POWER March 2016 - 47

FUNDAMENTALS
Monitoring and Treatment of
Closed-Loop Cooling Water
Systems
In part because they are closed-loop systems, it can be easy to ignore power
plant cooling water systems that support the reliable functioning of everything
from stator bars in the generator to critical pump bearings for
feed pumps and heat exchangers on air compressors. Proper monitoring
and maintenance of these water systems can help you avoid more-costly
repairs to the mechanical systems they cool.
David Daniels
T
here may be multiple closed-loop
cooling systems at your power plant.
Chances are good that they cool or
control temperature on some very critical
components. The two that are most likely to
exist are the so-called bearing cooling water
system (which takes care of more than just
bearings) and the stator cooling system, for
those plants that have a water-cooled stator.
Closed-loop cooling systems can also be
found in air coolers on the intakes of combustion
turbines.
By its very nature, when a closed-loop
system remains closed and operates properly
for an extended period of time, it is often forgotten-or
at least neglected. Small changes
in the chemistry or the flow rates and differential
pressures throughout the system may
not be noticed. However, once corrosion processes
get a foothold in these systems, it can
be very difficult to correct them. In the meantime,
critical data equipment may be damaged
to the point where it affects the ability
of the plant to operate.
We begin with some general principles
and practices for closed-loop cooling water
systems before looking at the stator cooling
water system, which is a special case.
Understanding Closed-Loop Cooling
Systems
Most power plants using closed-loop water
cooling for mechanical systems (rather than
for the steam cycle) have several subsystems.
The bearing cooling water system generally
provides cooling for critical pump bearings and
seals, hydrogen coolers for the generator, lube
oil, and air compressor coolers. Other closedloop
cooling systems can include chilled water
systems for air chillers used at the air inlet to the
gas turbines at a combined cycle power plant
and the chemistry sample panel.
March 2016 | POWER
www.powermag.com
47
A closed-loop cooling system can exchange
heat with the main cooling water
system in conventional tube and shell heat
exchangers or plate and frame heat exchangers.
Chilled water systems (air chillers) exchange
heat with the compressor, which in
turn uses a cooling tower to throw heat back
into the environment.
Generally, demineralized water is used for
closed-loop cooling water makeup, but chemical
treatments are required to prevent corrosion
and, in some systems, freezing. Most commonly,
the piping in a closed-loop system is
carbon steel. Heat exchange surfaces, such as
air chiller assemblies, may be copper or even
aluminum. Plate and frame heat exchangers
are often made of stainless steel plates. Care
and keeping of these systems requires that you
pay attention to all the metals.
In a closed-loop system, oxygen pitting is
the most common type of corrosion (Figure
1). Symptoms of oxygen pitting may be rusty
water or recurring maintenance on bearings
due to the abrasion caused by the corrosion
products against the seal surfaces.
In order for oxygen pitting to occur, there
must first be a deposit that covers a portion
of the metal surface, creating a differential
between the oxygen content underneath the
deposit and the oxygen content in the bulk
water. The oxygen-deficient area underneath
the deposit becomes the anode, and the area
1. Oxygen pitting in a closed-loop cooling water system. Courtesy: M&M
Engineering
http://www.powermag.com
POWER March 2016

Table of Contents for the Digital Edition of POWER March 2016
