Repeated Disaster Bushing Failure
Repeated Disaster Bushing Failure
(OP)
Hello everyone
We have a between bearings Union centrifugal pump in a high temperature (700deg F) high viscosity service.After shop overhaul the disaster bushing failed during startup.The bushing was fused against the seal sleeve.This disaster bush is made of bronze and has a diametral clearance of 0.035 thou.You would think that with heat, the disaster bushing would expand and rub the seal flange, but here we are seeing the bushing fusing onto the sleeve.Can hydraulic instability cause the shaft to deflect 0.017 thou causing the shaft to rub against the bush?
How critical is the ring clearances for a pump in this type of service in relation to hydraulic stability?
Thanks in advance
We have a between bearings Union centrifugal pump in a high temperature (700deg F) high viscosity service.After shop overhaul the disaster bushing failed during startup.The bushing was fused against the seal sleeve.This disaster bush is made of bronze and has a diametral clearance of 0.035 thou.You would think that with heat, the disaster bushing would expand and rub the seal flange, but here we are seeing the bushing fusing onto the sleeve.Can hydraulic instability cause the shaft to deflect 0.017 thou causing the shaft to rub against the bush?
How critical is the ring clearances for a pump in this type of service in relation to hydraulic stability?
Thanks in advance





RE: Repeated Disaster Bushing Failure
Mike Halloran
Pembroke Pines, FL, USA
RE: Repeated Disaster Bushing Failure
Concentricity and perpendicularity checks was carrried out during the overhaul and was found to be 0.003 thou.
RE: Repeated Disaster Bushing Failure
The word "Repeated" suggests that the pump was overhauled because the "disaster bushing" (whatever that is) died ... and it died again on startup.
Did anyone figure out what really failed in the first place?
I.e., from here it smells like your shop replaced all the parts they had spares for, and called it an overhaul ... but maybe it needed a part you didn't have. The maintenance guy who expresses the worst attitude about having to r&r the pump again may know or suspect which part that is.
Mike Halloran
Pembroke Pines, FL, USA
RE: Repeated Disaster Bushing Failure
Is most of the diametrical clearance above the shaft? I would expect the shaft to lift off the bearing sleeve on start-up, but don't know by how much.
Were the bearing brackets removed and replaced during the overhaul, possibly changing the elevation between bearing and seal register fit?
RE: Repeated Disaster Bushing Failure
RE: Repeated Disaster Bushing Failure
Mike Halloran :The component that I am referring to (Disaster bushing) is also referred to as the throttle bushing. No we didn't figure out what went wrong in the first place, these two failures took place within seconds in one instance, and about a couple of minutes in the other.
rzrbk :This pump has ball bearings,the brackets were adjusted and dowelled into position during concentricity checks during assembly.
Artisi :API states that the throttle bush clearance should be 0.025 thou, we are presently at 0.035thou, which is a generous clearance.I don't think we would want to increase this clearance anymore as we would be defeating its purpose, if we haven't already.
What about the point of the thermal expansion of bronze?, it is confusing to us that the throttle bushing would fuse on the sleeve and not on the seal flange.
RE: Repeated Disaster Bushing Failure
Making this bushing out of carbon would allow some contact during running, but would not really solve your problem.
Is this seal the same type as was running in the pump before the overhaul? (has satisfactory history before the recent problems)
RE: Repeated Disaster Bushing Failure
The 700F sevice would make me wonder about piping expansion and extra loads on the pump after heating up system. Check that the piping supports were re-installed correctly. Casing distortion from excessive piping loads usually first cause bearing failures, but in your situation it is an immediate failure. Some field techs will put dial indicators on casing or piping connections to show any casing movements that could cause loss of clearances inside of pump. All the concentricity checks on the work bench would mean nothing if the piping adds several thousand pounds of loads on the casing. If possible, check the shaft clearance by hand rotation for any rubbing, before starting pump but after filling piping to heat up the system.
Which brings up another point when you ask about ring clearance and hydraulic stability. Are you sure that fluid has filled the seal cavity? If the bushing is dry, then any rubbing could quickly result in enough heat to fuse parts.
RE: Repeated Disaster Bushing Failure
One other possibility occurs to me. If the shaft was bent during the first failure or first repair, there could be excessive shaft run-out. This would not show up on the seal chamber concentricity check since the dial indicator would still spin a true circle on the bearing centers. Did you check shaft run-out in the seal area?
Johnny Pellin