Page 52 - Know and Understand Centrifugal Pumps

Cavitation

With the pump disassembled on a shop table, the evidence of
turbulence cavitation appears like vaporization cavitation or inadequate
NPSHa. To deal with these problems, the technician must understand
the ‘Lost Art of Pipefitting’. There is a complete discussion on the
laws of submergence and turbulence in Chapter 17 of this book.

Review for preven ti ng cavitation

The general rule is
NPSHa > NPSHr + 3 fi

To increase the NPSHa
1. Raise the level in the suction vessel. This increases Hs.
2. Elevate the suction vessel. This increases Hs.

3. Lower the pump. This increases Hs.
4. Reduce the friction in the suction piping. This is probably the most
creative way to deal with cavitation. This reduces the Hf.

Use larger diameter suction pipe.
Change the pipe schedule. If there is a designated schedule, you
can bet it was based on discharge pressures and not suction
pressures.
Change to pipe with lower friction characteristics. Ex. Change
cast iron piping for PVC or even food grade stainless.
Move the pump closer to the suction vessel.
Convert globe valves into gate valves if possible.
Convert quarter turn butterfly valves into ball valves.
Re sure all ball valves are full port design.

Be sure all suction valves are totally open.
Reduce multiple elbows. If a system is designed with 9 or more
elbows in the suction piping, you can be sure that some of these
elbows are self-canceling. If so, then some elbows can be
eliminated.

Convert 2 or 3 close fitting elbows into a flexible ‘S’,
Convert ‘mitered 90” elbows’ and short radius elbows into long
radius elbows.

Keep suction pipe inside diameters clean and scale free.

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