Electric motor and Cavitation 2

[memo83]

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

 

[ScottyUK]

Measure the electrical parameters: current, voltage, power. Cavitation shouldn't cause a problem for the motor per se, but if the pump is too large for the motor or the supply voltage is low at the motor terminals then you could certainly get problems with the motor.


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If we learn from our mistakes I'm getting a great education!

 

[DickDV]

Yes, a little more information here would be nice. For example, about the first thing a pump guy looks for with cavitation is inadequate suction-side pressure on the pump. Have you checked that?

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!

 

[potteryshard]

Changing from an eroded, undersized impeller to a new shiny impleller has probably increased pump efficiency enough to overload the motor. It may be necessary to turn down the new impeller slightly to trim the load to motor ratings.

 

[burnt2x]

What potteryshard said is true. The easiest way to trim the pump load to match the drive motor rating is to decrease the impeller diameter.
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.

 

[jraef]

Bottom line however, the cavitation is not an electrical problem and cannot usually be fixed by electrical means (VFDs notwithstanding). This is probably a better question for the Pump forum. But here are a couple of quick primmers on the issue.

http://www.mcnallyinstitute.com/01-html/1-3.html

http://www.pumped101.com/

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[memo83]

Many thanks for your replies all of them had been taken in my consideration if there is more don't hesitate to post.

regards

 

[nawao]

Submersible pumps must be protected to detect dry running and cavitation. There are adequate protection relays to do so. See the link below

http://www.fanox.com/index.php?option=com_content&task=view&id=231&Itemid=363&lang=en

 

[ScottyUK]

Current monitoring is a poor way to detect dry running. There are similar products which apply a threshold to true power measurement, or use power factor as the measurand (an unladen motor having a low PF). Siemens and Dold are two brands that come to mind but there are many others.


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If we learn from our mistakes I'm getting a great education!

 

[rmw]

If the pump/motor combination was sized correctly prior to the cavitation destruction of the original impeller it should have been adequate for the replacement impeller regardless of metallurgy. IF, and I say IF that is, the replacement impeller was the same size as the original impeller.

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

 

[Compositepro]

There are other features of an impeller other than diameter that determine how much power it draws (e.g., vane angle and width). Only the manufacturer of the pump and or impeller can say that they are equivalent. The fact that your motor load is not the same probably means they are not equivalent.

 

[burnt2x]

There are other features of an impeller other than diameter that determine how much power it draws (e.g., vane angle and width). Only the manufacturer of the pump and or impeller can say that they are equivalent. The fact that your motor load is not the same probably means they are not equivalent.

Yep, as previously mentioned, replacement impeller is not the same as the original impeller. With pump load exceeding the motor rating, most people in the field would trim the impeller.
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.

 

[automatic2]

Dare I say, but just to be safe, check your motor rpm to the pump specs.

 

[memo83]

Dear friends,

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

 

[DickDV]

First, I suspect the motor rpm at 380V should be 1485, not 1385.

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?

 

[rmw]

- generation of air bubbles due to a very negative pressure at the suction eye of the pump

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