Choked Flow in Backpressure Valves 1

[b7031719]

The definition of choked flow in IEC 60534-2-1 is:

The condition where further increase in pressure differential at constant upstream pressure no
longer produces a corresponding increase in flow through the control valve is designated
“choked flow”.

My question is, does this apply for backpressure valves where the upstream pressure varies at constant downstream pressure? My sizing calculation for a backpressure application has used the choked flow pressure drop for the sizing pressure drop as per the standard. I am wondering if, for this case where downstream pressure is constant, should I just use the actual pressure drop as the sizing pressure drop?

 

[RVAmeche]

If the downstream pressure is constant, the upstream pressure is variable, and the resulting dP does not result in choked flow for that fluid, you should probably just use the worst case dP.

 

[b7031719]

Hi thanks for the reply. In this particular case I am interested in, the dP does result in choked flow.

 

[RVAmeche]

Then yes you have choked flow and should use whatever sonic flow equations that apply.

 

[b7031719]

I think the statement in IEC 60534-2-1 is misleading then, based on what you are saying. For backpressure valves the statement should read:

The condition where further increase in pressure differential at constant downstream pressure no
longer produces a corresponding increase in flow through the control valve is designated
“choked flow”.

 

[btrueblood]

The IEC statement is correct.

 

[b7031719]

Well there can only be one correct answer. Either IEC is correct for pressure reducing and backpressure service, that only upstream pressure changes can increase the flow beyond choked pressure drop, which implies there is no choked flow for backpressure service. Or the statement only applies only for pressure reducing applications and that for backpressure applications a downstream pressure change can increase the flow beyond choked pressure drop.

 

[btrueblood]

"which implies there is no choked flow for backpressure service. "

Doesn't imply that. The general statement is made for fixed orifice sizes. If the upstream pressure is constant, there is a downstream pressure at which choking occurs, and any decrease in downstream pressure will not increase flow. But changes in upstream pressure DO affect flow. Also changes in the valve position or minimum flow area (orifice size) can also affect flow.

Your backpressure regulating valve MAY (is generally designed to, over a certain operating range) operate to change the flow orifice size at varying differentials, and thus the above general statement must be modified to "MAY", depending on the design pressure ratio of the regulator elements, and you should probably refer to the manual/tech data sheet for the product. I can tell you for certain that there will be a downstream pressure and flow combination where your regulator will choke. You must consult the valve specs to determine if the maximum design flow rate for the valve is within your requirements. You should also be asking what then happens at your minimum flow, as cheaper regulators will likely chatter near shutoff, but that's a whole 'nother discussion...

 

[b7031719]

OK thank you for the reply. If the downstream pressure is constant, let's say atmospheric pressure, which is the case for my application. As the upstream pressure increases as will flow, until choked flow occurs then what? What if the upstream pressure then increases further?

 

[Compositepro]

A BPR operating with choked flow will still work normally by modulating the opening as required. It just is not affected by changes in downstream pressure. At higher upstream pressure the gas density increases so more mass can flow through a constant restriction. In choked flow the downstream pressure can no longer affect conditions in the orifice, is all.

 

[LittleInch]

What you describe is not normally called a back pressure valve. What you describe is just a control valve.

A BPV maintains a fixed upstream pressure.

If this is a gas then mass flow will still increase even in choked flow if upstream pressure increases.

If the upstream pressure is constant but downstream isn't then once you hit critical flow lowering the downstream pressure has no impact on mass flow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[b7031719]

Hi LittleInch thanks for the reply. I take your point I guess back pressure regulator is not the correct terminology at all and thanks for pointing this out.

I suppose really the question is why does the IEC standard require the choked flow pressure drop to be used for the sizing pressure drop? May be an example might help understand the process involved. If I have a pressure vessel relief valve for example, venting to atmosphere. The IEC method requires calculation of the choked flow pressure drop. The sizing pressure drop used to calculate the valve capacity is the lesser of choked flow pressure drop and actual pressure drop. If in this case the choked flow pressure drop is less than the actual pressure drop, why use choked flow pressure drop because the upstream pressure will be higher?