Gabriel,
We have noted a downward trend in all our Feedwater temperature
measurements, including thermocouples, although the other temperatures have
trended down only about half the amount the RTDs have. This could be
explained if the RTDs have drifted to a slightly lower than actual value, as
this would cause us to operate at a slightly lower reactor power, which
would result in an actual feedwater temp decrease.
We have had this RTD/FBM combination installed about 5 years. The RTDs are
in a fairly hot environment (125 deg F) in our steam tunnel, but the FBMs
are in our control room. There is a seasonal change in feedwater
temperature superimposed on top of the general downward trend due to
condenser circulation water (river temp) changes. The seasonal changes are
normal and expected - we typically vary +/- 5 MW electric from winter to
summer due to condenser backpressure changes.
This issue is just a part of an overall MW witch hunt we go through
occasionally. Even if we find out we are 2 degrees F lower than actual,
this would only translate to about 3 MW electric, but that could be worth a
lot of money in California this summer.
Based on CAR 7375, my unshakable faith in the accuracy of the FBM03s is
starting to crumble. I have been in denial that the four RTDs on two
separate FBMs would drift downward together, but .....
I appreciate the help.
Gaylon
----------
From: [EMAIL PROTECTED][SMTP:[EMAIL PROTECTED]]
Sent: Tuesday, May 22, 2001 11:43 AM
To: Foxboro DCS Mail List
Subject: Re: FBM03 Drift Characteristics
Gaylon, I can only offer initial thoughts.
How are you determining the drift over one year ? Is it that your MW
output has changed relative to a controlled water temperature ? If this is
the case are you sure the drift is caused by the temperature measurment end
of your controlled system ? i.e. have you verified the drift against another
measurment method ?
How long are you measuring this temperature with the same RTD/FM
combination ? You could be seeing drift due to RTD degradation/time,
degradation of wiring connections, or some form of "cooking" within the FBM
circuitry. Is there any change in the drift rate over the year which might
suggest seasonal ambient temperature changes not being correctly compensated
for ?.
Good luck,
Gabriel.
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