= Resistance coefficient = f (L/D), non-dimensional
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Pressure Drop in Piping System
To determine pressure drop in terms of psi:
ΔHρ
ΔH
ΔP =
=
144 υ
144
where:
ΔP
= Pressure drop, psi
υ
= Specific volume, ft3/lb
ρ
= Weight density, lbm/ft3
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Formulae Used
Darcy-Weisbach Equation
where:
ΔP = pressure, psf (pa)
f = Moody friction factor, dimensionless
L = Length of pipe, ft (m)
D = Inside diameter of pipe, ft (m)
ρ = Density, lb/ft3 (kg/m3)
V = Velocity, ft/s (m/s)
g = Gravity acceleration, 32.174 ft/s2 (9.807 m/s2)
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Formulae Used
Hazen-Williams flow formula
where:
V = Velocity, m/s
Ch = Hazen-Williams flow coefficient
d = Inside diameter of pipe, m
hL = Head loss, m
L = Length of pipe, m
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“K” Values For Valves
VALVE TYPE
PRESSURE CLASS
≤ 300
≥ 600
K
K
Globe
a. Conventional
Check
400f
600f
a. Conventional swing
50f
200f
b. Tilting disc
60f1
40f
8f
12f
Gate
a. Disc
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“K” Values For Valves
FITTING TYPE
Pipe Size
< 2"
K
>2"
K
90o Short Radius Elbow R = 1.0D
60f
20f
45O
Short Radius Elbow R = 1.0D
15f
10f
90O
Long Radius Elbow R = 1.5D
60f
14f
15f
10f
Standard Tee (Converging flow thru
branch)
70f
29f
Standard Tee (Diverging flow thru
branch)
70f
60f
Standard Tee (Converging flow thru
run)
32f
27f
Standard Tee (Diverging flow thru
run)
32f
12f
45O Long
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Radius Elbow R = 1.5D
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Extraction Steam Pressure Drop at Heater Nozzle
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Recommended Piping Velocity for
Steam
Steam Flow Maximum Velocity
Maximum Velocity
Saturated steam greater than 25 psia
(and superheated steam with less than
25°F superheat)
12,500 fpm (except as
covered by Notes 1 and 2
Superheated steam (with a minimum of 20,000 fpm (except as
25°F superheat)
covered by Note 2
Low-pressure wet steam (including
saturated steam up to 25 psia)
7,500 fpm
Notes:
1. When 90° elbows are present in the line, velocity should be limited to 9,000 fpm.
2. Steam piping to feedwater heaters should be designed such that velocity at the
entrance to the heater does not exceed 9,000 fpm
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Recommended Piping Velocity
System
Velocity
Feedwater Discharge
25 fps
Feedwater entering
feedwater heaters
12 fps
Heater Drain Piping
7 fps
General Service Piping
10 fps
City Service Piping
System
Velocity
Condensate Pump
Suction Piping
3 to 4 fps
Slurry
5 to 8 fps
Compressed Air
(Header)
20 fps or less
7 fps
30 fps
Circulating Water Piping
(Steel)*
Compressed Air
(Branch Line <50 ft)
8 to 14 fps
6 to 10 fps
Circulating Water Pipe
(Concrete)*
Oil with viscosities at
or below 2 Centipoise
7 to 10 fps
Fuel Gas Piping (With
Insulation to Reduce
Noise Level)
8,000 to
10,000 fpm
Oil with viscosities
above 2 Centipoise
6 fps or less
(the higher the
viscosity the
lower the
velocity)
* Based on economic considerations (piping cost, pumping cost, and valve closure impact), steel pipe
circulating water velocities up to 14 feet per second have been recommended. The maximum economic
cost effective velocity for concrete circulating water pipe is 10 fps
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Velocity Effect
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Higher velocity
Lower pipe size and higher-pressure drop and higher pumping
cost
Lower velocity
–
Higher pipe size resulting in increased piping system supply
and erection cost
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Oversized lines will require more space for layout
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Impact routing of other systems
Exit velocities to and from equipment
–
Expanders/reducers can be used in the piping system to
obtain the desired velocity local to the equipment
Pump suction piping
–
Lower velocity in order to ensure sufficient Net Positive
Suction Head Available (NPSHA) at pump suction to avoid
cavitations
Downstream of steam bypass valves
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Velocity will high.
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The material of construction shall be of Alloy Steel for about
20D length to overcome the problems of erosion
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“What Can Go Wrong”
¾ Piping material changes – Pipe sizes, wall
thickness and pressure drop affected
¾ Pipe routing changes – Pressure Drop
Calculation affected
¾ Pipe size changes – Pressure Drop and
velocity affected
¾ Pipe stress analysis changes – Pipe
routing affected which leads Pressure Drop
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