Broken pump shaft 1

[tuneball]

Hey guys,
I'm trying to determine what might of caused this pump shaft to break. I know by looking at the break, there's signs that can help determine what might of happened.
Pump is a Goulds 3405 8x10-17, horizontal split-case pump and the break occurred right at the impeller hub. Pumping clean water.

Any type of feedback would be great.

Thanks
Dan

 

[tuneball]

Here's another picture

 

[tuneball]

And another

 

[tuneball]

and another

 

[EdStainless]

In the last one, your fig 3 there are striations indicative of fatigue.
It looks like the fatigue started at the lower keyway, but given the facets I have to wonder about multiple initiation sites.
Cracks progressed until >80% of the shaft was cracked.
Since it is so discolored it is likely that it has been cracked for a long time.
Some process upset or stop/start probably caused the remaining material to fail in simple overload.

Have you ever had the pump apart?
What about the condition of the bearings?
How was the fit of the impeller on the shaft?
How many stops/starts does this pump see?
Has the pump ever jammed, does it pump debris, is there air in the line?


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P.E. Metallurgy, Plymouth Tube

 

[LittleInch]

Can you post the sectional drawing? Is this a BB pump?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[tuneball]

Thanks for the replies guys! Below are some answers to your questions. I will also find out a bit more next week when I'm on site.

Have you ever had the pump apart? We were only sent to rotating element, so I don't have the complete pump. I'm on site next week in which I will check the casings out and see if anything jumps out at me.
What about the condition of the bearings? We have the bearings, in which we will investigate
How was the fit of the impeller on the shaft? Not sure yet, will let you know
How many stops/starts does this pump see? I will find this out
Has the pump ever jammed, does it pump debris, is there air in the line? The liquid is softened water, so no debris

It is a BB pump, cross sectional attached.

Again, thanks for the replies.

Dan

 

[Artisi]

Assume it is the drive end of the shaft. Certainly a lot damage done at an earlier time, I would think it is / was an operation problem some time back, possibly running for extened periods well below minimum flowrate or even against a closed discharge resulting in high vibrations from blade pass frequency, and the same conditions recently leading to the failure. Any way to check the operating conditions of the unit over time, either logged or verbally from the operators?

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.)

 

[EdStainless]

I won't surprise me if you hear from an operator that every now and then the pump was very noisy.
Running way out of range can cause a lot of vibration.
I am expecting that the bearings will show it also.

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P.E. Metallurgy, Plymouth Tube

 

[mike1969]

we have seen problems like this occasionally when a pump has a bad check valve. pump spins backwards until motor is energized and then BANG major repair bill...

 

[MJCronin]

Has this pump been repeatedly pulled out of service, then reinstalled with the motor leads being reattached ?

This can result in periods of reverse rotation (and shaft failure) as the operations people do not discover this right away.

Did you purchase features on the shaft/impeller to prevent impeller loosening if there is reverse rotation ?

MJCronin
Sr. Process Engineer

 

[tuneball]

That's actually the first thing I asked, is if the pump perhaps spun backwards for any reason. I'm told it didn't, but that doesn't mean it actually didn't. They could've had a bad check (like mike1969 mentions), that they have no idea of.

 

[bimr]

Shaft breakage is typically a direct result of the operating conditions. Typical operating issues that will cause shaft breakage:

Vibration is most commonly caused by cavitation, critical speed, passing vane frequency, and operating outside the pump's best efficiency point. Pump bearings begin to wear, allowing the shaft to move laterally, causing the shaft to flex and eventually fail. The vibration harmonics increase the stress on the pump shaft.

Impeller imbalance creates problems while the pump is running, though the shaft will measure straight if stopped. Imbalanca is a source of vibration and will reduce machine, bearing, and mechanical seal life.

Poor installation, pipe strain, extreme belt tension and sheave misalignment on direct drive pumps all put undue stress on a pump's bearings and shaft

Serious damage can be caused by hydraulic shock. For example, when a check valve slams shut, interrupting the flow of fluid, a massive shock wave results. This shock wave reverses flow and travels back downstream. When the shock wave collides with a pump, assuming the shock is strong enough, the shaft could bend or break instantly or over time. Reverse flow causes a pump's impeller and shaft to turn backwards and the pump starts and will break a shaft. This scenario occurs when check valves upstream are not present, partially or completely plugged, or otherwise not functioning properly.

 

[EdStainless]

War story, used to build multistage centrifugal submersibles, think 100 stages or more and up to 14,000' of head.
When these would shut off it might take a while for the fluid to drain back, usually 10-15 min but sometimes an hour.
Our control panels had an interlock that looked at back EMF to inhibit restart. Some people just used a timer, and others just kept breaking shafts when they would hit the start button while the unit was rotating backwards at 50% speed.


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P.E. Metallurgy, Plymouth Tube