POWER May 2010 - 16

You should perform a leak-detection survey prior to a major
outage to plan work items and immediately after the outage to
check tightness on components that were disassembled. A survey
also can be conducted when a sudden increase in air in-leakage is
detected, based on condenser backpressure, decay tests, dissolved
oxygen concentration in the condensate, or air-removal rates.
Many leaks found through mass spectrometry can be stopped almost
immediately. A typical example is a drain valve that was inadvertently
left open. Other leaks, such as the gaskets around rupture
discs or manway covers, can be temporarily repaired until the next
scheduled shutdown. Leaks in pump-shaft or turbine-gland seals,
however, usually cannot be repaired online. Because it can determine
the rate of air in-leakage at any one leak site, mass spectrometry
can help a plant decide how long the repairs can be deferred.
Consider Online Cleaning of Generator Coolers
Managing risk-as opposed to completely avoiding it-means taking
calculated gambles to cut expenses and sustain availability.
Some maintenance groups are doing just that when they attack
heat-exchanger fouling in their hydrogen-cooled electric generators.
The hydrogen is cooled by service water in a tube-and-shell
heat exchanger, which, over time, becomes fouled with deposits.
Traditionally, plants have cleaned the deposits by shutting
down the plant and manually brushing the heat exchanger tubes.
This procedure is somewhat effective at restoring cooler performance,
but hard deposits that are not removed by brush eventually
began to build up. Besides, taking the units out of service
means the plant availability will take a hit.
HDS Ad_hr.pdf 2/7/07 10:02:02 AM
To improve on the traditional practice, innovative plants have
considered two methods: liquid-fill-and-circulate and foam cleaning.
The former raises concerns about spills, aggressive liquids
leaking through to the generator, and the handling and disposal
of waste liquid. Foam cleaning, by contrast, is faster and neater,
with more control of cleaning parameters such as flow, temperature,
and velocity. Plus, if hydrogen leaks occur during a foam
cleaning, the hydrogen will be carried safely outdoors by the
foam and detected immediately by a flammable gas analyzer.
The first step is to determine an appropriate concentration for
the foam cleaner by laboratory testing of deposit samples. Often,
straight hydrochloric acid is effective in dissolving the deposits,
but a formic/oxalic acid mixture may be selected because of its
lower aggressiveness to the Cu-Ni and steel materials. The cleaning
procedure begins by reducing the station's electrical load to
where one water-to-hydrogen cooler can be taken out of service;
typically, plants have four, and three can carry about 75% of the
load. Then the one heat exchanger is isolated, drained of service
water, and injected with the formic/oxalic acid foam. The entire
procedure takes about four hours for each cooler, with the foaminjection
stage taking only about one hour.
According to plant managers who have tried it, the procedure
is remarkably effective, reducing hydrogen temperatures in electric
generators by as much as 45F with no indications of hydrogen
leakage or excessive thermal stress on any of the cooler bundles.
In at least one case, generators that had been derated because of
temperature limitations were restored to full capacity. ■
-Dr. Robert Peltier, PE, is POWER's editor-in-chief.
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POWER May 2010

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