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Boiler Safety ‘Hot Buttons’
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.
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
A number of organizations have been established to regulate boiler operation and assist boiler operators in achieving a safe operation:
Also, nearly every state has regulatory agencies that oversee the operation of boilers.