Flow Control Valve

[more22]

I have the following situation:
We are trying to use an existing pump to transfer liquid to a tank 4 miles away from the pump. At times, the pumping rate is going to be less than the minimum flow required by the pump (a spill back line with a restriction orifice will protect the pump for this case).
I need to install a flow control valve on the pump discharge and need to know how to select the pressure drops for the min, normal, and max flowrate cases.
Thanks

 

[StoneCold]

more22
If you know the piping layout and you have the pump curve then the pressure drop accross the control valve is just going to be the developed head at a given flowrate minus the pressure drop of the piping. However maybe you are still in the design phase and you want to specify a minimum pressure drop across the control valve. So you might want to use 5 psi or 50% of the total pressure drop of the piping run at normal operating conditions which ever is greater. I looked through the fischer control valve sizing info and did not see this criteria listed but I have seen it somewhere else I just can not remember the reference.

Goodluck
StoneCold

 

[TD2K]

It's much easier since you have an existing pump. I assume it's an centrifugal since you are using an RO to provide minimum flow protection.

For min/normal and max flows, first calculate the discharge pressure from the pump. If you are taking suction from a tank, you can tweak your numbers by assuming the tank is full or empty as you want/need.

Then, for min/normal/max flows, calculate the line losses through the 4 miles of piping to the final destination. If you take the pressure loss plus the pressure at the far end (however that is set/maintained), that gives you the pressure needed at the outlet of your control valve. The difference between the pump discharge pressure and the pressure needed downstream of the control valve is the dP the valve will see for that flow.

 

[buzzie]

A general sizing criteria for control valves on the discharge of a centrifugal pump is as follows:
50% of the resistance loss or 15 psi whichever is greater. Note this is resistance loss and not fixed head requirements based on elevation change or destination pressure.

The 15 psi minimum is adjustable. If you have a small, well defined system hydraulically 5 psi minimum can be OK.

For a long pipeline I would use a minimum of between 15 and 25 psi to allow for long term fouling resistance increase in the piping. If this is FRP or plastic or lined pipe this conservative approach could be reconsidered.

In any case the control valve maximum Cv is generally selected as about two times that calculated from the pressure drop determined by the above method(s). This permits about a +10% flow margin on valve capacity above nominal design as the pump discharge pressure decreases as it runs out on its curve.

Regards