Fire case Relief 1

[sav455]

Hi All,

I am wondering if the relief rate form a PSV sized for fire case is dependent upon plant feed rate. Let's say we want run an existing plant at 130% capacity and do the debottleneck work.
Do we need to rerate the fire case relief also?? The equations in API 521 don't indicate the same.
Your insight/thoughts are welcome.

Thanks

 

[Jefka]

Without looking at the specific system, I can only offer general advice. There are two primary fire cases typically evaluated: vapor generation, gas expansion.

Fire - Vapor Generation
In this case you're typcially protecting vessels with a constant liquid level. Most vessels have an upper limit of anticipated liquid (set by high level switches or controllers). If you have a scrubber that normally sees 50 MMSCFD but is now seeing 65 MMSCFD, will the highest expected liquid level in that vessel change? Probably not. Perhaps you can verify that the liquid composition is not changing with the increased throughput; that is probably be the only significant change that could occur.

Fire - Gas Expansion
This case deals with gas-full pressure vessels. As with the vapor generation case, I would perhaps look to see if the increased 130% throughput leads to differing gas compositions in each vessel. If not, there's probably no reason to resize the valve.

Of course, if you swap out any vessels with larger-sized replacements, you will definitely need to resize those valves.

 

[ajs1972]

I think the answer is yes, because the wetted area in the vessels are changing, and the heat rates, too.

 

[sav455]

Thanks Jefka.

In this case we are not changing any vessel. The only thing which might change is the composition which will change the latent heat of boiling and hence the relief.Same goes with the gas case also.
I have a related querry in use of HYSYS depressuring utility for estimating the gas blowby case relief rate. Have you used it for relief estimations and if yes what has been your experience? In my case, it is predicting lower number than I got doing hand calculation (in excel) using Fisher control handbook method.I also selected Fisher method in HYSYS.

Sav

 

[sav455]

Hi Ajs,
I just posted my reply to Jefka's post only to see yours was posted also.
I am wondering if you could perhaps elaborte a little more about increase in wetted area and heat rate. I think the wetted area stays the same because it in a way is indicative of hold up volume in the vessel which would be heated in case of fire. The increase in plant rate will not change this unless the operating pressure (for gasses) or liquid levels are changed and also compositions change (which in turn lead to an increase in hold up volume and/or change in latent heat).
Thanks
Sav