Ring of bolts shearing off!

[jerf]

We have seen a sewage pump rated at 440 litres/second with a motor of P1 = 267 kW P2 = 250 kW with the ring of bolts sheared off along the flange that connects to volute to the pumpmotor casing. We are wondering where the forces to do this sort of damage could have come from. We are finding it hard to see how a blockage could do this sort of damage and wonder wheter anyone has seen this sort of thing before? The only other clue is that the pump was recently repaired and some of the bolts could have been replaced!

 

[Artisi]

I would be investigating volute / frame bolts not being torqued correctly following the rebuild, this could result in the volute casing / motor frame osillating back and forth leading to failure. A metalurgical analysis of the bolt failure might also shead some light on the problem.

International College
Naresuan University
Phitsanulok
Thailand

 

[PUMPDESIGNER]

Artisi might be on the right track.
Normally the number and size of the bolts at that volute to motor case or motor adapter plate is many times the actual required strength to resist hydraulic forces, but not knowing more about your pump I cannot say that for sure.

Typically, excessive hydraulic forces would damage bearings quickly, tear up the motor, shafting, etc., but not be capable of shearing those bolts.

So that would leave things like wrong bolts, wrong torque, etc.

Make no mistake, sometimes hydraulic thrust forces can be huge, but the problem most often shows up in bearings and shafts.

PUMPDESIGNER

 

[saxon]

jerf, In addtion to the above, also check to make sure that the bolts are the right specification and grade. Also, make sure that the bolt grade is the one marked on the bolt. There have been major problems as of late due to wrong bolt markings, such as grade 2 bolts marked as grade 8. This is especially true of bolts coming out of the Orient. Ever wonder how costs are reduced? Remember, to Purchasing agent, a bolt is a bolt.

Hope this helps.
saxon

 

[pablo02]

Another item to look at is how frequently and how many times these bolts have been torqued (and how much torque is involved) -- the metallurgical analysis mentioned by artisi should point to the cause -- saxon's comments should be considered by all...

 

[WGH]

I would agree that the bolting looks suspect - is the sewage of a saline nature? If so, check that the bolts are compatible. It wouldn't be the first time that bolts of an inappropriate grade had been fitted following a repair and they get severely weakened by corrosion, (galvanics, etc), in a short period of time.

 

[Berko]

Hi Jerf,

I had a similar problem a few years ago with 15 kW dry well submersibles (Raw Sewage transfer) shearing the volute/motor bolts.

In our case, the screenings in the treatment plant were removed during a plant upgrade.

This lead to severe pump blockages, which in one case put the impeller out of balance to such an extent, that the pump vibrated violently, the volute/motor bolts shook loose & then sheared.

The motor kept rotating & literally took off(only stopping when it twisted its power cable to a point were the copper wires inside sheared & cut the power to the motor).

The entire dry well flooded, as the associated wet well emptied with the help of gravity via the pump volute.

Never under-estimate what a blockage can do to a sewage pump !!

I hope this helps,

Berko

 

[tonyh]

I was not quite clear. Was it the bolts that sheared, or the ring? If the ring sheared and the bolts were intact, the other possibility is that the assembly allowed the bolts to put the ring into bending and you ahve mechanically failed the ring due to bolt loading.