Liquid capacity certification 1

[CJKruger]

Suppose you have a relief valve in the vapour space of a feed surge drum. Some of the relief cases are vapour (like fire), and some are liquid (like outlet fail close).

Now, suppose the largest orifice area is from a liquid relief case, does this relief valve then need liquid certification ?

 

[lizking]

Great question (if it hasn't been addressed already). It's my understanding that in your case (assuming ASME requirements) that you need a valve certified in both services. Or you could go the dual valve route if the difference in required valve size is significant. Note that many valves certified for vapor service are mistakenly assumed certified when used in a liqiud application. API 520 has two distinct sizing equations for liquid certified/non-certified applications.

 

[CJKruger]

lizking, thanks. Now suppose liquid relief was one of the cases, but not the governing case, do you still need liquid certification (in addition to vapour) ?

 

[lizking]

IMO, if the valve sizing proved adequate using the liquid non-certified equation, I would say you're probably OK. But, the key word is "probably"; I don't know for sure.

 

[CHD01]

For an ASME certified vessel, the relief valve needs to be liquid trim (I prefer using trim instead of certified) if the worst case is for a liquid. The relief valve name plate will be stamped in gpm of water. If the vapor case is the worst case, then assuming the same model relief valve is Ok for vapor or for liquid service, the valve trim should be for vapor service and the namplate for the valve would be stamped in scfm of air or in pph of steam depending which applies. If multiple valves are used then the top valve should be vapor trim and the bottom valve liquid trim. If 2-phase flow applies, the trim should be based on the predominate flow.

The more you learn, the less you are certain of.

 

[CJKruger]

lizking+CHD01, thanks for the replies.