Q - When is it recommended to start a pump with the discharge control valve
closed, and when is it with the discharge control valve opened?
A - A pump should be started in a manner that its required starting torque is at
the lowest possible.
Since torque is directly proportional to the brake horsepower (BHP), then a pump
should be started with the discharge control valve either closed, or opened,
depending on whether the lowest required BHP as shown in its performance
curve, is near the shut-off capacity (start the pump with the valve closed), or near
the end-of-curve (start with the valve opened.)
Generally, radial and mixed-flow type pumps have their lowest required BHP near
the shut-off capacity with the BHP increasing towards the higher flow rates, and
should therefore be started with the discharge valve closed.
In contrast, axial flow type pumps generally have their highest required BHP near
the shut-off capacity, with the BHP decreasing towards the higher flow rates, and
should therefore be started with the discharge valve opened.
When starting a unit with the discharge valve closed, the valve should be opened
gradually as soon as the driver reaches its full speed. Operating it over a longer
period of time with its discharge valve closed can result in unwanted, sometimes
destructive, temperature rise in the casing.
"Make it simple"