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Boiler
Safety ‘Hot Buttons’
Despite
technology advances, maintenance, inspection and
testing remain vital in ensuring boilers operate
efficiently and safely
— By
James Piper
Boilers
used to require nearly constant attention. Burners
required ongoing adjustments in response to
changes in loads, and operators had to constantly
monitor a range of operating parameters.
But times have changed. Improvements in boiler
design and the switch to digital control systems
have greatly improved boiler safety, reliability
and performance. Operating efficiency has
improved, and interruptions of service have been
all but eliminated. Today’s boilers are so
automated and reliable that they have become out
of sight and out of mind for many.
But being out of sight and out of mind has created
its own concerns and problems for maintenance and
engineering managers. It has helped create the
misconception that boilers do not need maintenance
attention.
In spite of all the improvements in boiler design
and construction, accidents still occur, and
boilers still fail. It is estimated that nearly
two-thirds of all boiler failures today result
from poor operating and maintenance practices. And
most unscheduled boiler shutdowns also result from
poor operating and maintenance practices.
A facility’s boilers represent a major
investment that must be protected. Service
interruptions disrupt building occupants and their
operations and can result in significant damage to
the building and its systems and contents. Even
more importantly, poor operating and maintenance
practices put everyone at risk. By paying close
attention to safety devices and maintenance
practices, maintenance and engineering personnel
can enhance boiler performance, reliability and
safety.
Critical
safety devices
The
safety relief valve is one of the most important
devices on any boiler. It is the boiler’s last
measure of protection against overpressure. It
must be adequately sized and of the correct
pressure rating for the boiler.
But getting a safe installation is only the
beginning. The safety relief valve also must be
inspected and tested regularly. Mud and scale from
the boiler can interfere with the operation of the
relief valve. Plugged discharge lines can prevent
proper operation or allow discharged water and
steam to come in contact with equipment or
operating personnel.
Lifting the test lever while the boiler is
operating will confirm its proper operation. At no
time should technicians test the valve by
increasing the pressure of the boiler to a level
higher than the safety-valve setting. They should
exercise caution when testing relief valves, as
steam or hot water will be discharged through the
valve at the operating pressure of the boiler.
Valves should be tested every time a boiler is
started and at the interval recommended by the
manufacturer.
Flame safeguard controls are designed to ensure
the proper lighting, operation and shutdown of a
boiler. Before a boiler can be started, the
control initiates a purge cycle to remove
potentially combustible gasses, reducing the
chances for an explosion. It verifies that there
is no flame before ignition and that the pilot or
ignition system is operating properly.
If any safety interlock is open, the flame
safeguard control prevents the boiler from
igniting. When the boiler is shut down, the
control also initiates a purge to eliminate
combustible gases from the boiler.
Proper operation of the flame safeguard control is
critical to the safe boiler operation. While the
control is reliable and designed to be fail-safe,
its failure can be catastrophic. Control operation
should be inspected at least weekly. Fuel shutoff
must be clean and complete.
Low-water cutoff controls are designed to prevent
boiler operation when there is insufficient water.
Operating a boiler under such conditions can
overheat steel components, causing them to warp or
rupture.
The controls are reliable, but they can fail. For
float-type controls, mud or scale can accumulate
in the float chamber, preventing it from properly
responding to changes in boiler water level.
Also, mechanical or electrical components in the
float assembly can fail, sending false low-water
alarms or failing to cut off the burner in the
event of low-water conditions. And probe-type
controls can fail due to the accumulation of
scale.
To prevent failure of the low-water cutoff
control, operators should blow down units
regularly to remove sludge and scale. At least
daily, operators should visually check the water
level in the boiler to confirm proper operation.
To ensure that the visual check of the boiler’s
water level is accurate, the water column must be
blown down regularly to remove accumulated sludge
and sediment.
Maintenance
Safety
devices prevent dangerous conditions from turning
into disasters, but only proper maintenance
practices prevent the development of dangerous
operating conditions in the first place. While
maintenance requirements vary by boiler size and
type, boiler age and condition, the quality of the
water, and the types of loads served by the
application, all boilers require common
maintenance activities.
One important activity required for long-term
performance and safety is a water-treatment
program. All water contains impurities that can
cause fouling of heat-transfer surfaces, spot
overheating of steel, corrosion, and damage to
other boiler components. Water-treatment programs
add chemicals to the boiler feedwater
to counteract the impact of these impurities, as
well as rust and other contaminants created within
the boiler and its distribution system.
When establishing a boiler water-treatment
program, it is best to work with someone who
specializes or has received special training in
water treatment programs. Programs must be
specifically tailored to match the quality of the
water being used in the boiler as well as
conditions found in the application. Programs also
must be capable of responding to changing
conditions with the boiler water source and in the
boiler system.
Another ongoing maintenance activity essential to
the performance and safety of a boiler is
regulating the quantity of boiler blowdown.
As makeup water is introduced into the boiler
system, it brings with it contaminants that can
precipitate onto heat-transfer surfaces, forming
scale that interferes with heat transfer, reduces
boiler efficiency and increases thermal stress.
Regularly bleeding off part of the water in the
boiler system keeps the level of contaminants
within limits. This blowdown
also helps maintain the concentration of chemicals
in the system at the desired level.
Technicians must tailor boiler blowdown
to boiler system conditions. The frequency and
quantity of blowdown
from the system depends on a number of site
specific factors, including the how much makeup
water must be added to the system, the quality of
that makeup water, and the rate of scale formation
within the system. Depending on the size of the
boiler, blowdown
systems might require inspection and adjustment as
often as once a day.
Another maintenance activity that technicians must
perform regularly is inspecting and cleaning both
the fire and water sides of the boiler to remove
soot and scale. They also should inspect
refractory materials for cracks and other signs of
deterioration and inspect burners for corrosion,
contaminants and wear.
Fuel and air linkages should be checked for proper
positioning and tension, and tubes should be
closely examined for leaks and cracks. Suspect
tubes should be examined using ultrasonic testing
to determine tube thickness.
Finally, all controls and safety devices should be
inspected, tested and calibrated, and the steam
drum should be cleaned and inspected.
For most applications, these activities should be
performed once a year.
Follow
the Light
One of the most commonly overlooked boiler safety
devices is the network of indicating lights and
alarms. Each indicator light and alarm is designed
to alert operators to specific conditions within
the boiler.
But to ensure their proper operation, each must be
tested. Bulbs do burn out, and alarms can become
disconnected, either accidentally or
intentionally. The result is that operators will
not receive an indication of a potentially unsafe
condition.
Most indicating lights and alarms are equipped
with a test button. At least once each week, more
frequently on larger installations, all lights and
alarms must be tested for proper operation.
—
James Piper
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