Question on radial thrust of pump

[Garou]

Hi All,

Am new here so would like a little advice regarding this.

In our plant, we have a non seal canned motor pump that recently had a failure where the shaft broke at the keyway connection to the impeller.

We suspect it might be caused by the radial thrust of the pump as it is operating at way below the BEP, left side of the pump curve, but above the minimum flow requirement.

However, checks with the pump vendor claims that as long as you operate above minimum flowrate, the pump rotor should auto thrust balance itself, hence radial thrust is not an issue.

Is this true?

Many thanks in advance.

 

[Artisi]

Why wouldn't it be true - is your statement about running above minimum flow true?

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.)

 

[hydtools]

Why would thrust cause a keyway failure? Torsion or bending would be more likely mode.

Ted

 

[Garou]

Oh no, please don not misunderstand. It’s just one of the possible causes we are brainstorming to find the most probable cause. Anyways, more details of this pump as follows:
1) Pump was taken out of service as it was reported there it was unable to deliver flow.
2) There was no high ampere trip prior to this.
3) We did check the suction strainer but it was clean.
I recognise that the failure is caused by torsion as the shaft breakage area shows signs of that.
4) Pump was running at half the rated flow for half a year prior to the failure but is above the minimum flow.

Can you all enlighten me on any other possible causes?

 

[Artisi]

Was there any signs of distress during the 6 months of operation, vibration, power fluctuations, uneven flow rate to the pump, is the inlet pipework of sound design - no bends, reducers, valves at the pump inlet, sufficient NPSHa, any entrained air in the supply, any pre-rotation of the inlet flow?

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.)

 

[Garou]

Hi Artisi,
There was no abnormal operating conditions of the pump during that period. And weekly patrols by the field man and maintenance team did not detect any abnormal running conditions of the pump. NPSHa is not an issue based on the tank level readings over that period.
The only difference was that this pump is of a different brand than the old pump and was changed in 2018. But it is still a canned motor pump. And there were no records of such damage encountered when using the old model pump.
There is a reducer just before the inlet suction though.
Just find it strange that such a phenomenon occurred after changing the pump brand. Could it be the cause? Although the pump was sized to match exactly as the previous pump.

 

[electricpete]

in pump terminology "radial thrust" is sort of an oxymoron because "thrust" normally implies axial direction (example "thrust bearing")

I assume op meant radial load or radial force. I think hydtools may have been misled by op's terminology


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[electricpete]

Got any pictures?



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[Garou]

Yes electricpete, I did mean radial force.
But was wondering what could be the cause of this force.
Attached below are the pictures of the shaft failure.

 

[hydtools]

My error. I read passed "radial" to the word "thrust".

Ted

 

[Artisi]

First guess, impeller fretting on shaft / against the keyway, the question is why - next guess, operating too far left on the curve /against a blocked system.
Believe nothing of what site staff tell you or what you "think" are the operating conditions.
For interest, what is the power input to the pump?

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.)

 

[hydtools]

The failure looks like torsion failure as indicated by the angular surface. Would not radial thrust more likely cause bending type failure with a surface nearly at right angle to the shaft centerline?

Ted

 

[Artisi]

Blade pass loading.

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.)

 

[electricpete]

The failure looks like torsion failure as indicated by the angular surface

I agree with the torsional failure idea. From looking at textooks, if the shaft is under torsional stress then the plane of maximum shear tensile stress is at 45 degrees (can’t really tell from pictures - does it look like 45 degrees op?). Slip leading to ductile failure occurs in the plane of maximum shear stress. I think most shafts are designed not to be brittle... Brittle failure occurs in the plane of maximum tensile stress. My reference [attached] shows this type of failure pattern would correspond to a brittle failure under torsion. Does it make sense that the shaft would be brittle? (what is material and operating temperature)?


Since the shaft diameter decreases significantly at the step, it would be a logical place to have torsional failure at the smaller portion of the shaft. I imagine that the torsional stress in the shaft is highest at the top of the impeller and lowest at the bottom of the impeller (using vertical pump terminology). So we might expect the failure to occur near top of the narrow portion / top of the impeller. Does the other side of that slanted crack (hidden from view in the photo) pass near the top of the impeller? Is the shaft step radiused and the impeller face radiused slighly larger?

Another question that comes to mind, did keyway fail first or shaft fail first:
[ol 1]
[li]Let’s say the shaft failed first, then the impeller would drop and grab onto a stationary part, the slanted shaft might continue to drive (by virtue of the slant) resulting in increased stress on the key / keyway. I can imagine that.[/li]
[li]Let’s flip the scenario and assume the key / keyway failed first. Then impeller might spin on the shaft. It wouldn’t go anywhere (to create increased stress on shaft) unless the impeller locknut also came out. And impeller seat on shaft doesn't seem to show any sign of spinning anyway. Seems a less likely scenario.[/li]
[/ol]

Was the impeller locknut still threaded onto the shaft in the as-found condition?
Got any pictures of the key or of the two failure surfaces of the shaft? How about the impeller and adjacent stationary surfaces (for indication what contact may have occurred… or did something get stuck between the impeller and stationary parts etc).

Edit - changed first paragraph. Added attachment from ASM Handbook Vol 11: "Fig. 7 Free-body diagrams showing orientation of normal stresses and shear stresses in a shaft and the single-overload fracture behavior of ductile and brittle materials. (a) Under simple tension. (b) Under torsion. (c) Under compression loading"

https://files.engineering.com/getfi...-40df-9eda-215e588e415d&file=ShaftFailure.pdf

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[hydtools]

I think that a crack was initiated at the keyway. It appears that the driving side of the keyway failed.
Continued load cycles caused the crack to grow leading to eventual shaft fatigue failure.

To the manufacturer's response re radial thrust balance remaining in effect. How does that occur without opposing volute outlets?

Ted

 

[electricpete]

I think that a crack was initiated at the keyway.

It could be. The highest torsional stress would be at the axial top of the impeller key (by the time we get to the bottom, all of the torque has been transferred to the impeller and there's no torsional stress at the bottom of the shaft beyond the bottom of the key). I guess it could be the failure initiated on the inside corner on the right side of the photo where it is pretty close to the top of straight portion of the keyway where the top of the key would be. It also appears that is the side of the keyway that would be transferring load

Do you think this represents a brittle failure? Is that normal for a pump shaft? (I'm pretty sure it's not normal for motor shafts but pumps may be different).

[Edit - deleted comments about membrane analogy]


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[electricpete]

hydtools - sorry I didn't read your post closely enough to notice you were talking about torsional fatigue and you already talked about initiating at the top corner of the keyway.

This link seems to support the idea that torsional fatigue would look like this (at least with respect to 45 degree angle and initiation at the keyway)

https://reliabilityweb.com/articles...ure_-_identification_diagnosis_and_prevention

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[Garou]

Hi All,

Just to update on the latest development. Electricpete and Hydtools, you both should be correct regarding the torsional failure of the shaft. The pump manufacturer just got back to us. They have verbally informed that their initial re-calculation found that the stresses at the keyway is very near the limit at the pump's rated flow. Hence, pump might have failed with small crack initiation followed by crack propagation over cycles of service stress before it failed totally as what you all have suggested?

However, the actual report from the pump manufacturer will only come out a week later so I will update once I receive it.

Many thanks to all who have replied in this thread and I have learned much just from this thread discussion alone.