Valve flow with given Cv and Inlet Pressure

[venividivici]

I am trying to determine the flow rate of gas (Methane T=80degF SG=0.6) for a valve that I have selected for my system. I know the inlet pressure (10psig) and the valve Cv (22.5). Looking at the flow equations for gas, it seems I have to first determine if the flow will be critical (choked) or non-critical. Though, to determine this I would first need to know the pressure drop across the valve - which I do not know because I do not know what the flow will be. Is there a way to determine flow of a gas based solely of the inlet pressure and valve Cv?

 

[katmar]

How did you determine the required Cv for the valve if you do not know the pressure drop or the flow rate?

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"An undefined problem has an infinite number of solutions"

 

[venividivici]

It is listed on the manufacturer's website.
Orifice Size (in): 1.25
Cv Flow Factor: 22.5

 

[katmar]

What I was actually asking was "why did you choose a valve with a Cv of 22.5?". Why didn't you choose one with a Cv of 600, or maybe 1.7?

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[venividivici]

I have a 2" line that this will be going into. For the type of solenoid valve that I am looking for (2-way general service 120VAC normally closed) I landed on ASCO's 8210 series. They offer these from 3/8" (Cv of 1.5) up to 2.5" (Cv of 45). I also needed the valve to have a minimal operating pressure differential of 0PSIG. With that being the case, the largest valve of this sort that they offer is an 1.5" (8210G056) with a Cv of 22.5.

 

[bljnv]

Operating pressure differential of 0 psig would not yield any flow across the valve.

 

[venividivici]

bljnv, this spec is the differential required for the valve to close. For the 2.5" with Cv=45, the minimum differential required to close is 5psig.

 

[georgeverghese]

There is salvation in chapter 4 of the GPSA - subsection on control valve sizing.
GPSA = Gas Processors Suppliers Association
You may also find this procedure in the Fisher Control Valve Handbook ?

 

[venividivici]

Thank you all for the responses.
Looking at this section of the GPSA, I would still need to know the pressure differential across the valve to size the valve.

 

[TD2K]

< Is there a way to determine flow of a gas based solely of the inlet pressure and valve Cv? >

No. With 0 dP, you have no flow. With atmospheric pressure on the outlet, the dP is 10 psi and you have one flow.

With a full vacuum on the outlet, you would have choked flow likely through the valve and have the maximum flow possible through the valve.

 

[georgeverghese]

Flow through the valve becomes independant of P2 when P2/P1 is approx less than 0.5 for methane at low pressure. So when P2 = (14.7+10)*0.5 = 12.3psia or less, flow would be constant.

If P2 > 0.5 x P1, then flow is dependant on both P1 and P2.

If you dont know what P2 is, there may be clues to this by looking at the system downstream of the control valve ( you may need to work this out by trial and error).

If you still cannot figure this out, post a P&I D with a short process description of this system in this thread, and we will try to help.

 

[gerhardl]

One way to get an idea is to try to set in 'guesstimate' ideas of max, minimum and avarage values and pre-calculate the flow data, adjust values and recalculate until you have an acceptable approximation of what you want.

 

[LittleInch]

I agree with TD2K. Is there any way - no. With the definition of CV using both flow and pressure drop you can't have two unknowns.

As above, look at the downstream system and see what is there.

First assume no pressure drop across the valve and calculate a flow. sub that into the valve Cv equation and find a pressure drop.

Half that figure, take it away from 10psig and repeat your flow calculation for the downstream system.

Repeat until you get reasonable convergence.

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

 

[venividivici]

Thank you everyone for the responses. There were some assumptions that I could have made that I was over looking (The process line that this let's into is essentially at atmospheric - which gives me P2).

One part I am still stuck on is determining if the flow will be choked or not even considering this assumption. I have heard it stated that if the pressure drop across an orifice is greater than half of the inlet pressure, then the flow will be choked. However, I have seen this expressed as below.

If dP>(14.7+Po)/2, Then Flow will be choked.

With that equation, I could have a differential of 9 PSIG, an inlet of 10 PSIG (which would satisfy the statement in my second paragraph for choked flow bc 9>5), but according to the above equation this flow would not be choked bc 9 is not greater than 12.4 PSIG. Am I missing something??

 

[LittleInch]

When it comes to choked flow you need to work in absolute pressures. Often this won't make much of a difference at say 150psig, but at 10 psig it does. What is often forgotten is that as p2 drops there is a point where flow does not increase. However if p2 stays constant mass flow will increase as p1 rises a density increases but velocity stays constant when choked flow occurs

Hence for your case, p1 is 24.5 psia, hence half of that is 12.5psia, or less than atmospheric pressure at sea level.

To get choked flow at p2 is atmospheric needs approx 30psia or 15psig.

Hope that makes sense.

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

 

[venividivici]

Thanks! Much appreciated