Faulty RTD on axial thrust bearing

[maypot]

Hi,

We are having a high temperature >300 deg C on the axial bearing of our 15 MW steam turbine . However, the high temperature is only noted when the m/c is stopped. It is worth mentioning that the machine is stopped every each month for maintenance purpose.The high temeprature is recorded when the m/c is stopped and off barring that is without oil circulation on the turbine. The temperature comes back to normal when the oil pump is restarted. Under running condition, that is steam at 450 deg C , the temperature is normal and stable and no alarm is recorded.
We are planning to replace th RTD during the next shutdown but I presume the RTD is fine.
Any comment.

Bob

 

[BigWallyB]

Bob
Couple of comments and/or questions.
If this were a T/C generating a mV I would wonder if the connections were reversed, but an RTD is bidirectional and power comes from another source. RTD's are resistors, so they require power to give a reading, and the system using them sometimes has burn-out protection (upscale/downscale) like thermocouples. Is there a chart showing the timing of these temperature excursions? Are they gradual increases or spike to 300C? What is the max limit on your reading (upscale burnout)? Is 300C close to the max limit? Is it possible the power is removed or broken during these down periods? Can you read the RTD using a DVM?

Regards

Life is what happens while we're making other plans.

Wally

 

[maypot]

Thanks Wally for your useful comments. The RTD is connected directly to a PLC through a RTD input card. The excursions only took place when the machine is stopped that is without steam in the turbine and it resumed back to normal reading^65 deg C when the oil pump is restarted. The rise in temperture is quite abrupt , a couple of seconds. The other temperature measurements on the card are stable during the sudden rise in temperature of the thrust bearing.

Bob

 

[BigWallyB]

Are there other units to compare this to? Seems to me that the mass you are talking about cannot possibly rise that quick? This is why they are looking to replace the RTD? Maybe that assumption is wrong with the lack of oil? When is the next shut down?
Have you tried the input card? I think your first assumption (RTD is fine) is valid for now. Could the RTD open up and cause this problem? (would the input rise or fall?) It could but the circumstances are suspect. Can you record the input from the RTD on a cheap recorder during the power down? (paralleled with the input card) A $200 ink-pen strip chart recorder can be an invaluable troubleshooting tool.

Not really an engineering problem (maintenance) but anybody else have ideas?

Life is what happens while we're making other plans.

Wally

 

[electricpete]

what is "m/c"?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[maypot]

m/c stands for machine. The turbine finally tripped on high temperature and the RTD has been replaced. However, there is nothing wrong with the RTD, the value of the resistance measured at different temperatures are fine and it is perfectly insulated from its body.
Further, the RTD has also been tested near and above its normal working range ( 90 deg C ) and again the resistance measured is fine. One could easily conclude that the problem is the RTD input card. Strange, the new RTD is working fine with the existing input card till now though a couple of hours of operation.

Bob