Electric motor and Cavitation
Electric motor and Cavitation
(OP)
Need your help??
110 kw SIEMENS electric motor driving water pump this pump is suffering from cavitation which damaged the impeller .the impeller had been replaced with stainless steel instead of cast iron then the electric motor get over heated.any idea about the effect of cavitation and motor loading??
Many thanks in advance
110 kw SIEMENS electric motor driving water pump this pump is suffering from cavitation which damaged the impeller .the impeller had been replaced with stainless steel instead of cast iron then the electric motor get over heated.any idea about the effect of cavitation and motor loading??
Many thanks in advance





RE: Electric motor and Cavitation
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If we learn from our mistakes I'm getting a great education!
RE: Electric motor and Cavitation
And, simply changing from cast iron to stainless on the impeller is not going to affect the motor. But, there are likely subtle differences. In fact, the cast impeller may well have been trimmed to give the pump the power curve that the motor can cope with. Without that trim on the stainless impeller, the motor is overloaded.
Next, some data please!
RE: Electric motor and Cavitation
RE: Electric motor and Cavitation
If only the impeller is replaced, most likely the replacement impeller is not the perfect replica of the original impeller.
The cavitation problem could have been a wrong suction diameter or a throtlled suction valve. If you have gauges, please measure the pressure at the suction flange of the pump and compare with the NPSH (net positive suction head) rating provided by the manufacturer for that particular pump. You have to convert the suction pressure into equivalent head to do this. Hope that helps.
RE: Electric motor and Cavitation
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RE: Electric motor and Cavitation
regards
RE: Electric motor and Cavitation
h
RE: Electric motor and Cavitation
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Electric motor and Cavitation
So you have two choices.
Check the operating records to see if the motor operation was problematic when the original impeller was new, or verify that the replacement impeller was correctly sized.
If the original impeller was sufficiently trashed when it was replaced, it may have been difficult to determine the original dimensions. Few pump impellers are selected as full size - a general No-No among ME's. I suspect that the replacement impeller was a full sized impeller intended to be trimmed to the OD of the original impeller, but it wasn't done at the time it was installed.
And... cavitation is a symptom of the problem, not the problem. You should go after the real cause of the cavitation.
rmw
RE: Electric motor and Cavitation
RE: Electric motor and Cavitation
OTOH, we know that cavitation is a function of the NPSH of that specific pump - generation of air bubbles due to a very negative pressure at the suction eye of the pump and the resulting abrupt collapse of these bubbles as the pressure changes to positive near the impeller vanes pitting the metal impinged by the collapsing bubble. Assuming the liquid pumped is not abrassive to the impeller, the suspect is the choking suction area or too negative a suction head. Relocating the pump to a lower elevation can sometimes solve these problems. Hope that helps. Or visit the mechanical engineering forum for verification of ideas here.
RE: Electric motor and Cavitation
RE: Electric motor and Cavitation
please find the below name plate of the motor:
SIEMENS 1L4 310 B3 315S
v 380 D 205Amp p.F 0.86 1385 RPM
418 D 190AMp p.F 0.84 1488
360 D 215Amp p.F 0.86 1488
IA/IN=6 MA/Mn=2.3
KT coolant temp 50 Degree Celsius IP 54
measured current 140AMP
supply voltage 392 volt
Manny thanks for your assistance
M.A.Jeidi
RE: Electric motor and Cavitation
Second, at 140amps, I don't see why the motor should be running hot. Or, is it? Was the pump cavitating when the amps were measured?
RE: Electric motor and Cavitation
The pressure may not be negative at all, in fact it could well be positive, but if cavitation is present, it indicates that the suction pressure is below the vapor pressure of the fluid being pumped, regardless of the operating pressure. In such a case the operating pressure is not the issue, the vapor pressure is.
But... that said, pumps normally have higher npsh requirements at their higher flow rates rather than lower flows (if properly sized and selected) so the presence of cavitation could well indicate that the pump is running way out on its curve, passing more flow than it was originally designed for, and higher HP draw would be a result of that. The pump/motor set may not have been designed to be non overloading.
So check your pump curve for what the flow was supposed to have been and what the HP required at the design flow was supposed to have been.
The pump may be doing what it is being asked to do, pump it's heart out to the detriment of the motor and the impeller. It will pump as much flow as you have motor to pump it if permitted to do so.
As someone above has suggested, you may want to move the topic (or ask another question - but not a double post - on the Pump Forum or a Mechanical Engineer's forum. The guys and gals that lurk there eat and breath this stuff.
rmw