Pressure drop across a control valve

[deccansher]

Hi,

For a flow control valve, having a fixed upstream pressure and downstream pressure, how does the pressure drop across the control valve vary with change in flow...or does the velocity change and pressure drop is constant?

 

[quark]

BigInch,

Yes, it is as simple as that and somehow I got distracted by the constant pressure difference of gravity flow (I mean your 18th Dec post). Pressure drop by the valve is the significant factor when the valve is directly bolted to the tank.

Reena,

Like I mentioned earlier, at the full rated flow of an opening (straight conversion of static head to velocity head), the pressure drop in a 1m piping is quite significant and is about 65% for a 1" line.

 

[deccansher]

Hi all,

Thanks for the responses. I am glad the question was not as simple as i thought it would be....anywyas.....continuing the discussion further...i had two more thoughts-

1. For sizing a valve...the Cv is determined for a constant pressure drop across a valve?

2. for a good orifice, doesnt that pressure finally recover itself to about 100% of the original (theroetically possible?)

So where is the pressure drop now across the orifice? So why would you have a varying pressure drop across the valve?

 

[25362]

If there is a pressure drop with a straight pipe section, why shouldn't it occur with a flow restriction ?

 

[BigInch]

There can be some pressure recovery downstream of a control valve. Some of the increased velocity head within the valve restriction can be converted into pressure head, if the velocity is slowed sufficiently upon exiting the valve. Check the valve's "recovery coefficient".

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[quark]

The drop in static pressure across the orifice due to high velocity (that results from reduced area) can be regained in the pipeline if there is no change in pipe diameter before and after the orifice. This is called as static regain.

Secondly, there is permanent pressure drop due to restriction characteristic of the orifice. This can't be regained.

 

[hanon]

Take care in calculating pressure drops thru valves. Valves nevers behaves as the manufactures curves indicates if there is a great pressure drops in the pipe or equipment (exchangers,...) (DPl) over the pressure drop in the valve (DPv). The greater is the ratio DPl/DPv the bigger is the difference between the installed charateristic of the valve and the inherent (manufacturer) charateristic (linear, isoporcentual,...). I mean for characteristic the relation between %opening and flowrate: the inherent characteristic of a valve is achived if DPv is constant over all the range of flowrates but this is almost never true because of increasing DPl with flowrate. This is the reason why valves sometimes don´t gives more flow with higher openings over 70%, 80%... while the inherent curves say they could give it. It depends on the system where the valves is installed (system curve for DP vs. Q). A good reference of this fact can be found in control book from Smith-Corripio or in internet in many webs as for example

http://www.chemengr.ucsb.edu/~ceweb/faculty/seborg/teaching/SEM_2_slides/Chapter_9.ppt