Page 51 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

w Joint rings on suction piping.
w Flange face sheet gaskets at pipe joints.
w O-rings and threaded fittings on instrumentation in the suction
piping.
w O-rings and other secondary seals on single mechanical seals.
w The faces of single mechanical seals.
2. Air can also enter into the pump from bubbles and air pockets in the
suction piping.
3. Products that foam can introduce air into the pump.
The evidence of this type of cavitation manifests itself like vaporization
cavitation, or inadequate NPSHa. The damage to the impeller appears
like vaporization cavitation. However, the solution is different.
To prevent this type of cavitation, you need to seal all points of entrance
and escape.
1. Tighten all flange faces and gaskets.
2. Tighten all pump packing rings and all valve stem packings on
suction piping.
3. Keep the velocity of the fluid in the suction piping at less than 8 ft
per second. It may be necessary to increase the diameter of the pipe.

4. Consider using dual mechanical seals with a forced circulation
barrier fluid (not induced with a pumping ring) between both sets
of faces on:
w Vertical pumps.
w Lift pumps and pumps in suction lift conditions.
w Pumps in vacuum.
w Pumps operating to the right of their best efficiency point
(BEP). This is explained in Chapters 7 and 8.

Turbulence cavitation
This is cavitation due to turbulence caused by the following:
1. Formation of vortexes in the suction flow.

2. Inadequate piping, sharp elbows, restrictions, connections, filters
and strainers in the suction.
3. The waterfall effect in suction vessels.

4. Violating or not respecting the submergence laws.

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