I work in a 160 MW coal fired Power Plant. The turbine is a condensing turbine with a surface condenser. After our last outage it was noticed that one of our three backpressure indications started reading 2 inHg different than the others. The one that is reading different is taken 30 feet higher than the others. Our Technicians believe that the difference in elevation (30 ft) is enough to give us the 2 inHg difference. Seems like a lot to me. Any comments?
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Backpressure Calculation 1
- Thread starter ksprague
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what changed? why would "your technicians" think the difference in elevation would make a difference now when it didn't before? Sounds like you need to whip out some thermo books and do some calcs to see if they're right. And if they are, you should probably buy them a "beverage of their choice" or two - most I&C techs would leave it up the engineer to figure out, especially if he's new. Patricia Lougheed
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Sounds odd for an explanation if it just started showing this difference. Are these gauges or transmitters?
Have you tried swapping them with one of the other units to see if you still get the difference?
Have you checked the static head difference to see if it accounts for a 2 in Hg difference?
Have you tried swapping them with one of the other units to see if you still get the difference?
Have you checked the static head difference to see if it accounts for a 2 in Hg difference?
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- #4
VPL The only change was that the transmitter in question was calibrated during the shutdown. So (in there words, not mine) it was always out of calibration. I have the books whipped out but I'm not having much luck with the calculations. Never said I was a good engineer! LOL
TD2K Two of the indicators are transmitters including the indication in question. The other is a hard piped mercury barometer. And like I said above, I'm having trouble with the calculations.
Thanks for both of your inputs. Have a great new year!
TD2K Two of the indicators are transmitters including the indication in question. The other is a hard piped mercury barometer. And like I said above, I'm having trouble with the calculations.
Thanks for both of your inputs. Have a great new year!
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- #5
The 2 in Hg (= 0.98 psi !!!) difference in the readings IS ABSOLUTELY NOT due to a 30 foot static head of steam.
For example, even at the relatively high exhaust pressure of 2.00 psia:
30 ft * 0.00588 lb/ft^3 ==> 0.0012 psid
Your exhaust pressure (and density) is probably less, and the elevation would therefore cause even less difference in the readings.
TD2K has a good question/suggestion that you should switch the instrumentation and compare those readings as a way to troubleshoot this oddity.
It is also possible that there is a difference in the "water legs" on the sensing lines leading to your instruments. (The water leg is the accumulation of condensate in pressure sensing connections on steam piping.) This must be properly accounted for when zeroing the instruments.
What is bothersome is that there was (apparently) a "sudden" change in the measurements. As VPL asked - What changed after the outage?
On a 160 MW turbine, you had better know to within 0.1 in HgA what is your exhaust pressure; the optimal performance of the turbine will be at (or near) the "design exhaust pressure".
For example, even at the relatively high exhaust pressure of 2.00 psia:
30 ft * 0.00588 lb/ft^3 ==> 0.0012 psid
Your exhaust pressure (and density) is probably less, and the elevation would therefore cause even less difference in the readings.
TD2K has a good question/suggestion that you should switch the instrumentation and compare those readings as a way to troubleshoot this oddity.
It is also possible that there is a difference in the "water legs" on the sensing lines leading to your instruments. (The water leg is the accumulation of condensate in pressure sensing connections on steam piping.) This must be properly accounted for when zeroing the instruments.
What is bothersome is that there was (apparently) a "sudden" change in the measurements. As VPL asked - What changed after the outage?
On a 160 MW turbine, you had better know to within 0.1 in HgA what is your exhaust pressure; the optimal performance of the turbine will be at (or near) the "design exhaust pressure".
One of these days, I'll master US customary units (probably the day after the US goes metric). Throwing "g" into my example equation above gives 0.04 psid (rather than 0.0012).
But my point is still the same...
Another correction to my earlier post is that "you had better know to within 0.1 in Hg (not HgA) what is your exhaust pressure"
For a steam turbine with a water cooled surface condenser, the nominal exhaust pressure (not usually called back pressure for condensing turbines) could be in the range of 1.5 - 4.0 in HgA. An uncertainty of 2 in HgA is very significant. It is in your own best interest to resolve this.
But my point is still the same...
Another correction to my earlier post is that "you had better know to within 0.1 in Hg (not HgA) what is your exhaust pressure"
For a steam turbine with a water cooled surface condenser, the nominal exhaust pressure (not usually called back pressure for condensing turbines) could be in the range of 1.5 - 4.0 in HgA. An uncertainty of 2 in HgA is very significant. It is in your own best interest to resolve this.
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