Relief device required? 2

[dennisr]

I have been scrutinising the codes to find the answer to the following question.

According to API-STD-521 sect 4.1, pressure-relieving devices are installed to ensure that a process system or any of its components is not subjected to pressures that exceed the maximum allowable accumulated pressure.

However is a relief device required, when the potential overpressure exceeds the maximum allowable working pressure (design pressure), but is less than the maximum allowable accumulated pressure? According to the codes the required relief flow becomes zero, so then why install a relief device?

 

[don1980]

dennisr...your question is actually two separate questions rolled into one. The answer is apparent when you separate them.

1) Is it necessary to size a relief device (or, how do you size a device) when the pressure excursion exceeds MAWP but is less than the allowable accumulation? The answer is that no sizing calculations are required for such a scenario. Any size PSV will surfice.

2) Does a pressure vessel require overpressure protection if there are no credible causes of overpressure? The answer is yes. Refer to ASME Sec VIII UG-125(a). "...all pressure vessels within the scope of this Division, irrespective of size or pressure, shall be provided with overpressure protection in accordance with the requirements of UG-125 through UG 138 and/or overpressure protection by system design per UG-140."

So, if there are no causes for overpressure, you still need to install a relief device or apply UG-140. If you choose to install a PSV in such cases, then you can simply pick any size PSV you like.

 

[dennisr]

Don1980
Thank-you for the reply. Its always interesting to discuss relief valve issues with experienced people.

You obviously know your standards, and based on your reply I was inspired to re-visit the ASME code.

I believe that the answer lies with ASME UG-140. Before introduction of this section (in 2008), the UG-125 was dictating that relief protection should be provided on all pressure vessels. As a result a lot of "courtisy" relief valve have been installed in the past, even if no cause of overpresure was identified.

By the introduction of UG-140 these "courtisy" valves (with zero flow) are no longer required. According to UG-140 point b) (not air, water steam service) a relief device is not required provided that:

- Normal operating pressure is below MAWP
- Credible overpressure is less than 116% of MAWP
- Overpressure analysis required (as per API)
- HAZOP is done

UG-140 b) agrees with API-STD-521, section 4.1 stating that: "Pressure-relieving devices are installed to ensure that a process system or any of its components is not subjected to pressures that exceed the maximum allowable accumulated pressure"

Conclusion:
Relief device are no longer required when the credible overpressure is not exceeding the maximum allowable accumulated pressure (i.e. MAWP+16%).

 

[sheiko]

Hi dennisr,
Just by curiosity, is it a real world problem? Or is it jist a theoretical question?
I ask because i am curious to know in which scenario you have succeeded in quantifying and calculating a MAAP?

"We don't believe things because they are true, things are true because we believe them."

 

[dennisr]

Sheiko

Your point is not clear to me???

The issue is definitely part of the real world.

MAAP = MAWP + 16%

When MAWP is not defined you may consider design pressure.

 

[sheiko]

Sorry for being unclear (english is not my native language). Let me try to explain.
First, accumulation can be 10%, 16% or 21% depending on the scenario.
After, my problem is that you mention a potential overpressure scenario in which the pressure would fall in between the MAWP and the MAAP, but i don't succeed in seeing in which case (fire, blocked outlet, gas expansiin?...) you had to calculate this pressure to arrive to the conclusion that it is higher than the MAWP? As far as i know, a PSV is needed when there is a credible scenario, and in order to determine the credibility of the a scenario, there are the consideration of the MAWP of the various items in your system and the calculation of the relieving flowrate, but usually not any pressure calculation. So could you please tell us what was this scenario?
Hope i am clearer now. However i may have misunderstood your OP. In that case sorry for your time wasted.

"We don't believe things because they are true, things are true because we believe them."

 

[don1980]

dennisr...the conclusion you drew from my comments is technically correct, but some people might mis-apply this. Let me clarify this point.

UG-140 is indeed an acceptable alternative to the use of a common relief device, but good judgment must always be used in deciding when to use this option. By default, an engineer's starting position should be that all pressure vessels need to be protected by a relief device. That is true regardless of whether or not he/she sees any credible scenarios. Always remember that there is a huge difference between a small PSV and no PSV at all. Treat UG-140 as a special-purpose option rather than a general-purpose option. UG-140 can only be used after doing a careful risk assessment, and after concluding that those risks are acceptable and well managed. In most cases that means that there are other layers of protection, and the consequences of failure are low. Use UG-140 by exception. Start with the assumption that all vessels need a relief device, and then use good judgment to decide whether or not UG-140 makes sense for a particular application.