Contents of Lecture 7 (Part 1)
Introduction to piping design in industry
Piping Engineer
Role, responsibilities and cost breakdown
Piping design components
Pipe
Definitions and factors associated with manufacture
Method of manufacture
Sizes and schedules
Optimum pipe diameter methods
Types of fittings and flanges
Rules for piping design
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Piping Engineer
To become a competent piping engineer
Requires many years of experience
A talent for creative thinking
Piping Design Engineer
Who is he/she?
o
A person who designs piping systems
Each design job
o
o
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Calls on their knowledge
And their ingenuity
And also their horse sense / gut feeling / intuition
The piping designer’s job comprises of
25% knowledge
25% experience
50% horse sense
Why do we have to look into this topic?
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Process Piping Design
Refining and petrochemical industries
Piping constitutes the major expenditure in comparison to all
design disciplines
Cost breakdown
50% engineering design cost
25% material cost of the plant
20-25% of the labour cost in the field
Inept piping design in the office
Could increase the cost of a plant
Process piping design comprises of
Piping, structural, electrical and vessels
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Process Piping Design
Piping
Flow diagrams
Model making
Insulation and painting
Piping material take-off
Piping material control
Instrument selection
Piping design itself
No single book can cover the entirety of piping design
What do we get from books?
Fundamental aspects and how to apply them to become a designer
The piping MTO or material take-off is a list of all the piping items required to purchase to fabricate and
construct the design to complete the demand of the project. This list includes all piping items like a pipe,
piping fittings, valves, flanges, blind flange, spacer & blank, gasket, fasteners, and the special parts like a
strainer, steam trap, flame arrester, rupture disc, bellow, sight glass, hoses, sample cooler, etc.
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Definitions of Pipe and Applications
What is a pipe?
It is a long tube or conduit that may be made of different materials
of construction
What for it is used?
For transporting water, gas, oil or other fluids, possibly along with
particulate solids
Where can you find them?
Almost any place
From piping in an automobile to the complicated maze of piping in
a process plant
Each process plant presents a new challenge to the designer
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Process Piping Requirements
For example
Consider two crude units, one handling 50,000 barrels a day and the
other 100,000 barrels a day
There is similarity between these units
But, there is no duplication as the piping requirements are different
A piping designer
May have worked on 2 to 3 crude units in his/her entire career
In a refinery or petrochemical process
There are literally hundreds of different types of units
Each type presents a different challenge
For these reasons, a piping designer requires personal judgement in
addition to fundamental knowledge
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Piping and Materials of Construction
Piping
Means not only pipe
Also includes the fittings, flanges, valves and other
items
What are they made of?
Metallic
o
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Ferrous: derived from iron, most commonly used in process
piping, e.g. carbon steel, SS, chrome steel, cast iron, etc.
Non-ferrous: e.g. aluminium
Non-metallic: e.g. plastic, ceramic, glass, etc.
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Methods of Manufacture of Pipe
Determining factors in the selection of the pipe manufacturing process
Pipe diameter
Wall thickness
Material specification
Delivery requirements
Piping engineer or designer
Must recognise the method of manufacture and its related mill
tolerance before calculating minimum wall thickness required for
piping
Various methods of manufacture will also determine the length of the
delivered pipe
o
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E.g. commonly, pipe is made in single random length (SRL), which is around 5-7
metres
In double random length (DRL), which is around 11-13 metres
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Methods of Manufacture of Pipe
Steel piping
Made by lap-welding, spiral welding, butt welding and seamless
methods
Welded pipe
Made from flat plates which are rolled to form round shapes
Edges are then welded together to form a longitudinal weld
Disadvantages of longitudinal welding
o
o
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Reduces pressure-containing characteristics of pipe
Reduces allowable stress if ANSI (American National Standards Institute) piping
code is used
Imposes a joint efficiency <100%
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Methods of Manufacture of Pipe
Seamless piping
100% Joint Efficiency (no longitudinal joint)
Welded pipe
Can attain 100% Joint Efficiency
In small sizes
Seamless piping is quite often as economical as welded if
100% joint efficiency is specified
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Pipe Diameters, Thickness and Schedules
Pipe and tubing
Are they the same? No, they are not.
