impeller nuts coming off
impeller nuts coming off
(OP)
On several centrifugal end suction pump of varying manufacturers, design conditions, etc., have recent problem of impeller nuts coming off shaft, followed by impeller, with associated damage to casing, impeller, shaft, etc.
Nuts are threaded opposite normal pump rotation direction and have retention devices (typically set screws). Check valves have been inspected and pumps are not known to run in reverse. Metallurgies are consistent ruling out loosening due to differences in thermal growth (several of the pumps are high temp-700 deg F range). Have looked for recommendations from OEMS on nut torquing values with common response being "tight is tight".
Compounding this problem, after couple of these events on one of the subject pumps, we modified the impeller nut to incorporate additional locking screws. Next time pump failed the shaft broke at the shoulder at the base of the thread relief.
Anyone seen this problem before? Any recommendations on nut torquing and locking devices?
Nuts are threaded opposite normal pump rotation direction and have retention devices (typically set screws). Check valves have been inspected and pumps are not known to run in reverse. Metallurgies are consistent ruling out loosening due to differences in thermal growth (several of the pumps are high temp-700 deg F range). Have looked for recommendations from OEMS on nut torquing values with common response being "tight is tight".
Compounding this problem, after couple of these events on one of the subject pumps, we modified the impeller nut to incorporate additional locking screws. Next time pump failed the shaft broke at the shoulder at the base of the thread relief.
Anyone seen this problem before? Any recommendations on nut torquing and locking devices?





RE: impeller nuts coming off
RE: impeller nuts coming off
Have you checked the impeller/shaft fit- possibily excesseive clearance resulting in freting of the impeller.
RE: impeller nuts coming off
Impellers with nuts can't be bump checked for rotation. If it's right it's good; if it's wrong it's hell.
rmw
RE: impeller nuts coming off
RE: impeller nuts coming off
If keyed on impellers - the locking screw or nut cannot unscrew either as there is no torque applied to the locking device.
RE: impeller nuts coming off
Your initial post implies that this problem recently appeared after a significant run with no problems, and on several different pump types and services. This implies that something has changed, be it process conditions, fluid properties, Maintenance practices, parts suppliers,or Operation practices.
RE: impeller nuts coming off
RE: impeller nuts coming off
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RE: impeller nuts coming off
Another thought. Are these pumps powered by adjustable speed drives (adjustable frequency drive with an induction motor)? Some of these drives can produce very nasty, fairly high frequency torsional shock loadings. Some early drives were bad enough to break motor or pump/fan shafts (usually fatigue failures, but there were some outright failures with high inertia driven loads). Has there been any indication of relative motion between the pump impellers and their shafts (wear on keys, keyways, shaft surface, or impeller bore)? This could account for the nuts being either loosened to tightened from torsional vibrations induced by the drive.
RE: impeller nuts coming off
What type of coupling now, and before?
How tight are the nuts being tightened?
The shaft and impeller and keys all must fit properly.
"Looks good" is just about meaningless for determining fits.
If the torque is indeed proven appropriate then Loctite and belleville spring washers come to mind.
RE: impeller nuts coming off
I am not aware of locking nuts EVER having fell off, however, we did used to put some 3mm slots in the nuts to allow us to 'peen' surrounding metal with centre punch to cause slight interference just in case - in 14 years, never heard of such a problem, so assumed it worked just fine !! Hope this helps ?
RE: impeller nuts coming off
RE: impeller nuts coming off
RE: impeller nuts coming off
If the pumps haven't been run the wrong way which is what appears from your post then is it possible that the thrust
from the impeller is undoing the nut and in fact when you
improved the locking of the nut the shaft itself failed.
Why not have the pump shaft analysed did the shaft fail in torsion or (tension along the principle planes) caused by the torsional load. Have a look at the pump shaft failure if the shaft ends are square to the diameter then the shaft failed in torsion. If however the shaft ends have lips on them running at roughly 45 degrees to the shaft end this would indicate tensile failure on the principle planes due to the torsional shear.Either way looking at the shaft failure may give some clue as to what forces are acting and causing the problem.
Additionally I don't agree with the pump supplier saying tight is tight its meaningless without knowing the forces
the nut is trying to work against.
regards
desertfox
RE: impeller nuts coming off
If tapered is the "fit" correct.
If parallel is there excessive clearance?
If threaded are the threads of correct form and parallel.
An impeller/shaft that has correct / bore/ taper / thread and fit plus a locking nut with nut retention that is coming "loose" would suggest a problem other than the impeller / shaft retaining system.
You need to expand your investigation into the failure and not just look at the "nut coming loose"