How to determine PRV set pressure 1

[phoenixmoca]

Hi, all,

As we all know PRV set pressure should be determined based on the design pressure or MAWP of the protected items.
But my question is whether the above saying is a general guideline or there are specific guidelines for determining the PSV set pressure applied to protect sepcific items such as pressure vessel, piping fired vessel and so on.

 

[e43u8]

phoenixmoca,

There is no specific guidance other than it's most optimum status to consider the set pressure at design pressure (or MAWP if is known) or as close as possible to it to avoid interferring maximum operating pressure with set pressure which might lead to premature opening of the PSV and also to avoid considering bigger PSV due to lower set presure.

 

[mk3223]

IMO, it's the specific guideline as stated in the API RP 520 Section 1.2.3 and illustrated in Fig.1.

 

[don1980]

Word choice is important here. There's no guidance on where a PRV "should" be set. Instead, the design/fabrication codes & standards for various types of pressure equipment set constraints on the maximum allowable relieving pressure, and for some equipment (e.g. pressure vessels) these codes/standards also set constraints on the max allowable set pressure for relief devices. Within these constraints, the user determines where the PRV "should" be set based on the details of the specific application.

Codes & standards for pressure equipment other than pressure vessels (e.g. piping) don't always constrain the maximum relief set pressure (e.g. ASME B31.3 Piping Code). Instead, they place limits on the peak allowable pressure during emergency conditions. For example, ASME B31.3 pipe code just sets the peak allowable pressure limits, without specifying a maximum set pressure for the relief device (note that this code doesn't allow the normal operating pressure to exceed the design pressure). That's why liquid thermal expansion PRVs on piping are sometimes set at a pressure above the design pressure of the pipe, and that's perfectly acceptable as long as the peak allowable pressure limits are met.

Determining where a PRV "should" be set is always a case-by-case assessment for the user to decide.

 

[phoenixmoca]

Hi, don1980
Thank you for your detailed explanation which is very useful for me to understand the PSV set pressure.
But what I want to know is whether PSV set pressure should be settled by design pressure of protected items or by their MAWP(Maxium Allowalbe Working Pressure).
As I understand it is common to settle PSV set pressure base on MAWP rather than design pressure in foreign design institutes. But in China it is settled by design pressure but not by MAWP though the protected pressure vessels are made according to ASME standards.

 

[e43u8]

Hi,

If the MAWP of the protected pressure vessel is unknown; the PSV set pressure has to be settled by design pressure or lower, but if the MAWP is known the set pressure can be settled by MAWP.
There is one issue based on which the MAWP of a protected pressure vessel being degraded through the time due to local or uniform thickness reduction; hence it's better and more conservative to settle the PSV set pressure a little bit lower than MAWP.

 

[mk3223]

IMO, you need to verify if using the design pressure as the PSV set pressure is the local standard requirement in your city, or just a typical approach of the conservative design for the project.

ASME and API are the Codes recognized globally. From my experience, the PSV set pressure with MAWP is acceptable for the equipment protection in China.

 

[don1980]

When a new vessel is being purchased, one of the details specified by the process engineer is the required pressure capability of the vessel - that's the design pressure. At this point all you know is a design T&P and a metallurgy. When the specification gets to the fabrication shop, the shop engineers look at the requirements and calculate the necessary thickness for the walls, heads, and nozzle necks. These components come in discrete thickness increments from suppliers. The fab shop purchases the closest size that meets the minimum specified requirements for design T&P. Since each component is a little thicker than actually necessary, the fabricated vessel will almost always tolerate a little more pressure than the design pressure. ASME requires the fab shop to take the actual thicknesses and perform a pressure capability calculation for the assembled vessel (using a prescribed ASME calculation procedure). The result is the MAWP.

So, when process engineers are specifying vessels, all they know is the design pressure and not the MAWP, but they know that the MAWP value (once it's calculated) will be at least as high as the design pressure. For new vessels, the process engineer generally needs to calculate the relief sizing before the MAWP is known, so they use the design pressure. If the relief design is being done after the fab shop has calculated the MAWP, then process engineers use the MAWP to calculate the relief sizing. Once the MAWP value is known, there's no longer any reason to continue using the design pressure. All relief device set pressure limits are based on MAWP and not design pressure. Design pressure is just a convenient number to use when one doesn't yet know the MAWP.

 

[phoenixmoca]

Hi, friends
Thanks a lot for what you have explained. It helps me a lot understand the relationship between design pressure and MAWP when PSV set pressure is being set.
The reason why this post was initiated is I am preparing to develop a best practice titled as How to settle PSV set pressure with the purpose of clarifying the difference between design pressure and MAWP when settling PSV pressure. But when starting the work I found that there is so little references about this topic. So I asked for help in the Techincal Group.
So thank you very much for what you hanve done and look forward to getting more and more information on the topic from you.

 

[don1980]

phoenixmoca - I think I now understand the source of your confusion. All global codes limit the peak allowable set pressure for relief devices based on the design pressure capability of the vessel. However, different codes/standards use different words/terms when referring to a vessel's design pressure capability. ASME uses the phrase "maximum allowable working pressure" (MAWP). This term has a specific meaning, and technically it only applies to ASME pressure vessels. In common parlance, "MAWP" has been used by engineers throughout the world to refer to the limiting pressure for all types of equipment (piping, low-pressure tanks, etc.). Technically, MAWP doesn't apply to this other equipment, but we usually know what is meant when engineers use "MAWP" in that way.

Other pressure vessel standards (other than ASME Sec VIII) use different words/terms for the vessel's design pressure capability. Chinese standard GB-150 uses "design pressure", and European standard ISO 4126 uses "maximum allowable pressure" (abbreviated "PS").

It's important to recognize that different standards use different words/phrases to refer to the same thing (the design pressure capability of a vessel). And some of these terms, such as "design pressure" have different meanings depending on the context in which it is used. For example, in my previous post (above) I explained the distinction between "design pressure" and "MAWP" in the context of an engineering project. But note that "design pressure" can have a different meaning when used in other contexts, such as that in the Chinese standard GB-150. When GB-150 refers to "design pressure" it has the same general meaning as "MAWP" and "PS". That is, GB150 isn't limiting "design pressure" to the same meaning as that described in my earlier post.

So, there's no need to write a best practice document explaining when certain standards require a lower limit on the set pressure (lower than "MAWP"). That's because there are actually no standards which have such requirements. Instead, just explain the fact that different standards use different terms/words to refer to the same thing - the limiting pressure capability of the vessel.

 

[phoenixmoca]

Thank you very much, DON1980.
What you have done, just like the beacon, totally cleared my confusion about the basis of PSV set pressure. So it is very important to discuss with others as well as to have a good command of professional standards or rules.
Thanks again, all friends.