POWER December 2020 - 30

OPERATIONS & MAINTENANCE
Getting Ahead of Costly Heat
Exchanger Failures in the Power
Generation Industry
Heat exchangers are important in all thermal power plants and in many other
applications. If not properly operated and maintained, a variety of failures can
occur, resulting in downtime, reduced efficiency, and higher costs. However, there
are a number of steps you can take to keep heat exchangers in top condition.
John Boyer and Jim Klimek
H
eat exchangers are used in many
critical processes to protect other
valuable manufacturing equipment,
optimize energy consumption,
and reduce associated operating costs.
A properly selected, installed, and maintained
heat exchanger can help enhance
the reliability and efficiency of a fluid
system.
However, although heat exchangers
provide high operating efficiency, they
are exposed to specific risks in power
plants, which can lead to costly downtime
if left untreated. There are three
common types of heat exchanger failures
that can occur:
■ Mechanical failures
■ Scale, mud, and algae fouling
■ Chemically induced corrosion
Mechanical Failures
Mechanical failures can take many different
forms including, but not limited to,
metal erosion, steam or water hammer,
and thermal expansion and cycling.
Metal Erosion. Excessive fluid velocity
on either the shell or tube side of
the heat exchanger can cause damaging
erosion, potentially removing the tube
material's protective film, exposing fresh
metal to corrosion. The areas most prone
to erosion are the U-bend of U-tube heat
exchangers (Figure 1) and tube entrances.
Tube entrance areas can experience
material loss when excessive high-velocity
fluid from a nozzle is divided into
much smaller streams as it enters the
heat exchanger. When excessive velocity
occurs at the entrance area of tubes, it
typically produces a horseshoe-shaped
erosion pattern.
The maximum recommended velocity
in the tubes and entrance nozzle depends
on many variables, including tube
30
1. A U-tube bundle is shown here being fabricated at a mechanical workshop. Source: Shutterstock
material, fluid handled, and temperature.
Materials such as steel, stainless steel,
and copper-nickel withstand higher tube
velocities than copper.
Erosion problems on the outside of
tubes can occur with impingement of
wet, high-velocity gases, such as steam.
Wet gas impingement is controlled by
oversizing inlet nozzles or by placing
impingement baffles in the inlet nozzle
projection area. Typical shell-side nozzle
velocity limits to prevent impingement
erosion on the outside of tubes can be
established with the following:
density x velocity x 2 ≤ 1,500 lbm/ft2-second
where density is entered in lbm/ft3, and
velocity is the shell nozzle velocity in ft/
second.
Steam or Water Hammering. Pressure
surges or shock waves caused by
the sudden and rapid acceleration or deceleration
of a liquid can cause steam or
water hammer. The resulting pressure
surges can reach 20,000 psi, which is
high enough to rupture or collapse the
tubing in a heat exchanger.
In a water/steam heating application,
damaging pressure surges can result
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in an interruption to the flow of cooling
water. The stagnant cooling water is
heated beyond its boiling point to generate
steam, and the resumption of the
flow causes a sudden condensing of
the steam, which produces a damaging
pressure surge or water hammer.
Therefore, cooling water flow should
always be started before heat is applied
to a heat exchanger. Modulating control
valves are also preferable to fluid flow
control valves, which open or close suddenly
and cause water hammer. If condensable
fluids are handled in either the
shell or tubes, vacuum breaker vents can
help prevent steam hammer damage resulting
from condensate accumulation.
The installation of properly sized
steam traps with return lines can help
eliminate steam hammer by preventing
condensation from accumulating in the
shell. Plant managers should also ensure
that the lines are pitched to a condensate
receiver or condensate return pump.
Thermal Expansion and Cycling. Accumulated
stresses associated with repeated
thermal cycling or expansion can
result in tube failure. This is more likely
to occur with a straight-tube fixed-tubesheet
design, where the tubing is fixed
POWER | December 2020

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POWER December 2020

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