Safety Bulletin No.1009
Equipment Type: Hot Water Boilers
Hazard: Leaking systems can create dangerous conditions
But it's just a little drip...
It could be around a hand hole, or maybe somewhere in the system a valve packing is leaking.
The most common leak is a "weeping" relief valve. Many sites have relief valves dripping or leaking, but since they are piped to a drain, they don't worry much about it, and take the "out of site, out of mind" approach.
This is a problem in both hot water and steam boiler systems. Hot water heating systems are not designed to have much make-up water. These systems don't typically need or get regular water analysis. The loss from any leak must be made up somewhere, and often it is through the automatic fill valve. This make-up water is untreated and can be extremely corrosive to the boiler. The fresh water brings in oxygen and minerals that will end up wreaking havoc on your boiler over time.
The minerals will insulate tubes and make hot spots that can warp or fail. The oxygen can make for pitting and tube corrosion. The next thing you know your boiler that should be good for 20-30 years is ready to fail in five.
If a shortened boiler life is all that happens, that's not bad. Your failure may just be leaks and expensive tube repairs. These same mineral deposits can corrode and seize the relief valve shut. If the relief valve doesn't work when it's supposed to, you can have a catastrophic failure (i.e. explosion).
To prevent failure, you should:
- 1. Fix all hot water boiler system leaks in a timely manner, (big or small).
2. Conduct periodic water sampling, even from closed systems. Make sure that oxygen and contaminant levels don't indicate the presence of untreated water in your system.
3. Consider make-up water meters. We try to design make-up water meters into all of the automatic fill systems of hot water boilers. The meter needs to be read and logged. Unusual water use can mean a problem. Certain volumes used over time may mean treatment with oxygen scavengers and dispersants may be needed.
4. Safety relief valve testing. Relief valves must be tested regularly. It's code, it's law, and it's really important. Be careful to do it properly so the valve reseats and you don't get hurt. Be mindful of where the discharges are.
Even though they are very simple, when compared to steam boilers, hot water boilers and their unique issues also need proper care and respect. The accompanying information about a small water heater that killed six kids and took out a substantial part of a school in Oklahoma in 1992 that follows will explain how pressure vessels don't have to be big to hurt you.
Boiler and Water Heater Safety: Are You Properly Storing Your Explosives?Explosions from even simple equipment like water heaters can be destructive and/or deadly. The aftermath with its tragic consequences can serve as a warning to all of us involved in operating and maintaining equipment. Shedding light on the underlying causes behind the explosion an elementary school provides an opportunity to review our own testing, repair and preventative maintenance schedules.
There is a lot of learning that can take place by reviewing a terrible disaster that left six children and one teacher dead 20 years ago. A water heater explosion changed the town and a lot of families forever.
This happened in an elementary school on a normal day in 1982. It was shortly after noon, in a busy school cafeteria, in a small town in Oklahoma. Children were seated at tables, enjoying lunch when a concrete wall, which separated the lunchroom from the kitchen, blew in, as an 80-gallon water heater exploded, and launched itself skyward. The children seated nearest the wall were crushed and killed as concrete and steel were propelled from the epicenter of the blast. It was a horrific scene. In all, seven were killed and 36 lay injured.
Tragic warning signs at this school screamed out loudly to those trained to listen. Sadly, most building managers and facilities staff would never have heard them. For example, would you think that when people complain that the water is too hot in the sinks, it could be a sign that you are about to have an explosion in your building? What about that safety relief valve that keeps dripping, or the little gas leak? It was a combination of issues as subtle as these that contributed to the death of these children.
What Happened?The first employees arrived at the school at 7 a.m. on that fateful day. They included the cafeteria workers, who noticed that the domestic hot water was much hotter than normal. The custodian was called, and the gas water heater was shut down, to await the arrival of a technician. The technician's fix was to replace the gas valve and re-light the water heater. The technician returned within the hour and noted that the water heater seemed to be working normally.
The cafeteria workers soon noticed that the water temperature was again much too hot and getting hotter. They placed another call for service, which tragically went unanswered. At 12:13 p.m., the explosion ripped through the school. The chief boiler inspector at the time was on the scene 15 minutes after the catastrophe.
How Did This Happen?The inspector indicated that the problem water heater sat in disrepair for three or four years. The controls had been tampered with, the safety valve was in the wrong place, and the temperature probe had been removed. Oklahoma's boiler inspection law at the time covered high-pressure steam boilers, but not smaller equipment such as water heaters. This was not a situation unique to Oklahoma. Most states do not provide much in the way of inspections for certain classes of combustion equipment, even if it is in places of public assembly.
This meant that the school system itself was responsible for determining what would constitute adequate inspection, maintenance and repair of the water heater. This is an often-overlooked responsibility, as most facility managers do not understand what they are responsible for when it comes to combustion equipment.
Could This Have Been Prevented?This was an accident that could have been prevented if the proper procedures and inspections had been put in place. If there was a preventive maintenance schedule, it was ineffective. Critical safety items were installed incorrectly, disabled, and/or never tested. Controls were tampered with and sensing probes removed. New repair parts were not used and the condition of the used replacement gas valve was not known. After installation, the functioning of the gas valve may not have been adequately confirmed to ensure it was cycling properly. The technician doing the work may not have inspected the entire unit to make sure that the safety relief valve was installed correctly, let alone to verify its operation.
The full investigation identified additional causes, including a safety relief valve that had been altered. The safety relief valve was a pressure-temperature model. The temperature probe had been cut down to fit an elbow that was installed at the unit. This disabled the temperature relief capability of the unit.
