3 replies to this topic

[eastorca]

Dear everyone,

It is feasible if I estimate 0,3 bar for pressure drop across a PCV (wide open 90%). If not, how much is OK?
flowrate = 6.5 MMSCFD

Thank you

Edited by eastorca, 02 June 2011 - 02:38 AM.

[kkala]

It is feasible if I estimate 0,3 bar for pressure drop across a PCV (wide open 90%). If not, how much is OK? flowrate = 6.5 MMSCFD

A PCV is just a control valve. Criteria for ΔP at (max) normal operating flow differ somehow from practice to practice; according to one of them, ΔP is the greatest of
0.7 bar
20% of frictional ΔP of the line
10% of operating pressure
Valves are sized to pass (max) normal flow at 75-80% of their opening (ie their travel); 90% seems too high for a new valve, if it means opening at (max) normal flow.
But you may want ΔP at 90% opening for some other reason (which?). In this case 0.3 bar seems to roughly correspond to 0.7 bar above.
Above concerns preliminary sizing, vendor's data give a more precise figure, when available.

[Kat]

As fas as I know, the pressure drop across the valve equal to one-half of the variable system pressure drop exclusive of the control valve; in other words, one-third of the total variable system drop including the control valve.
But it is the case of a single control valve.
Do you guys have any idea how to specify the pressure drops through a series, let say, 2 control valves in a gas pipeline?
Kind regards,
Kat

Edited by Kat, 05 June 2011 - 10:57 AM.

[djack77494]

The pressure drop you take across a PCV (or any other control valve for that matter) can very widely vary. If you have only a little pressure "to spare", then you'll design for a very low pressure drop. For example, in vacuum systems, the allowable dP may be in the order of just a few mm of mercury. If you're letting down high pressure nitrogen to near atmospheric pressure, you may have a pressure drop of many bar (or psi), even hundreds. The "rules of thumb" mentioned in other posts apply mainly to pumped circuits and may or may not be applicable to your situation. For extremely high pressure drops, it may even be necessary to split the work between multiple control valves, since it may not be possible to achieve reasonable pressure control of blanket gas in a tank operating at a few inches of water pressure if your blanket gas is available at high pressure. If in doubt, talk to your instrument engineer.