WHITE PAPER / PIPE STRESS ANALYSIS

FINDING THE RIGHT FLEXIBILITY FOR PIPING SYSTEMS

BY Phil Zsiga, PE

The cost of a high-pressure gas line failure is great. That is why pipes and their supports must be designed

to withstand the many stresses that can threaten their structural integrity. The challenge is to design a

piping system with suffi cient fl exibility to distribute stresses, without overdesigning the system and adding unnecessarily to its cost. Pipe stress analysis can help.

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From an engineering perspective, a piping system is,

at its root, a group of beams that connect together to

form a shape and transport a liquid or gas. Whether

installed above, below or near grade, these beams are

subject to a variety of continually changing stresses

and strains. Some, caused by changes in pressure,

weight and temperature, are associated with normal

operating loads. Others come from the occasional

loads created by wind, earthquakes and water hammer.

In a worst-case scenario, these stresses can lead to

catastrophic pipe failure that results in loss of life.

Even in less serious situations, the cost of repairs, lost

product and cleanup can be substantial, especially

when a pipe system’s performance relies on — and its

failure impacts — adjacent equipment and systems.

That is why it’s important for designers to understand

how a piping system can be expected to behave

when subjected to stresses and strains, and then

to use those findings to inform the design process.

Most rely on pipe stress analysis for this purpose.

Pipe stress analysis serves a number of important

functions. By predicting the stresses a pipeline is

likely to face, it helps engineers design pipe and

fittings that can withstand them. These analyses are

also necessary for calculating design loads for pipe

supports and restraints. Because the stresses on

adjacent equipment can impact piping systems, these

analyses also help see that nozzle loadings on attached

equipment and pressure vessel stresses at piping

connections are also designed within allowable levels.

Pipe stress analysis, therefore, goes beyond evaluating

the stresses on the pipes themselves and also considers

the forces and moments on equipment flanges that

connect to the pipe. The location, type, forces and

moments that act on pipe support structures are also

considered. Calculating these stresses becomes more

challenging as pipe size grows, with scale and loads

increasing exponentially as diameter increases.

DESIGNING OPTIMAL PIPE SYSTEM FLEXIBILITY To withstand these stresses and strains, it is

necessary to know how much a pipe might be

expected to expand and contract, and then design

that amount of flexibility into a piping system.

The question is: How much flexibility is enough?

A piping system with insufficient flexibility is at risk

of cracks, breaks and failure. A piping system that

is too flexible will not only cost more to fabricate

and install; it may also be more prone to vibration.

The pipeline’s resistance to wind, seismic and other

occasional loads may also weaken. Because excess

flexibility is not usually taken into account in hydraulic

analysis, it can potentially lead to a drop in pressure

and a pump being starved of sufficient flow, producing

a phenomenon known as pump cavitation.

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The truth is, no amount of analysis can calculate pipe

stresses and strains precisely. To improve accuracy,

it is often necessary to separate a piping system’s

beam elements — which are relatively large, compared

to typical finite elements in a finite element analysis

(FEA) — into smaller components. Through a series

of complex calculations, pipe stress analysis can

then achieve a relatively close approximation.

These complex calculations can be expedited and

simplified with the use of computer-aided stress

analysis programs, such as Bentley AutoPIPE and

Intergraph Caesar II. These software programs

create three-dimensional pipe stress computer

models that approximate a piping system’s behavior

under various conditions. ASME B31, the code

for pressure piping system design, fabrication,

installation, testing and certification, includes safety

margins that allow for these approximations.

The models created by computer-aided stress analysis

programs can be very helpful for quick analysis of

complex systems. However, they are only as accurate

as the information entered into them. Obtaining an

accurate model is contingent upon an engineer’s

ability to set correct boundary conditions.

Even the highest-quality software, in other words,

cannot assure the validity of results. That requires

experienced engineering.

THE ART AND SCIENCE OF PIPING SYSTEM DESIGN In practice, an engineer’s level of experience can

have a significant impact on piping system design.

Experienced engineers understand the criticality of

building flexibility into piping systems. If they err, it

is by designing more flexibility into a system than

absolutely necessary, compared to less experienced

engineers, who tend to design too little.

Engineers have multiple ways to add flexibility

to a piping system. Among them:

Expansion joints — One way to absorb heat-related

pipe expansion and contraction is to add expansion

joints to connections. Expansion joints can also help

to isolate vibration and limit lateral pipe movement

that might otherwise reduce the loads on equipment

nozzles. Because expansion joints have the potential

to develop leaks, however, designers often look for

other pipe configuration solutions, especially when

the footprint is large enough to accommodate them.

Pipe routing — Curved pipes will deform when a bending

force is applied to them. Including bends in a piping

system, in other words, increases longitudinal bending

stress and flexibility. The use of Z-bends, L-bends and

expansion loops along the pipe route can provide

the additional flexibility needed to absorb thermal

movement. How much it can absorb is determined

primarily by leg length and the number of bends it

includes, although some system elements, such as

flanges welded to elbows and a succession of elbows not

separated by pipe spools, can impact these calculations.

