High HP Differential in Steam Turbine

I

Thread Starter

izharkhan101

Hello everyone

I hope you are in good health.

We are facing problem with the HP differential of our Steam turbine. Recently our turbine has been on partial load for quite some time due to the demand of national grid, we have been changing over the turbine from full load to partial load continuously from last month.

After the continuous operation on partial load, values of differential has increased to â 1.8 mm at full load i-e 105 MW (inlet temperature of main steam being 535 C) which used to be less than -1 mm in past.

Currently, the Alarm setting of HP differential is at -2.5 mm and trip has been set at -2.8 mm. From the OEM provided quality hold points documents we found that the recommended clearance between stationary diaphragm and rotor (for HP) is around 7 mm (min).

All other operational and mechanical parameters are running in normal condition.

I need to know the following:

1) What are the basis of setting alarm value at -2.5 mm if the clearance is higher which is around 7 mm (min)?

2) What are the recommended checks which should be carried out to analyze the increased HP differential expansion issue?

3) What precautions should be taken to avoid this issue in the future?

Will appreciate any response from anyone.

Thank you.
 
I have some experience with GE and Westinghouse turbines, as an Operations Supervisor.

I would ask a few questions:

Was the unit in continuous base load operation prior to the part-load operation, and was the expansion normal then? If so, I would suspect the unit is merely reacting to the part load operation (the inlet steam temps will be lower due to the throttling of the inlet valves) -it will take some time - 6-8 hrs at the part load for the expansion to stabilize at a different value. If the exp does not return to previous values after 8 hrs of base load operation, then there may be other causes at work.

Do your reheat/intercept vales begin to close when you are at part load? There may be some effect from increased 'windage' in the HP section if so, causing a 'rotor long' condition-resulting in increased diff exp.

Has anything changed environmentally with the turbine insulation? ie. water leaking into the turbine from failed lagging, or just cooler weather coupled with the part load operation? they go together.
A long shot here, but still a cause; when was the last time the turbine pedestal was lubricated?

There could be something going on with the steam sealing system here also that you could check.

I've about exhausted any other causes that I can think of---I'm sure there are others more knowledgeable than I that can give valuable feedback.
 
Check the slideways for free expansion of casing. has there been any significant change in steam admission pressure/temp observable at interstages? Anything indicating steam path degradation? what about turbine extractions, drains and condenser-vacuum?

That 7mm must be sum of minimum clearances on both sides of moving row. you can't play with that clearance as it can be catastrophic.

Amarjyoti
 
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