As someone that has been on both sides of this. I’ve often seen problems crop up resulting from a pipe stress analysis. Often the piping designer and the pipe stress engineer are different persons in the same organization or the pipe stress engineer is external.
Most piping designers are dependent on communication with the pipe stress engineer to correctly design and go from a concept to a finished and calculated design. Sometimes you can’t contact the pipe stress engineer for example if the calculation has been done years ago. How do you know what the critical areas are to update a design?
Reading pipe stress reports is a great skill to have, to have a better understanding of the critical areas of the piping and to extract the right information you need at that moment.
1. Familiarize yourself with the Coordinate & Node system.
Every pipe stress program works with a coordinate system, where numbered nodes are placed. These nodes can be manually numbered or automatically and they are points in a 3d coordinate system.
Reports usually come with Pictures that show where each node is placed, This is important to be able to read the report itself. It should also show the coordinate system.
Ask for this report if you haven’t received it with the report. It is possible if nodes have coordinates to figure out where they are exactly placed, but this is difficult.
(while in cad software this can differ, the standard convention for pipe stress is that the Y coordinate is up/down, while the X and Z coordinates are in the horizontal plane)
2. Check cases
Reports should have input values on the different cases somewhere in the front of the document. this is going to be filled with things like materials, design pressures, temperatures and external loadings. They are also combined into different case combinations. Write down the most relevant case combinations with case numbers and what they represent. you will use this while looking through.
The often-used load combinations:
- Primary or sustained.
These have the Weight of piping/fluids, pressure and manual external loads.
This is an important load combination that does not take into account any temperature difference and should always clear.
2. Secondary or Temperature
This takes the difference between the highest and lowest temperature, together with any displacements of for example equipment.
This combination will show you the effect of thermal expansion/contraction on the piping without anything else (like weight). The allowable stress for this is higher than for primary.
3. Occasional> Wind/Seismic
This is for any “occasional” loads, they are generally combinations that combine primary, secondary and wind/seismic. with a different set of allowable stresses.
4. Combined or OPE(Operating)
This combination shows the system as it is working, it is a combination of primary and secondary and is the most important for evaluating pipe supports. It will also be the most important for evaluating any movement.
3. Look at support forces and movements
Deciding where and what kind of pipe supports to use is a process that needs agreement between the piping designer and pipe stress engineer. Here are some points to look at when going through.
First, check these values for the Operating or combined cases and then others to get an idea of where the forces come from. If the force on a support is high in operating and Secondary cases, but low in the Primary case. That shows that the force is caused by thermal expansion/contraction.
- Look at forces for each support and the direction. You will have to make sure the supports in those places have adequate Structural steel to hold those forces.
2. Very high support forces can still be ok in a pipe stress report, but it would be costly to build this in reality. That would be a good moment to discuss with the pipe stress engineer for other options.
3. Very low support forces could mean that you can remove those supports and save money by having fewer supports and structural steel. Be sure to check any wind/occasional loads however, sometimes supports are placed to not move off of a pipe bridge under wind load for example.
4. Check the type of supports used:
4.1 The type of support is generally selected by the pipe stress engineer for code compliance, but not always for what is easier to set up. A support that needs to stop translation but not rotation is quite a bit more tricky to make than a fixed support or a guide.
4.2 Often in pipe stress software, a guide is programmed as a support that allows movement in the direction of the pipe (axial). but not to the side. To allow for axial movement in reality, there are small gaps placed, so the pipe is not friction fit. This also means that for example in a vertical pipe, a pipe with multiple guides will rattle around quite a bit.
4.3 Spring hangers are very specific in usage and require a more precise calculation. You need to know quite well what the weight is of the components close to these. So be sure to communicate with the pipe stress engineer the weights of special components like control valves and Flow transmitters, as they can throw-off spring hanger selection the most.
4.4 Spring hangers will also need a specific height put into the pipe stress software. A very short spring hanger will stop the pipe from expanding more than a long one.
5. Check nodal movements of piping and supports.
5.1 Especially with large movements, it is important to check the movements of piping. For example, a long pipeline that has sliding supports and a large movement of 70mm, should have a long pipe shoe, so it doesn’t fall off of the structural beam it is standing on.
5.2 Also sometimes with large movements, the pipe can crash into cable trays, structural parts or even vessels. Those parts are not in the pipe stress program so a pipe stress engineer won’t see them.
6. Check nozzle loads with allowable loads. This is something that the pipe stress engineer should check. But everyone makes mistakes and if you are the only person going through the report then it would be a good idea.
7.0 Check special inputs. These can be displacements of equipment like vessels or connected piping. in high or low-temperature piping systems, the equipment itself moves around as well. You can check if this is input correctly by looking at the nodal movement of an equipment node. A tower that heats up, should have a positive movement on each nozzle.
My checklist:
- – Check for the correct design code (like EN 13480 or B31.3)
- – Check correct temperatures and pressures in input data
- – Check the 3d view for any obvious differences with my model.
- – Check Correct seismic loads (in G or m/s2) and wind forces (Force or speed)
- – Check If present, hydrostatic density and fluid level.
- – Spot check Wall thickness and material code
- – Check nozzle loads with allowable loads
- – Check each pipe support forces while looking at the structural 3d model (200N on a structural beam is fine, but 50.000N would require a check by the structural engineer)
- – Check for large nodal displacements (see if piping is constrained correctly and if supports fall off anywhere)