2 replies to this topic

[antator]

This may well be covered elsewhere in the forums but I haven't been able to find it so feel free to point me in the correct direction if I've missed it.

I wondering how a control valve can increase the allowable pressure drop before choked conditions are met?

I've been calculating the maximum gas flow through a control valve in order to check a relief valve sizing scenario. To do this I've been using the ANSI/ISA Flow Equations for Sizing Control Valves.

In these equations x=deltaP/P1=1-(P2/P1) and x<=Fk*Xt
Typical values of Xt that I have found range from c.a. 0.3 - 0.9

If we assume we are looking at air so the heat capacity ratio,k = 1.4 then Fk=1 and x is totally dependent on Xt. If Xt =0.9 then x=0.9 and flow through the valve becomes choked at P2/P1=0.1

If I assume the gas in question is ideal I can apply to following condition at choked flow P1/P2=[ ( k + 1 ) / 2 ] ^{k / ( k - 1 )}. This suggests that P2/P1~0.5 at choked conditions.

Can anyone explain how a control valve can increase the allowable pressure drop before choked conditions are met?

Thanks

[ankur2061]

Antator,

Refer the attachment regarding the sizing of valves by Parker. It may provide you the answer you are looking for.

Regards,
Ankur

Attached Files

•  Parker_Sizing_of_Valves.pdf   470.84KB   365 downloads

[antator]

Thanks for the reply.

I still find it curious that Xt values up to c.a. 0.9 are possible given that through an orifice I would normally expect choked flow to occur at approximately 0.5 asssuming ideal behaviour. I guess I will have to accept this for the time being.