Pressure-relieving devices

1. Safety valve requirements for steam boilers are: a. Each steam boiler must have one or more American Society of Mechanical Engineers approved safety valves of the spring-pop type adjusted and sealed to discharge at a pressure not to exceed fifteen pounds per square inch [103 kilopascals]. Seals must be attached in a manner to prevent the valve from being taken apart without breaking the seal. The safety valves must be arranged so that they cannot be reset to relieve at a higher pressure than the maximum allowable working pressure of the boiler. For iron-and-steel-bodied valves exceeding two inch [50.8 millimeter] pipe size, the drain hole or holes must be tapped not less than three-eighths inch [9.53 millimeter] pipe size. b. Each safety valve three-quarter inch [10.05 millimeters] diameter or over, used on a steam boiler, must have a substantial device that will positively lift the disk from its seat at least one-sixteenth inch [1.59 millimeters] when there is no pressure in the boiler. The seats and disks must be of suitable material to resist corrosion. c. A safety valve for a steam boiler may not be smaller than three-quarter inch [19.05 millimeters] unless the boiler and radiating surfaces consist of a self-contained unit. A safety valve may not be larger than four and one-half inches [114.3 millimeters]. The inlet opening must have an inside diameter equal to, or greater than, the seat diameter. d. The minimum relieving capacity of valve or valves is governed by the capacity marking on the boiler. e. The minimum valve capacity in pounds per hour is the greater of that determined by dividing the maximum British thermal units output at the boiler nozzle obtained by the firing of any fuel for which the unit is installed by one thousand, or is determined on the basis of the pounds of steam generated per hour per square foot of boiler heating surface. (One British thermal unit equals 1.055 x 10 to the third power joules.) MINIMUM POUNDS OF STEAM PER HOUR PER SQUARE FOOT OF HEATING SURFACE Boiler Heating Surface Firetube Boilers Watertube Boilers Hand-fired 5 6 Stoker-fired 7 8 Oil, gas, or pulverized fuel-fired 8 10 f. Safety valves must be installed with the valve spindle in the vertical position. Discharge piping, to a safe location, may be required by the inspector. 2. When a boiler is fired only by a gas having a heat value not in excess of two hundred British thermal units per cubic feet [745.58 x 10 to the fourth power joules per cubic meter], the minimum safety valve or safety relief valve relieving capacity may be based on the values given for hand-fired boilers above. 3. The safety valve or safety relief valve relieving capacity for electric boilers is three and one-half pounds [3692.5 joules] per hour per kilowatt input. a. The safety valve capacity for each steam boiler must be such that with the fuel-burning equipment installed and operated at maximum capacity the pressure cannot rise more than five pounds per square inch [34.47 kilopascals] above the maximum allowable working pressure. b. When operating conditions are changed, or additional boiler heating surface is installed, the valve capacity must be increased, if necessary, to meet the new conditions, the additional valves required, on account of changed conditions, may be installed on the outlet piping provided there is no intervening valve. 4. Safety relief valve requirements for hot water boilers are: a. Each hot water heating boiler must have at least one American society of mechanical engineers approved pressure relief valve set to relieve at or below the maximum allowable working pressure of the boiler. Each hot water supply boiler must have at least one officially rated safety relief valve or at least one American society of mechanical engineers approved pressure-temperature relief valve of the automatic-reseating type set to relieve at or below the maximum allowable working pressure of the boiler. Pressure relief valves officially rated as to capacity must have pop action when tested by steam. When more than one safety relief valve is used on either hot water heating or hot water supply boilers, the additional valve or valves must be officially rated and may be set within a range not to exceed six pounds per square inch [41.47 kilopascals] above the maximum allowable working pressure of the boiler up to and including sixty pounds per square inch [413.69 kilopascals] and ten percent for those having a maximum allowable working pressure exceeding sixty pounds per square inch [413.69 kilopascals]. Safety relief valves must be spring loaded without disk guides on the pressure side of the valve. Safety relief valves must be arranged so that they cannot be reset to relieve at a higher pressure than the maximum permitted by this subdivision. b. Each safety relief valve must have a substantial device that will positively lift the disk from its seat at least one-sixteenth inch [1.59 millimeters] when there is no pressure on the boiler. c. Materials subject to deterioration or vulcanization when subject to saturated steam temperature corresponding to capacity test pressure may not be used for any part. d. A safety relief valve may not be smaller than three-quarter inch [19.05 millimeters] nor larger than four and one-half inch [114.3 millimeter] standard pipe size, except that boilers having a heat input not greater than fifteen thousand British thermal units per hour [15.38 x 10 to the seventh power joules] may be equipped with a rated safety relief valve of one-half inch [12.7 millimeter] standard pipe size. The inlet opening must have an inside diameter approximately equal to, or greater than, the seat diameter. The minimum opening through any part of the valve may not be less than one-quarter inch [6.35 millimeters] diameter or its equivalent area. e. The required steam-relieving capacity, in pounds per hour, of the pressure-relieving device or devices on a boiler must be determined by dividing the maximum output in British thermal units at the boiler nozzle obtained by the firing of any fuel for which the unit is designed by one thousand or by multiplying the square feet of heating surface by five. In every case, the requirements of subdivision g must be met. (One British thermal unit equals 1.055 x 10 to the third power joules.) f. When operating conditions are changed, or additional boiler heating surface is installed, the valve capacity must be increased, if necessary, to meet the new conditions and be in accordance with subdivision g. The additional valves required, on account of changed conditions, may be installed on the outlet piping provided there is no intervening valve. g. Safety relief valve capacity for each boiler must be such that with maximum heat input the pressure cannot rise more than six pounds per square inch [41.37 kilopascals] above the maximum allowable working pressure for pressures up to and including sixty pounds per square inch [413.69 kilopascals] and ten percent for maximum allowable working pressures over sixty pounds per square inch [413.69 kilopascals]. h. Safety relief valves must be installed with the spindle in the vertical position. Discharge piping, to a safe location, must be installed.