Time Required for System Evacuation

The relation above is theoretical, and does not take into account any inleakage while pumping. It is recommended that a liberal multiplier of perhaps 2 or 3 be used to estimate closer to actual time requirements. 

An alternate relation for calculating evacuation time is from the Heat Exchange Institute [11]  

Alternate Pumpdown to a Vacuum Using a Mechanical Pump 

For large process systems of vessels, piping, and other equipment, the downtime required to evacuate the system before it is at the pressure (vacuum) level and then to maintain its desired vacuum condition, can become an important consideration during start-up, repair, and restart operations. 

It is difficult to determine the time required to evacuate any particular vessel or process system including piping down to a particular pressure level below atmospheric. When using a constant displacement vacuum pump this is estimated by O Neil [31]  

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