POWER June 2012 - 77
CONDENSERS
ing can occur in the plate-and-frame heat
exchanger. If this happens, the ability of
the heat exchanger to cool the seal water
is much reduced, which in turn means the
seal water temperature will rise above the
saturation temperature of the operating
condenser's vacuum, the liquid seal will be
lost, and the condenser vacuum will quickly
be lost. If the condenser vacuum rises
too high, the steam turbine, and potentially
the entire plant, must be taken off-line.
Given the importance of the seal water
temperature to reliable plant operation,
you would think that online monitoring of
that temperature and logging by the plant
historian would be mandatory. However,
it's my experience that very few plants actually
monitor seal water temperature. Recent
events experienced by the plant staff
affirmed their decision to digitally monitor
and log data in a plant historian, again
OSIsoft's PI system. Online air in-leakage
monitoring is not currently available at
this plant, so a real-time comparison between
air removal and seal water temperature
was not possible.
Prior to May 7, 2011, the seal water
temperature had risen to over 130F (Figure
5), high enough that the plant staff was
concerned that the steam turbine would
trip off line (Figure 6). At the time, the
pond water was approximately 85F. In the
past, the typical approach temperature was
about 4F to 5F, so the expected seal water
temperature was about 90F.
Given the plant's prior experience with
the vacuum pump heat exchangers, the
decision was made to acid clean the exchangers
in the hope that the seal water
temperature would then decrease. The heat
exchanger cleaning was successful, and
the effect was an immediate reduction of
the seal water temperature and, therefore,
improved vacuum conditions inside the
condenser.
The seal water temperature did not suddenly
increase; instead, it rose gradually,
beginning late the previous year (Figure
7). The plant did not have a consistent
condenser performance-monitoring protocol
in place at that time, so the temperature
rise was not spotted until the steam turbine
backpressure began to rise. Digitally
monitoring the seal water temperature
would have allowed the plant to schedule
a heat exchanger cleaning during a routine
planned outage rather than handling the
problem as an emergency.
As a result of this incident, the plant
staff developed an appreciation for the
importance of condenser performance
monitoring and, specifically, the critical
importance of seal water temperature meaJune
2012 | POWER
If the condenser vacuum rises too high, the
steam turbine, and potentially the entire
plant, must be taken off-line.
surement and logging for trend analysis.
Because this particular cooling water
is so difficult to control without antiscalant,
the plant is now considering use
of an anti-scalant for all cooling systems.
An anti-scalant, combined with continued
monitoring of the seal water temperature
and other important condenser parameters,
7. Spotting trends. The plant staff was unaware of the slow seal water temperature rise
until the steam turbine backpressure rise threatened to trip the steam turbine. Vacuum pump seal
water temperature is important plant data that should be closely monitored. Source: Nalco Co.
150
140
130
120
110
100
90
80
70
8. Weighty fill. Biofouling of a cooling tower increases the weight of the tower fill. Injecting
a single slug of biocide temporarily decreased the biofouling, although it soon returned. Source:
Nalco Co.
270
260
250
240
230
210
200
www.powermag.com
77
Weight of cooling tower fill (lb)
Vacuum pump seal water temperature (F)
6/6/11
5/17/11
4/27/11
4/7/11
3/18/11
2/26/11
2/6/11
1/17/11
12/28/10
7/21/10
12/8/10
7/11/10
7/1/10
6/21/10
6/11/10
6/1/10
5/22/10
http://www.powermag.com
POWER June 2012
Table of Contents for the Digital Edition of POWER June 2012
Contents
POWER June 2012 - Cover1
POWER June 2012 - Cover2
POWER June 2012 - Contents
POWER June 2012 - 2
POWER June 2012 - 3
POWER June 2012 - 4
POWER June 2012 - 5
POWER June 2012 - 6
POWER June 2012 - 7
POWER June 2012 - 8
POWER June 2012 - 9
POWER June 2012 - 10
POWER June 2012 - 11
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POWER June 2012 - 17
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