VTP Pump major failure

[Rocket2M]

Hi guys,

I'm currently having a problem with my VTP. Pumping service is for seawater with high static head application (about 13 to 19 m depending on tidal). The impeller material is nickel-aluminium-bronze (NAB) while the shaft and coupling material is monel k400. Pump design is closed, diffuser type with radial flow impeller and consist of 2 stage impeller. Sodium hypochlorite is injected into the well every week for 20L. We recently pull this out as we received report of no flow pumping and found lineshaft has parted near the coupling section (key type) and spider has torsioned, splitted from its flange. Found minor cavitation on inlet vane of 2nd stage and minor pitting on outlet vane, not sure due to cavitation or not. Whole line shaft was found pitting and runout out of tolerance. Found out-of-straightness as well. The spider bearing design is having two sleeve, one monel on the shaft, and the other is vesconite in between monel sleeve and NAB spider bearing. No vibration monitoring and protection is installed as this pump is originally only used to provide seawater in case of emergency and is engine driven by right angle gearbox. For the past few months however, this pump has been routinely run to overboard line (which is sized for pump minimum flow). Seek expert opinion on what could be contributing to the development of this case. For me I personally think that running near low flow region caused higher vibration and accelerated bearing wear of the pump, subsequently causing wobbling and pump running out of concentricity and causing torsion of the spider. Afterwards as spider has failed, lineshaft has no radial support for deflection and parted. Please let me know what do you think

 

[LittleInch]

Any more details of the pump you can share ( model, type, drawings, pump curve and what flow was it running at?

Min flow for long periods is never a good plan, you get flows and internal recirculation which can play havoc with your pump.

I think your supposition makes sense, but would be good to understand what sort of length of shaft you have between bearings and what the actual vs rated flow was

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

 

[MJCronin]

These are expensive pumps .... Call in a pumping consultant, and pay his fee ...

Do not rely on the ad-hoc comments of a bunch of cranky internet strangers for this complex issue ....

Good luck

Please keep us posted on your final resolution of this problem ...

We all like to learn ...

MJCronin
Sr. Process Engineer

 

[TugboatEng]

Hey now, don't scare the OP away. We can all learn things from posts like this. I'm moat curious about the pitting on the shaft. That looks like stainless steel and not monel.

 

[Artisi]

I'm with LittleInch, a bit more tech. detail might help. I also agree (to a point) with MJCronin, have you involved the manufacturer?

Interesting that the bearing support spider has failed, that would indicate something is way-out of tune with the operation together with the shaft not straight, would take considerable force to bend a shaft of that diameter?

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.)

 

[JS_]

Out of curiosity,

Do you have more pictures?

What size of shafting?

How much HP? & RPM?

What is length of of pump assembly? and column pipe length?

Usually,from my experience, spider bearings and or pump bowl bearing lock up when you have a dry run or not enough liquid to lube the bearings and flush the debris out of them...

 

[EdStainless]

How far below the intakes is the Hypo being injected?
The wear on the impellers does not scare me, but low flow is not healthy.
Did this shaft fail at a machined groove? It looks like there is a step there.
Yes it failed from vibration and fatigue, but what caused the vibration.
That is what you need have help to figure out.


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

 

[Nutzman]

Is Monel a suitable material for torque transmission? I've had court cases along these lines. Some of the "Stainless sttel" materials are not suited to torque transmission and tend to cork screw, thereby wreaking the spiders.

 

[TugboatEng]

Monel isn't particularly strong so you have to use a lot more of it vs a shaft made from 17-4PH.

 

[Nutzman]

Looking closer at that shaft failure. That is cyclic fatigue. The hair thin circular lines is the give away.