2 replies to this topic

[iqballhud]

Hi,

If the gas quantity will be transferred from upstream to downstream of control valve, then the pressure in the downstream control valve will be increased.

 

How to calculate (as dynamic calculation) downstream pressure (gas) from control valve if the upstream control valve having data as below:

P (bar.a) = 70; T (^oC) = 40; MW =20.1; Density (kg/m3) = 60.

And this is the downstream control valve data, volume (m3) = 1350. This is included the pipe and vessel.

 

To calculate gas flow through control valve, can use this formula:

QG = Cv*816*P1 / (Sg * T)^0.5

 

Thanks in advance for your helps.

Regards,

Iqballhud

[iqballhud]

If the total volume gas flow on the downstream valve is 300 m3, how much pressure will be increase?

[MrShorty]

I think this kind of problem needs some careful communication. For example -- when you say that "total volume gas flow on the downstream valve is 300 m3", exactly what does that mean? A gas will always fill its container, so, if your container downstream of the valuve is 1350 m3, then we might say that there is always 1350 m3 of gas downstream of the valve. Or perhaps this nebulous 300 m3 really means something like 300 "standard" m3 where a standard m3 (Sm3) is defined as the quantity of an ideal gas in 1 m3 at 1 bar and 25 C (or whatever your definition of a Sm3 is).

 

I expect that the heart of this problem is "basic" equation of state (EOS) stuff PV=znRT. When you say that you have 300 m3 of gas pass through the value, you put that into your EOS (P*300=znRT) and figure out what should be in the other variables of the equation. (I would expect you to solve for n at this stage of the problem). With that quantity (moles) of gas entering the container, enter that into your EOS, and solve for P at T.

 

I will add that, the way you described the problem in the first post, I thought that you might be trying to solve this as differential equations. From your EOS, you should be able to find dP/dn, and your control valve equation should give an expression for dn/dt. Chain rule should allow you to get dP/dt=dP/dn*dn/dt.

 

In either case, the heart of the problem is the EOS, so naturally, the simplest solutions for this problem will be those that assume ideal gas (z=1). Even if you decide that the assumption of an ideal gas is not valid, it might be a good, simple exercise to solve the problem assuming ideal, just to fix the overall computational procedure.