View Full Version : K, Kv and Cv clarification - H2OMAP

Cliff Dredge
March 2, 2016, 02:34 PM

Do the minor losses for valves require the K resistance coefficient? Wanted some clarification of K and Kv (flow coefficient) and Cv which I'm pretty sure is imperial.

I've got these conversion formulas:

K=0.001604x(d4/Kv2) Diameter in millimetres.

Cliff Dredge.

Patrick Moore
March 3, 2016, 03:56 PM

This can often confuse a lot of people because valve manufacturers typically publish Cv and Kv values, but these are not the same as the minor loss K value. To complicate things further the Minor Loss K is sometime called the Resistance coefficient which can add further confusion.

To understand the differences between The minor Loss K and the Cv or Kv value it is best to refer to a fluid dynamics text as the units are critical when converting between the K and Cv or K and Kv values so as to get the correct conversion formulas. This link describes the differences from a fluid hydraulics standpoint and may be of assistance. http://www.forum.pipeflowcalculations.com/viewtopic.php?f=3&t=378

The model however only uses the minor loss coefficient K and the headloss for minor loss is calculated as Hl_ml = K*V^2/2G where K is unitless v is the velocity and G is gravity at 32.2 ft^2/s^2 in US units. Units are converted so that the headloss is consistent with the Head units (ft or meters) your model is using.

The ML value is typically not required as input for any valve, but if it is not used, the model can have the potential where it can differ from actual field behavior at high flow values. For instance a 2 inch PRV in the model will allow very high flow through the valve even though the actual valve in the real world will limit the flow due to the internal headloss at higher flows. Using a minor loss coefficient of 5 for an angle valve or 10 for a globe valve is often enough to restrict the valve maximum flow for a PRV to more reasonable values as the minor loss headloss at higher flows will exceed the available head and the valve will convert to act like an open pipe with flow controlled by the minor loss headloss.

Here is a typical minor loss coefficient table form the help file:
(click fro larger image if necessary)

One last note to be aware of as well. We have recently found that EPANET has a few unusual things it can do with minor losses on valves for certain valve types which causes the similar behavior in H2OMap Water and InfoWater. For example when using a FCV, EPANET only appears to account for the minor loss headloss when the valve status is "open" (i.e. it is acting like a pipe) and ignores the minor loss headloss when the valve is active. This can allow unrealistic flows through the valve at very high settings under certain circumstances. TCVs in EPANET will also ignore the K minor loss value on the valve unless the valve status is OPEN as well. For PRV's though EPANET appears to correctly account for the minor loss at all times. But given these recent findings, it may often be wise to verify valve operation when the minor losses are significant.