BB5 Multi-stage centrifugal pump

[Meshry]

In our water injection facility; BB5 multistage pump is installed; every time only NDE mechanical seal fails. DE is okay. any specific reason?
this pump is equipped with 53B mechanical seal piping.
I am expecting some reverse pressure from pump discharge while pump is not running (may be the check valve is passing). however, this should affect both DE and NDE seals!!
please help me to understand why only the NDE mechanical seal fails.

Thanks

 

[quadtracker]

When does the seal begin to leak after you repaired the pump seal?

 

[Meshry]

Quadtracker,
this is almost 3 months after seal replacement.

 

[quadtracker]

api plan 53b includes a pre-charged bladder to keep barrier fluid pressure to the seal, is your system setup like that? If so is the bladder pressure at the proper pressure. Check with your mechanical seal manufacture for pressure specs based on your operating condition. I think that seal plan may even allow an external pressure source instead of a bladder accumulator. In any case check for proper pressure on system. Plus check condition of barrier fluid change as needed.

 

[shashankTurbomachine]

BB5 has DE and NDE end. The problem is in NDE side.
The seal plan is 53B on both the side.

There shall be uneven load on NDE side. This also depends upon no. of stages of pump and based on that there shall be enough distance at throttle bush on NDE side and last impeller and if that is not maintained which is your case, seal is failing.

A well phrased question has answers in itself!!!

 

[quadtracker]

I have rebuilt many multi-stage horizontal split centrifugal pumps including complete disassembly of rotor and reassembly of the pump and when in operations did many pump switches on several units in the plant processes over the years. I am very familiar with that style of pump. given the length of operating time with the seal I would assume assembly was correct. The seal plan tubing may or may not be separate for each seal. Unknown at this time.

 

[quadtracker]

The balance pipe from NDE high pressure side connects to the low pressure side near the suction of this vertically split (Barrel) multi-stage pump to balance the hydraulic forces created by the pump. The Kingsbury thrust bearing controls the thrust created by the remaining hydraulic forces. The throttle bushing clearances can be washed around the shaft allowing more fluid pressure exposed to the seal but the throttle bushing sleeve and impeller distance should never change because there keeper rings and keys on the shaft that along with the fit on the shaft keeps the locations permanent. the other non rotating part of the throttle bushing is kept in place by capscrews and fit according to drawing. I don't understand this distance change that was mentioned.