Calculating efficiency loss from a tube leak

[Rpsfinest]

We had a tube leak in the LP Evaporator section of the HRSG. I know the pressure and temperature in that section. I also know the size of the tube leak. (lbs/hr) How can I calculate a btu loss from the leak that I can use to show an adjustment to our heat rate? Basically I just put Fuel Burned (mmBTU) over net MWs out. Can anyone provide asistance?

 

[metengr]

Don't worry about heat rate effects. What are you doing to the rest of the LP evaporator section in the HRSG would be a bigger concern. These type of tube leaks can manifest themselves into a real mess in terms of collateral damage to the remaining tube bundle. Don't run like this for too long because repairs will be significant.

 

[EdStainless]

Yes, you can figure out the energy lost and figure that you will need to burn that much more fuel + your efficency. You should also account for lost condensate.

But Metengr pointed out the major cost. A leak like this can take out an entire bank of tubes.

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http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[FredRosse]

In one simple scenario, the leak is taking away heat that would not otherwise escape. If it is considered just an isolated leak, and the plant does not have regenerative feedwater heating, the energy loss is = Mass Flowrate X (Enthalpy of Leak - Enthalpy of Feedwater entering the Economizer).

However things are generally much more complicated if the leak is spraying water on other heat transfer surfaces within the HRSG. Apart from the physical damage potential of the leak, it will tend to drastically reduce the heat absorption in adjacent economizer tubes which are being sprayed by the leak. Addressing this by analysis is very difficult.

You probably get a reasonably accurate indication of Heat Rate impacts by using station metering of fuel flow, depending on the calibration of your flowmeters.