Centrifugal pumps can operate over a wide range of capacities, from near zero flow to well beyond the rated capacity. However, there are certain minimum operating flows which must be imposed on centrifugal pumps for either hydraulic or mechanical reasons. Four limiting factors must be considered: radial thrust, temperature rise, internal recirculation, and shape of the power curve.
For sustained operation, it is important to adhere to the minimum flow limits recommended by the pump manufacturer.
The thermodynamic problem that arises when a centrifugal pump is operated at extremely reduced flows is caused by the heating up of the liquid handled.
The difference between the power consumed and the waterpower developed represents the power losses in the pump, except for a small amount lost in the pump bearings. These power losses are converted to heat and transferred to the liquid passing through the pump.
If the pump were to operate against a completely closed valve, the power losses would be equal to the shutoff power. Since there would be no flow through the pump, all this power would go into heating the small quantity of liquid contained in the pump casing. The pump casing would heat up, and a certain amount of heat would be dissipated by radiation and convection to the atmosphere.
However, because the temperature rise in the liquid pumped could be quite rapid, it is generally safer to ignore the dissipation of heat through radiation and the absorption of heat by the casing. Calculations for determining the temperature rise in the liquid are available from Kelair.
The maximum permissible temperature rise in a centrifugal pump varies over a wide range, depending on the type of service and installation. The minimum capacity based on thermodynamic considerations is then established as that capacity at which the temperature rise is the maximum permitted.
There are also hydraulic considerations which may affect the minimum flow at which a centrifugal pump can operate. In recent years, correlation has been developed between operation at low flows and the appearance of hydraulic pulsations both in the suction and in the discharge of centrifugal impellers.
It has been proven that these pulsations are caused by the development of an internal recirculation at the inlet and discharge of an impeller at certain flows below the best efficiency capacity. The pump manufacturer’s recommendations on minimum flows dictated by these considerations should always be followed.
The NPSHR curve becomes increasingly unstable at low flows. As a rule of thumb, do not operate pumps at flowrates lower than that equivalent to the left-hand end of the NPSHR curve. This rule must be considered in conjunction with other issues detailed in this section.
Ensuring you have the right pump for the right application involves a thorough set of considerations. Consult a knowledgeable pump supplier like Kelair Pumps to discuss your pumping requirements.
Image: Kelair Pumps
