Startup of centrifugal pumps in flashing or cryogenic liquid service

There are generally two operations during which flashing may occur in the pump  

During plant commissioning, or after a prolonged plant shutdown when the whole system has been depressured to atmospheric conditions. The pumps are not part of the pressurized system, as the unit is just being brought up to operating pressure. 

During actual plant operation, when it may become necessary to bring a spare pump online. Although in some cold locations the spare is kept full of liquid, this is not the general practice (usually, the spare is kept purged and depressured) because of problems resulting from vaporization of the volatile liquid. 

The extent of flashing depends upon the fluid being pumped, but flashing has two basic effects. First, it creates a vapor-lock situation where vapor generation prevents head buildup and causes cavitation, and the pump cannot be started up. Second, in the case of light liquefied products (both hydrocarbon and nonhydrocarbon), flashing within the pump can create very low temperatures and cause auto-refrigeration. 

The following method for starting up centrifugal pumps in flashing or cryo- 

---