To empty a tank on occasion, I'm planning to install a PD pump, pumping about 30-60 gpm of warm water through a 2" line. At the end of the line my circuit will join into a larger 50 psig line. I'll add a check valve for when my line's not in use. Right now I'm doing the hydraulics and selecting the pump.

The problem is, I will probably have to tie my 2" line into the 50 psig line via an existing 3/4" diaphragm valve. I've got no model number/data for this valve, so thought I would just get out Crane 410 and estimate the pressure drop with K-factors. Except there is no K-factor for diaphragm valves in Crane, nor anywhere else I've looked.

Does anyone have a K-factor for diaphragm valves, or any other suggestion to estimate it? Should I just get three vendor catalogs and take the highest pressure drop from them? The valve is SS, SCH80 and probably a Teflon diaphragm.

For comparison, at 60 GPM a 3/4" globe valve is >60 psi loss, whereas a gate is ~2 psi. So it's not insignificant how I do this.

Most manufacturers of flow components can provide you with the component capacity in terms of a Cv value from wich you can calculate the K value from K=891*d^4/(Cv^2) (see Crane page 3.4). 

So, no way you can change that valve?

K or Cv values are highly dependent on what type of diaphram valve you have.  A full opening type diaphram valve, when fully open, has little resistance at all if any.

Another approach might be to work with the pump.  Since its a PD, if you have the power available and the strength, you can get the pressure.  Select it to the high side and you can pump through any valve you need to. Just an idea.

For a 3/4" weir type diaphragm valve I use a K of 4.5.  If you have a teflon diaphragm it is likely to be a weir type valve because teflon does not have the flexibility of the other elastomers. Forgive me for stating the obvious, but remember to apply this K to the velocity in the 3/4" line and not that in the 2" line.

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