Normal and Rated Capacities 1

[EngMecBra]

If we select a pump by rated capacity x diff. head, how the pump operates in the same head with normal capacity (normal operating point)?
The performance curve shows that if the operating point goes from rated capacity to operating capacity (right to left in performance pump curve), the diff. head will be increased.
How does it occur in the practice?
Thanks!

 

[rutherford703]

In reality, it's true. florate decrease will cause the pump discharge pressure higher because you have to increase the system pressure drop such as flow control valve to control the flowrate.

 

[bimr]

The point on the curve where the flow and head match the application's requirement is known as the duty point or rated capacity.

Normal capacity corresponds to the flowrate given by a PFD/Material balance.

A centrifugal pump always operates at the point on it's performance curve where its head matches the resistance in the pipeline.

In the real world, a centrifugal pump operates all over the place, depending on a flow need – and the intent is that operators will not operate the pump too far outside the recommended envelope of operation, which is typically between 70 to 120% of BEP point.

A pump curve slope generally moves upward from the duty point from right to left on the pump curve. In practice if the system head curve is underestimated and the system head is actually larger than the duty point head, the pump operating point will shift to the left along the pump curve.

http://www.gouldspumps.com/cpf_0009.html

 

[BigInch]

The pump curve is half the answer. You need the system ccurve now. Superimpose the pump curve on the system curve by adding suction head to the pump curve. The operation point is at the intersection of the two curves.

What would you be doing, if you knew that you could not fail?

 

[Pumpsonly]

The rated capacity is used by the purchaser to compare pump performance between suppliers and always higher than the normal capacity.In operation a control valve or a pump discharge valve is used to adjust the flow rate according to the process demand by changing the system resistance curve of the discharge piping.This is called throttling control.
Another commonly used method is by varying the pump speed which changes the pump diff. head.
The intersection of the pump Q-H curve and the system curve will be the flow rate.

See attached for better understanding

 

[Pumpsonly]

attachment was left out.....

 

[fel3]

[1] If you have (a) a fixed system curve and (b) a constant speed centrifugal pump, operation will occur at the point on your Q-H plot where the system curve and pump curve intersect. This is your duty point. Constant or nearly contant system curves are common, for example, in agriculture where irrigation water is pumped from a large lake or regulated canal and free-discharged into an irrigation ditch.

[2] If you have (a) a varying system curve and (b) a constant speed centrifugal pump, operation will occur along the "line" of the pump curve between the upper and lower system curves. Varying system curves are common in muncipal water systems where water levels in the storage tanks on the suction and discharge sides of the tank fluctuate and the water demands between those tanks and the pump fluctuate. This requires analyzing the system for multiple conditions, including high suction tank level to low discharge tank level, low suction tank level to high discharge tank level, and these conditions for different water demand levels and other operational issues. For some systems I have generated as many as 16 system cuves to find the actual envelope, though normally I need only 2 to 4 curves.

[3] If you have (a) a fixed system curve and (b) a variable centrifugal pump, then operation will be along the "line" of the system curve between the upper and lower operating points of the pump. I don't have any examples for this type of system.

[4] Finally, if you have (a) a varying system curve and (b) a variable centrifugal pump, then operation will within a region bounded by the upper and lower system curves and the upper and lower pump curves. This is also common in municipal water systems.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[EngMecBra]

Thanks!