Worst Case Scenario on Compressed Air Tank 3

[RJB32482]

We have an compressed air tank (only about 35 gallons in size) and it does not have a relief valve on it. My assumption is that the worst case scenario would be the tank is overpressurized by the inlet flow of air to the tank. If I put the relief valve right on the vessel, how would I calculate the required capacity of the relief valve?

Thank You

 

[EGT01]

What goes in must come out and worst case is everything has to go through the relief valve.

What is the source of you inlet flow?

 

[RJB32482]

I don't exactly know the flowrate of the air into the vessel. Wouldn't the flow out of the valve be the theoretical flow of air at the MAWP (80 psig in this case) comingout of a 2" hole in the tank?

Is a fire case creditable in this case since the air is already a vapor?

Thanks.

 

[EGT01]

RJB32482,

With all due respect, it would seem that you have little experience with relief systems so I strongly urge you to seek the advice and guidance of a more experienced associate within your group. The subject of properly designing and selecting a relief system is too broad and important to rely just on a forum for assistance. The questions you ask or only a small part of what you need to know.

The "required" relief capacity (inlet flow of air) and the valve's "rated" capacity are two different things. A valve's rated capacity can be determined by equations provided by the manufacturer or, for the particular case of air, most relief valve manufacturers publish tables that indicate the valve's capacity at various set points.

How you determine the required capacity depends on the source of air flow into the vessel. Does it come directly from a compressor, does it come from a pressure let down station supplied by a higher pressure system...etc.

As for whether a fire case is credible or not depends on whether there is anything in the area of your vessel that could support a fire. What happens to the vessel and its contents when exposed to a fire will be somewhat different when the contents are vapor rather than liquid. That's a complete other subject.

 

[RJB32482]

EGT01,
You are correct, I am pretty new at looking for relief system scenarios. Just looking at if fire would be a credable scenario for this tank. Please expand on the subject of the fire case for air.

Thanks Again. I will look at the inlet air flow.

 

[EGT01]

RJB32482,

The contents of the vessel have nothing to do with whether fire is a credible scenario. Generally speaking, if the vessel in question is within 25 feet vertically or 20 - 30 feet horizontally from a source that can support a significant fire, then fire would be considered a credible scenario.

For a vessel filled with vapor (or gas), as the vessel and its contents are heated by exposure to the fire, the internal pressure of the vessel will increase by virtue of the ideal gas laws and can result in overpressuring the vessel.

For further information on the subject, American Petroleum Institute Recommended Practice 521 (API RP-521) is suggested.

 

[RJB32482]

Thanks for the information. I'm gaining a good bit of knowledge on the subject. So looking at the scenarios listed in API 521, it does not look like the tank could be overpressured (since plant air pressure is below MAWP) and a sustainable fire could not happen in the area. So no other scenarios (blocked outlet, operator error)could occur. Should every vessel still have a relief valve (somehow air pressure did get higher than the MAWP)?

Thanks

 

[EGT01]

Sounds like you've been doing some homework.... keep up the good work! If you are not familiar with API RP-520 Parts I and II, you will eventually want to look at them as they are a good reference covering the mechanical operation and sizing equations associated with relief devices and considerations for their associated piping.

Regarding whether every vessel should have a relief valve, pressure relief device requirements are usually associated with the Code or Standards to which the vessel is built. Local governing authorities (local city, your plant, etc.) may have additional requirements. So we need to backup a bit and you need to determine what code was used for your tank.

ASME Section VIII is a common pressure vessel code in the US but you need to confirm if it applies in your case. API 520 and 521 are intended as supplements to ASME Section VIII.

But let's assume that ASME Section VIII applies, then you may want to get this reference which includes excerpts from ASME Section VIII as well as being a good relief valve reference in general...

http://www.tycoflowcontrol-na.com/ld/CROMC-0296-US.pdf

Now look at UG-125a and UG-125g...ASME says a relief device is required for all pressure vessels following ASME Section VIII but accepts that the relief device does not have to be installed directly on the vessel. Similar language can be found in API 521....look in the appendix section that covers Special System Design Considerations where they discuss having a single relief valve protecting several pieces of equipment.

One thing you may want to double check is your claim that the plant air is below MAWP. The normal operating pressure of the plant air system may be below MAWP but what regulates or limits the maximum pressure for the plant air, is it a relief valve and is it set at or below MAWP of your air tank?