Pressure Regulators in Compressible Flow Calculations

[RJB32482]

Hello,

Is there any way to convert a Cv or Cg for a pressure regulator for compressible flow to a resistance coefficient in pressure drop/flow calculations? Or do you just have to do the following:
- determine the pressure in the pipieline up to the regulator
- determine the pressure drop of the regulator
- determine the pressure drop from the flow needed from the outlet of the regulator to the source. If it is OK, then good. If not, the you need to move the regulator closer to the source.

Thanks. I have not found anything in literature where you can just convert Cv or Cg coefficients to K or equivalent length values.

 

[PEDARRIN2]

Per Crane TP-410,

Cv = 29.9 d^2/(K)^.5

However, when I do pressure drop calculations for natural gas with regulators, I usually split the system into two systems, one upstream of the regulator and one dounstream since the regulator is designed to intentionally/consitently lower a pressure to a certain value irrespective of the flow (as long as it is within the flow parameters of the regulator spring/orifice/etc.)

Putting the regulator closer to the source would only be advantageous if the pressure drop in the system was such that, by the time the flow got to the regulator, the pressure was too close to the set point of the regulator for it to function correctly. If you locate the regulator too close to the source, you also may not have enough pressure at your equipment at the end. Of course, you can mitigate this by pipe sizing to minimize pressure loss.

 

[RJB32482]

Thanks for the reply. I'll have to look at my Crane book. I have the newest version and could not find it.

I made the comment since I thought if you had a regulator that dropped pressure from 100 PSIG to 3.5 inches W.C. you would need less piping of a certain size to get the target flow there. Now you could always upsize the piping to make up for the difference.

Am I correct? For example, if there was 1" piping you would get less flow if the regulator was 10 ft from the end destination instead of 200 ft
(assume same length of piping and size before the regulator).

 

[PEDARRIN2]

No, you would get the same flow because you would have the same consumption at the end (boiler, generator, etc.) but the pressure at the inlet of the farther located regulator would be less than the closer located regulator since the piping has a set pressure drop unit length based upon the flow through it.

Say you start at 100 psig and you place the regulator close to the source. The inlet pressure to the regulator might be 95 psig. If you placed it farther away, the inlet pressure might be 85 psig. If designed correctly, the regulator would still reduce the pressure down to 3.5" w.c. at either inlet pressure.

Flow and pressure drop are dependant on each other.

Within reason, you can get any design flow through any pipe size, only the pressure loss varies. 1 cfm will flow through a 10" pipe or a 1" pipe, but the pressure drop is different.

Likewise, you can get any design pressure drop through any pipe size, only the flow varies. A 1" pipe with a design pressure drop of 1 psig/100 feet will flow a lot less than the 10" pipe at the same pressure loss.