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Pumping Condensable Vapors

Background

Water vapor and other condensable vapors present a problem for users of vacuum pumps. As these vapors pass through the vacuum pump they are compressed. This compression causes the vapors to condense into liquids or solid deposits. In oil-sealed rotary vane pumps these condensates get in the oil and are distributed throughout the pump leading to corrosion and eventual failure. Water and other contaminates in the oil can destroy the lubricating properties leading to pump seizure.

Processes with Low Vapor loads

For low levels of vapor pumping regular use of the gas ballast and changing the oil when it begins to change color will keep the pump in good running condition. The gas ballast is an air passage into the exhaust stage of the pump that is opened and closed by means of a knob or lever on the pump. When opened the gas ballast dilutes the vapor concentration in the pump and helps prevent condensation so that the vapors are carried out the pump exhaust. The gas ballast also makes the pump run hotter which can purge previously condensed substances from the pump oil. In moderate vapor pumping situations typical practice is to isolate the pump from the system and run the pump with the gas ballast open for a half hour to an hour to reduce the amount of condensable gases in the pump.

Processes with Higher Vapor Loads

For processes that produce higher levels of condensable gases, special measures must be taken. Continuous use of the gas ballast may be necessary. In many oil-sealed pumps this will quickly reduce the oil level because the increased air flow through the pump carries oil out the exhaust. To avoid the cost and frequent maintenance of continually refilling the oil an oil return system may be used. This system is connected to the pump exhaust and consists of a vessel and an oil-coalescing filter. The filter separates the oil from the exhaust stream and the oil collects in the vessel. The vessel is periodically drained by means of a float or solenoid valve and a return line to the suction side of the pump.

Processes with Extremely High Vapor Loads

Extremely high vapor loads require devices that prevent condensable from entering the pump. Condensate traps in the inlet line of the pump condense gases and remove them from the vacuum stream before they enter the pump. Condensate traps work by adsorption of vapors to material placed in the vacuum stream, by cooling surfaces placed in the vacuum stream or by a combination of both. Condensate traps require regular servicing to drain the condensed liquids. Many traps are clear or have sight glasses to indicate when draining is required.

Considerations

Some things to consider when installing accessories

  • The effect on pumping speed
  • Installation of bypass plumbing to enable servicing during operation>
  • Selecting the right trap for the type of substances pumped

Carefully planned incorporation of the right accessories can prevent failures, increase maintenance intervals and lower maintenance costs.

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Document # PTB-T35-0004