Low-Water Cutoff: A Maintenance Must
Professional technical writer, producer and host of The Amazing Science Emporium
Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN.
From the birth of the first commercial steam engine in 1699, operators respected a boiler's explosive power. Heat one cubic foot of water to boiling in a container, and it rapidly expands into 1,600 cubic feet of steam.
"That's a significant instantaneous release of a lot of energy . . . and it's got to go somewhere," understates John Cecilia, P.E. Mr. Cecilia is the marketing manager for ITT McDonnell & Miller, one of the foremost manufacturers of boiler controls worldwide.
In the old days - roughly the first two centuries steam engines were in use - critical safety functions were monitored and controlled manually. Someone sat all day, watching fire, steam pressure and water levels, then adjusted the fire, air and water as needed. Today, boilers boast automatic controls to perform many of these functions.
"[Automatic controls are] always there, always vigilant," Mr. Cecilia says. "The results with them are so much better than the results without them."
According to the National Board's 1995 Incident Report, more than 1,300 accidents reported last year resulted from low-water cutoff device failure. That's nearly double the second-place category of operator error/poor maintenance. Fortunately, no injury resulted from all those incidents involving low-water cutoff device failure.
Such statistics don't make automatic controls look like much of an improvement to the untrained eye. Behind the statistics, though, lies one important fact: the low-water cutoff device itself never causes a low-water condition in a boiler. The device's job is simply to prevent low-water condition once it detects liquid levels dropping. The cause of the water drop lies elsewhere.
The cause of low-water cutoff device failure usually lies elsewhere, too. Rarely does this controller malfunction because of design or manufacturing flaws. These are relatively simple mechanisms; their purpose is to shut off fuel to the fire when the water level drops below a safe operating condition. This is accomplished either mechanically or electronically.
"The mechanical cutoff device is similar to a toilet tank," explains Chris Thomson, ITT McDonnell & Miller's product engineering manager. "The position of a float opens and closes switches. In the electronic version, a probe is mounted into the boiler; it uses the electrical conductivity of the water itself" to determine safe water levels.
Both techniques merely detect a low-water condition caused by some other problem in the boiler. For that reason, inspectors must look beyond a cutoff device failure to find the root cause of a boiler's low-water condition.
"Low-water cutoff devices are like anti-lock brakes," Mr. Thomson says. "They help prevent accidents."
Yet, even with their simple designs, the devices need basic maintenance to properly prevent those accidents.
"The problem of low-water cutoff device failure is due to either lack of maintenance, incorrect maintenance or tweaking and doing things to them," explains Mr. Cecilia.
Mr. Thomson agrees. "In virtually all of the cases I'm involved with, when a low-water cutoff device is blamed, poor maintenance is the cause."
"It's not just a low-water cutoff device issue," adds Mr. Cecilia. "There's no such thing as a maintenance-free device invented by man. There's no getting away from that."
According to the 1995 Incident Report, more than two-thirds of the accidents resulting from the failure of low-water cutoff devices occurred in the Heating Boilers Steam category - a category generally associated with users such as apartment building owners.
Although it's the big, high-pressure industrial boilers that have the most damaging potential, industry tends to keep these units on rigorous and regular preventive maintenance schedules. Problems are caught before they lead to accidents.
Boiler use, then, is an inspector's first clue in looking for potential low-water cutoff device failures. According to ITT McDonnell & Miller's field experience, commercial use, as in apartment buildings and smaller office complexes, is statistically more likely to be problem-prone.
The reason for high incidence in this category is simple. Often, little if any maintenance is performed on this equipment between inspections. In some cases, inspections are not jurisdictionally mandated for the equipment at all or the inspections may be performed by unqualified persons.
"People don't want to deal with timely inspections," John Cecilia says. "It's like anything else; some people don't want to change the oil in their car, either."
"A lot of folks take the boiler issue in apartment buildings for granted. The boiler industry is a background business; the heat's there!" adds Mr. Thomson. "Then, when an accident does happen, they blame the boiler inspectors, saying 'This shouldn't happen; it was inspected three months ago.' But there's been no maintenance since then!" Where regulations do not require review, many boilers never receive any kind of attention.
"They're only treated when they're broken," Mr. Cecilia responds. "I've seen some situations where they've been untouched for 15 or 20 years. Think about a pump in your automobile; if you left it alone that long, what would it do?"
Maintenance is clearly another factor for inspectors to focus upon in their prevention efforts. Check the records of the unit under inspection (see table below). Focus on the frequency of blowdowns and other maintenance. As Mr. Cecilia puts it, simply look for a lack of attention.
Sediment is another warning sign. And observe the boiler's operation; it should run smoothly, without any components hanging up or operating erratically.
"Boiler inspectors also have their own procedures, which are very thorough," adds Mr. Thomson. "Depending on where the boiler fits into the code, some boilers require redundant controls. In general, codes are more strict on larger, higher pressure boilers."
It's important to remember, Mr. Cecilia points out, that "the whole idea of boiler inspection is often based on minimums. Keep in mind that more is better. Not just more inspections, but more maintenance."
Mr. Thomson offers plenty of advice inspectors can forward to boiler operators, too.
The single most important preventive measure they can take is a regular blowdown and cleaning. "For the smaller stuff," he says, do it "maybe on a weekly basis. Certainly on an annual basis would be a dismantling and inspection of the boiler and controls - the whole thing.
"It's a pretty foolproof guide. I've never heard of an incident where people who've followed all the procedures, followed all the codes, have had an accident."
Boiler owners should also try to find qualified, experienced operators and service providers. Some localities have no substantial regulatory process, allowing anyone to repair boiler controls.
"My 11-year-old nephew could legally make repairs!" Mr. Cecilia says.
Commercial operators, in particular, should also request more frequent inspections. "Many codes only require a once-a-year inspection," Mr. Thomson points out, "but a lot can happen in that time."
Finally, both engineers recommend operators follow their own common sense.
"We've had incidents where people just weren't using common sense . . . like stacking things on top of boilers," Mr. Thomson says.
Mr. Cecilia points to a common case. "A lot of codes have clearance requirements, for example. All these codes and all these rules have been developed over the years, sometimes after bad things have happened of one kind or another. If you follow the rules, everything's going to work fine."
For both Chris Thomson and John Cecilia, solving the high incidence of low-water cutoff device failures is really a matter of public education. They would like to see attitudes toward boilers come forward from an "in the background" status into the limelight of other public safety issues.
Such recognition is important because this equipment is found in nearly every building Americans occupy - from the water heaters in suburban basements to the power generators of industry. When these boilers and pressure vessels are not inspected and maintained properly, needless destruction can result.
Chris Thomson has even dreamed up an ad to kick off such a safety campaign. While the headline would blast a sense of danger into the reader, the ad would offer safety and security with one prime piece of advice:
Just do a little maintenance.
Preventing Low-Water Cutoff Device Failures
Despite their high incidence rate, low-water cutoff device failures are remarkably easy to prevent. Here are a few warning signs to which inspectors should be alert:
- Commercial use of a boiler, especially in an apartment/office setting
- Incomplete (or lack of any) maintenance record
- Physical signs of poor maintenance
- Very low blowdown frequency
- Sediment buildup
- Rough or erratic function in boiler components
Landlords and other commercial boiler owners want to avoid accidents in their facilities. Regularly following these prevention points will help them keep their boilers trouble-free:
- Weekly blowdown and cleaning
- Yearly dismantling and inspection of boilers
- More frequent inspections than local codes require
- Repairs performed only by qualified, experienced personnel
- Rigorous adherence to the manufacturer's maintenance schedule
- Attention to common sense
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.