Question

What is the purpose of a throttling valve? What are the implications of having a throttling valve prior to the pump inlet? Explain in terms of NPSH.

Answer 1

Throttling valves are a type of valve that can be used to start, stop and regulate the flow of fluid through a rotodynamic pump. When the flow of a pump is regulated using a throttling valve, the system curve is changed. The operating point moves to the left on the pump curve when the flow is decreased.

Throttling valves are one way of controlling flow rates, by throttling flow directly or in a bypass line. Variable speed operation is an alternative method of controlling the flow of a system.

-If the discharge of a centrifugal pump is throttled, then its suction lift will increase (by considering liquid level in suction pond/sump is same as before). When i say suction lift, i am directly pointing out the Static Suction Pressure of the pump.

Because of throttling operation, flowrate of centrifugal pump will decrease. Pump "suction" is from the same source as before so static pressure in the suction pond/sump is same. As we are assuming liquid level inside sump is same as before, hydrostatic pressure is also same. Now only variables to calculate the static suction pressure of the pump are velocity head (kinetic energy) loss and frictional loss. Due to decrease in flowrate velocity head loss as well as frictional loss decreases, by applying the Bernoulli's equation, pump static suction pressure will increase.

As a result of this, NPSH will increase.