Distance between supports — Another way designers

add flexibility is by increasing the distance between

pipe supports. When a force is applied to a beam, it

bends in proportion to the length of the beam. A slight

increase in the beam’s length will greatly increase

the degree to which it will bend. In other words, the

longer the pipe, the greater its inherent flexibility.

OTHER DESIGN CONSIDERATIONSOver time, piping that is under constant or intermittent

stress can experience fatigue failure from repeated daily

operation. When making design decisions, engineers

must be able to estimate the maximum stresses in a

pipeline and the fatigue failure of its component or joints.

Each ASME piping code has a table or tables of

Stress Intensification Factors (SIFs) for fatigue failures

of common piping system components. Validated

by fatigue tests, SIFs measure the stresses seen in

bends or elbows, compared to those on a straight

pipe of the same diameter and thickness when

subjected to the same bending. Because SIFs vary

among codes, it’s important to know and follow the

code that is applicable to a given application.

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PIPING SUPPORT DESIGNThere are four general categories of pipe supports.

Vertical stops (V-stops) provide basic pipe support.

While they keep pipe from falling down or sagging,

they do not restrict pipe from moving in axial or

lateral directions or allowing lift off of the support.

Guide supports restrict significant deflection in

lateral directions, while allowing axial movement

and some lift off. Line stops, which can be combined

with guide supports, restrict axial movement

to a set amount. Finally, anchor supports are

designed to restrict all pipe movement.

Designers follow these basic guidelines when

determining the placement of pipe supports:

• Process Industry Practices (PIP) calls for vertical

supports to be placed close enough together to

limit sag deflection to 5/8 inch. The support spacing

requirements of the American National Standards

Institute’s MSS-SP-58 is more conservative.

Before making placement recommendations,

engineers should learn the owner’s standards,

which they can then verify against the code

governing the project. While some owners prefer

greater spacing between supports, vibration can

become a concern at higher sag deflections.

• Supports should be placed near elbows

to limit pressure deflection.

• Guides should be included with every

third vertical or line support. Guide

placement should not limit flexibility.

• Expansion loops should be considered on straight

runs of pipe that are 250 feet or greater, depending

on the product and ambient temperatures.

• Supports should be placed, as needed,

near Z-bends and L-bends along the

route without inhibiting flexibility.

• To minimize costs, supports should be grouped,

whenever possible, to create pipe racks.

• Adjacent pipe supports should be checked

for potential below-grade obstructions

that might impede constructability.

• Computer-aided stress analysis should be used,

when applicable, to verify placements. The

flexibility of the pipe supports themselves should

be a factor in any stress analysis. Any isolation

between the piping and pipe support should also

be taken into account, as should the friction factor

between the piping and the isolation materials,

which can include wraps, I-rods and wear pads.

EQUIPMENT NOZZLE LOADING CONSIDERATIONSA piping system includes the tanks and other

equipment it connects to. Excessive forces, moments

or deflections at the connections can impact the entire

system, resulting in leaks at flanges, damage to vessel

walls or sensitive equipment, pump misalignment

and disproportionate loading of skid supports.

Nozzles are a particularly weak link. That is why it is

critical to check the American Petroleum Institute

(API) and/or manufacturer standards for the allowable

loads on equipment nozzles. The flexibility of the

nozzle and the equipment should also be considered.

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CONCLUSION A designer’s challenge is to design a piping system

with enough flexibility to withstand stresses, without

overdesigning it. This is achieved by coordinating

design for the piping and support system between

mechanical and structural engineers from the project’s

onset. Analysis of the system and the loads it will impact

should, along with geotechnical investigations, be among

the lead items on the design team’s to-do list. Code

requirements should be a primary consideration. Plans

for future line additions should also be factored in early.

It is much less costly to design for future expansion

from the beginning, rather than to come back later.

Pipe stress analysis should begin with the very first

equipment layout. Large equipment should be arranged

to allow access, as well as room for flexibility. Equipment

should set far enough from the pipe racks to allow

for thermal expansion of the piping in the rack.

Rule of thumb calculations for Z-bends, L-bends and

expansion loops should be included in early layouts,

which should be completed before proceeding with

other structural design. The pipe route should also

be coordinated with other disciplines, with stress

analysis taken into consideration at every phase.

Piping system design remains a deceptively complex

challenge. Engineers of all experience levels can

benefit from reviewing designs from the past,

including both those that have worked well, as

well as those that have presented challenges.

The lessons those designs have to teach, in

combination with computer-aided pipe stress

analysis and an integrated design approach,

provide the basis for piping system success.

BIOGRAPHY

PHIL ZSIGA, PE, is a senior mechanical engineer

in the Terminals Group at Burns & McDonnell. His

responsibilities include overseeing the development of

piping and installation diagrams, site plans, equipment

specifications and pipe stress analysis for systems.

Phil received a bachelor’s degree in mechanical

engineering from the University of Oklahoma.

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