Coolant pump - minor problem

[Peter Sanders]

Hi

One of the schools lathes has a limited flow of coolant. I pulled out the tank and pump to clean/check etc.

The pump is a submersible type, powered by a small 3ph motor.

There was nothing obvious and virtually no crud anywhere. I cleaned out the line from the pump to the outlet, minimal blockage there. The impeller looks good as does the pump housing. The inside of the diecast pump housing does have some mimimal pitting/corrosion.

After this clean and check, I fitted the pump in the tank and ran the pump (under 4" depth of coolant) through its short length of hose back into the tank and watched the flow. Everything looked good with a good flow of coolant.

I let this run for about three minutes and checked again. The flow was about half the starting flow. The longer I left it running the more the flow decreased until it reached a substantialliy diminished rate.

Any ideas why?

Could the coolant be "airated" and these bubbles affect the flow? Something like a cavitating prop? The coolant being used is a water soluble oil based coolant.

The coolant tank was only half full during the test/check, maybe it would work better if the tank was full?

Thank you...

[wierdscience]

It's possible the cavitation being caused by the pits is whipping air into the coolant which will reduce the flow.Do you see any fine foaming if you look close?

Simple fix would be to degrease the casting and fill the pits with a good epoxy then see what it does.

[JoeFin]

Airbubbles indicate cavitation

Chances are the impeller is too worn to effect proper seal / fit. You can try fiddling with it trying to get a proper fit but it is much easier just to fleabay a new pump. For some reason people shy away from any thing that says "Chemical Feed" yet you can find fractional Hp, intrinsically sealed for $20 - $30

[Peter Sanders]

Hi

There is a thin foam on the coolant surface. I initially thought this was just due to the coolant flowing back into the tank.

There is a split pin that attaches the drive tube from the motor to the impeller shaft. This split pin is quite large and the legs of the pin are bent at right angles (probably the original manufacture). This could also be generating air bubbles in the coolant especially if the level is low. I'll replace/refit the split pin properly.

What is the normal seal or gap between the housing and the impeller. I have not examined the distance between the impeller hub and the housing but I imagine it would need to be very close to improve the efficiency (suction) of the pump.

[macona]

I think you are just getting too much air in the coolant. Run the coolant through the lathe as it would being used. By the time it gets back to the tank it should be pretty air-free.

[Peter Sanders]

Hi

I think you are just getting too much air in the coolant. Run the coolant through the lathe as it would being used. By the time it gets back to the tank it should be pretty air-free.

Thanks for the suggestion, you may very well be correct. I would prefer though, to try and obtain the correct result before reinstalling the tank etc, so I don't have to pull it to bits again The designers/assembly team used 3x 30mm long cap screws to hold the 2mm thick tank mountings in place one of which is in an almost inaccessible place I think I'll have to shorten them before reinstalling.

I will fix that split pin first then put the system back in place and see how it performs.

[J Tiers]

Dunno about that "air theory"........

Submersible pump........... where does the 'air" come from down in the coolant? You can't just "make" it...... and dissolved air shouldn't be enough.

if any air is involved, it must be sucked down the shaft housing to the submerged portion of the pump. But then you should see lots of bubbles/foam, not just a little. With a partly filled tank, that could happen, but would happen right away, or very soon.

if the tank gets enough air to reduce flow from aeration, there should be bubbles all over.

if it starts with good flow, it has the capability. So there may be something else going on, like trapping of one big bubble inside the pump somewhere, the pump actually slowing down, a piece of crud or several that get sucked up on the inlet and fall off when flow stops, etc.

[derekm]

Hi

...

The coolant tank was only half full during the test/check, maybe it would work better if the tank was full?

Thank you...

I had exactly this situation
you need it to be full ~ In operation when the fluid is running around the suds tray the level is even lower in the tank and then you get cavitation after a few minutes running.
either that or the impeller is slipping on the shaft when it warms up.