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Explaining the Limitations and Benefits of Operating Two Pumps in Series

By Welcome / October 25, 2018

The term Pumping in Series means that 1 pump acts as a booster to feed another pump.  The purpose of pumps operating in series is that, at the same flowrates, the Head (differential pressure) of both pumps is additive.  So, each of these 2 Pumps in Series is lower cost, lower power, and potentially more reliable as a system than a single larger pump that produces the same heads as the total of the two smaller Series pumps.

Operating Two identical Pumps in Parallel

The schematic of 2 identical pumps connected in Series is shown below:

 

centrifugal pumps in series

As you can see in the above schematic, the Pump-B Suction pressure is the Discharge pressure of Pump-A.  So, it is important that Pump-B is rated for the higher Suction and Discharge Pressures.   Even though Pump-B is identical to Pump-A in differential pressure and flow rate, the higher suction pressure it experiences means that care must be taken to select a pump that can meet the higher working pressure.   Mechanical seals and thrust bearings must also be reviewed on the B pump when operating in series.

The correct starting technique, once the Pump-A and Pump-B and the interconnected piping are primed full of the pumping liquid, is to first start Pump-A which is the booster pump feeding Pump-B.  Then start Pump-B with its discharge throttle valve previously opened to approximately 30-40%, which should then be gradually opened, to ensure that Pump-A & Pump-B produce the Rated flowrate.

The schematic below shows the Head & Flow curves plotted for Pump-A, and the Series Curve which is plotted by totaling the Heads of Pump-A and Pump-B at the same Flowrates.  The actual Rated flowrate of Pump-A and Pump-B is where the System Resistance Curve crosses the Series Curve as shown below, not where it crosses the individual Pump Curves:

centrifugal pump in parallel

When Pumps are operating in Series, the effects of Pump failure is more serious than Pumps which are set to operate in Parallel.

If the failed pump is Pump-B, then Pump-A would suddenly have a serious System Resistance causing its flowrate to be very low or even zero.  This resistance might be caused by the system or by the stationary Pump-B.

But if it is Pump-A that fails, then this would eliminate the suction flow to Pump-B, which would result in cavitation or even loss of prime if Pump-B wasn’t immediately stopped.

A means of protecting Pump-A and Pump-B against damage due to a pump failure, would be to connect each Motor driver with a PCM (Pump Control Monitor).  This would detect when a Pump failed and would stop the Motor if its power was either very low, or due to seizure of Wear Rings in the pump, became very high.  So, immediately, when 1 Motor was stopped, the other PCM would stop the other Motor, to prevent damage to the pump that was still operating ok.

Operating Two Different Sized Pumps in Series

When operating in Series, the size difference of the 2 Pumps means that Pump-B can be a larger pump producing higher Heads, or can even be a multi-stage pump producing much higher Heads.  But it is essential that the Flowrates of both Pump-B and Pump-A must be the same.

The Series setup is shown below:

 

pumps in parallel equation

Once again, the Rated Flowrate is the flowrate where the System Resistance Curve crosses the combined Series Head-Flow curve, as shown below:

To evaluate the Overall Series Pumps Efficiencies, the following calculations should be carried out:

pumps in parallel

Now you have a general understanding of how to operate two pumps in series.  Stay tuned for our upcoming training blogs that explore more aspects of pump application, design and maintenance.

 

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