SYSTEM CURVES
System curve represents the resistance, or the losses the pump has to overcome. These include the friction losses, losses through fittings, valves and heat exchangers. Thus the variation of the system resistance or head can be plotted against the flow on the graph in the form of a system curve.
Let us learn more about these pump curves on this page.
Let us consider the following piping system.
The system consists of plug valve, isolation valve, unions, piping system and a coil. These all contribute to system losses. This is termed as resistance.
The system resistance varies with flow with the following relation :
where K is some constant and Q is the rate of flow of water through=h the system.
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Generally most piping systems have control valves installed to regulate the system pressure loss in order to balance the piping system.
Thus for a given system, as the resistance increases, the system curve shifts upwards and shifts downwards when the system resistance decreases.
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This is depicted in the diagram given below:
In the aforementioned graph, the system resistance increases in case of the orange line and decreases in the case of the grey line.
Pump operation in a system:
Thus, the pump has to overcome the pressure head developed in the piping at the given flow rate and hence provide the required flow.
In actuality, the point of Intersection of the pump and the system curve tells us how the pump operates when installed in the system.
This is termed as the 'PUMP OPERATION POINT'.
PUMP SELECTION:
Let us first discuss the Best Efficiency Point. It is defined as the point on the pump curve where the pump operates at maximum possible efficacy for the given pump dimensions. It is usually around 75% efficient. Operation at the BEP ensures maximum pump life and reduced maintenance.
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The pump BEP is marked in the given figure. It is ideal to match the pump operating point and BEP for maximum benefits.
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In practice, operating point is allowed to be within 10% range of the BEP point on the pump curve, and still the pump gives a sensible performance. Care must be taken to ensure pump operating point doesn’t exceed these limitations to avoid undue losses.
CAUTION:
Operating the pump too far right or lift may cause rise to undue problems and must be avoided.
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Operating the pump too far to the left , ( or at less than 60% of BEP flow), often may lead to very low flow through the pump. This leads to the pump not being able to dissipate enough heat through the flowing water and eventually heats up the pump and motor. Recirculation or backflow may also be an unnecessary complication.
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Operating too far to the right, (more than 120% of BEP flow), the flow rate is abnormally high. This more often than not results in a low pressure in the pump, or increases the NPSR. And we all know from the above sections that this leads to cavitation in the pump and hence expensive damages. These higher velocities also lead to unpleasant sounds in the surrounding areas.