Each year, hundreds of accidents are reported nationally involving steam and hot water heating boilers in businesses, public buildings, and other facilities. The majority of these incidents are attributed to malfunctioning low water cutoffs, operator error, poor maintenance, and/or corrosion.
Hartford Steam Boiler has published a number of articles on general boiler maintenance over the years. This article contains updated information on critical safety and control devices, and recommendations to keep a boiler in proper working condition.
Properly functioning control or safety devices are absolutely essential for any boiler. The only way you can be confident they will work when called upon to do so is to perform required maintenance and testing on a regular basis.
What are these control and safety devices? Some of the more obvious ones are discussed here, with basic recommendations for testing and maintenance. These are not the only items on a boiler that contribute to its proper operation, but they are some of the primary ones.
This discussion of testing and maintenance procedures is not exhaustive — consult your boiler manufacturer, insurer, or state boiler authorities for more information on procedures and requirements.
Safety ValvesOften considered the primary safety feature on a boiler, the safety valve should really be thought of as thelastline of defense. If something goes wrong, the safety valve is designed to relieve all the pressure that can be generated within the boiler.
Although it is essential, a safety valve can also give you a false sense of security that encourages letting the testing and maintenance schedules slide. Keep in mind that the same conditions that make other safety devices malfunction can also affect the safety valve.
Every steam and hot water heating boiler must have at least one safety or safety relief valve of sufficient relieving capacity to meet or exceed the maximum burner output.
The ability of a safety valve to perform its intended function can be affected by several things, such as internal corrosion or restricted flow, which can prevent the valve from functioning as designed. Internal corrosion is probably the most common cause of “freezing” or binding of safety/relief valves. This condition is generally caused by slight leaking or “simmering” due to improper seating of the valve disk and is a condition that should be corrected without delay.
To ensure that a valve’s mechanism will operate properly, the try-lever should be lifted once a month and the valve set pressure tested annually. If a valve will not operate or does not reseat properly when tested, the boiler must be shut down immediately and the valve repaired or replaced.
The safety or safety relief valve must be set to open at or below the maximum allowable working pressure established by the manufacturer. This is the maximum pressure at which the designers have determined the boiler can be safely operated. The maximum allowable working pressure is listed on the required boiler nameplate or stamping.
It is not good practice to operate a boiler too close to the valve setting. Operating too close to the set pressure will cause these valves to leak slightly, resulting in an internal corrosion buildup that will eventually prevent the valve from operating.
Water Level Control, Low Water Fuel CutoffsThese devices perform two separate functions, but are often combined into a single unit. This method is economical, providing both a water level control function and the safety feature of a low water fuel cutoff device.
We recommend, however, that both steam and hot water boilers always have two separate devices — a primary and a secondary low water fuel cutoff. They should be attached to the boiler through separate openings to prevent a restriction in the connecting piping from disabling both devices. Many jurisdictions require two such devices on steam boilers.
Piping should be kept open and free of scale or sludge buildup at all times. Properly installed piping will use “cross tees,” so the piping can be easily cleaned and inspected. A simple indicator that trouble may be developing in piping connections may show up when the float chamber of the low water fuel cutoff is flushed out or drained.
The water level should quickly return to normal in the gauge glass when the drain valve is closed. A slow return is a good indication that the connecting piping to the boiler is being restricted.
The most common water level control and low water fuel cutoff devices consist of two main components, a float chamber and an electrical switch operated by a float in the float chamber. A malfunction in either prevents the cutoff device from operating. Malfunctions in the float chamber generally result from neglect. Tampering and age most often cause those in the switch and associated wiring.
As the water level in the boiler drops, there is a corresponding drop in the float. When the float reaches a preset position, it activates an electrical switch that shuts off the burner. Sludge and sediment accumulate in the bottom of the float chamber, and, if not regularly flushed out, will build up, preventing the float from dropping down to the shut off level.
Note that flushing the float chamber should not be considered as a test of the low water cutout.
Be Careful When TestingLow water fuel cutoffs should be checked periodically for proper operation during the period when the boiler is operating. Since this test requires lowering the boiler water to the minimum safe operating level, qualified personnel should use extreme caution.
Never allow the water level to drop out of sight in the water gauge glass. This test should be done daily for steam boilers operating at more than 15 psig and weekly for those operating at less than 15 psig. In addition, a slow drain test should be done semi-annually on steam boilers operating at more than 15 psig.
In addition to these periodic tests of the low water device, the float chamber on the water level control and/or the low water fuel cutoff should be thoroughly flushed to remove any accumulated sediment.
At least once a year, water level controls and low water fuel cutoff devices should be disassembled, cleaned, and checked. These devices are an important part of boiler safety. Unless you are thoroughly familiar with them, have an experienced technician perform this type of maintenance.
The electrical switches and wiring are generally quite reliable and require little ongoing maintenance. At least once a year, the switches should be cleaned and any dust or dirt removed. The covers should be kept tightly in place except when opened for cleaning. If used and maintained properly, these switches are virtually trouble free.
