Raw water pumps disappear after a few years

[Skogsgurra]

I had - or rather have - an interesting case with cast iron raw water pumps around 100 ft down in a gravel ridge. There are around 50 of them and size varies from around 8 to 80 HP. They are all fed from VFD:s and grid voltage is 400 V 50 Hz, grounded neutral.

Some of the pumps have eroded so pumping efficiency is below acceptable limits. Water Chemistry, cavitation, sand erosion have all been ruled out and I was asked to have a look at the problem. I have had Electric erosion in AC Circuits Before. It happens when there is streaming water that takes the positive metal ions away from the Surface, so it is entirely possible to have electrolysis also with AC. School experiments use DC, but that probably has historical reasons. Also, it would be difficult to illustrate "the bang" if O2 and H2 were produced in the same test tube. I digress, but needed to say that so you don't concentrate on less important details.

Sine filters have been tested, but the ones used has capacitors between motor phases and not from phases to GND. So the Common Mode current is not effectively reduced. Somewhat, but not as well as we hoped for.

We have tested nanochrystalline (amorphous cores) and HiLite cores. They do reduce CM currents and things look promising. Seem to work well and losses are a lot less than in a sine filter.

BUT: IF THE PROBLEM ISN'T ELECTRIC EROSION - ALL THIS WILL BE IN VAIN. And since it takes several years to find out if the measures help or not, I turn to the expertise on Eng-Tips for tips, verification, experience and whatever there may be. Thanks for helping out on this puzzling problem.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[Artisi]

Gunnar, is the problem on all wetted areas or just the rotational and associated bits?
A few photos would be great, if only for general interest.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[MikeHalloran]

Some of my business friends run boats that take a hundred or so folks at a time out of Alaskan fjords to search for whales and look at glaciers.

In the interest of showmanship, they are obligated to make a pass close to the glacier face, where the water may be shallow, and glacial silt gets sucked into their raw water pumps.

They have described glacial silt as like talcum powder, except it's gray/black and highly abrasive. It will go through a sand filter easily, so a boat's normal raw water strainers are ineffective against it.


Can you get a sample, like half a liter or so, of water from some representative pumps' actual environment, preferably some that hasn't gone through the pump and whatever filtration they use, or second choice, just diverted from the pumps' discharge? Store it in clear glass and let it sit and settle for a week, then you'll have an idea of what your customer might be facing if it's not an electrical problem.



Mike Halloran
Pembroke Pines, FL, USA

 

[EdStainless]

And what about a detailed water chemistry?
Both ions and dissolved gasses, pH and whatever else you have.
If it is from stray current you should be able to put a lead into the water and onto a pump, and measure it with a oscilloscope, both waveform and voltage. I really doubt that it is stray current unless your system ground is not near true ground.

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P.E. Metallurgy, Plymouth Tube