All Rights Reserved. Disclaimer Please read the following carefully: This software and this manual have been developed and checked for correctness and accuracy by SST Systems, Inc. Users must carry out all necessary tests to assure the proper functioning of the software and the applicability of its results. All information presented by the software is for review, interpretation, approval and application by a Registered Professional Engineer. SST Systems, Inc. Thank you for licensing CAEPIPE pronounced kay-pipe , the simple yet powerful software for solving a variety of piping design and stress analysis problems in several industries such as energy, process and aerospace.
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SST Systems, Inc. All Rights Reserved. To partly remedy this problem, we provide a simple tutorial on the basics of piping stress analysis.
It is common practice worldwide for piping designers to route piping by considering mainly space, process and flow constraints such as pressure drop and other requirements arising from constructability, operability and reparability.
So, when as designed piping systems are handed-off to pipe stress engineers for detailed analysis, they soon realize that the systems are stif and suggest routing changes to make the systems more flexible. The piping designers, in turn, make changes to routing and send the revised layout to the pipe stress engineers to check for 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 worsened in recent years by increased operating pressures and temperatures in order to increase plant output; increased operating pressures require thicker pipe walls, which, in turn, increase piping stifnesses further!
Such increased operating temperatures applied on stifer 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.
Types of Loads. Piping systems experience diferent types of loadings, categorized into three basic loading types Sustained, Thermal and Occasional loads. Sustained Loads These mainly consist of internal pressure and dead-weight. Dead- weight is from the weight of pipes, fittings, components such as valves, operating fluid or test fluid, insulation, cladding, lining, etc.
Additionally, internal pressure gives rise to axial stresses in the pipe wall. A pipes deadweight causes the pipe to bend generally downward between supports and nozzles, producing axial stresses in the pipe wall also called bending stresses which vary linearly 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. For the calculated actual 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 deadweight are: Variable spring hangers Constant support hangers Rod hangers Resting steel supports Rod hangers and resting steel supports fully restrain downward pipe movement but permit pipe to lift up.
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 Loads Expansion Loads These refer to the cyclic thermal expansion or contraction of piping as it goes from one thermal state to another for example, from shut- down to normal operation and then back to shut-down. If, on the other hand, 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.
For example, if two equipment nozzles which are to be connected by piping are in line, then the straight pipe connecting these nozzles will be very stif. 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 given above and steps e and f given below, such nozzle loads can be reduced.
Occasional Loads This third type of loads is imposed on piping systems by occasional events such as earthquake, wind or a fluid hammer. To carry sustained loads, normally vertical supports are required. For thermal loads, having no supports gives zero stresses.
So, fewer the number of supports, lower the thermal stresses. Axial restraints and intermediate anchors are recommended only to direct thermal growth away from equipment nozzles. Once all the data is in, Analyze. Now, review Results. General Procedure. Here is a step-by-step procedure. Step 1 Review the thermal stress contour plot first. The plot is color- coded such that blue region denotes areas with the least stress ratios where stress ratio equals to actual computed stress divided by material allowable stress , green region with higher stress ratios, yellow region with even higher stress ratios, and red region with the highest stress ratios.
Intermediate areas between these distinct colors will be of bluish-green, greenish- yellow and orange colors. The goal will be to arrive at a layout that avoids orange and red zones in thermal stress plot so that there is suficient thermal margin left for performing a detailed piping analysis when the layout is finalized at the 3D-design stage.
You may wish to avoid even the yellow zone in the stress contour plot so as to provide additional thermal margin for future use. Since thermal stresses generated are directly dependent on how stif or flexible the layout is, in order to reduce thermal stresses, it may be necessary to make the layout flexible by including bends, offsets, loops etc. So, the first step is to make sure thermal stress ratios remain within blue to yellow range and not get into orange and red zones.
For more flexible layout, even yellow zone can be avoided. Step 3 After finalizing piping layout under Steps 1 and 2 for thermal loading, the next task is to support the system vertically to carry its deadweight under operating condition. In this connection, first review sustained stress ratio contour plot generated by deadweight and pressure for the system without any vertical supports excepting those provided by equipment nozzles, shown in color codes from blue to green to yellow to red as in Step 2 above.
Step 4 In case sustained stresses exceed yellow zone in one or more areas of the piping system, study the deformed shape for sustained load case in order to understand how the piping responds to its own deadweight: next, identify pipe locations in the 3D model where the pipe can be vertically supported by the support types listed above. Based on this information, it is possible to vertically support the piping such that sustained stresses do not exceed yellow zone.
