submersible sump pump part deux - does it rotate in reverse when flow is reversed? 1

[electricpete]

This is the same pump described in my previous thread thread407-494946
1/2 HP, 115 Volt, 60 Hz, 3.87″ Impeller, 2″ NPT Discharge, 20′ Cord, 3/4″ Solids.
(No, we haven't changed anything since last thread)

We "backflush" or "backwash" this pump with pressurized water source applied to the pump discharge once a week in an attempt to clear out debris which we perceive (rightly or wrongly) is causing the chronic reliability problems. Throttling of the water source is manual. Duration is not specified.

Today another one of these pumps was removed today due to reported starting problems.
We examined it on the bench.
There is no foreign material present in the impeller (just like last time).
There are conflicting reports on whether the pump rotated freely at this point in time.
We energized the motor with 115vac and it just hummed.
Starting capacitor tested expected capacitance (that test was done later but it seems more relevant to present here).
Then we removed the lower cap/flange and retried start and this time it successully it started (we immediately stopped it).
We noted indication of rub between the impeller and the lower cap and also a chip on the "trailing edge" of the impeller (slides 1 and 2).
Bearings seem to be in pretty good shape.

We noted that the vendor literature (slide 2) says "“Backflow can allow the pump to turbine backwards and may cause premature seal and/or bearing wear”. I'm not sure exactly why that is, the best I can figure is the speed is unpredictable during backwash so it may be more likely that you spin the pump at a resonant speed for awhile which might cause damage.

That last one sure had some damaged bearings (looks more like bad quality but maybe backwash contributed who knows). This one had no damaged bearings but there was that chip on the impeller that we're wondering about.

It was theorized that
[ul]
[li]1. perhaps the chip at that particular location (trailing edge of the impeller) was more likely to occur when the pump is rotating backwards.[/li]
[li]2. backwards rotation apparently happens during backwash as stated by the vendor.[/li]
[/ul]


Questions

[ol 1]
[li]Do you believe claim 1?[/li]
[li]Do you believe claim 2? It's hard for me to visualize looking at this impeller. Looking from the bottom at the open face, it normally rotates CW but it would rotate CCW if we put high pressure water into the [normal] discharge port and allowed that water to flow through the pump into the sump?[/li]
[li]Is it your experience that backwashing a pump can cause damage? Is the explanation similar to what I said above (spinning at resonance) or something else?[/li]
[/ol]


Link
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(2B)+(2B)' ?

 

[EdStainless]

Yes, with backflow the pump spins backwards.
Some bearing and seal designs have a 'rotation direction' requirement. They don't like going backwards.
And depending on the speed the vibration can be very different.
Enough to cause damage.
I would also suggest a sensor on back EMF to inhibit energizing the pump while it is spinning backwards.
This is how you break shafts and lots of other parts.
We made pumps that went into wells, so often the discharge line as 10,000' long, straight up.
When they shut one off (or it tripped) it was spin backwards for a very long time (hours).
We actually had test rigs for looking at this situation.

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P.E. Metallurgy, consulting work welcomed

 

[3DDave]

Doesn't that amount of rust count as FOD? It looks like it just grew until it filled the gap.

The cracked corner is probably just because it's the worst place on the sand casting - thin and the bottom of a thin run.

 

[TugboatEng]

What is the horsepower of this pump. I don't know do any bearings or seals that are sensitive to reverse rotation in the 0-100hp range.

The missing piece is not corroded. This likely broke during disassembly. Removing impellers from small horsepower pumps is a very... crude operation.

Now, the break is shiny and doesn't look like cast iron. Is the impeller material anything but cast iron?

For the rubbing, I don't see any metal showing. There will always be some rubbing at some points due to corrosion if you keep buying iron pumps. It will corrode until it rubs and then clearance itself.

 

[3DDave]

Second line: 1/2 HP

 

[electricpete]

Yes, with backflow the pump spins backwards.

I can visualize that easier for an axial flow pump (or mixed flow pump). It's harder for me to visualize the mechanism for that with a radial flow pump like this. Looking from the bottom the impeller rotates CCW in pumping mode. Looking at that impeller, does it make sense to you that high pressure water coming into the outlet would make it rotate CW?

3DDave and TugBoat - I'm not saying there is anything conclusive shown in the photos. More like circumstantial. The bigger thing is that we're backwashing when the vendor tells us not to. It sort of shifts the burden of proof about what the impeller might be telling us. I'd like to focus on those 3 questions (at least to start with)

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(2B)+(2B)' ?

 

[TugboatEng]

How often and how long do these pumps run? Typically impeller to housing clearance is around 0.012-0.15" for a pump of this size. That will take some time to close up. If they run daily that usually keeps things clear. If they sit for weeks at a time that can cause problems.

Maybe you should add a timer relay that runs the pump for a minute each day instead of back flushing. This might be sufficient to keep corrosion under control. The pump will tolerate short run times when deadheaded or with a partially flooded suction.

The reverse rotation should be expected. It's just like the exhaust turbine in a turbocharger. Flow enters on the OD and leaves through the center.

 

[swazimatt]

I am not an electrical engineer, but doesn't a pump motor spinning as a result of backflow or siphon becomes a generator, so where does that electricity go? will that cause issues with the control panel?

 

[electricpete]

> The reverse rotation should be expected. It's just like the exhaust turbine in a turbocharger. Flow enters on the OD and leaves through the center.

I'm not that familiar with turbochargers but I gather that is also a radial flow device (not axial or mixed)?

> I am not an electrical engineer, but doesn't a pump motor spinning as a result of backflow or siphon becomes a generator, so where does that electricity go? will that cause issues with the control panel?

When we backflush there is no power applied and the motor is isolated from the power system. In that condition the electrical circuit is pretty much irrelevant to the problem (the spinning is produced by fluid and resisted by mechanical friction and fluid friction and potentially by rotational/polar inertia). Even if there was a bit of residual magnetism in the rotor that would give rise to a voltage induced the stator but no electric power would flow in wye configuration since there is no path for current to flow outside the motor.





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(2B)+(2B)' ?

 

[EdStainless]

Mixed or radial flow, the only difference is how fast they will spin with back flow.
If you don't want to measure the back EMF then put a timer in the controls so that after backflush you can't start the pump for a few minutes.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Artisi]

Would think a single channel impeller of this size would be very unlikely to rotate from reverse flow.
For a multi blade impeller pump to run backwards it depend on the break-away torque of the unit and the torque available from reverse flow must be greater.

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.)