Canned motor pumps can transfer or boost volatile liquids under conditions of low NPSHA by modifying the standard design to permit circulation through the motor section without vaporizing the liquid in the bearings. Two types of modifications are available:
The internal circulation flow path for a canned motor pump modified for reverse circulation starts at the pump casing and flows to the rear of the pump. After the fluid exits the rear of the pump, it is returned to the suction vessel. The front bearing housing or pump casing adapter is modified to allow for the circulation fluid to enter the motor section. The rear bearing housing has a port for piping back to the suction vessel.
For pressurized circulation, a standard pump is modified by installing an auxiliary impeller on the rear of the motor shaft to increase the pressure in the motor section while it passes over the bearings. As the circulated fluid leaves the motor section, it is directed behind the impeller where the pressure is close to the discharge pressure. By returning the recirculation liquid to the high pressure area of the pump, the entire motor section or primary containment barrier is â€œpressurized,” preventing the fluid from vaporizing even though it is at a higher temperature than the suction temperature.
Pressurized circulation eliminates the need for piping back to the suction vessel and allows the fluid to be filtered prior to entering the motor cavity.