Pipeline Stress Analysis
06.05
It is common practice worldwide that piping
designers/layout personnel route pipes with consideration given mainly to space
constraints, process and flow constraints (such as pressure drop) and other
requirements arising from constructability, operability and reparability.
Unfortunately, often pipe stress requirements are not sufficiently considered
while routing and supporting piping systems, especially in providing adequate
flexibility to absorb expansion/contraction of pipes due to thermal loads. So,
when "as designed" piping systems are given to pipe stress engineers
for analysis, they soon realize that the layout is "stiff" and
suggest routing changes to make the layout more flexible. The piping designers,
in turn, make routing changes and send the revised layout to the pipe stress
engineers to check compliance again.
Such "back and forth" design iterations between layout and stress departments continue until a suitable layout and support scheme is arrived at, resulting in significant increase in project execution time, which, in turn, increases project costs.
This delay in project execution is further aggravated in recent years as operating pressures and temperatures are increased in operating plants to increase plant output; increased operating pressures increase pipe wall thickness, which, in turn, increase piping stiffness further; increased operating temperatures, applied on such "stiffer" systems, increase pipe thermal stresses and support loads. So, it is all the more important to make the piping layout flexible at the time of routing by piping designers.
The "Design by Color" product "checkSTRESS" from SST (different from CAEPIPE) for AutoCAD, CATIA, Autoplant, PDMS, Cadmatic, etc. can be used to substantially reduce the number of design iterations between the piping layout and stress departments, resulting in huge time savings during design.
Such "back and forth" design iterations between layout and stress departments continue until a suitable layout and support scheme is arrived at, resulting in significant increase in project execution time, which, in turn, increases project costs.
This delay in project execution is further aggravated in recent years as operating pressures and temperatures are increased in operating plants to increase plant output; increased operating pressures increase pipe wall thickness, which, in turn, increase piping stiffness further; increased operating temperatures, applied on such "stiffer" systems, increase pipe thermal stresses and support loads. So, it is all the more important to make the piping layout flexible at the time of routing by piping designers.
The "Design by Color" product "checkSTRESS" from SST (different from CAEPIPE) for AutoCAD, CATIA, Autoplant, PDMS, Cadmatic, etc. can be used to substantially reduce the number of design iterations between the piping layout and stress departments, resulting in huge time savings during design.
Basic Pipe Stress Concepts for
Piping Designers
Piping systems experience different loadings,
categorized into three basic loading types, namely Sustained, Thermal and
Occasional loads.
Sustained Load:
It mainly consists of internal pressure and dead-weight. Dead-weight is from weight of pipes, fittings, components such as valves, operating fluid, test fluid, insulation, cladding, lining etc.
Internal design/operating pressure develops uniform circumferential stresses in the pipe wall, based on which pipe wall thickness is determined during the process/P&ID stage of plant design such that "failure by rupture" is avoided. In addition, internal pressure develops axial stresses in the pipe wall. These axial pressure stresses vary only with pressure, pipe diameter and wall thickness, all three of which are pre-set at the P&ID stage and hence these axial pressure stresses cannot be reduced by changing the piping layout or the support scheme.
On the other hand, dead-weight causes the pipe to bend (generally downward) between supports and nozzles, producing axial stresses in the pipe wall (also called "bending stresses"); these bending stresses linearly vary across the pipe cross-section, being tensile at either the top or bottom surface and compressive at the other surface. If the piping system is not supported in the vertical direction (i.e., in the gravity direction) excepting at equipment nozzles, bending of the pipe due to dead-weight may develop excessive stresses in the pipe and impose large loads on equipment nozzles, thereby increasing the susceptibility to "failure by collapse".
Various international piping codes impose limits, also called "allowable stresses for sustained loads", on these axial stresses generated by dead-weight and pressure in order to avoid "failure by collapse".
For the calculated axial stresses to be below such allowable stresses for sustained loads, it may be necessary to support the piping system vertically. Typical vertical supports to carry dead-weight are:
Sustained Load:
It mainly consists of internal pressure and dead-weight. Dead-weight is from weight of pipes, fittings, components such as valves, operating fluid, test fluid, insulation, cladding, lining etc.
