What are series and parallel centrifugal pumps? What do they do?

In actual use, a water treatment pump station often requires multiple centrifugal pumps to operate together to provide the required flow and pressure for the water treatment system. The connection and operation modes between them are parallel and series. So what are the functions and differences of parallel and series operation of water pumps?

Water from two or more pumps flows into a common outlet pipe, which is called parallel operation of pumps. In irrigation projects, multiple centrifugal pumps usually draw water from the same water source and deliver it to the same irrigation system. Parallel operation of pumps means that multiple pumps share a single outlet pipe. Each pump is equipped with an independent check valve. After the pumps are operated in parallel, the flow rate at the same head will be superimposed.

That is: Qsum = Q pump 1 + Q pump 2 + Q pump 3 + ... + Q pump n

Centrifugal pump parallel operation characteristics

1. The water supply can be increased, and the flow rate in the water supply main is equal to the sum of the water output of each parallel pump.
2. The flow rate and head of the pump station can be adjusted by the number of water pumps started and stopped, so as to achieve the purpose of energy saving and safe water supply. For example, when designing a water intake pump station, the flow rate is considered according to the maximum daily average hourly flow rate of the city, and the head is considered according to the low water level of the river. Therefore, in actual operation, due to changes in river water levels and changes in water consumption in the urban pipeline network, the start and stop of the water intake pump station unit will inevitably be involved. In addition, the start and stop of the pump station unit also need to be adjusted.
3. When one of the pumps working in parallel is damaged, the other pumps can continue to supply water. Therefore, the parallel water supply of pumps improves the flexibility of the pump station operation and the reliability of water supply, and is the most common operation mode of the pump station.

Centrifugal pump parallel operation is suitable for the following situations

1. The flow rate of a single centrifugal pump is too small to meet the needs of the irrigation system.
2. When the water delivery distance is long or the terrain conditions are complex, and the amount of earthwork excavation for the pipeline is large, in order to reduce investment, the water delivery pipelines of multiple pumps can be merged into one.
3. The well network in the northern region connects the outlet pipes of multiple water pumps together to facilitate the allocation and rational use of water resources.
4. The head of the pumps running in parallel should be equal or the same, and it is best to use pumps of the same model, which is also convenient for operation management and maintenance. When multiple pumps of the same model are connected in parallel, if the distance between the inlet pipe and the outlet connection point of each pump is short, the total flow after parallel operation is equal to the sum of the flow rates of each pump, and the head remains unchanged.

The series operation of centrifugal pumps means connecting the outlet pipe of the previous (first-stage) pump to the inlet pipe of the next (second-stage) pump, and the last (final-stage) pump supplies water to the irrigation system. This mode of operation is suitable for pumping stations with higher lifts, or when the water delivery distance is long and the lift of one pump cannot meet the requirements, and there is no suitable high-lift pump. In actual use, it is rare to see three or more pumps in series, and generally, two pumps are connected in series.

The series connection of centrifugal pumps mainly solves the problem of insufficient lift. After the pumps are connected in series, the flow rate remains unchanged, and the lift is the sum of the lifts of the two pumps. In order to avoid an insufficient water supply from the downstream pump to the upstream pump, the flow rate of the downstream pump is usually adjusted to the optimal state to ensure sufficient water supply from the upstream pump. The schematic diagram is as follows:

In the figure, the outlet of pump "D" and the inlet of pump "E" are connected in series through a pipeline. After the pumps are connected in series, the medium first enters the inlet of pump "D". Through the operation of pump "E", the medium is pushed to the inlet of pump "E" and transported to where it is needed. Pump series connection is essentially an extension of step delivery. What is step delivery? It means that the downstream water level is too low, and the position to be introduced is too high, and one pump cannot "complete the mission" at all.

For series operation, the outlet pressure of the N-1 pump (the loss between pumps needs to be subtracted for long-distance series connection) is the inlet pressure of the nth pump. Therefore, there are certain requirements for the pressure, bearings and shaft seals of the series pumps, otherwise it will cause shell rupture, shaft seal damage, bearing heating, etc. As in the parallel case, if one or more pumps are shut down, the operating conditions of the remaining pumps will also change.

For two centrifugal pumps in series, the flow rate should be equal or not much different, and the head should be matched. The low-lift pump should be installed before the high-lift pump, and the low-lift pump should supply water to the high-lift pump to prevent the low-lift pump from being subjected to excessive pressure. If two pumps of the same model are connected in series, the pressure of the latter pump will double, and the pressure bearing capacity of the pump or the pressure-bearing capacity of the booster pump must be checked. If the strength is not enough, reinforcement measures should be taken, otherwise, it is not suitable for series connection.

If the flow rates of the two sets of series pumps are different, the large-flow pump should be installed in the front, and the small-flow pump should pump water from the front, so that the inlet pressure of the small-flow pump is large, which can avoid cavitation.