Low Vacuum During Low Load of Steam Turbine

Both of the above answers had good things to get you started. Curious as to the source of your cooling water and whether anything could have entered your supply line while you were shut down for PM. It could be possible if you are drawing from a fresh water supply that during the inactivity due to your shutdown that something could have set up in the intake. This should be evident if your water circulating pumps aren't getting sufficient suction pressure or are cavitating.

I assume that you have checked for sealing steam to your turbine labyrinths or that you have positive steam whispers coming out and not air entering the labyrinth.

We tried changed over the main steam ejectors. Checked the packing seal steam pressure. No abnormalities found. We rule out cooling water because the pumps are normal and the inlet pressure to the condenser are normal. No air was trapped at the water box of the inlet side of the condenser.

Thanks for the feedback. Since you have checked all the obvious points from above, it's time to look for less obvious secondary effects.

Aside from the reading on the vacuum gauge, what other parameters on the steam and/or vacuum side are different than before? Can you get your machine to full load without any other parameters appearing off normal? Have there been any noticeable changes in any of the steam conditions? Did you flush the boiler and subsequently verify that the water chemistry is correct? There aren't too many things that affect the vacuum if everything else is ok except a leak, poor vacuum control, or the turbine exiting steam conditions being way off spec.

It might be a good time to put an ordinary direct acting vacuum gauge on a different condenser port and verify that your other gauge and its tranducers are working properly, especially if the machine takes load despite the reading on the suspect gauge, and no other instrumentation/conditions appear abnormal or indicate a problem.

The unit can reached to full load with all parameters normal. The vacuum reading is the same from other unit with identical rating. Only when the load is at around 50 MW the problem occur. The reading at control room and local gauge are correct no problem with the instrumentation. The boiler water parameters are normal. Is it possible that the intrusion of air is at the extraction going to heaters especially the low pressure side extractions since at low load it becomes vacuum? We still did not isolate one of the extractions at low pressure side.

I'm pretty sure that this has been checked but have you checked the condensate levels as the load increases i.e. condensate pumps working properly, level in condensate well staying at proper mid glass range. If level goes up over the tubes your ability to condense is less due to less exposed area of your condenser tubes. Much the same as if some of the tubes got plugged due dirt or sea life entering your cooling water supply which since your inlet pressure is normal wouldn't be the case.

Don't quite follow your concern as to "low pressure side extractions" since I would assume that this is a positive pressure even if only 10-15 psig for water heating.

Dear Mr. Spinco,

You have referred,

"have you checked the condensate levels as the load increases i.e. condensate pumps working properly, level in condensate well staying at proper mid glass range. If level goes up over the tubes your ability to condense is less due to less exposed area of your condenser tubes."

Your point about the LEVEL of the CONDENSATE is VERY IMPORTANT and if it goes up - the Tubes for Heat transfer is SUBMERGED, hence Condensation is AFFECTED.

But if the CONDENSATE LEVEL GOES UP, the ALARM will be sounding, and the Turbine Protection System for HIGH CONDENSATE LEVEL should TRIP. If the Protection System is Mal-Functioning, it is to be set-right, as it is NOT ADVISABLE TO RUN THE TURBINE WITH HIGH LEVEL CONDENSATE.

DHAYANANDHAN.S

Pl confirm how many LP extractions are there. At Low Load the Extraction may be under Vaccum & the air may be sucked through the drain line. We also have faced similar issue at one of the site

Regards

Hi fri ,

so how did you fix this problem?

dear Frnd,

As condenser vacuum decrease at low load, you should check your all LP heaters and low pressure flash tank connected to condenser,there must be air ingress from that area.

Or else use bundle of incense stick in that area and follow the path of stick smoke to find air ingress area.