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Liquid ring vacuum pump Working Principle & Uses

What is a liquid ring vacuum pump?

A Liquid-Ring Vacuum Pump (LRVP) is a rotating positive-displacement pump.

A liquid ring vacuum pump or compressor is a rotary device. They operate according to the  positive displacement principle. A liquid acts as a piston in these machines. The liquid in most applications is usually water, so that’s why they’re also known as water piston or liquid piston rotary pumps and compressors. In similar fashion to other pumps, liquid ring vacuum pumps have bladed impellers attached to a central hub.

Construction of Liquid Ring Vacuum Pumps:

Liquid ring pumps

As the above diagram shows, in a liquid-ring vacuum pump, the pumped gases or vapours (air is a mixture of gases that can be pumped) are sucked into the pump-motor unit via the Inlet and discharge connections. The impeller with blades is housed in a cylindrical housing. The impeller is positioned eccentrically within the housing. Additionally, the housing contains an operating or sealing or service liquid. The operating liquid is fed into the system through the operating liquid port and discharged together with the pumped gases through the discharge connection.

How does a liquid vacuum pump work?

Water is the common operating medium for liquid ring vacuum pumps. When an eccentrically installed impeller rotates in the casing partly filled with liquid, the liquid inside is also put into motion and accelerated.

The liquid inside the cylinder is rotated by the impelling force of the impeller and then forms the so-called liquid ring, which also rotates. The centrifugal force causes this ring to revolve concentrically within the housing and eccentrically to the impeller.

Gas flows in the chamber between the single vanes and the liquid ring. Because the impeller is eccentrically installed, the spaces are larger and the process gas is drawn into the suction slot.

The space decreases as the impeller continues to rotate, so the gas is compressed and discharged through the discharge slot. The vacuum pump can be operated in closed or open loop circuits with water recirculation.

The following happens when the impeller completes a complete rotation.:

  • The liquid ring is closest to the rotor axis at the upper vertex (point A) and fills the impeller cells completely with operating liquid.
  • During the first half rotation, the moving liquid ring acts as a piston in the suction stroke because of the lifting of impeller hub by liquid ring. The inlet port allows the pumped gases to be sucked in by increasing the volume of the cells.
  • The impeller moves down until the impeller reaches the lower vertex (point B). The space in the cells is greatest in the lower vertex because these cells are nearly devoid of operating liquid.
  • During the second half rotation, the liquid is moving toward the hub (the moving liquid acts like a piston during its discharge stroke). The space in the cells decreases, resulting in compressed gases that are discharged through the discharge port.

As a result, during every rotation, the gases are drawn from the inlet port and compressed, then discharged from the discharge port. Since there are so many blades in the machine, the operation is continuous and non-pulsating.

When the inlet of a vacuum pump or compressor is connected to a closed system and the outlet to the atmosphere, it will collect gases from the system and blow them away into the atmosphere, acting as a vacuum pump. The machine is a compressor when the inlet is connected to the atmosphere and the outlet to a closed system. The machine draws the air (gases) from the atmosphere and discharges it into the system and will act as a compressor.

Liquid ring vacuum pumps are available in two different types of designs –

  1. The single-stage liquid ring pump.: In a single rotation, the pump pulls back a vacuum (suction), and then releases it back to atmosphere (compression). Single-stage pumps can produce a vacuum as low as 35 mm Hg or 25 Torr.
  2. The two-stage liquid ring pump.: A two-stage vacuum pump, on the other hand, is just two single-stage pumps operating in a series. The first pump sends its fluid out to the suction port of the second pump.

Two-stage pumps are more efficient at very high vacuum levels, with most designed to produce a vacuum of up to 4 Torr. Two-stage pumps can handle solvents better at much higher vacuum levels.


Pump manufacturers and other companies in the field supply many accessories to liquid ring vacuum pumps. An application’s specific requirements, type of operation, and control scheme dictates which items are necessary for what. The following list includes some of the more commonly used items: 

  • Non-return or Check valves restrict the flow of gas and liquids when the pump stops. The non-return valve is mounted on the inlet connection of the pump. It looks much like a valve with a plate seat. 
  • The inlet vacuum valve prevents cavitation in the pump. The vacuum relief valve must open for atmospheric air to flow into the pump if the suction pressure is lower than a certain setting. Most vacuum relief valves, regardless of their price, are based on atmospheric pressure and need to be calibrated periodically. 
  • The operating liquid and gas discharged from the pump are separated by a liquid separator. Generally, the separator can be mounted either on the floor, on a baseplate with the pump, or supported by the discharge piping of once-through systems.
  • Gas Ejectors offer suction pressures that are lower than what is capable of the liquid ring vacuum pump when operated alone. Gas ejectors can be attached to pumps so that the inlet pressure can be as low as 4 mbar or 0.058 psi.

The functioning of a gas ejector is similar to that of a steam ejector. As a motive force, air or recycled gas from the separators works to compress the process gas from the inlet pressure of the system up to that of the gas inlet of the liquid ring vacuum pump.

What is a liquid ring vacuum pump used for?

  • Aircraft Industry.: Used for processes such as carburettor testing, altitude chamber work, vacuum filling of fluids, vacuum chucking for holding and handling parts.
  • Automobile Industry: Used for engine testing, carburettor testing, vacuum filling of power steering and brake fluid.
  • Brewing Industry: Using the pump as a compressor to air agitate yeast and malt during the fermentation process; vacuum de-aeration of makeup water; bottle filling under vacuum.
  • Chemical Industry: Used to vacuum on chemical reactors and other processes, vacuum deaeration of caustic solutions, vacuum exhausting on resin kettles, vacuum filtration, the distillation of synthetic resins and fatty acids, de-aeration of solids and liquids.
  • Concrete Industry: Vacuum lifting and deaeration of precast concrete slabs.
  • Dairy Industry and Dairy Products: Used for deodorization of milk under vacuum; flash cooling milk under vacuum; liquid ring compressors are used to agitate and aerate milk; vacuum for milking machines.
  • Gas Specialties: Vacuum drying of oxygen cylinders; emancipation of old gas cylinders using vacuum before filling.
  • Petroleum Industry: vapour recovery from storage tanks, priming of pumps, vacuum cleaning and filling of barrels, gas compression compressor for carbon dioxide, and furnace for sulfur dioxide–if sulfur dioxide is present, bronze pumps are required.
  • Water Treatment Industry: Air agitation to destroy bacteria; de-aeration to prevent corrosion within distribution systems and provide clear water; flue gas compression to recover carbon dioxide for water treatment systems; vacuum de-aeration to remove available gases before treatment; vacuum distillation.

When choosing a vacuum system, one of the most important factors to consider is the sealing vapour pressure, since it is directly affected by the pump’s end vacuum and capacity. Make sure you ask potential suppliers to provide you with a full performance curve for the liquid ring vacuum pump when combined with the primary solvent.China Vacuum pump offers premium quality liquid ring vacuum pump at best price in Industry. For more details contact us.



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