Gas Pressure Spike due to Valve Closing(A EE in need)

[KontrolFreak]

1. 1000 ft 20" gas (methane) pipeline flowing 1250 scfm @ 5-10" W.C.
2. Valve closes
3. How high will the pressure "spike" ?
4. 2 - 8 Bio Gas digesters = huge volume in dome.

 

[zdas04]

What is the pressure of the source? After the transients, the pressure in the pipeline will equal the source pressure since all friction stops when the flow stops (just like current when you open a switch).

But it seems like you are probably looking for the pressure spike from the gas hitting the valve and bouncing back. I know those spikes exist and that they are measurable, but your density is so low at those pressures that the there is a ton of room for the gas to just absorb the pressure wave without it ever spiking even a very sensitive pressure gauge. The gas is just too "fluffy" at those pressures, the momentum is very low.

There are some FEA programs that could try to model this, but I don't know how good a job they would do at 0.36 psig or 2.5 kPa (don't you just love Uconeer?). My experience with FEA is that its ability to reflect real-world events rapidly diminishes in pipe flow below 2 bar.

David

 

[KontrolFreak]

Definitely looking for the pressure spike. I am looking for a quick estimation equation / program / calculation.

I just can not see that affecting the system pressure that much.

I had a Chem E quoting PV=nRT blah blah blah and he thought it would be like a fluid water hammer and wreck everything.

Rest of the story is that there are 5 flare pressure relief regulators between the valve(compressor) and the digesters.

Thanks Zdas!

 

[zekeman]

I'll check this further, but I believe that the pressure will be approximately the stagnation pressure which is the socalled
total pressure.
You can get it by using the continuity equation wherein the enthalpy + V^2/2g is conserved and if the V goes toward zero, you can the final enthalpy increase by the V^2/2g term and thus the temperature and pressure increase accordingly. If you next use adiabatic isentropic compression you get a pressure, which is always greater than the actual since friction and heat loss would make it irreversible and thus mitigate it.

 

[sailoday28]

KontrolFreak
The max pressure rise(N/M^2) will be equal will equal the product of the fluid velocity (m/s)times density (KG/m^3)times sound speed (M/S).

 

[zdas04]

Saloday,
That feels right. The first two terms are momentum, the last one is the rate of dissipation of the shock wave. I just put this problem into MathCad and got a very reasonable number (0.394 psi or 10.1 inH20 or 2.7 kPa). So you could expect the pressure to go from 10 inH20 to around 20 inH2O.

David