3 replies to this topic

[nickchen]

I am using the CV curve from control valve vendor to calculate the liquid flowrate, I found the actual flowrate is higher than predicted.
I know the CV value of control valve was generated with water, for viscosity fluid, a factor should be used for correction, and actual CV will less than calculated CV.
The service I have calculated is lower viscosity (0.2cP), does it means I will gain a higher CV than calculated. How to correct the CV value of control valve for a very low viscosity fluid?

[kkala]

Look at http://www.documentation.emersonprocess.com/groups/public/documents/reference/d351798x012_11.pdf for correction due to liquid viscosity. Figure 2 is the main one for this. Suppose that C'v=2 without correction, Q=1 gpm, Vgs=1 cSt, then corrected Cv~1.05C'v. Correction factor will be even lower at lower viscosity. If valve Re number is higher than 3500, correction factor is 1.1 or lower.

Edited by kkala, 15 May 2012 - 03:26 PM.

[nickchen]

Thanks kkala! the documents you provided is for high viscosity fluid correction, the actual CV will decreased when the kinetic viscosity higher than 1cst.
My case is the kinematic viscosity very low (0.2cst) so that the Re number is high than 5000000, I suppose there must be a lower correction factor, i.e. 0.9

[kkala]

Text says "although the majority of valve applications will involve fluids where viscosity corrections can be ignored, or where the corrections are relatively small, fluid viscosity should be considered in each valve selection".
Even though the nomograph does not extrapolate below 1 cSt, the text does not seem to place any limitation on viscosity. Valve Re counts, to my interpretation, and high Re means fully developed turbulent flow, hence correction factor = 1.0 according to the correction factor diagram in function of valve Re. Correction factor is not significant for Re > 3500 (post No 2).
The example of post No 1 presents a correction factor of ~1.05 (not 1.0) for viscosity=1 cSt.
It is noted that K constant for valves in laminar flow is higher than the relevant value in turbulent flow (Perry, 7th ed, Fluid and particle dynamics, Tables 6-4 / 6-5), which complies to a correction factor >1.0 for the control valves, when valve Re is low (rear case). This correction factor approaches 1.0 for Re numbers representing turbulent flow.

Edited by kkala, 16 May 2012 - 02:52 PM.