Explaining the Limitations and Benefits of Operating Two Pumps in Series

The term “Pumping in Series” means that one pump acts as a booster to feed another pump. The purpose of operating pumps in series is that, at the same flow rates, the Head (differential pressure) of both pumps is additive. As a result, two smaller pumps in series may offer a lower cost, lower power, and potentially more reliable solution than a single larger pump designed to produce the same head as the combined output of the two pumps.

Operating Two Identical Pumps in Series

The schematic of two identical pumps connected in series is shown below:

As shown in the schematic, the suction pressure of Pump-B is the discharge pressure of Pump-A. Therefore, 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 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 Pump-B when operating in series.

Starting Sequence for Pumps in Series

The correct starting technique, once Pump-A and Pump-B and the interconnected piping are primed full of the pumping liquid, is as follows:

1. First, start Pump-A, which is the booster pump feeding Pump-B.
2. Then start Pump-B with its discharge throttle valve opened to approximately 30-40%.
3. Gradually open the discharge valve to ensure that both Pump-A and Pump-B achieve the rated flow rate.

The schematic below shows the Head & Flow curves plotted for Pump-A, along with the Series Curve, which is plotted by totaling the heads of Pump-A and Pump-B at the same flow rates. The actual rated flow rate of Pump-A and Pump-B is determined by where the System Resistance Curve crosses the Series Curve, as shown below:

Effects of Pump Failure in Series Operation

When industrial pumps operate in series, the impact of a pump failure is more significant than with pumps operating in parallel. Here’s what to expect in case of failure:

• If Pump-B fails, Pump-A will face serious system resistance, causing its flow rate to drop significantly or even to zero. This resistance might be caused by the system or by the stationary Pump-B.
• If Pump-A fails, it will stop suction flow to Pump-B, which may lead to cavitation or even loss of prime if Pump-B isn’t immediately stopped.

Protection Against Pump Failure

To prevent damage due to pump failure, connect each motor driver with a PCM (Power Control Monitor). This system detects when a pump fails and stops the motor if power is too low or, due to wear ring seizure, too high. Immediately stopping both motors when one pump fails can prevent damage to the remaining operational pump.

Operating Two Different-Sized Pumps in Series

When pumps of different sizes are used in series, Pump-B can be larger and produce higher heads or even be a multi-stage pump for much higher head applications. However, both Pump-B and Pump-A must maintain the same flow rate for proper function.

The series setup is shown below:

Once again, the rated flow rate is where the System Resistance Curve crosses the combined Series Head-Flow curve, as shown in the following graph.

Calculating Overall Series Pump Efficiency

To evaluate the overall efficiency of series pumps, perform calculations based on the combined head and flow rate to ensure optimal efficiency in this configuration.

Common Applications for Series Operation

• High-pressure applications – Series configurations are useful in applications requiring high discharge pressure, such as water distribution in municipal systems.
• Industrial processes – Series pumps provide flexibility in processes that may involve variable pressure requirements, such as chemical transfer.
• Mining and drilling – High-pressure pumping is critical in extraction and drilling operations, where maintaining consistent pressure is crucial.

Want to Learn More? Contact PumpWorks!

Now that you have a general understanding of how to operate two pumps in series, stay tuned for our upcoming training blogs that will explore more aspects of pump application, design, and maintenance. Contact PumpWorks today for guidance on your pump systems, or visit our website to explore our range of high-performance pumps.