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Cavitations 1-3

The McNally Institute

Cavitations means that cavities are

forming in the liquid that we arepumping. When these cavities form at

the suction of the pump several thingshappen all at once.

We experience a loss in capacity.

We can no longer build the same

head pressure!

The efficiency drops.

The cavities or bubbles will

collapse when they pass into thehigher regions of pressure

causing noise" vibration" anddamage to many of the

components.

The cavities form for five basic reasons and it is common practice to lump all

of them into the general classification of cavitations. This is an error because

we will learn that to correct each of these conditions we must understandwhy they occur and how to fix them. #ere they are in no particular order$

%apori&ation 'ir ingestion

Internal recirculation

(low turbulence

The %ane )assing *yndrome

Vaporization' fluid vapori&es when its pressure gets too low" or its temperature too high.

'll centrifugal pumps have a required head pressure! at the suction side ofthe pump to prevent this vapori&ation. This head requirement is supplied to

us by the pump manufacturer and is calculated with the assumption thatfresh water at +, degrees (ahrenheit - degrees Centigrade! is the fluidbeing pumped.

*ince there are losses in the piping leading from the source to the suction ofthe pump we must determine the head after these losses are calculated.

'nother way to say this is that a Net )ositive *uction #ead is /equired

N.).*.#./.! to prevent the fluid from vapori&ing.

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We ta0e the Net )ositive *uction #ead 'vailable N.).*.#.'.! subtract the%apor )ressure of the product we are pumping" and this number must be

equal to or greater than the Net )ositive *uction #ead /equired.To cure vapori&ation problems you must either increase the suction head"

lower the fluid temperature" or decrease the N.).*.#. /equired. We shall

loo0 at each possibility$Increase the suction head

/aise the liquid level in the tan0

/aise the tan0

)ut the pump in a pit

/educe the piping losses. These losses occur for a variety of reasons

that include$

o The system was designed incorrectly. There are too many

fittings and1or the piping is too small in diameter.

o ' pipe liner has collapsed.

o *olids have built up on the inside of the pipe.o The suction pipe collapsed when it was run over by a heavy

vehicle.

o ' suction strainer is clogged.

o 2e sure the tan0 vent is open and not obstructed. %ents can

free&e in cold weather

o *omething is stuc0 in the pipe" It either grew there or was left

during the last time the system was opened . Maybe a chec0

valve is bro0en and the seat is stuc0 in the pipe.

o The inside of the pipe" or a fitting has corroded.

o ' bigger pump has been installed and the existing system has

too much loss for the increased capacity.

o ' globe valve was used to replace a gate valve.

o ' heating 3ac0et has fro&en and collapsed the pipe.

o ' gas0et is protruding into the piping.

o The pump speed has increased.

Install a booster pump

)ressuri&e the tan0

4ower the fluid temperature

In3ecting a small amount of cooler fluid at the suction is often

practical. Insulate the piping from the sun5s rays.

2e careful of discharge recirculation lines" they can heat up the suction

fluid.

/educe the N.).*.#. /equired

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6se a double suction pump. This can reduce the N.).*.#./. by as

much as -78 or in some cases it will allow you to raise the pump

speed by 9:8

6se a lower speed pump

6se a pump with a larger impeller eye opening.

If possible install an Inducer. These inducers can cut N.).*.#./. byalmost ;8.

6se several smaller pumps. Three half capacity pumps can be cheaper

than one large pump plus a spare. This will also conserve energy at

lighter loads.

It is a general rule of thumb that hot water and gas free hydrocarbons

can use up to ;8 of normal cold water N.).*.#. requirements" or :feet < meters!" whichever is smaller. I would suggest you use this as

a safety margin rather than design for it.

Air ingestion' centrifugal pump can handle .;8 air by volume. 't +8 air the results can

be disastrous. 'ir gets into as system in several ways that include$

Through the stuffing box. This occurs in any pac0ed pump that lifts

liquid" pumps from a condenser" evaporator or any piece of equipment

that runs in vacuum.

%alves above the water line.

Through lea0ing flanges

%ortexing fluid.

' bypass line has been installed too close to the suction.

The suction inlet pipe is out of fluid. This can occur when the level gets

too low or there is a false reading on the gauge because the float is

stuc0 on a corroded rod.

2oth vapori&ation and air ingestion have an affect on the pump. The bubbles

collapse as they pass from the eye of the pump to the higher pressure side

of the impeller. 'ir ingestion seldom causes damage to the impeller orcasing. The main effect of air ingestion is loss of capacity.

'lthough air ingestion and vapori&ation both occur they have separatesolutions. 'ir ingestion is not as severe as vapori&ation and seldom causes

damage" but it does lower the capacity of the pump.

Internal RecirculationThis condition is visible on the leading edge of the impeller" and will usually

be found at the discharge tip wor0ing its way bac0 to the suction. It can alsobe found at the suction eye of the pump.

's the name implies the fluid recirculates increasing its velocity until itvapori&es and then collapses in the surrounding higher pressure. This has

always been a problem with low N)*# pumps and the term *)=CI(IC

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*6CTI>N *)==? was coined to give you a guide in determining how closeyou have to operate to the 2.=.). of a pump to prevent the problem.

The higher the number the smaller the window in which you have tooperate. The numbers range between

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'll pumps are running.

The upstream conditions should have a minimum straight run of ten

pipe diameters to provide uniform flow to the suction bells.

=ach pump capacity must be less than :;" gpm..

2ac0 wall clearance distance to the centerline of the pump must be at

least .7; of the suction diameter. 2ottom clearance should be approximately .