A Boiler: The Explosive Potential of a Bomb
This article by the late William Axtman was originally published in the winter 1996 National Board BULLETIN. Some 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 and addenda of the National Board Inspection Code for current requirements.
Over the years, various estimates of the potential energy of a boiler or pressure-vessel explosion have been presented, with the intent of demonstrating the danger of this equipment if not properly maintained. Fire and rescue squads have entirely too much familiarity with the explosive potential of even the small pressure vessel in everyone’s home or apartment, the hot-water heater – a device that sometimes is under the regulatory jurisdiction of no organization whatsoever.
It is because hot-water heaters are usually so reliable that many consider them benign. But consider this: If you could capture all the energy released when a 30-gallon home hot-water tank flashes into explosive failure at 332°F, you would have enough force to send the average car (weighing 2,500 pounds) to a height of nearly 125 feet – or more than the height of a 14-story apartment building – starting with a lift-off velocity of 85 miles per hour!
When this amount of explosive force is unleashed in a basement or utility closet, the shock wave created by the explosion carries dirt, debris, ceiling, pipe, darkness, and burning temperatures in all directions with tremendous force. The result is devastating damage to anything in the vicinity. Whole houses can be leveled. Nearby cars can be tossed around like playthings. Furniture, even several rooms from the point of the explosion, will become airborne. People in the same building or on a nearby sidewalk or parking lot can be injured or killed. All of this is due to the power unleashed in a shock wave of energy as the pressure vessel explodes. In a fraction of a second, as hot water snaps to steam, everything turns to darkness and destruction.
Fortunately, such disasters are rare, considering the huge number of boilers and pressure vessels in use worldwide. So the general public only hears about the occasional explosive disaster associated with a pressure-vessel failure. However, the explosive potential is ever present and very real, especially if regular maintenance and repair are ignored.
Standard physics laws of temperature and pressure are used to measure the explosive potential of boilers and pressure vessels. New calculations show, for instance, that if liberated in a rupture, a 30-gallon hot-water tank at 90 psig (or 104.7 psia) has approximately 314,095 foot-pounds of energy to flash its water into steam at 332°F. Translate that force potential into real-world terms and you have the example of the car being rocketed into the air as noted above. Another way to look at this 300,000 foot-pound figure is that it equals the explosive force of 0.16 pounds of nitroglycerin.
Yet another approach to examine the potential explosive force is the flashpoint conversion factor for water to steam. Here the increase in volume is a factor of approximately 1,600 to 1. So when water or a similar dense liquid that normally needs one cubic foot for containment suddenly needs 1,600 cubic feet, explosion occurs. That’s like taking the volume of an S-gallon bathroom trash can and causing it to fill a 12- by 11-foot living room with an 8-foot ceiling in a split second. Such a dose of explosive power propagates the shock wave that inevitably does most of the damage to property and humans.
Several operational situations can bring on the violent, deafening blasts that result when a buildup of pressure is suddenly released. A common cause of explosion is the runaway firing condition that takes place when the boiler’s burner does not shut off. In this case, when the hot-water heater is connected to the city water supply on one side and the building hot-water service on the other, the check valve will shut off and no back pressure will go into the city side. Instead, the tank will take the pressure. As a result, the temperature in the tank will rise. And this temperature rise will compromise the tank structure, weakening the metal around the bottom of the tank and causing a sudden failure.
Of course, a properly functioning pressure and temperature relief valve would prevent such a failure. But a properly installed and maintained safety valve is not a universal feature of pressurized vessels.
Another type of explosion is caused by superheated liquid. Here the resulting vapor explosion occurs when a liquid transforms quickly into a gas, causing a rapid pressure increase. Actually, a typical malfunction can result when normally benign, saturated steam is forced out of its temperature/pressure equilibrium by a combination of valves shutting and extra energy being introduced into the system. These conditions disrupt the temperature/pressure balance, causing the fluid (steam) system to seek out a new equilibrium.
When this occurs in a closed vessel, as the liquid accidentally heats above its boiling point, the frantic and almost instantaneous physics of the system trying to re-establish equilibrium can result in explosion. Often, part of the vessel breaks away because the liquid is vaporizing to gas so rapidly. The rarefaction wave moves within the vessel and the resulting compression wave violently forces the liquid particles out through the vessel breach. This sequence of events spells split-second disaster.
Firefighters and other well-trained emergency personnel follow important protocols for identifying flows of superheated material. They are trained to assume a mine-field-type situation exists if they hear a leak. Everyone stops moving immediately. Someone at a safe peripheral location at the incident scene cautiously holds up a broom handle or 2- by 4-inch board and waves it across the breach where the invisible but searing steam might be. If present, the steam will literally cut the wood. If a human hand were to reach up unsuspectingly, trying to detect the steam, it too, could be cut by superheated steam! Instead, the length of wood indicates the area of the leak – or, preferably, the clear path around the leak – and the personnel can be shown a path to safety.
Preventing catastrophic failure of any boiler or pressure vessel has always been a concern of the National Board. Boilers in dry cleaning operations, pressurized steam cleaners in jewelry shops, air compressors used by painters, pressurized air systems used with power hydraulic lifts in auto repair shops, even instrument sterilizers in dentists’ offices – all are capable of causing injury due to explosion. On one hand, they rarely are inspected for proper mechanical performance and structural integrity. On the other hand, their operators are occasionally guilty of rigging the wiring or controls intended to prevent potentially explosive operational problems. And as a complicating factor, each state or province has its own inspection rules, and most apply only to commercial operations of a certain size or to water heaters or steam heat systems in dwellings that hold four or more housing units.
The National Board is conspicuously aware of the explosion hazard of boilers and pressure vessels. For most of this century, the public, law enforcement agencies, emergency response teams, and industry standards groups have looked to the National Board for assistance in setting guidelines for boiler safety. One of those areas of concern is the actual force generated by explosive failure – and how best to increase public awareness and safety practices to control this.