loss of pressure head 1

[padovano]

the problem is that if diesel oil is passed through a pipeline equiped with different control curves and valves the flow rate is lower compared to a similar pipeline with the same number of curves and control valves.I calculated the pressure loss of the two pipelines and discovered that the pressure loss on the pipeline minus that due to the control valves where the same.
I would like to know if this difference in flow rate can only be due to the control valves.

 

[BigInch]

How similar is, "Similar"?

Same inside diameter, elevations, roughness, density of diesel, fittings, viscosity of diesel, inlet pressure, outlet pressure, inlet temperature, outlet temperature?

Check the valve Cv vs Position at the valve's %Open, then you'll know what the pressure drop across the valves should be.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[padovano]

can you throw some light about the valve cv. can you please give me any short formular that I can use to relate it with pressure loss.

 

[BigInch]

Many valve manufacturers publish curves typical of their valves that show the Flow Coefficient, Cv = gpm/(psi * SG^0.5) vs. % Open. A valve with a Cv of 1 will flow 1 gpm when the differential pressure drop is 1 psi.

To give you some ideas, I just searched for valve Cv and found a discussion of Cv and the metric equivalent Kv, and these tables here,

http://www.crispinvalve.com/kflosite/cvvalues.htm

http://www.pdcvalve.com/sizing/PDF/engineeringinfo.PDF

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[padovano]

below is a small table. can you please tell me other than the %opening of the valve how I can adjust the valves such that they have the same outlet pressure,hence pressure loss.
thanks

VALVES P1 P2 P1-P2 FLOW RATE CV
FV64 6 1,5 4,5 51,8m3/h 25,5
FV65 6 4,3 1,7 51,8m3/h 41,6

 

[BigInch]

Correction to formula above is,
Cv = gpm/(psi * SG)^0.5

The only way do it without changing the valve %open and the corresponding Cv, is to change one of the two inlet pressures, so that the downstream pressures are equal. BUT, If you do that, you will probably also be changing the flowrate in the upstream pipeline segments. I think you probably don't want to do that, but here goes,

Solving the equation to have P2 for both valves = 1.5 requires that valve FV65 have an upstream pressure, P1 = 2.64

Solving the equation to have P2 for both valves = 4.5 requires that valve FV64 have an upstream pressure, P1 = 5.76

I think it is some other slight dissimilarity that is causing the different pressure drops, perhaps the interior valve trims are different, but I really can't say with the info you've provided.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[padovano]

thank once more for the valuable information. Can I use the valve % opening to solve this problem? or can I use the valve cv if I assume the valve trims are the same?.

 

[BigInch]

You'll have to look at the specific valve's Cv vs %Open or position curve, as there are several different curve families.. linear, equal%, modified parabolic, quick opening ..

That will give you an idea on what Cv the valve will have at various %Opens, then tie that back to actuator travel.



BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[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. This is the reason why valves sometimes don´t gives more flow with higher openings over 70%, 80% while the inherent curves say the 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