PSV Discharge Line (ASME Sec. VIII vs B31.1)

[fizzers]

ASME Sec. VIII UG-135(f) does not disallow a smaller discharge line size than the relief valve outlet, and only states that "the size of the discharge lines shall be such that any pressure that may exist or develop will not reduce the relieving capacity of the pressure relief devices below that required to properly protect the vessel, or adversely affect the proper operation of the pressure relief devices."

However, B31.1 Clause 122.6.2 explicitly disallows this.

My question is, which code should be followed? Sec. VIII or B31.1?

 

[JoeWong88]

If the PSV discharge line size smaller than PSV outlet, do you experience CHOKE flow in the discharge line ?

When come to code and safety, general understanding is most stringent code requirement shall be adopted.

JoeWong
Chemical & Process Technology

 

[fizzers]

thanks Joe. Our calculation shows no choke flow even with the smaller discharge line.

If I choose to use a discharge line that is smaller in size than the relief valve outlet, calculations may be needed to demonstrate that the backpressure generated does not adversely affect the operation of the relief valve.

Why is there such discrepancy between Sec. VIII and B31.1?

 

[nickelkid]

I do not believe Section VIII is intended to be used for the design of piping, in your case B31.1 is. See U-1(e) in Section VIII for clarification.

 

[fizzers]

However, sizing of the PSV discharge line has a direct impact on the proper operation of the PSV, which then falls under Sec. VIII.

 

[nickelkid]

Does following the piping code have a negative effect on the PSV operation?

 

[fizzers]

No, I don't think so.
But I have issues with installation if using 1" piping in discharge line. The relief valve outlet is 1".

 

[nickelkid]

I do not think it would be valid to supersede a piping code with an interpretation of the vessel code.

Ultimately the owner of the piping system needs to be involved with, fully understand, and approve any deviations from the piping code. The owner will need to understand the "issues" and understand the liability associated with deviating from the design code.

 

[insult2injury]

Your installation is too tight for a 1" line?

If the discharge line could be smaller, are you sure you have the correct psv?

I2I

 

[JoeWong88]

fizzers,
1" line normally results very high pressure drop...it caused hgih backpressure to your PSV and supressed secondary choke in the discharge line.

Quite suspicious with your situation...:
i) Is the vent line discharge directly to ATM or into large flare header ?

ii) What type of PSV you have ?

iii) Do you considered rated flow or required flow for pressure drop/mach no calculation ?

iv) What is the problem with 1" line apart from code issue?

I am just thinking if 1" is not a problem, i don't think it is worth for you to spent your time for this "little monkey". (sorry if this is not the case...)




JoeWong
Chemical & Process Technology

 

[lizking]

Note that 31.1 is Power Piping. The RV discharge guidelines are the same as stated in ASME Section 1- Power Boilers (PG-71.3).

 

[EGT01]

In my opinion, lizking is on the right track. To answer fizzers question... which code should be followed? The answer is the code that applies.