NAC455C.228. Power boilers: Safety valves.

     1. The use of weighted-lever safety valves or safety valves having the seat or disc of cast iron is prohibited. Valves of this type or construction must be replaced by direct spring-loaded, pop-type valves that conform to the requirements of section I of the ASME Boiler and Pressure Vessel Code, as adopted by reference in NAC 455C.108.

     2. Each boiler must have at least one safety valve certified by the American Society of Mechanical Engineers or the National Board and, if it has more than 500 square feet of water-heating surface or an input of electric power of more than 1,100 kilowatts, it must have two or more such safety valves.

     3. The valve must be connected to the boiler independent of any other connection and attached as close as possible to the boiler, without unnecessary intervening pipes or fittings.

     4. A valve of any description must not be placed between the safety valve and the boiler, or on the discharge pipe between the safety valve discharge and the atmosphere. A discharge pipe must be at least the full size of the discharge of the safety valve and fitted with an open drain to prevent water from lodging in the upper part of the safety valve or discharge pipe. If an elbow is placed on a safety valve or discharge pipe, it must be located close to the outlet of the safety valve or discharge pipe and must be anchored and supported securely. All safety discharges must be so located or piped as to be carried clear of walkways or platforms.

     5. The capacity of the safety valve of each boiler must be such that the safety valve will discharge all the steam which can be generated by the boiler without allowing the pressure to which any valve is set to rise more than 6 percent above the working pressure if the steam is discharged or 6 percent above the maximum allowable working pressure of the boiler, whichever is less.

     6. One or more safety valves on every boiler must be set at or below the maximum allowable working pressure. The remaining valves may be set within a range of 3 percent above the maximum allowable working pressure, but the range of the setting of all the safety valves on a boiler may not exceed 10 percent of the highest pressure to which any valve is set.

     7. If two or more boilers operating at different pressures and settings of the safety valve are interconnected, the lower pressure boilers or interconnected piping must be equipped with safety valves of sufficient capacity to prevent overpressure, considering the maximum generating capacity of all boilers.

     8. In those cases where the boiler is supplied with feed water directly from water mains without the use of feeding apparatus other than return traps, a safety valve must not be set at a pressure greater than 94 percent of the lowest pressure obtained in the supply main that feeds the boiler.

     9. The relieving capacity of the safety valves on any boiler must be checked by one of the following methods, and if found to be insufficient, additional valves must be provided:

     (a) By making an accumulation test, which consists of shutting off all other steam discharge outlets from the boiler and forcing the fires to the maximum. The safety valve capacity must be sufficient to discharge all the steam that can be generated by the boiler without allowing the pressure to rise more than 6 percent above the highest pressure at which any valve is set and in no case to rise more than 6 percent above the maximum allowable working pressure of the boiler. This method must not be used on a boiler with a superheater or reheater or on a high-pressure, high-temperature water boiler.

     (b) By measuring the maximum amount of fuel that can be burned and computing the corresponding capacity for evaporation or generation of steam upon the basis of the heating value of this fuel. These computations must be made as set forth in the appendix of section I of the ASME Boiler and Pressure Vessel Code, as adopted by reference in NAC 455C.108.

     (c) By measuring the maximum amount of feed water that can be evaporated.

     10. If either of the methods outlined in paragraph (b) or (c) of subsection 9 is employed, the sum of the safety valve capacities must be equal to or greater than the maximum evaporative capacity or the maximum steam generating capacity of the boiler.

     11. The capacity rating of a:

     (a) Safety valve must be expressed in pounds per hour.

     (b) Relief valve must be expressed in British thermal units per hour.

     (Added to NAC by Div. of Industrial Relations by R101-02, eff. 12-15-2004)