Error 4
Submersible pumps, line shaft turbine pumps, and centrifugal pumps with flooded or pressurized suctions, are not vulnerable to cavitation.
Answer
This is a very common area of misunderstanding due to the lack of general knowledge about cavitation, how pumps work, and how pumps are implemented. This author has seen pumps fed from pressurized sources such as flooded suctions and pumps submerged underwater in a well or wet well, destroyed by cavitation resulting in the loss of many thousands of dollars. All pump applications should be done by a qualified expert to ensure selection of the correct pump including careful design of the intake installation to avoid cavitation.
The curve below illustrates one reason why many pumps fail prematurely due to cavitation.
The pump curve above is for a real pump. This pump can be easily misapplied by a careless or unknowledgeable person.
The NPSH calculations below are for the pump in the graph above, installed at Sea Level, submerged 5 feet under water at 90 degrees F:
BEP Flow rate = 1,150 gpm @ 168 Feet TDH
NPSHR @ 1,150 gpm = 46 feet TDH
NPSHA = 37.3 feet TDH
NPSHA = Atmospheric Pressure of 33.9 feet + 5 feet Submergence - 1.6 feet vapor pressure for water at 90 degrees F. (33.9 + 5 - 1.6 feet = 37.3 feet NPSHA)
NPSHA is deficient by 8.7 feet TDH
Severe Suction Cavitation would occur in this pump because NPSHR exceeds NPSHA by 8.7 feet.
Related Subjects on this Website:
Glossary - Suction Specific Speed
