This article is to introduce cooling purge and cleaning purge for a dry screw vacuum pump.
It is recommended to purge a dry screw vacuum pump before process operation and after the end of the process. The cleaning is to remove the gas and vapour that remain in the screw pump during the compression process. When the screw pump is restarted, such residual substances may lead to motor overload and eventually damage the screw or pump body.
The cleaning process includes cooling and cleaning purge.
1. Cooling purge
The purpose of the cooling purge is to cool the inside of the pump body and the pump screw. The screw rotates to pump in, compress and pump out the air during operation. The gas temperature rises rapidly in this process. The generated heat shall be removed immediately to avoid possible pump damage.
Usually the atmospheric air is used as the cooling medium. When the air is not suitable for the process, then other media such as nitrogen is recommended. For a standard screw vacuum pump, a connection for this operation is provided near the exhaust port of the pump body.
2. Cleaning purge
Cleaning purge is necessary to clean the pump chamber at the end of process operation. Before switching off the pump, close the main valve on inlet pipeline, purge the pump with nitrogen or clean air for about 30 minutes in order to remove viscous process material or process gas from the inner surface of the pump. This operation is particularly important when the screw pump is used to displace corrosive or toxic gas, or sticky material like resins.
Steam flushing is effective to remove residuals of gas material, particularly reactive materials such as oligomers, monomers, polymers, resins etc. The deposits forming inside may cause increased motor load to and motor current which may damage the pump. In case corrosive gases accumulate inside the pump, the coating of the pump’s passage may be corroded and affect the pump negatively.
Two materials are commonly used for flushing. One is non-condensable inert gas, such as nitrogen, argon, etc and the one is steam. Again, the proper purge timing is before process operation and after it. Both cleaning are essential. Whatever an inert gas or steam is used, the pressure shall be kept at 1kg/cm2. Too high pressure damages oil seals while low pressure doesn’t work at all.
1) Generally speaking, the most effective material for cleaning is 99.99% dry air or nitrogen.
2) If the above materials cannot be used to flush the vacuum chamber effectively, it is recommended to use a cleaning liquid that have a decomposing effect on deposits (as long as they are not corrosive).
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