Cavitation 1

[ab12cd]

Hi

We have a purified water storage tank, volume 2000L and the pump starts to cavitate when the level drops below 1500L.

Could someone give information on how to solve this problem or a prodecure to follow to rectify this problem

Thanks

 

[Guidoo]

You get cavitation when Net Pump Suction Head (NPSH) available drops below NPSH required for the pump. So in order to prevent cavitation you have to:
- increase NPSH available and/or
- decrease NPSH required.

You can increase NPSH available by:
- increase elevation difference between vessel and pump
- decrease pressure drop in pump suction line (e.g. by increasing pipe diameter, reducing line length, number of elbows etc. or reducing the flowrate)
- increase pressure in vessel (not possible when vessel is open to atmosphere)

You can decrease NPSH required by:
- installing another pump with less NPSH required

A procedure you could use is as follows:
1) find out what NPSH required of current pump is
2) calculate NPSH available (= pressure in vessel + elevation difference at low liquid level - pressure drop between vessel and pump suction, all expressed in same units as NPSH required (e.g. meters of liquid))
3) check whether you can increase NPSH available so that current pump can still be used. How to increase the NPSH available (by increasing vessel elevation or pressure or decreasing pressure drop in pump suction) depends on specific circumstances at your plant, and on whether NPSH available is significantly reduced by (excessive) pressure drop in suction
4) if it is not possible to increase NPSH available sufficiently, see whether you can obtain other pump with lower NPSH required

 

[25362]

From the heading, "Cavitation", one is led to think about centrifugal pumps. If the case is so, kindly remember that it is suction head, not volume, what these pumps need to operate away from cavitation. So, please submit a bit more of information, on the physical conditions of the storage tank, flow rates, discharge pressures, kinf d of pump, and so forth, to enable suggesting possible solutions to the problem.

 

[saxon]

ab12cd, In addition to the above comments, you may have a problem of vortexing (drawing air into the suction line and pump that sounds just like cavitation). Take a close look at the the tank and the location of the pump suction nozzle on the tank. Simply relocating the nozzle to the tank bottom may solve your problem. If not, you may need to install an anti-vortexing strainer in the tank at the beginning of the suction line; or a pump respec. may be in order.

Hope this helps.
saxon

 

[MJCronin]

ab12cd,

All of the above comments are good, however, you should ask yourself, what has changed in the process ?????

Has the transfer pump been rebuilt recently ? Was a higher speed electric motor installed ?

The NPSHr for a 3550 rpm motor is typically twice that of a 1750 rpm motor on the same pump

Sometimes, the wrong impeller is used, the impeller was installed with a "ding" or dent or a misalignment could push the operation into the unstable range

Another source of "cavitation" is an increase in the liquid temperature.....is the liquid being strored at a higher temperature ?

Just my opinion.....

GIVE US MORE DETAILS !!!

MJC

 

[ixchawla]

Generally, water pump should not have npsh problem as the vapour pressure is much below the operating pressure. You may try the following:
1. Check suction strainer blockage- clean and reinstall
2. Location of pump suction wrt tank inlet and layout of the suction line, vortex breaker(as suggested by sexon)
3. Suction velocity (line size)- High velocity can cause air entrapment.