However, if abused they can be a prime cause of boiler accidents. During the annual cleaning, the wiring should be examined for signs that insulation is cracking. All connections should be tight.
Don’t Bypass The SwitchesIt is not unusual for a maintenance worker to remove the cover and install a “jumper” wire to prevent the switch from operating. This starts out as a temporary convenience, often to “fix” a boiler that keeps shutting off on low water while being operated at high demand or as a temporary means to test other circuits in the control system.
This bypass can easily become a permanent and dangerous condition. A boiler that regularly shuts down indicates a very serious problem that could lead to a catastrophic accident.
A jumper wire should never be permanently installed in a low water device. Only a qualified technician should use a jumper to test another circuit.
The Fuel SystemThe fuel system, particularly the burner, requires periodic cleaning and routine maintenance. Failure to maintain the equipment in good working order could result in higher fuel costs, the loss of heat transfer, or even a furnace explosion.
Modern fuel systems are very complex assemblies, consisting of both electronic and mechanical components. Over a period of time, many things may go wrong — ignition transformers deteriorate or fail, ignition electrodes burn and become coated, fuel strainers and burner equipment become clogged, fuel valves become dirty and leak, air/fuel ratios drift out of adjustment, flame scanners become dirty.
Many users wisely contract with their gas company or oil service company to periodically check and maintain their burner equipment.
Properly maintained equipment should be safe and reliable, but devices installed to ensure safe operation are sometimes viewed as an inconvenience. The personnel who operate the boiler may tamper with or adjust these devices, thereby compromising operation of the boiler.
The safety feature most often adjusted is the burner purge cycle, designed to prevent furnace explosions caused by a buildup of unburned fuel in the furnace chamber.
The cycle length is determined by the equipment manufacturer to purge fuel from a leaking fuel valve or an unsuccessful ignition sequence.
It is annoying to have a boiler fail to ignite and then wait for the burner to go through another complete purge cycle. You may be tempted to shorten or even bypass the cycle. Don’t! Doing so greatly increases the chances of a serious furnace explosion.
Clean And Maintain The Water Gauge GlassThe importance of proper cleaning and maintenance of the water gauge glass, or sight glass, cannot be stressed enough.
The water gauge glass on a steam boiler enables the operator to visually observe and verify the actual water level in the boiler. If not properly cleaned and maintained, however, a gauge glass can seem to show there is sufficient water, when the boiler is actually operating in a low water condition.
A stain or coating can develop on the inside of the glass where it is in contact with boiling water. After a time, this stain gives the appearance of water in the boiler, especially when the glass is completely full or empty of water.
Another problem that can be the indirect cause of accidents is for the connection lines to the gauge glass to become clogged and show normal water levels when water may be low. The piping connecting the gauge glass to the boiler should be cleaned and inspected regularly to ensure it remains clear.
One final problem should be mentioned. Often, a boiler is operated with the isolation valves to the gauge glass closed because the glass has been broken, or is leaking. Take the time to replace the glass, even if the boiler must be shut down. That inconvenience is nothing compared to the damage that may result from operating a boiler without a gauge glass.
Some operators routinely replace the glass and seals during annual maintenance because it is so important to verify the actual water level.
Stack Temperature GaugeA stack temperature gauge is normally installed on a boiler to indicate the temperature of the flue gas leaving the boiler. A high stack temperature indicates that the tubes may be getting a buildup of soot or scale.
Also, the baffling inside the boiler may have deteriorated or burned through, allowing gases to bypass heat transfer surfaces in the boiler.
These conditions generally develop slowly over a long period of time, slow enough so the person who operates the boiler can become accustomed to the gradually rising temperature. Approximately 1 percent in boiler thermal efficiency is lost for a 40 degree F increase in stack temperature.
Boiler Logs Are ImportantThe majority of boiler accidents can be prevented. One of the most effective tools is the proper use of operating and maintenance logs. Boiler logs are the best method to ensure a boiler is receiving the required attention and provide a continuous record of the boiler’s operation, maintenance, and testing.
Because a boiler’s operating conditions change slowly over time, a log is the best way to detect significant changes that may otherwise go unnoticed.
If a boiler is to be kept in good operating condition, someone who tends to the boiler must be responsible for its operation and maintenance. This person should have a good understanding of boiler operation and safety devices. Maintenance and testing should be performed and recorded in the log on a regularly scheduled basis.
The responsible individual should initial the log to verify each operation performed, who performed it, and when it was done.
Brian W. Moore, P.E., is a principal engineer and boiler specialist for the Hartford Steam Boiler Inspection and Insurance Company. He earned B.S. and M.S. degrees in mechanical engineering from the Massachusetts Institute of Technology, and an M.B.A from the University of Connecticut. Moore has more than 25 years of experience in the boiler business.
Publication date: 09/15/2003