Series and parallel installation scheme of pumps

Series Installation

The series installation scheme of pumps is used in two cases:

• One pump cannot provide the required pressure, and replacing it with a more powerful one is impractical.
• During the technological process, there is a need for a short-term increase in head, and installing a more powerful pump with a frequency regulator is impractical.

Series connection of two pumps with identical characteristics allows creating a mode with doubled pressure at the same flow rate [m³/h] provided by a single pump.

In the case of series installation of two pumps in a closed circulation loop, such as a heating or cooling system or for circulation through a heat exchanger, you will not achieve doubled pressure. In this case, the operating point of the pumps will be at the intersection of the pressure characteristics of the two pumps and the hydraulic pressure loss curve of the circulation loop depending on the liquid flow rate. Thus, you will achieve a mode with some increase in pump head [m] and some increase in flow rate [m³/h] in the circulation loop.

Series installation of pumps with different characteristics should be calculated as there may be a mode in which the more powerful pump creates too high a flow rate, turning the less powerful pump into a hydraulic resistance in the flow path.

Parallel Installation

The parallel installation scheme of pumps is used in the following cases:

• One pump cannot provide the required flow rate, and replacing it with a more powerful one is impractical.
• During the technological process, there is a need for a short-term increase in flow rate, and installing a single more powerful pump with a frequency regulator is impractical.
• It is necessary to incrementally increase the flow rate, which is often used in water supply pump stations with high peak consumption.
• To increase system reliability. In this case, the second pump serves as a backup.

Parallel connection of two identical pumps leads to a mode with doubled flow rate while the head remains the same as when a single pump operates.

Parallel connection of pumps with different characteristics to a single discharge manifold should be carefully calculated as there may be an operating mode in which the more powerful pump creates too high a pressure in the discharge pipe, which the less powerful pump cannot counteract, allowing reverse flow.

To prevent reverse flow, a check valve must be installed on the discharge pipe of each parallel-connected pump, and pumps with identical characteristics are recommended.

Regardless of the pump installation scheme in the circulation loop, remember that the head losses [m] in the loop quadratically depend on the flow rate [m³/h] in the loop.

For example, to double the flow rate (pump flow rate [m³/h]) in the circulation loop, the pumps need to generate four times the head. Doubling the head generated by the pumps will increase the flow rate in the circulation loop by 1.4 times (1.4 is the square root of two).

Therefore, in the illustrations above, connecting two pumps in series and parallel did not lead to significant increases in flow rate in the circulation loop. The hydraulic characteristic of the circulation loop on the pump's head-flow curve is depicted as a parabola.

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