We have a STG of 25MW capacity running in parallel with state electricity board. Recently we are observing a very interesting phenomenon, where in turbine axial bearing temp (Non active side is running high). The temp goes higher when the load is raised to full load and even it trips on high axial bearing temp. If out of three uncontrolled extractions are stopped the temp drops and STG can run on full load. The axial shift goes negative with full load and all extractions in line. All other parameters of STG set is within permissible limits.I am unable to under stand how does the extractions folw is affecting rotor float to this extent. Can any one suggest any further checks to understand this problem. Many of us are thinking at is may be due to saulting of blades in LP side (The M/c is Single Cylinder), however the specific steam consumption and wheel chamber pressure are within prescribed limits.

Regards,
Rangacharya,

 

It seems that your STG is not managing to balance out the axial forces when the extractions are active, thus forcing the axial bearing to do more work that it was designed for.

Assuming that your STG is a reaction turbine, each stage of the turbine will produce an axial force in the direction of flow of steam proportional to the steam flow passing through that stage. The resultant axial force from all stages is balanced out approximately by a balance wheel located at the high pressure side of the rotor, whose job is to create a similar force to that created by the steam on the blading but in the opposite direction. The axial bearing will then have to cater only for the resultant force only, which would be small.

Now when you extract steam from the steam turbine, a lower axial force is generated by the stages after the extraction, as less steam is passing through them. Normally the design of the turbine caters for these variations, as long as you stay within the allowable extraction rates.

In your case it seems that you are extracting more steam than is allowed, thus resulting in an unbalance of forces to such an extent that the resultant force is exerting too much force on the axial bearing.

Check the design parameters of your STG and see whether you are exceeding allowable extraction flows, especially in view of the fact that your extractions are uncontrolled, so there is nothing on the turbine to limit them.

Some question:
1.have you changed anything in the complete process, which may be creating this problem, i.e. more extraction flow (check the secondary processes where the extracted steam is used)?

2. Has this problem cropped up recently, or have you been living with it for some time?

 

Dear Jojo,

Thanks for your elaborated response, first to your two questions,
1) There is no change done in recent times
2) The problem has cropped up recently i.e. six months.

One more point i have forgotten to mention in my post is, the rotor float goes negative with drop in turbine inlet pressure there by increasing the non active side thrust bearing temp.
I fully agree with you about the rotor float to -Ve side with excess steam drawl from extrns, however There is no chance of any such excess drawl as they are connected to feed water heaters and de-aerator. One more thing what i have mentioned is if we stop any one extraction then the rotor float becomes normal and thrust brg temp drops down.

I hope you continue to discuss the issue

Regards,

 

hye
Is there any balancing drum in your turbine
sure there will be..
I think there is excessive steam leakage through balancing drum and that is not able to maintain proper axial balance of rotor
Or may be any restriction in balancing line
at 22MW machine balance line will be small,around 1 or 2 inch so this line possibly can clog
finally turbine is back pressure type or condensing type
if back pressure then see the back pressure parameters

 

M. Rangacharya... have you compiled a parameters matrix listing steam-related flow, temperature, and pressure, bearing temperature & vibration, attached piping displacement, etc, of all pertinent variables?

Regards, Phil Corso

 

M. Rangacharya, now we know that it is a steam turbine with conventional extractions to feedwater heaters.

When was the last time that you inspected the feedwater heaters? Did you do a check on the temperature rise across each heater? There could be the possibility that one of the lower pressure heaters is not heating enough the feedwater passing through, putting more load on the higher pressure heaters. This will cause more steam to be extracted out of the higher pressure extractions thus unbalancing the steam turbine forces with respect to design. I have met a case once where one HP heater on a 60MW steam turbine was damaged internally (water was short-circuiting rather than passing through heater tubes due to an eroded baffle plate), sending 'cold' water to the next HP heater. This second heater still managed to attain the required final temperature but at the expense of too high an extraction of steam to it, thus unbalancing the steam turbine. Compare your temperatures across the heating cycle with those obtained during commissioning (see heat balance diagrams), and see where the differences lie.

By the way when you reduce the inlet steam pressure, you are reducing both the forces created by the steam turbine proper and the forces created by the balance wheel. However you will not know the rate of change, so that test may be mis-leading.

 

Dear Jojo,

your point is very valid, let me cross check this, however i would like to inform you that the axial float and thrust bearing temp becomes normal if any one of the extractions steam flow is stopped.

Is there any possibility of fault deposition in LP side blades causing this problem. I am trying to collect some more data in this regard, let us see, i will keep this forum posted about the progress.

 

I do not see a reason how deposits on the LP side blades may cause this problem. In any case I would expect that deposits increase friction across the blade surface and thus increase axial forces not decrease them.

Keep in mind that whenever you stop an extraction, you are increasing axial forces towards the LP side, and given this reduces thrust bearing temperatures, we need to find the reason why the steam turbine and balance wheel are ending with too high a force towards the HP side when all extractions are operating.

By the way, assuming your steam turbine is a condensing turbine, are your vacuum levels at the exhaust hood within allowable limits, or are you running with lower vacuum levels than design, i.e. higher exhaust hood temperatures than design?