Finding the Right Flexibility for Piping Systems/media/files/insights...WHITE PAPER / PIPE STRESS...
Transcript of Finding the Right Flexibility for Piping Systems/media/files/insights...WHITE PAPER / PIPE STRESS...
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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