Fire vs Thermal Relief

[maddocks]

Client has a vessel in the plant that has been un-protected for years now. Upon reviewing the flowsheets and going through the plant I noticed the abscence of proteection. Client now wants a PSV installed. As this is a Recycle Vapour KO drum, how do I decide if I need a fire sized PSV or a simple thermal one?

 

[Montemayor]

maddocks:

A "simple thermal" pressure relief valve works only on contained liquids (or non-compressible fluids). It is not applicable to gases or vapors. Therefore it does not apply to your scenarios - unless you need to consider the case where the entire KO drum is 100% liquid-filled and subjected to a fire case while it is blocked-in. I don't think that this is one of your credible scenarios.

Briefly, you have to establish the identity of all the possible, credible over-pressure scenarios that can occur on the KO drum. Once you have identified them (including the fire case), then you have to quantify the amount of PSV capacity required in each scenario to avoid an over-pressurization of the KO drum. The worse-case scenario is your design case for the PSV. That is how you decide the size of the PSV required.

 

[maddocks]

Thanks for the quick reply. I generally agree with you but the vessel is actually rated at 155 Psig with upstream equipment rated lower at 140 Psig. Consequently, I think the vessel only requires over-pressure protection in the event of a fire or pressure rise due to block-in condition. Since the vessel is normally manually drained several times a day, I'm uncertain as to the sizing criteria needed.

 

[SeanB]

IF you normally maintain a level in the drum, you definitely have to consider the Fire Case scenario. The other cases you will have to evaluate as well. For example, is there a control valve upstream? If so, do you have a vapor blow through case? You will have to evaluate all cases to determine which are credible and the largestr relief load will determine the size required for your PSV.

 

[Montemayor]

The draining of the vessel or its frequency has no bearing on the need or the size of a PSV. If the vessel is an ASME vessel, then it requires a PSV - the only question is what size?

The fire case has to be considered as to whether it is credible. If you have an essentially gas-filled vessel at the time of the fire, then you should consider a need to de-pressurize - especially if the gas is hazardous. Once enveloped and depending on the duration of the fire, the vessel could fail structurally and its effect could be very dangerous to equipment and humans in the area. A PSV will not save the vessel in that case because the gas pressure does not increase appreciably as compared to when an internal liquid is vaporizing ("boiling").

 

[Degas]

For ALL
I have a pressure vessel( filter) operating with water, flow 200m³/h and pressure 20kgf/cm² and need install a PSV to protect it and attend local requirements.
The PSV flow design need to be 10% of the flow? or other value must be considered?
Where can I find standard number( API, ASME or other)that define this percentage?
Is available a freedownload for the PSV calculations?

 

[Ashereng]

Degas,

You should start a new thread/post, and not highjack this one.

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Albert Einstein
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