Accurate selection of the pump drive and calculation of operating cost or feasibility studies require the knowledge of the drive power required for the pump to operate at the required Duty Point. The shaft power requirement or the power input of the pump are, as the hydraulic performance, also shown in a graph.
The drive motor is sized to suit that point if the pump is to operate non-overloading over it's feasible capacity range.
Glandless Pumps are always equipped with motors capable of covering the total curve range. It is thus possible to reduce the number of pump types and thus to ensure a convenient stock keeping on spare parts.
If the specified duty point of a pump (Glanded Pump) is located on the left hand portion of the duty curve with a correspondingly lower power input.
It is feasible to select a smaller motor size. In such cases however there is a possibility of overloading the motor if the actual duty point allows a higher flow-rate than that calculated (a more flat system curve).
As in practice there is always the danger of the duty point having shifted it must be recommended to select the motor to drive a glanded pump with power reserve of approx. 5 to 20% above that theoretically required.
To determine the operating costs of a pump you must distinguish between the shaft power requirement P2 (kW) of the pump, often equal to the installed rated motor power, and the actual power input to the drive motor P1 (kW).
The latter, P1, is the basis for determining the operating costs. In the event that only the shaft power, P2, is given it is possible to determine the actual power input by dividing P2 by the efficiency of the motor.
An electric power input P1 is stated where pump and drive motor are encapsulated in a single unit such as a glandless pump. However, both P1 and P2 values are normally stated.
The required shaft power P2 is generally given where pump and motor are coupled such as glanded pumps. This is necessary to allow the use of different types of motors with differing power inputs
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