Internal design/operating pressure develops uniform circumferential stresses in the pipe wall, based on which pipe wall thickness is determined during the process/P&ID stage of plant design such that "failure by rupture" is avoided. In addition, internal pressure develops axial stresses in the pipe wall. These axial pressure stresses vary only with pressure, pipe diameter and wall thickness, all three of which are pre-set at the P&ID stage and hence these axial pressure stresses cannot be reduced by changing the piping layout or the support scheme.
On the other hand, dead-weight causes the pipe to bend (generally downward) between supports and nozzles, producing axial stresses in the pipe wall (also called "bending stresses"); these bending stresses linearly vary across the pipe cross-section, being tensile at either the top or bottom surface and compressive at the other surface. If the piping system is not supported in the vertical direction (i.e., in the gravity direction) excepting at equipment nozzles, bending of the pipe due to dead-weight may develop excessive stresses in the pipe and impose large loads on equipment nozzles, thereby increasing the susceptibility to "failure by collapse".
Various international piping codes impose limits, also called "allowable stresses for sustained loads", on these axial stresses generated by dead-weight and pressure in order to avoid "failure by collapse".
For the calculated axial stresses to be below such allowable stresses for sustained loads, it may be necessary to support the piping system vertically. Typical vertical supports to carry dead-weight are:
a. Resting steel supports,
b. Rod hangers,
c. Variable spring hangers, and
d. Constant support hangers.
Both rod hangers and resting steel supports fully restrain
downward pipe movement but permit pipe to lift up at such supports. If pipe
lifts up at any of the rod hangers / resting supports during operating
condition, then that support does not carry any pipe weight and hence will not
serve its purpose.
Two examples are presented in this Tutorial to illustrate how piping can be supported by spring hangers and resting steel supports to comply with the code requirements for sustained loads.
Thermal Load (also referred as Expansion Load):
It refers to the "cyclic" thermal expansion/contraction of piping as the system goes from one thermal state to another thermal state (for example, from "shut-down" to "normal operations" and then back to "shut-down"). If the piping system is not restrained in the thermal growth/contraction directions (for example, in the axial direction of a straight pipe), then for such cyclic thermal load, the pipe expands/contracts freely; in this case, no internal forces, moments and resulting stresses and strains are generated in the piping.
On the other hand, if the pipe is "restrained" in the directions it wants to thermally deform (such as at equipment nozzles and pipe supports), such constraint on free thermal deformation generates cyclic thermal stresses and strains throughout the system as the system goes from one thermal state to another. When such calculated thermal stress ranges exceed the "allowable thermal stress range" specified by various international piping codes, then the system is susceptible to "failure by fatigue". So, in order to avoid "fatigue failure" due to cyclic thermal loads, the piping system should be made flexible (and not stiff). This is normally accomplished as follows:
Two examples are presented in this Tutorial to illustrate how piping can be supported by spring hangers and resting steel supports to comply with the code requirements for sustained loads.
Thermal Load (also referred as Expansion Load):
It refers to the "cyclic" thermal expansion/contraction of piping as the system goes from one thermal state to another thermal state (for example, from "shut-down" to "normal operations" and then back to "shut-down"). If the piping system is not restrained in the thermal growth/contraction directions (for example, in the axial direction of a straight pipe), then for such cyclic thermal load, the pipe expands/contracts freely; in this case, no internal forces, moments and resulting stresses and strains are generated in the piping.
On the other hand, if the pipe is "restrained" in the directions it wants to thermally deform (such as at equipment nozzles and pipe supports), such constraint on free thermal deformation generates cyclic thermal stresses and strains throughout the system as the system goes from one thermal state to another. When such calculated thermal stress ranges exceed the "allowable thermal stress range" specified by various international piping codes, then the system is susceptible to "failure by fatigue". So, in order to avoid "fatigue failure" due to cyclic thermal loads, the piping system should be made flexible (and not stiff). This is normally accomplished as follows:
a. Introduce bends/elbows in the
layout, as bends/ elbows "ovalize" when bent by end-moments, which
increases piping flexibility.
b. Introduce as much
"offsets" as possible between equipment nozzles (which are normally
modeled as anchors in pipe stress analysis).
