In this Article, I have Explained Why discharge valve closed during starting of centrifugal pump ? Or Why centrifugal pump started with closed discharge valve ?
let us first understand through characteristics curve of centrifugal pump
A centrifugal pumps usually started with discharge valves shut.
Considering the properties of centrifugal pump by operating characteristic curve it is clearly found that; when the amount of water delivered is zero, the power available is at minimum.
So discharge valves valve remains shut off throughout the beginning to take care of low power demand by the pump on its motor having low power output at the beginning.
This helps stabilizing pumps at the starting. When the pump is stable the discharge valves opened and high power avaibility meets the high power demand; which might be not possible for the motor to fulfill for the starting 4-5 seconds.
It is a very common practice to start large capacity centrifugal pump with the closed discharge valve .
If the characteristic curve for a centrifugal pump are examined it will be seen that when the quantity of water discharged is zero the power required by the pump is zero or very small amount.
by starting the pump with discharge valve closed the power demand made by the pump on the pump motor is kept to very minimum after the pump has started and the momentary high motor current demand has stabilized the discharge valve is opened.
Why centrifugal pump started with closed Discharge valve
By closing the discharge valve, the starting current can be reduced.
As we know, at the start of any motor, the current will be high. When we start the pump with an open discharge valve, the discharge head will act on the pump, i.e. more resistance, so that the motor has to give more starting torque to the pump, which means more current is drawn by the motor.
If we start the pump with discharge valve closed, it means there no discharge head and minimum resistance to the pump and so current drawn is minimum. Although the current taken during startup of motor is more than normal, but by closing the discharge valve of centrifugal pump we can avoid extra load act on the pump.
If there is a check valve, then the discharge valve can be opened. Without having a check valve, we need to have the discharge valve closed if there is pressure on the discharge side of centrifugal pump at the time of startup.
If we think differently,If there is a pressure on the pump’s discharge side,before starting it may flow back through the pump,causing a backward spin and can draw more current , resulting damage to the pump.Thus can be prevented by keeping the discharge valve of centrifugal pump closed when starting.