Article and photos by Gregory Havel
Pressure vessels–including boilers, water heaters, compressed air tanks, and water pressure tanks–operate under internal pressure greater than the atmospheric pressure, and are required by codes to be equipped with pressure relief valves, also known as safety valves.
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The pressure relief valve in Photo 1 is on a hot water heating boiler. A properly installed pressure relief valve will be connected at the top of the boiler in an upright position (unless approved for use in other positions), using pipe and fittings of the same size as the inlet of the relief valve. No other valves are permitted between the relief valve and the boiler. The pressure relief valve on a steam boiler is similar. Boilers with more than 500 square feet (46.452 m2) of heating surface may be required to have more than one pressure relief valve.
Pressure relief valves for boilers are rated on their capacity to relieve pressure, based on the maximum allowable working pressure of the boiler and on the rated heat input of the boiler in British thermal units (BTUs) or kilojoules (1 BTU = 1.055 kilojoules). The calculations account for the expansion of water when heated and for the even greater expansion when it converts into steam. If the pressure inside the boiler rises above the set pressure of the relief valve, the relief valve will open, reduce the pressure, and close again.
The relief valve setting must be at or below the maximum allowable working pressure of the boiler. Its relieving capacity must be equal to or greater than the boiler’s maximum rated heat input. The relief valve in Photo 1 is set to open at 30 pounds per square inch (psi) (2.109 kg/cm2), and is installed on a boiler with a maximum allowable working pressure of 30 psi. The relief valve in Photo 1 is rated at 1 million BTU per hour, whereas the boiler and burner are rated at 900,000 BTU per hour.
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Water heaters require a temperature-pressure relief valve (Photo 2). This is similar to the pressure relief valve on a boiler, except that it also has a temperature probe that extends several inches into the water heater through the inlet connection on the valve. In addition to opening to reduce high pressure in the water heater, these relief valves will open if the temperature in the water heater rises above 210º F (99º C) because of a failure of the operating controls.
The temperature-pressure relief valve in Photo 2 is on a 50-gallon residential water heater with a pilot-light natural gas burner rated at 190,000 BTU per hour and a maximum working pressure of 160 psi. The relief valve is set to open at 150 psi (10.546 kg/cm2) or 210º F (99º C), and is designed for a maximum heat input of 205,000 BTU per hour.
Pressure and temperature-pressure relief valves for residential and most commercial applications are calibrated, set, and sealed at the factory. They are not adjustable. They are required to have a permanently attached tag (usually metal) stating the manufacturer’s name, size, model number, settings, capacity, and the standards with which the valve complies.
Relief valves have a manual-operation lever at the top, so that they can be tested as recommended by the manufacturer and as required by fire codes and mechanical standards. Testing is required to ensure that the relief valve functions properly and has not become corroded or obstructed by mineral deposits. If a relief valve leaks after a test and will not seat properly after two or three attempts, replace it. The test intervals are usually quarterly, monthly, or weekly, depending on the type of valve, its application, and code requirements. The boiler or water heater operator’s manual will show the proper test interval and procedure, as will the manufacturer’s tag on the relief valve.
The discharge from a relief valve must be piped to a safe location near the floor, preferably near a drain or catch basin that will not be damaged by hot water. Valves are not permitted between the relief valve and the discharge end of the pipe connected to it.
Pressure relief valves for compressed air tanks and domestic water tanks are similar to the pressure relief valves for boilers. Relief valves for compressed air tanks are rated for an operating pressure equal to or less than the rated working pressure of the air tank and, in cubic feet (cubic meters) per-minute capacity, equal to or greater than the air compressor. Relief valves for domestic water tanks are rated for an operating pressure greater than the water system’s operating pressure and less than the rated working pressure of the tanks and, in gallons (liters) per minute, equal to or greater than the rating of the water pump.
Firefighters need to be concerned about the safe operation of relief valves. Under normal conditions, a properly functioning relief valve will prevent injuries to anyone in the area if the appliance malfunctions. However, the heat from a room-and-contents or structure fire will produce many times more heat than the burner in the water heater or boiler. This, in turn, will produce steam and hot water (or heating of compressed air with a corresponding increase in air pressure) faster than it can be discharged by the relief valve and may cause the failure or explosion of the boiler or water or air storage tank.
This is the reason for the requirement in National Fire Protection Association (NFPA) 1403,
Standard on Live Fire Training Evolutions, 2007 Edition, Chapter 4 “Acquired Structures.” Section 4.2.9 calls for closed containers (like boilers, tanks, and water heaters) to be drained and removed from the structure or vented with an inert atmosphere and left in place. An acceptable method of venting a closed container is to remove pressure from the boiler, tank, or water heater; drain it; remove the pressure relief valve; disconnect all piping including the fuel pipe; and leave it in place, empty, and with the many vent openings provided by disconnected piping.
Standard on Live Fire Training Evolutions, 2007 Edition, Chapter 4 “Acquired Structures.” Section 4.2.9 calls for closed containers (like boilers, tanks, and water heaters) to be drained and removed from the structure or vented with an inert atmosphere and left in place. An acceptable method of venting a closed container is to remove pressure from the boiler, tank, or water heater; drain it; remove the pressure relief valve; disconnect all piping including the fuel pipe; and leave it in place, empty, and with the many vent openings provided by disconnected piping.
For more information, search the Internet for “safety valve,” “pressure relief valve,” and “temperature-pressure relief valve,” and review the following:
• NFPA 54,
National Fuel Gas Code, 2009 Edition (ANSI Z223.1—2009), especially section 10.3.6
National Fuel Gas Code, 2009 Edition (ANSI Z223.1—2009), especially section 10.3.6
• American Society of Mechanical Engineers (ASME)
Boiler and Pressure Vessel Code.
Boiler and Pressure Vessel Code.
• NFPA 1403,
Standard on Live Fire Training Evolutions, 2007 Edition.
Standard on Live Fire Training Evolutions, 2007 Edition.
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Subjects: Building construction for firefighters, safety valves on tanks, relief valves
