In this article we see about the Liquid Ring Vacuum Pump-Cavitation and Prevention Methods. Usually the cavitation will reduce the design life of the vacuum pump and cause premature failure. Hence it is always imports to predicted the cavitation occur in the vacuum pump and to take necessary steps to prevent it.
How Cavitation will happen Vacuum Pump?
In liquid ring vacuum pump the cavitation is occurs as a result of vaporization of the seal liquid. It is due to the vacuum pump operate at the vapor pressure region of seal liquid.
If the vacuum pump run at the vapor pressure region, bubbles will form in the seal liquid. As the bubble move from low pressure region to high pressure region, the bubble is imploded and create a void space. This will cause the seal liquid to rush and fill the void space. As a result of this an abnormal noise and extensive erosion or pitting of the vacuum pump internals is produced. This phenomena is called as cavitation.
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Reason for Cavitation occurs in Vacuum Pump
The following are the major reason for cavitation takes place in vacuum pumps. They are
- The operating pressure is equivalent to (or below) the vapor pressure of the sealing liquid
- Sudden fluctuation or change of pump operating point from high vacuum to low vacuum.
- Sealing liquid temperature rise.
Methods to eliminate cavitation in Vacuum Pump
The following methods are used to eliminate the cavitation in vacuum pump.
Seal liquid Cooler
By using seal liquid cooler, the temperature of seal liquid is maintain below it boiling points. There by the flashing of seal liquid is prevented and maintain below its vapor pressure region. Refer below figure for the cooler arrangement.
Low Vapor Pressure Seal Liquids
Based on the operating pressure of Vacuum pump, the seal liquid shall be selected must having low vapor pressure property. This will prevent the seal liquid from flashing and causing cavitation. The seal liquid should be compatible with process gas mixture and the materials of construction of vacuum pump.
Inlet Operating Pressure
Increase the inlet operating pressure (if this pressure will be tolerated by the process) of vacuum pump beyond the vapor pressure region of seal liquid then cavitation will be prevented. Usually this method is ot preferred, because changing the inlet operating pressure, the downstream of vacuum pump will get affected.
Loading of Vacuum Pump
If the loaded less than 50% percent of the design capacity, the pump may be operating at a lower absolute pressure. This may lead to increase in seal liquid temperature and cause cavitation. A vacuum relief valve or air bleed valve can be used to introduce an additional load so the pump will be able to operate closer to the design capacity (also design pressure)
Suction Pressure Booster arrangement
Normally the suction pressure of vacuum pump is boosted by installing a booster pump or ejector arrangement in the upstream of the liquid ring vacuum pump. The booster will compress and raise the absolute pressure of the vacuum pump suction.
When compare with booster pumps, ejectors are less capital and maintenance cost due tono moving parts. An ejector works by using pressure energy of the motive to increase the velocity of the entrained gas load. This produces a higher absolute pressure at the outlet of the ejector. Two types of ejectors are a steam ejector which uses pressurized steam, and an atmospheric air jet which uses atmospheric air as the motive.