pump cavitation
pump cavitation
(OP)
I am trying to figure out the reasons behind apparant cavitation and failed impeller on double suction split case centrifugal pumps. NPSH available is way higher than requested. Pumps are fed from a common 48" suction manifold. The only reason for the cavitation I can come up with appears to be a relativly short suction piping from 48"x48"x16" "T", through 16" isolation valve and 16"x12" reduction directly connected to a 12" suction flange. Could that be a reson for such a violent impeller failure. Any ideas?





RE: pump cavitation
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RE: pump cavitation
Suction piping for double suction pumps should not be installed so that there is an elbow close to the suction nozzle except when this elbow is in a plane at right angles to the pump shaft. There is always uneven flow in an elbow and this unequal flow causes more water to enter one side of the impeller than the other side. The unequal flow may cause impeller damage due to cavitation or flow separation.
Suction piping should have 3-4 diameters of straight pipe between the pump and the elbow prior to the pump. The reducer may be included within this length. Long radius elbows should be used on the suction side.
The solution is to somehow straighten the flow entering the pump with a flow divider (straightener) of some sort inside the pipe. Not sure that this is possible.
Another option is to use a 45 Deg Y instead of the Tee.
Another option is to increase the diameter of the 48" header to reduce the velocity and flow distortions in the header
A full port isolation valve would be preferable to something like a butterfly valve as well.
Refer to the Hydraulic Institute Standards for arrangement of piping.
RE: pump cavitation
Thank you all
RE: pump cavitation
If the impellers show evidence of cavitation, you can bet the ranch you don't have enough straight length of pipe before the pump. Recommended straight runs are 5 D minimum to 10 D, especially after a 90 or T. Actually velocity head (v^2/2g) helps NPSHA, although its seldom considered, so your low velocity doesn't help that situation any either.
http://virtualpipeline.spaces.msn.com
RE: pump cavitation
In Florida ground waters there is a lot of dissolved H2S that tends to come out of solution when you start pumping.
Are you injecting any chemicals like chlorine or ammonia in front of these pumps for disinfection/residual control?
In either of these two cases, the problem could be corrosion, not cavitation.
Bduane
RE: pump cavitation
Where in the piping is the 2 ft/sec flow measured?
Are all of the pumps experiencing the damage?
Is the last pump also experiencing the damage? The last pump should have an elbow and the flow through the pump should be different than the ones with tees. Although you still have a double suction pump with uneven flow in an elbow.
RE: pump cavitation
RE: pump cavitation
http://virtualpipeline.spaces.msn.com
RE: pump cavitation
RE: pump cavitation
I think I made a mistake saying that it was 48" suction when in fact it is a 36" common suction. There are also three "low pressure effluentpumps" drawing water off the same suction but they have not been a target of this project. There are four pumps in question, three of which run coninuously in parallel. Only one of four pumps has somewhat better suction piping arrangement (several feet of straight line prior to suction flange). Three pumps provide anywhere from 4,000 to 6,000 gpm at measured system pressure around 300 feet TDH. When I took instantenuous measurements of system's flow and pressure it appears that pumps are pumping 30% less flow (for the recorded system pressure) than they should, based on the existing pumps' design curve.
RE: pump cavitation
http://virtualpipeline.spaces.msn.com
RE: pump cavitation
But it's all crystal ball gazing at the moment without any hard evidence.
RE: pump cavitation
The standard describes the need for physical modelling if the design does not comply with criteria such as straight lengths on pump suctions. Also you may care to check the Pumping Station Handbook by Sanks. In particular the section on "Blunders to Avoid'. One of which describes your scenario.
Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
www.waterhammer.bigblog.com.au
RE: pump cavitation
If you have plenty of NPSH then it might be entirely possible that air is being introduced into the system prior to pumping. Do you have any mechanism of air relief?