Sizing Needle Valves

[Kimusubi]

I am trying to size a needle valve to make sure that it can allow the required flow rate at the fully open position (0.025 m^3/s). For a given Cv you should be able to calculate the maximum flow rate through the needle valve provided that you know the temperature of the gas and the pressure drop across the valve, but my problem is that I do not know what the outlet pressure of the needle valve is going to be ahead of time. Wouldn't P2 (outlet pressure) be dependent upon the needle valve itself? I seem to be confused on this point which is preventing me to get the correct size for the valve.

The flow conditions are:

Medium: Air
Upstream Pressure: 2 atm
Maximum flow rate required: 0.025 m^3/s
T = 300 K


Any help would be highly appreciated.

 

[Latexman]

Does the air end up going to atmosphere somewhere downstream of the needle valve? If so, the pressure at the end of the line is atmospheric pressure. Then, calculate the "back pressure" at the needle valve outlet knowing atmospheric pressure and the pressure drop of the flow through the downstream configuration.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[Kimusubi]

It does eventually exit out into the atmosphere, but there is a lot of piping/lines before that. The line eventually combines with a separate methane line that goes through a combustion chamber and then finally exits into the atmosphere. It seems unlikely that the pressure at the outlet of the needle valve would be equal to the atmospheric value.

 

[MikeHalloran]

>>>It seems unlikely that the pressure at the outlet of the needle valve would be equal to the atmospheric value. <<<

In that case you need to model the entire fluid circuit in order to calculate the Cv you can tolerate for just the needle valve.



Mike Halloran
Pembroke Pines, FL, USA

 

[moltenmetal]

The answer depends on whether the outlet pressure is likely to be more than 1/2 of the absolute inlet pressure. If the inlet pressure is more than 2x the likely outlet pressure, further reductions in outlet pressure will not affect the flow- the flow will not increase. The reason is that the flow will be choked (sonic) in the needle valve's trim. So if the inlet pressure is high enough that flow will be choked, you don't need any detailed evaluation of the downstream pressure in order to accurately estimate the flow.

 

[Latexman]

"Upstream Pressure: 2 atm" with the eventual exit pressure of 1 atm after "a lot of piping/lines before that". Seems unlikely "the inlet pressure is more than 2x the likely outlet pressure". It's probably not choked, so the downstream pipe and fitting will have to be characterized and estimated, unless the system is existing and a pressure guage can be installed at the outlet of whatever valve is there now.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[moltenmetal]

I read the 2 atm as being 30 psig, not 30 psia- if it's 30 psia, flow is NOT choked obviously and a detailed analysis will be needed. But if it is 30 psig and discharging into what sounds like a burner train? Pressures in there are quite low, and flow through the valve could still be choked.

 

[Latexman]

Yep, it depends on what the OP meant. I admit I mentally defaulted to the "atmospheres are always absolute pressure" where I come from, but it's not necessarily universal.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[Kimusubi]

Upstream pressure is 2 atm (absolute) - so around 28 psia. I did find some relations in regards to the pressure loss based on the Cv (from the Handbook of Valve Selection). I am not sure on how accurate it is, but it states that:

dP = z*0.5*rho*V^2 (where z is resistance coefficient)
z = (899*d^4)/Cv^2 (where d is the orifice diameter in inches)