The water heater's burner would not shut off. The safety relief valve, now altered, could not relieve the pressure. When this happens, the water in the tank starts storing lots of energy. As the temperature increases, the tank itself can start to tear. At failure, all of this water expands 1,700 times its volume in an instant. This creates a pressure pulse that blows out walls and moves anything in its path. A hundred gallons of water instantly flashing into steam is like setting off 10 sticks of dynamite.
The unit had also not been maintained properly, and the staff had not been properly trained in maintenance techniques. Most facilities do not have personnel properly trained in combustion equipment maintenance. Most sites also do not follow proper interlock and safety testing guidelines, even though laws mandate them. Boiler safety laws have been passed by a number of states to help this. Boiler inspections that call for combustion interlock testing to be carried out only exist in 26 states and in some additional specific municipalities. In these states, jurisdictional inspectors ask to see evidence of this gas train and safety interlock testing. However, it is beyond their work scope to do any of this testing.
In most other states, boiler inspection laws call for inspecting only the pressure-vessel part of each boiler system and not for looking at combustion issues at all. In many cases, facilities and maintenance managers think that a boiler inspection is a full and complete look at everything, when in fact it usually is not.
Boiler inspectors (often hired by insurance companies or employees of the state) have their hands tied when it comes to what they can ask someone to do. What they are inspecting for is often limited to exactly the letter of the law. For example, in many cases, they can only evaluate equipment for its code compliance when it was installed.
There's typically no screening for how far away from the most recent codes the old "grandfathered" technology is. This kind of inspection sometimes means that you could "technically" be in compliance using archaic and antiquated equipment that is 50 or more years old and has little in the way of modern safety features.
What Is Interlock Testing? Why Does It Matter?Burning fuels can be useful so long as it is a controlled process. Controlled means that combustion takes place where you want it, when you want it, and at the rate you want it. The complicated looking series of valves, wires and switches that comprise the gas train installed on gas-fired equipment is what attempts to do this.
Gas trains help to keep gas out of the combustion chamber when no combustion is taking place through a series of tight, specially designed shut-off valves that are spring-loaded to close. These are the safety shut-off and blocking valves. Larger gas trains require dual valves, and some also have a vent between these for added safety. The specific configuration that you have depends on your insurance and local code requirements. Gas trains also have a number of components that try to ensure that safe light-offs take place and that shutdowns occur immediately if anything goes wrong during the operation of the equipment. They do this with a series of pressure switches that look for too high or too low of gas pressures being sent to the burner. They typically also have switches to make sure that airflows are correct for purging residual combustibles prior to light-off and that airflow is correct during operation.
Flame-sensing components also usually exist to make sure that flames are present when they are supposed to be and not present at the wrong time.
Other components for sensing that the fuel valve is at low-fire position prior to light-off may be present along with furnace pressure switches, high-temperature limits, and/or water level cut-outs (depending on the type of equipment.)
All of these components are logically linked or interlocked to a burner management system controller, or BMS. The BMS is the brain that supervises and sequences all of the light-off efforts and sits and watches as the combustion processes take place. BMS systems manage the timing and adequacy of the purge prior to light-off and the time intervals allowed for getting pilots and main flames lit.
All of this equipment is required by law to be checked on a regular basis, but with maintenance budgets among the first to be cut, proper checkouts and testing are seldom performed. Codes and manufacturers define what these frequencies are for different types of equipment. Frequencies of required testing range from daily observation of flames (assuming you know what to look for), to annual checks for block and bleed valve tightness testing requirements. It is in this frequency area that we find many problems in facilities today.
Auditors at Combustion Safety, Inc. have evaluated thousands of pieces of combustion equipment. They usually find that few sites are aware of regular testing requirements specified by codes. In most cases they find that sites do some level of testing semi-annually or annually. The level of comprehensiveness varies depending on who is in charge and that person's knowledge of the equipment or systems. It is rarely what is required by law.
Operations and the human element are the biggest safety issue. The National Board of Pressure Vessel Inspectors statistics for boiler incidents from 1992 through 1998 show that 40% of all deaths, 37% of all injuries, and 31% of all accidents are caused by human error or poor maintenance.
Gas Explosions Can Be AvoidedNatural gas and combustion equipment safety continues to be a black art to most maintenance staffs and even engineers. Most sites have personnel that are not adequately trained in the safe start-up/shutdown of equipment, daily operations, or the proper testing and maintenance. A recent survey of users found that less than 10% actually perform manufacturer or code-recommended preventive maintenance, including testing of critically important safety interlocks. The combination of these two circumstances can spell disaster, and has in numerous facilities. When assessing your site's circumstances consider the following:
- 1. Most of the explosions and fire incidents, by far, are due to human error. All of the safeties and interlock equipment in the world will not help if you attempt to short-circuit or jumper-out safety controls. There is no possible substitute for proper training.
2. Start-up and shutdown are your biggest risks. You need good written and clear procedures so that everything is very simple and straightforward.
3. Make sure that you do regular and complete interlock testing. Jurisdictional inspectors cannot be at your facility every day. Combustion safety and testing needs to be part of your organization's culture.
It may take a lot of effort and changes in your site's culture to start testing and maintaining this equipment properly.
The bottom line is that implementing comprehensive combustion equipment safety programs will save lives.
Note: The state boiler inspector listed four main causes:
Lack of proper controls and safety devices,
Lack of proper maintenance,
Improperly trained maintenance personnel, and
Failure to inspect on a regular basis.