Tubing is specified by its outside diameter
o
E.g. 4 inch tubing has a 4 inch OD
In the case of piping, 4 inch pipe has a 4.5 inch OD
o
This is usually specified as 4 inch IPS (Iron Pipe Size) for pipe
o
Can also be defined by specifying 4 inch schedule 40
o
Schedule number defines the “OD” and “nominal” wall thickness for IPS piping
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Nominal wall thickness is the average wall thickness of the manufactured pipe not the minimum wall thickness
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Pipe Diameters, Thickness and Schedules
Standard pipe is made in a discrete number of sizes that are
designated by nominal diameters in inches (IPS)
Depending on the size, up to 14 different wall thicknesses are
made with same outside diameter
Identification is done using SCHEDULE NUMBERS
Schedule number = 1000P/S
where:
P = internal pressure, psig
S = allowable working stress, psi
Schedule 40 is the most common one
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Pipe Diameters, Thickness and Schedules
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Pipe Diameters, Thickness and Schedules
Sizes of 1.25, 2.5, 3.5 and 5 inches
Normally considered as non-commercially manufactured
Usually not specified by a piping design engineer
Equipment manufacturers may employ these sizes
o
o
The piping designer will have to attach a flange or reducer to this
connection
And immediately increase to the next larger size for his/her piping
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Optimum Pipe Diameter
Capital investment in process piping
25-40% of the plant investment
Power consumption for pumping
o
Depends on the line size
This is a significant fraction of the total cost of utilities
As the diameter of a line increases
The capital cost of piping goes up (Why?)
Usually accompanied by decrease in consumption of utilities and
costs of pumps and drivers (Why?)
o
Because of reduced friction
What do we need to do?
Find the optimum balance between operating cost and capital cost
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Optimum Pipe Diameter
How do we obtain optimum line sizes?
For small capacities and short lines, optimum values can be obtained on the
basis of typical velocities or pressure drops (dealt with in Lecture 5 Part 1)
What do we do in the following scenario?
Large capacities
Long lines
Expensive material of construction
Highly viscous material
o
Selection has to be based on a complete economic analysis
How to take care of viscous materials?
o
Possibly by heating the fluid to alter its properties – principally its viscosity and
consequently the power requirement for pumping
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Annual Water Pumping Costs
Source: Energy Tips – Pumping Systems, Industrial Technologies Program Pumping Systems Tip Sheet #9 • October 2005, US Department of Energy
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Optimum Pipe Diameter
For installed costs of piping and pumping equipment
Adequate information must be available
Supplier quotations are desirable in most cases
Published correlations may be adequate
Are there any simplifications in deriving the optimum diameters?
Yes – ignoring the costs of pumps and drivers
Since they are essentially insensitive to pipe diameter near the optimum value
Two short-cut rules (Peters and Timmerhaus) for optimum diameters for steel
pipes of 1 inch size or greater
D = 3.9 Q 0.45 ρ 0.13 ( for turbulent flow)
D = 3.0 Q 0.36 ρ 0.18 ( for la min ar flow)
Nomographs based on
these equations are
given in Figure
D is in inches; Q in cuft/sec; ρ in lb/cuft; µ in cP
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Optimum Pipe Diameter
The factors involved in the derivation of the two formulae are
Power cost to be $0.055/kWh
Friction loss due to fittings is 35% that of the straight pipe length
Annual fixed charges are 20% of installation cost
Pump efficiency is 50%
Cost of 1 inch IPS schedule 40 pipe is $0.45/ft
Other detailed studies of line optimization
Happel and Jordan, Chemical Process Economics, Dekker, New
York, 1975
Skelland, 1967
o
This reference looks at problems in simultaneous optimization of pipe
diameter and pumping temperature in laminar flow
Mohinder K. Nayyar, Piping Handbook, McGraw-Hill, New York,
1998
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Economic Pipe Diameter
Liquids
Gases
Perry’s 5th edition:
Nomograph - created using cost
data from the early 1960s. Since
only relative cost data are
important, the economic
diameter should not change
significantly over time.
To use, draw a straight line
between the flow rate (in
gallons per minute for liquids or
cubic feet per minute for gases)
to the density (top for liquids,
bottom for gases). Where this
line crosses the middle scale
gives the economic diameter of
Schedule 40 steel pipe.
Smaller diameters should be
used for more expensive piping
materials, larger diameters for
more expensive pumps or
compressors.
