I think opinions in posts will be more converging, if
irprocess can give more relevant information, such as:
1. Purpose of the control valve, where downstream line goes ( a scheme is very useful).
2. Necessity to keep valve outlet pressure at 0.5 kg/cm2 g (per data sheet) or not. Are there other low pressure lines connected to downstream line?
3. Rough composition of the fuel gas handled?
4. Approximate physical length (total equivalent better) of downstream line, operating pressure at destination.
5. What is the mentioned "drilled disc"?
6. Brief description of situation before and after the problem.
Need of requested clarifications may get clearer in the following.
DB Shah: Upstream 2" line; v = 51 m/s, versus max allowable 122/SQRT(ρ) = 122/SQRT(6.95) = 46 m/s per local practices for HC vapors (ρ=density in kg/m3). ΔPf = 16 psi / 100 ft, too high. You are right that upstream line diameter should also increase.
Downstream line; Quick estimate by kkala, assumed a short 4" pipe (less than 100 m equiv length) to flare or atmosphere ("unless resulting frictional ΔP is unacceptable, suggesting higher line diameter"). So only the Mach limitation was considered, plus that ΔPf should not exceed available ΔP. For a normal gas line, a 6" pipe would be assumed, long length could increase this diameter.
Destination to flare was supposed because 0.5 barg downstream of valve looked small for any use (e.g. burners).
ankur2061: Can you suggest value for compressibility factor and sound velocity to check adopted ones? (1.0 and 428 m/s). Gas critical pressure = 44.8 barg (valve data sheet). To my opinion values adopted are adequate, but no check is bad.
S.AHMAD: It has been asked whether downstream pressure can increase, or it has some purpose to remain at 0.5 kg/cm2 g. Downstream pressure cannot go much higher than 4 barg at normal flow, for the new control valve should have adequate pressure drop for control (say pressure = 5 or 4 barg).
Can you advice need of two downstream globe valves or HCVs (at some distance?) instead of one? I understand this is more efficient, but reason may be more specific.
Hoping of settlement of this hot topic
Edited by kkala, 24 October 2011 - 03:43 PM.