Combined operation of pumps

Combined operation of pumps is advisable in the following cases:

• One pump cannot provide the required flow or pressure, and replacing it with a more powerful one is impossible or impractical.
• According to the technological process requirements, there is a need for a peak increase in flow or pressure during operation.
• It is necessary to gradually increase flow or pressure, which is often used in pumping stations of water supply systems with high peak demands.
• High system reliability requirements must be met.

Series arrangement

Series pump installation scheme is used when a single pump cannot provide the required pressure. In a series connection of two identical pumps, their pressure [m] is added while the flow rate [m³/h] remains the same as when one pump is operating.

A series connection of pumps with different flow rates may result in a condition where the pump with the lower flow rate becomes a hydraulic resistance in the path of the water flow, unable to handle the volume supplied by the more powerful pump. Therefore, it is recommended to install pumps of equal capacity or carefully verify the installation's operation using calculations for the specific system.

Parallel arrangement

Parallel pump installation scheme — is used in systems with a gradual increase in flow rate and in cases where a single pump cannot meet the specified parameters. In a parallel installation of two identical pumps, their flow rates [m³/h] are added while the pressure [m] remains the same as when one pump is operating.

If there is a possible mode where only one pump operates, a check valve must be installed on the discharge pipe of each pump.

Parallel connection of multiple pumps with different capacities to a single discharge manifold may result in a situation where the more powerful pump creates pressure in the manifold that the less powerful pump cannot withstand, allowing backflow. To avoid such conditions, it is recommended to install pumps of the same brand or thoroughly calculate the operation of two pumps with different capacities for the specific system.

Combined operation of pumps in a circulation loop

Regardless of the pump installation scheme in a closed circulation loop (heating, cooling systems, or circulation through a heat exchanger), it should be noted that head losses [m] in the circulation loop quadratically depend on the flow rate [m³/h] in this loop. The hydraulic characteristic of the circulation loop on the pump head-flow characteristic is represented as a parabola.

That is, 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 produced by the pumps will increase the flow rate in the circulation loop by 1.4 times (1.4 is the square root of two).

The operating point during the combined operation of pumps in a circulation loop will be at the intersection of the combined head characteristic of the pumps and the dependency of hydraulic pressure losses in the circulation loop on the flow rate of the liquid being pumped by these pumps. Thus, you will achieve a mode with some increase in the pumps' head [m] and a corresponding increase in the flow rate [m³/h] in the circulation loop.

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