Considerations for the selection and design of reverse osmosis high-pressure pumps

To maintain the reverse osmosis water production rate and keep the operating pressure within the allowable limit, it is necessary to control the outlet pressure of the high-pressure pump. The common types of high-pressure pumps used in reverse osmosis are centrifugal pumps and plunger pumps.

① Centrifugal pump

Centrifugal pumps are suitable for systems that require lower pressure and higher flow. High-pressure pumps are generally used for low operating pressure reverse osmosis systems below 2.0 MPa. When using a pump with a constant-speed motor, a throttle valve is often installed on the pump outlet pipeline to control the outlet flow and pressure.

For reverse osmosis and nanofiltration systems with significant feed water temperature variations, it is recommended to use a pump with a variable frequency drive to adjust the motor speed in order to reduce energy consumption. However, variable frequency pumps are more expensive.

② Plunger pump

The operating principle of a high-pressure plunger pump relies on the displacement of a solid wall in an extrusion manner, making it suitable for systems that require high pressure and low flow.

This type of pump is often used in seawater desalination systems above 5.0 MPa and high-pressure reverse osmosis systems.

Because the discharge pressure of a plunger pump is too high, a throttle valve cannot be used to control the outlet flow and pressure. Instead, the pressure and flow are regulated by a backpressure prevent valve installed at the pump outlet.

Generally, a plunger pump comes with its own backpressure prevent valve, but the valve is set according to the pump’s own pressure, which is usually much higher than the maximum allowable pressure of the membrane element.

A pressure relief valve should be installed at the pump outlet to prevent excess pressure from impacting the reverse osmosis membrane.

Additionally, if the pump itself is not equipped with a pressure damping device, a damping device should be installed at the outlet to reduce pressure pulses.

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