PARALLEL PUMP OPERATION
How to calculate Q-H curve of parallel operating pumps?
How to define the operating point of every pump working parallel?
What will happen if one pump to turn off? How other pumps will operate?
What system curve is more acceptable for parallel operation?
What can happen if pumps with different Q-H characteristics operate in parallel?
How will pumps operate if one of them have speed control?
Interesting fact!
Some users think that if to add another pump to existing pump the overall flow of the two pumps working in parallel will be equal to a flow of one pump multiplied on two and flow of three pumps working in parallel is equal to a flow of one pump multiplied on three and so on. Very often this is not right. The explanation is below.
The H-Q curve of pumps operating in parallel can be built by adding a value of flow at the same head. This procedure should be made for several points.
Operating point. The operating point of pumps working in parallel depends on the system curve especially the share of static head in characteristic.
Parallel pump operation in a system with friction losses
Operating of three pumps in parallel in the piping system with the mainly friction losses.
You can see in the graph that the adding of the every next pump increases overall flow on the less value of a flow - Q1, Q2, Q3.
Parallel pump operation in a system with a mainly static head
Operating of three pumps in parallel in the piping system with the mostly static head. Adding the every next pump increases the overall flow approximately on the same value of flow.
Conclusion
Parallel pumps operation and flow control by the changing of a number of working pumps is more suitable for pumping system with the static head.
The important question is
At what operation point will work every pump at parallel operation? The control algorithm of a parallel pump operation (when to turn on or off pumps depending on the changing of system curve) should provide operation of every pump within the operating range.
What is useful to keep in mind about the parallel pump operation?
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Pumps in parallel operation with the same head.
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To receive the flow of every pump it is necessary to divide the overall flow on the number of pumps.
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In the graph, it is necessary to project the duty point on the vertical axis and at the intersection with Q-H curve of the pump. This point will be the operating point of a single pump.
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The overall flow will depend on the shape of the system curve especially the share of a static head in a system curve.
What will happen if one pump stops during the parallel operation?
For example
Three pumps operate in parallel. Let's suppose that one pump stops. The duty point will be on the intersection of two pumps Q-H curve with the system curve.
If one pump left in operation the duty point will be in the area of overloading
At the operation of three pumps, every pump operates at the area of underload. If two pumps turn off single pump will operate in the area of overloading.
Constant operation of several parallel pumps is the evidence of ineffective operation of a pumping system.
Requirments to pump Q-H curve.
Flat curves are not suitable for parallel operation.
Pump for the parallel operation should have a steep stable curve.
If the Q-H curve is a flat slight change of the system curve will cause the movement of the operating point toward the less flow. See below the illustration.
If the pump has a flat curve even slight changes of the head causes large changes of flow.
Possible problems with the pumps parallel operation
Identical pumps can have different Q-H due to tolerances, deviations and degradations. This can cause pump operation far from the BEP.
There are no two identical pumps. Every pump is manufactured with some tolerances. It means one pump will generate more head than another.
Pump degradation chages pump characteristics and the pump will have lower head than other installed in parallel.
If the pump has a flat curve even slight changes of the head causes large changes of flow.