Design Pressure or Operating Pressure?

[ewpat]

When designing or reviewing a piping system I have always used the design pressure rather than the operating pressure in calculations because this will result in the highest stresses developed during system operation. In addition it will produce the largest displacements as well as support and anchor loads.

Now I am being told that I'm over designing the system and that the code calls for stresses to be calculated based on the operating condition. If this is the case why have design conditions

I have looked through most of the B31.1 Power Piping Code but have not found anything that addresses which pressure should be used.

Any assistance will be greatly appreciated.

 

[XL83NL]

I can only tell you my POV with reference to B31.3 piping systems which I mainly work on (occasionally we do have EN 13480).
I always use design pressure. Reason is that we have a design pressure on the PID, in the spec break, which means that our systems are designed to that limit. From a B31.3 perspective one could discuss whether that’s overkill/conservative, but it keeps out any future discussion when our client wants to change operating conditions of their plants while staying within the range of the spec break on PID, e.g. if they want to use a new catalyst.
The exception I make on this is that nozzle loads and flange loads (to assess potential for leakage), and occasional loads such as wind and earthquake, I use max operating conditions.
Furthermore, be aware to assess the impact certain systems, such as pump bypasses may have, that could result in scenarios not considered merely by design pressure. Review B31.3 appendix S.
The wording design pressure and design temperature won’t be found in B31.3 afaik.
Not sure how all this would convert to B31.1 though.

Huub

 

[RVAmeche]

I've run into this issue as well. For me, the issue mainly comes from us being supplied a pipe spec that far exceeds a particular systems needs, simply because it's an existing pipe spec. So the design temperature & pressure may be much higher than this particular system and the client doesn't want to make a new pipe spec.

I think the technically correct answer is if you're using a spec with only one set of design temperature & pressure values, you should use that in your analysis.

 

[Latexman]

From an old process guys perspective, use design pressure. You never know what that pipe will be used for next year. It will be used over and over until it’s demolished.

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[KevinNZ]

for B31,1 you must use design pressure in the hoop and sustained stress calculations, but you can use operating when you consider occasional loads. You can use the pressure in the system when the occasional occurs.

 

[MJCronin]

Ok, fine.... Here is my two cents...

This whole discussion gets back to the design practices of establishing both the Design and Maximum Operating Pressures FOR THE SPECIFIC PIPING SYSTEM ...

Back in the ancient days, when the crust of the earth was cooling, for any project, we used to have piping system line lists. These lists were tabulated with each and every line number and each pipe had a specific Design PRessure and Max operating Pressure.

The Max Operating Pressure was established by the Process Engineer from the maximum credible pressure that the system could generate under any operating condition. Pump dead head conditions would be an example. The associated relief valve setpoint would be another mechanism to establish this ...

The Design Pressure was a bump upward from that MOP... usually 25 or 50 psig above. This was a matter of judgement.

Yes, I realize that it is easier to get the Design pressure from the piping line spec. But this will be a much higher number ... Andt this spec is developed based on any possible usage for that particular piping /flange/Valve combination.

I believe that a review of how the specific pressure are developed and used would help your problem.

Piping line lists are very useful for many other project purposes as the project drives to completion...

MJCronin
Sr. Process Engineer

 

[0707]

In my opinion, if you are designing, you must do your calculations based on design conditions as per correspondent codes. After that, when your unit is working is another history. To have your unit keep on running in safe condition, you should think in terms of MAWP(Maximum Allowable Working Pressure).

luis

 

[gerhardl]

In my opinion this is quite simple. You have two different systems that will set limits to the use of a piping system:

1. A design pressure is a design pressure. If a piping system is built according to a design pressure, this is the pressure all mechanical components must be calculated for. If not, the complete system is simply not 'designed' after the given specification, and can not be given the certification that it is designed after the specification.

2. Then you will of course have additional options and variations for parts of, or your whole system, adapted and suited to your requirements and local rules and requirements All to be certified and (or in any case) documented as part of the technical information and user guide/limitations for the systems. As for instance:

a) Tested and used for a lower pressure than the standard test included in design specification.
b) Designed after (any) lower pressure.
c) Some components for a higher or lower pressure. (If lower than design pressure the whole system will be lower)
d) Part of the system for a lower design pressure. (Quite common for hydroelectric powerplants, water mains etc.)

Just remember that this is the end users decision, made on basis of local rules and practical and economical
adaptions for the actual use, all based on max possible pressure.

 

[ewpat]

Thank you to all that replied.

 

[1503-44]

In addition to what others have said above,
If you are the process engineer, use any one you want.
If you are the piping engineer, use design pressure. That's the biggest number of the two.
If you are the piping engineer and anyone disagrees with your use of design pressure for stress-based design and selection of pipe, get them to change the design pressure to a different number.
If you are not the process or the piping engineer, go find her.



“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[ntgd]

another point need to concern, the work pressure is the one pipe work stably, but we need to consider instantaneous pressure, caused in the open or close process. So require a safety coefficient.

 

[LittleInch]

If you're using B31.1 this implies it is power piping, which is a little different from B31.3 methodology.

Sections 101 and 102 have a lot of information on what pressures etc should be used when and where.

It would help a lot if you could provide some examples of what you mean.

B 31.1 uses the terminology "Maximum sustained Operating Pressure" - MSOP - and states "Internal design pressure" should not be less than this pressure. There is an allowance of between 15 to 20% ofr occasional overstress conditions.

Now why it appears that your design pressure is much higher than "operating" pressure is not clear or described.

what you sometimes get is a piping spec which works at the maximum pressure of a given flange class rating and temperature on the basis that this covers everything in that class rating and you don't need to bother with multiple different wall thicknesses.

Not sure if that's what you mean as the original question isn't very clear.


What you use to establish pipiing loads though can be amended on the basis that the MSOP is a certain figure and that there are protection devices to stop it exceeding that in normal operation. This is usually shown on the line list. If not there it needs to be in the stress report, but has to be clear what you used.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MJCronin]

I believe that LittleInch and I are on the same page ....

Now why it appears that your design pressure is much higher than "operating" pressure is not clear or described.
What you sometimes get is a piping spec which works at the maximum pressure of a given flange class rating and temperature on the basis that this covers everything in that class rating and you don't need to bother with multiple different wall thicknesses.
Not sure if that's what you mean as the original question isn't very clear.

What I am suggesting is going back to an ancient mechanism called the "Piping Line List" that kept everything straight...

My opinion only ...

MJCronin
Sr. Process Engineer

 

[1503-44]

A "pipe list" or "piping spec" doesn't solve the problem. It is most likely to be THE problem. You have to select the design pressure to find which ANSI class, material and pipe wall the piping spec tells you to use. So really you need to find out why the design pressure is apparently so high and get the person responsible for setting that to change the line's design pressure and then get it entered in the plant's revised line list, if you have one.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"