Safety valve is a valve designed to open or close automatically to relieve excess pressure in the pressure vessel to which it is attached.
Spring loaded safety valve is a safety valve fitted with a spring which normally holds the valves disk in a closed position against the seat and allows it to open or close at pre-determined pressure. Spring loaded safety valves are characterized by pop action.
Pressure loaded pilot actuated safety valve is one which is held in the closed position by steam pressure and controlled in operation by a pilot actuator valve.
. Spring loaded, pilot actuated safety valve is one In which a spring is used in the conventional way to hold the disk against the seat, but which has a piston attached to the spindle and enclosed within a cylinder, which when subjected to a limiting or set pressure, unbalances the spring load thereby opening the valve.
Spring loaded pilot valve is a conventional safety valve designed to actuate another spring loaded safety valve through a pressure transmitting line led from the body of the pilot valve.
. Lever Safety valve is a safety valve having the disk held on its seat by the action of a weight hung on a lever which is pivoted on a fulcrum.
Relief valve is a valve constructed on the general design of safety valves to relieve excess pressure. It may be used for service where any special certifications not needed.
(a) Each boiler and its super heater shall be provided with one or more approved safety valves designed, constructed and flow tested in conformance. Boilers of more than 500 square feet of water heating surface but without super heaters shall be fitted with at least two safety valves, and boilers with integral super heaters shall have at least two safety valves fitted to the super heater outlet. On river steam vessels whose boilers are connected in batteries without means of shutting off one boiler from another, each and equipped with not less than two safet valves of equal size.
(b)Drum safety valves shall be set to relieve at a pressure not in excess of that allowed by the certificate of inspection. Where for any reason this is lower than the pressure for which the boiler was originally designed, an accumulation test, as provided for in paragraph (g) of this section, shall be conducted to insure that the relieving capacity of the safety valve is sufficient for the lower pressure.
(c) The setting of the super heater safety valve shall not exceed the design pressure of the machinery plus the pressure drop in the piping from the super heater outlet to the machinery. To prevent damage to the super heater, the drum safety valves shall be set at a pressure not less than that to the super heater safety valve setting plus 5 pounds minimum plus approximately the normal load pressure drop through intervening valves between the super heater safety valve and boiler, the relieving capacity of the super heater safety valve rated in accordance with the requirements. May be included in determining the total required relieving capacity of the boiler, provided the drum safety valves have at least 75 percent of the aggregate relieving capacity required. The foregoing requirements do not apply when drum pilot actuated super heater safety valves are used. Installations employing pilot actuated safety valves will be given special consideration by the commandant.
(d) In the event the maximum steam generating capacity of the boiler is increased by any means, the relieving capacity of the safety valves shall be checked by an inspector and if determined to be necessary, valves of increased relieving capacity shall be installed.
(e) The total rated relieving capacity of drum and super heater safety valves as certified by the valve manufacturer shall not be less than the maximum generating capacity of the boiler as certified by the boiler manufacturer. The safety valves shall be capable of discharging all the steam that can be generated without allowing the pressure to rise more than 6 percent above the set pressure of the valve.
(f) (1) The boiler manufacturer may certify the normal as well as the maximum generating capacity of the boiler, In the absence of such certification the minimum safety valve relieving capacity shall be determined upon the basis of the pounds of steam generated per hour per square foot of boiler heating surface and the water-wall heating surface, as given in the following table.
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