PERFORMANCE CURVES OF PUMPS

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Performance Curves

Head ~ Flow   H~Q

 The static Head is the difference between suction head  and delivered head. As the suction Head changes then the Static Head changes.

When the pump is operating the liquid will be moving within the pipework and so a loss due to friction will occur

Dynamic Head  H = H_D - H_S - H_F

Positive displacement.

A positive displacement pump encloses a volume of fluid and physically displaces it to the pump discharge. The flow of the pump is therefore constant depending on the speed.

 A reduction in flow will occur as the head increases due to internal leakage. This slip is greater for rotary pumps than reciprocating pumps.

Centrifugal.

For a centrifugal pump, which uses the conservation of energy principle, which changes velocity energy into pressure energy, as the flow rate, Q, decreases then the differential pressure head, H, increases.

This would curve a performance curve ofA centrifugal pump incurs head losses due to friction, the fact that the fluid changes direction travelling through the pump, shock, and because of clearances within the pump, leakage. These losses vary with both head and flow. A large head, low flow means that friction losses will be small but leakage losses high. Low head, high flow means that friction losses will be high but leakage losses will be low

Operating centrifugal pumps in series means that the pressure head of the second pump adds to the pressure head of the first. Due to system and pump losses the total head will be less than the sum of the individual heads. The flow rate through the second pump can only equal tOperating centrifugal pumps in parallel means that the flow of the second pump adds to the flow of the first. Due to system and pump losses the total flow will be less than the sum of the individual flows. The pressure head of both pumps can only equal that of the lower value.

 that of the first pump

Power requirements are at a minimum when the flow is zero.

Efficiency = Useful work out / work supplied. This is at a maximum when the losses are at a minimum