Axial Shift Related Problem in 250MW Steam Turbine

Without some better background, I assume you had an after market dealer reverse engineer one of the last stage bucket rows of your double flow LP. He used a pinned dovetail versus the present design. You assembled the unit, synchronized and got some load on, failed the thrust bearing.

You suspect that the LSB from the vendor is causing an unbalanced thrust load. All other things being equal that may be a valid concern. I would question the advisability of replacing one last stage of an LP double flow unless you had failed the stage in operation and had an urgent need for the unit generation.

Since the dovetail has been modified that indicates that the rotor has had some significant machine work, things tend to get complicated from that point forward.

I think that if the LP is your problem, you need to get the OEM involved whether he wants to get involved or not. You may have to pay him for an engineering study.

Odds are it is just a coincidence and the bearing failed for another reason. I don't see any way converting the dovetail of an LP could increase loading, even if you located the new dovetail off axially. I can see improper design of the bucket profile causing some significant problems with thrust.

I would expect that if the LP bucket design was the culprit you would have noticed an increase in crossover pressure.

But like I said get a really good look at that bearing and don't just point fingers.

My second question was that HP&IP having extra balance disc in 250MW NCG Turbine by M/S BHEL make.pl. give idea.

If this is a reaction type turbine it will have some form of balance piston, disc etc. in the single flow sections. Some of them are not easily identifiable if not specifically called out on the turbine outline, so I am at a loss to comment on something I can't see.

Are you saying that there is "an extra balance disc" or should there be an extra balance disc? If this is a double flow LP reaction section then the thrust components should be pretty much in balance since the opposing directions should balance assuming an equal number of stages in both directions. There should be somewhat of a balancing disc on both ends generally cross connected to maintain an equal pressure on both ends. The axial thrust component should not be as significant as on the HP single flow end which is probably primarily impulse staging.

Since this a power generation unit there would be no need as in Marine turbines with Reverse Impulse stages in the LP section to worry about the axial thrust under Astern power. I would also question what happened that you have a unit with extensive modification of the blade root system or did it just come to you that way originally? There are often some locking blades where the root system is different since it is the last one in. That scenario of different roots and extensive machining would seem to affect your radial balance and not your axial balance. If the unit was disassembled was the thrust bearing reassembled properly, because if not, the axial movement would be significant under that scenario.

with your feed back, it is difficult to diagnose the real culprit. After modification of LP blades, how long your turbine had run before failing. Is your thrust gland fins/labyrinths are OK. BHEL turbines do have some problems related to balancing drum glands. I am not convergent with 250MW turbines, The balancing lines connected from balancing drum leak off to LP stage, can show the increased temperature if the balancing drum gland had failed. You should further details to see more inside for diagnosis.

Your's is a brand new turbine (which was not clear from the post and hence the misconception of the post #1 about RE).

I think the E&C staff must be still there? Any way the Unit-5 and Unit-6 must be similar turbines for you. So had there been any specific design problem that would have been in U-5 too.

I don't understand the meaning of "this modification done in one side only another side is normal"

Does it meen these are half fir tree or half the blades are fir tree? The OEM will be infact able to give more light on this. However I sincerely don't believe that half firs will be giving axial shifts.