4 replies to this topic

[Lisia]

Hello

 

I didn't find similar topics that answer my question and would like to know your opinion. I am calculating PSVs for fire case in unwetted (small) vessels and I have found extremely low rates of relief (<100 kg / h) and  very high temperatures. In this context 21% of overpressure do not seem to make much sense to me. 

 

Is it reasonable to consider smaller overpressures in these cases? Do you have any recommended value in these situations?

 

 

Thank you!

 

 

[fallah]

 

I didn't find similar topics that answer my question and would like to know your opinion. I am calculating PSVs for fire case in unwetted (small) vessels and I have found extremely low rates of relief (<100 kg / h) and  very high temperatures. In this context 21% of overpressure do not seem to make much sense to me. 

 

Is it reasonable to consider smaller overpressures in these cases? Do you have any recommended value in these situations?

 

 

Lisia,

 

The 21% overpressure is a general value might not so affect the PSV size for extremly low relief rate especially in unwetted small vessels; while in wetted vessels such higher overpressure leading to smaller PSV size. Then yes, in most cases of extremely low relief rate considering 10% overpressure  may lead to the same PSV size as with the 21% overpressure.

 

It's not to be forgotten many designers use the PSV fire case (with no other credible scnarios) in smallest size just for code compliance due to a belief based on the fact that relevant vessels cannot withstand subsequent failure and PSV cannot be activated before vessel failure.

Edited by fallah, 27 September 2016 - 11:50 AM.

[Pilesar]

For unwetted vessels exposed to fire, the very high temperatures you are finding are usually such that the vessel wall will rupture before a relief device is activated. API 521 has a discussion of this which you should familiarize yourself with. It is the boiling liquid in the vessel that keeps the walls cool Without a liquid to take away the heat, metal walls in a fire get too hot in a very few minutes. Relief valves for unwetted vessels are not much protection in a fire. It is reasonable to choose another relief scenario for PSV sizing other than fire case.

[RUBABU]

fallah

 

" It's not to be forgotten many designers use the PSV fire case (with no other credible scenarios) in smallest size just for code compliance due to a belief based on the fact that relevant vessels cannot withstand subsequent failure and PSV cannot be activated before vessel failure "

 

Hydrocarbon fuel gas system consists of fuel gas from pipeline, slug catcher, fuel gas scrubber, filter Coalescer followed by fuel gas compressor and gas turbine. Inlet and Outlet pipelines of Filter Coalescer are provided with 3/4" D 1"  PSVs.

 

HC liquid would accumulate in the bottom portion 12 inch dia and 9.5 feet long pipe. PSV sizing for Fire Case is matching with API Size 'D' (considering liquid in the bottom pipeline portion). PSV sizing for blocked discharge (governing case) is handled by another PSV in the inlet header piping to filter Coalescer.

 

Can I assume the filter coalescer vendor has provided PSV considering fire case (smallest size) just for code compliance ? asking this after seeing your comment in quotes above. is it a typical design practice ? 

 

filter coalescer brochure is attached please.

 

Attached Files

•  model-pgp-peach-gemini-purasep-coalescing-filters.pdf   4.91MB   69 downloads

[fallah]

 

" It's not to be forgotten many designers use the PSV fire case (with no other credible scenarios) in smallest size just for code compliance due to a belief based on the fact that relevant vessels cannot withstand subsequent failure and PSV cannot be activated before vessel failure "

 

Hydrocarbon fuel gas system consists of fuel gas from pipeline, slug catcher, fuel gas scrubber, filter Coalescer followed by fuel gas compressor and gas turbine. Inlet and Outlet pipelines of Filter Coalescer are provided with 3/4" D 1"  PSVs.

 

HC liquid would accumulate in the bottom portion 12 inch dia and 9.5 feet long pipe. PSV sizing for Fire Case is matching with API Size 'D' (considering liquid in the bottom pipeline portion). PSV sizing for blocked discharge (governing case) is handled by another PSV in the inlet header piping to filter Coalescer.

 

Can I assume the filter coalescer vendor has provided PSV considering fire case (smallest size) just for code compliance ? asking this after seeing your comment in quotes above. is it a typical design practice ? 

 

 

RUBABU,

 

Because there is at least one another overpressure scenario, i.e. blocked outlet, your case is different than i mentioned in which their is no credible scenario other than fire case; means the PSV for blocked discharge can handle the fire case also provided that there would be reasonable difference between relief loads to avoid chattering.

Anyway, it might the small PSV has been considered as supplemental for the external heat source, here external fire.