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Optimum Pipe Diameter Correlations
For turbulent flow:
Where di,optimum is the diameter in m, G is the flowrate in kg/s, and 𝞺𝞺 is the
fluid's density in kg/m3
Source: Towler & Sinnott, p269
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Fittings and Flanges
Welded fittings are manufactured to match the companion type
It is not mandatory that the fitting and the pipe have the same
thickness
Pipes of several schedules - stocked
Fittings of several schedules - not stocked
Fittings are usually specified as
o
o
o
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Standard weight
Extra strong
Schedule 160
Double extra strong
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Fittings and Flanges
It is usually advantageous
To specify a fitting schedule of the next higher available weight
E.g. for 14 inch schedule 10 (0.250" wall thickness), the standard
weight would be specified, which has a 0.375" wall
For 14 inch schedule 40 (0.438" wall thickness), the extra strong
fitting would be specified, which has a 0.5" wall
For pipe sizes 2" and below, welded fittings are usually not used
For low pressure, non-critical service, screwed fittings are specified
For high pressures and most process systems, socket welding
fittings are employed
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Do’s and Dont’s for Pipe Designers for Fittings
Use eccentric reducers only where absolutely necessary
Why? They cost almost twice as much as concentric reducers
Reducing elbows are a cost saving for sizes 8 inch and
below for the large end in carbon steel materials. Avoid
their use in alloy materials.
Avoid the use of the 90° elbow at the end of a long pipe run
The short-radius elbow causes additional pressure drop in a
piping system
Use it only where close connections are needed
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General Do’s for Pipe
• Use Pipes and Fittings from same manufacturer.
• Install according to the Installation instructions and follow recommended safe
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work practices.
Keep Pipe and Fittings in original packaging until needed and store pipes in
covered areas.
Use tools designed for use with plastic pipe and fittings.
Take correct precautions while installing pipes and fittings above 2″ in
accordance with Industry recommendations.
Remove dirt from pipe & fittings. Clean pipe & fittings with clean cloth.
Cut off min. 25 mm beyond the edge of the crack in case any crack is
discovered on the pipe.
Cut the pipe as square (perpendicular) as possible before making a joint.
Ensure no sharp edges in contact with the fittings surface while inserting the
pipe.
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General Do’s for Pipe Designers
• Ensure proper alignment of pipe & fitting to avoid stress on the
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joints.
Ensure installation is done in such a way that there are no
chances of air entrapment.
Provide vertical & horizontal supports as recommended.
Use solvent cement only as thread sealant.
Insulate hot water pipes exposed to the atmosphere.
Always conduct hydraulic pressure testing after installation to
detect any leaks and faults.
Wait for appropriate cure time before pressure testing. Fill lines
slowly and bleed air from the system prior to pressure testing.
Provide expansion loops on hot water lines.
Paint pipe (water based paint) exposed to sunlight.
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General Dont’s for Pipe Designers
• Do not Use Metal Hooks or Nails to support/hold or put pressure on
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the pipes. Do not use straps & hangers with rough or sharp edges.
Do not tighten the straps over the pipes.
Never expose the pipe to Open Flame while trying to bend it.
Do not drop pipes on edges from heights. Do not drop heavy objects
on pipes or walk on pipes.
Do not use Fusion Compound for PVC or any other plastics for
joining CPVC pipes & Fittings.
Do not dilute the Fusion Compound with Thinners/ MTO or any
other liquid etc.
Do not use air or gases for pressure testing.
Do not use any other petroleum or solvent-based sealant, adhesive,
lubricant or fire stop material on CPVC/PVC pipes and fittings.
Do not use CPVC/PVC Pipes & Fittings for pneumatic applications.
Do not use plastic threaded fittings for hot water above 60°C.
Do not thread CPVC pipes.
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Best Practices of Process Piping
• The Process Piping Best Practices Series: Layout and
Design (processengineer.com)
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What have we learnt from this lecture?
Overview of piping design in industry
Familiarity with the role of the piping design engineer
and the cost breakdown in piping design
Piping design components
Learnt the factors associated with pipe manufacture
covering
Methods of manufacture
Sizes and schedules
Methods to estimate optimum pipe diameter
Types of fittings and flanges
Rules for piping design
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