Control (liquid dump) valve in 2/3 phase separators; Cavitation/flashing and dP across valve

[engr2GW]

Hello,

Many times when you try to size a valve using sizing tools by the manufacturers, e.g. Kimsize for Kimray, Norrisize for Norriseal, Fishre etc. you will see red flag of critical flow. I have heard that sometimes it might say critical flow, but not bad to use with the dP. Is there a rule of thumb as to when to worry about that?
the cases I'm sizing for are
1. 350 to 30 psig for 500bbl/day oil and then 30psig to 4psig for the same oil rate. This is a two inch valve with trim size from 0.75 to 0.25 inches depending on component materiall, and snap acting
I have an options of
350 to 50 psig and then 50 psig to 4psig before storage tanks.
I ran all this scenario and found critical flow, is it critical in actual operation, or is there a rule of thumb, any experience/ideas/advice??
The trim in sight now is Carbide:
thanks for your help

As much as possible, do it right the first time...

 

[LittleInch]

Have a look at this site,

http://blackmonk.co.uk/2009/11/13/how-to-size-a-liquid-control-valve/

Tells you a lot about it, but if you hit critical flow you`re either not going to get the flow you want or the liquid is starting to cavitate somewhere in the valve. If your duty is constant as opposed to a short term duty, e.g. starting a flow from a reducing pressure like a pipeline start then you might be ok, but if constant then you won't have much of a valve or trim left after a short time and it will be rather noisy.

I'm not sure why you're trying to do the vendors job for them. They know much more about what their valves and trims can do than you ever will. If I was you I would try and figure out what your max and min requirements are, write them in a data sheet and send to the vendors. The tools are there so that you can get some idea of size and duty or if your inputs change on an existing valve and you want to try and see if it needs changing or not.

By the way 4 psi doesn't sound enough to fill much of a tank to me, but you will find valves to do your duty, but critical flow is a real issue and can't just be ignored if you want your system to work or last.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way