If pipe lifts up at any of these resting supports during operating condition, then that support does not carry any pipe weight and hence will not serve its purpose. Similarly, at rod hanger locations, the tendency of piping should be to deform downward for operating load case, so that the rod hangers carry the pipe weight under tension. On the other hand, if pipe lifts up at any of the rod hangers, then that rod hanger goes into compression thereby not carrying the weight of the piping during operating condition.
Step 5 You should perform Steps 1 to 4 for all piping systems of the project. Systems, for which the layout and support schemes are finalized, are ready for detailed analyses and stress report preparation. It is most likely that the layout and support schemes finalized during steps 14 meet all other pipe stress requirements such as meeting nozzle allowable loads and hardly require any further iteration s.
The pipe, made of A53 Grade A carbon steel, is heated to F. This problem illustrates the use of expansion loops to reduce thermal stresses.
Figure 1A Layout. After modeling this layout in CAEPIPE, upon analysis, you will find that the pipe between nodes 10 and 20 grows thermally to the right towards node 20, while pipe between nodes 30 and 20 grows up towards node 20, as illustrated in Fig. Figure 1B - Thermal Deflection. This thermal deformation generates large thermal stresses orange and red zones in the bend at node 20 and at anchor node 30, as shown in Fig. This layout will have to be rerouted. Let us try the rerouting as shown in Fig.
Figure 1D - Rerouting. So, thermal growth of X-directional pipes between nodes 10 and 14 and then between 18 and 20 as well as the growth of Z- directional pipe between nodes 30 and 20 are absorbed by the three bends at nodes 14, 18 and The corresponding stress contour plots for thermal and sustained load cases are shown in Fig.
Figure 1E - Code-compliant thermal case. Figure 2B - Thermal Deformation Plot. This, in turn, produces large thermal stresses i. In other words, the thermal growth of pipe between nodes 40 and is mostly absorbed by the 4 loop and very little by the 8 loop, defeating the very purpose of the 8 loop.
In order to alleviate thermal stresses in the 4 loop, introduce an intermediate anchor at node 95 immediately after the second reducer, so that the thermal growth of straight pipe from node 95 to node is absorbed by the 8 loop, while the thermal expansion of straight pipe between nodes 40 and 95 is absorbed by the 4 loop, thereby making both loops achieve their intended purpose.
The corresponding thermal displacement and thermal stress contour plots are given in Fig. The deformed geometry due to the thermal load Fig. The intermediate anchor at node 95 restrains the vertical riser between nodes and 70 from thermally growing upward towards node As a result, this riser grows downward producing large bending moments and stresses at and around equipment nozzle at node Since the intermediate anchor efectively restrains upward growth of this vertical riser node 70, we see large localized thermal stress at the welding tee.
See thermal stress contour plot shown in Fig. The resulting deformed geometry plot in Fig. Figures 3F and 3G show the thermal and sustained stress contour plots in this case sustained stress is due to only deadweight as pressure is zero , confirming a code-compliant system for both load cases.
It is observed that the weight of i. On the other hand, the pump nozzle at node 40 carries the weight of the horizontal line from node 20 to node 40, iii. The deformation response for deadweight, in turn, generates large forces and moments and hence large sustained stresses at nozzle nodes 5 and 40 as shown in Fig. When one pump is operating, the other one is on standby.
Figure 5B - Thermal Deformation Plot. This causes the pipe between nodes and to bend at the tee producing high strains and hence thermal stresses locally at the tee node , as shown in Fig. We then shift the second part downstream towards the two pumps, resulting in the modified layout shown in Fig. From the thermal deformation plot for this revised layout shown in Fig. The branch pipe between nodes and acts as a rigid stick resulting in lower thermal stresses in that branch pipe as seen in Fig.
These opposing deflections rotate the interconnecting pipe between nodes 90 and like a horizontal see-saw in the horizontal XZ plane, resulting in lower thermal stresses in this region, as observed in Fig. Although the thermal stress criterion has been met, the weight stresses exceed the sustained stress allowables, as illustrated by many red and orange areas in the sustained stress contour plot given in Fig 5G.
Vertical resting supports are therefore introduced as shown in Fig. The recalculated sustained stress i. We hope you feel confident now in playing with CAEPIPE by creating simple models and conducting several what-if studies on them, as alluded to in the examples above. If you have questions, please feel free to send them to us support sstusa. If you have not downloaded the free pipe stress analysis software, visit www.
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