For example, if two equipment nozzles (which are to be connected by a pipeline) are in line, then the straight pipe connecting these nozzles is "very stiff". On the other hand, if the two equipment are located with an "offset", then their nozzles will have to be connected by an "L-shaped" pipeline which includes a bend/elbow; such "L-shaped" pipeline is much more flexible than the straight pipeline mentioned above.
For example, if two equipment nozzles (which are to be connected by a pipeline) are in line, then the straight pipe connecting these nozzles is "very stiff". On the other hand, if the two equipment are located with an "offset", then their nozzles will have to be connected by an "L-shaped" pipeline which includes a bend/elbow; such "L-shaped" pipeline is much more flexible than the straight pipeline mentioned above.
c. Introduce expansion loops (with
each loop consisting of four bends/elbows) to absorb thermal
growth/contraction.
d. Lastly, introduce expansion
joints such as bellows, slip joints etc., if warranted.
In addition to generating thermal stress ranges in the piping system, cyclic thermal loads impose loads on static and rotating equipment nozzles. By following one or more of the steps from (a) to (d) above and steps (e) and (f) listed below, such nozzle loads can be reduced.
In addition to generating thermal stress ranges in the piping system, cyclic thermal loads impose loads on static and rotating equipment nozzles. By following one or more of the steps from (a) to (d) above and steps (e) and (f) listed below, such nozzle loads can be reduced.
e. Introduce "axial
restraints" (which restrain pipe in its axial direction) at appropriate
locations such that thermal growth/contraction is directed away from equipment
nozzles, especially critical ones.
f.
Introduce
"intermediate anchors" (which restrain pipe movement in the three
translational and three rotational directions) at appropriate locations such
that thermal deformation is absorbed by regions (such as expansion loops) away
from equipment nozzles.
A few example layouts are presented below to
illustrate how loops/offsets, axial restraints and intermediate anchors are
used to reduce thermal stresses in piping (and resulting nozzle loads).
Occasional Load:
This type of load is imposed on piping by occasional events such as earthquake, wind etc. To protect piping from wind (which normally blows in horizontal plane), it is normal practice to attach "lateral supports" to piping systems. During an earthquake, the earth may also move vertically. To protect piping against both horizontal and vertical movement during earthquake, some of the resting supports may be made as "integral 2-way vertical and lateral restraints".
Fortunately, to carry sustained loads, normally vertical supports (as those listed under the Section titled "Sustained Load" above) are required. To withstand static seismic ‘g’ loads, "integral 2-way vertical and lateral restraints" are required. Generally, some of the vertical weight supports can be modified as "integral 2-way vertical and lateral restraints". On the other hand, for thermal loads, zero supports give zero stresses. So, thermal stresses and equipment nozzle loads will normally decrease as the number of supports goes down. Axial restraints and intermediate anchors are recommended only to direct thermal growth away from equipment nozzles.
Occasional Load:
This type of load is imposed on piping by occasional events such as earthquake, wind etc. To protect piping from wind (which normally blows in horizontal plane), it is normal practice to attach "lateral supports" to piping systems. During an earthquake, the earth may also move vertically. To protect piping against both horizontal and vertical movement during earthquake, some of the resting supports may be made as "integral 2-way vertical and lateral restraints".
Fortunately, to carry sustained loads, normally vertical supports (as those listed under the Section titled "Sustained Load" above) are required. To withstand static seismic ‘g’ loads, "integral 2-way vertical and lateral restraints" are required. Generally, some of the vertical weight supports can be modified as "integral 2-way vertical and lateral restraints". On the other hand, for thermal loads, zero supports give zero stresses. So, thermal stresses and equipment nozzle loads will normally decrease as the number of supports goes down. Axial restraints and intermediate anchors are recommended only to direct thermal growth away from equipment nozzles.
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