Root cause for seizure between balance bushing and balance sleeve on BB4 tie rod multi state

[Mech5656]

Hello All,
We had a seized shaft on a boiler feed pump. After disassembly, we found that balance bushing was stuck to balance sleeve. After extra work, we were able to remove them. Nothing on impellers and diffusers was seized. I have attached pictures. Can you help me identify the root cause? What can be done to avoid this in future? Thanks in advance.

 

[JJPellin]

How many stages? How is the balance line routed? How long since last overhaul? Does it run continuously?

That is the tightest clearance inside the pump. Anything that could cause distortion could cause a rub here. Pipe strain. Poor assembly practices. Assembly error. Closed block valve on the balance line. Some of these pumps with a larger number of stages have special pie-shaped case sections to account for rotor sag.

Johnny Pellin

 

[Mech5656]

JJpellin:
Thank you for your response. It is a 8 stage pump and was repaired last time 12 years ago.

Based on what I think, there was debris stuck between balance bushing and sleeve.

The clearances on impeller rings and diffuser bushings were higher so debris passed through them but got stuck in balance bushing because of tighter clearance. This is my opinion.

 

[JJPellin]

That is credible. 8 stages should not require a wedge shaped case section. If it ran for 12 years, it was probably not an assembly error. Are there block valves in the balance line? If the pump was operated below minimum flow the heat in the balance bushing could get hot enough to flash to steam. Steam does not provide enough Lomakin effect to prevent a rub.

Johnny Pellin

 

[Mech5656]

Thanks for your replies. I know it has been a while but I wanted to come back and tell what I found. The best answer I have got and what I believe is the case, based on my research is: the pump was turned off and turned on again too quickly. The temperatures were not settled and system did not come to equilibrium with surrounding, when the pump was turned on again. I don’t believe anymore that debris got stuck between sleeve and balance bushing.

 

[Maheshpowerplant]

Yes, same problem is happened with us also.The root cause was, after pump preventive maintenance for 4 hours, suction valve of the pump opened & pump started immediately. As soon as pump started pump got seized, as there was no enoufg time to accomodate expansions due to hot water.
Other causes for pump seize

Reverse rotation of pumps
Foreign materials
No suction/cavitation
Stuck up of ARV during start up..

Also refer this for more QnA on high head multistage pumps

https://www.powerplantandcalculations.com/2020/06/boiler-feed-pumps-bfp-questions-answers.html