Specialty Item Safety & Safety-Relief Valves
Safety valves (SV) and safety relief valves (SRV) prevent unsafe pressure conditions in air, steam, hot water, refrigeration, and other fluid systems. An SV is a pressure relief device that is designed to open rapidly, or pop open, when the inlet pressure is greater than the set pressure of the SV. An SV is normally used on air or steam systems. An SRV is designed to open in proportion to the amount of excess pressure over the set opening pressure of the SRV. An SRV is used primarily on hot water or other liquid systems. The ASME Code requires that all hot water and steam boilers be mounted with an SRV to maintain pressure at or below the maximum allowable working pressure of the boiler.
Spring Loaded Type SVs and SRVs
SVs and SRVs may be constructed using a directly loaded spring. This spring loaded feature of SVs and SRVs allows for reclosing when the proper operating pressure is restored. These SVs and SRVs may also have an open gravity drain through the casing below the valve seat to allow proper drainage. The springs in the SVs and SRVs are restricted to a tolerance of 5% above or below the pressure marked on the valves. All adjustments made to SVs and SRVs to maintain the tolerance must be made by an authorized person, usually the manufacturer. SVs and SRVs are sealed with a tag identifying who made the adjustments, the set pressure capacity, and the date. SVs and SRVs certified for a steam system, designed according to Section I and Section IV of the ASME Boiler and Pressure Vessel Code, must have the official Code Symbol Stamp on the valve.
Non Reclosing Type SVs
There are also non reclosing, pressure relief devices, such as a rupture disk, which are actuated by inlet pressure. A rupture disk will burst when pressure is above the rated disk value. There are several rupture disk designs: plain flat, pre bulged, or reverse buckling. The materials used for a rupture disk may be either brittle or ductile metal which is not required to conform to ASME Standards. The portion of the non reclosing type of SV that must meet ASME Standards is the rupture disk holder. The rupture disk holder encloses and clamps the rupture disk into position. Rupture disk holders will have some form of identification stating ASME compliance.
Fusible Plug Type SV
A fusible plug is a form of an SV. The plug has an alloy core which melts at a predetermined temperature corresponding to a pressure that is safe for the vessel on which it is installed. In the event of fire, a fusible plug will respond to temperature only, and will not protect against excessively high pressure alone.
System Design Requirements Regarding SVs and SRVs
The sizing of the pressure relieving devices must be properly determined to maintain safe working conditions of the system. The devices for a hot water system must be able to release the maximum quantity that the system can generate without allowing a rise in pressure of 16% above the allowable working pressure. If the pressure vessel could be exposed to a fire hazard, the pressure relieving devices must be able to release the maximum quantity that the system can generate without allowing a rise in pressure of 21% above the allowable working pressure.
Manifold Arrangement—For some circumstances, a manifold type SRV may be used. This valve will work equally well in discharging water during normal operation and steam during emergencies. This safety device is a combination of two SRVs, one for steam and one for water. Since steam occupies 1,600 times the volume for the same pound of water, a larger SRV is required for steam due to the expansion, and a smaller SRV handles the water.
Steam Boilers—Steam Boilers have additional requirements. All steam boilers require at least one SV or SRV. If the capacity of the boiler is large enough, two or more devices may be required. The safety relief devices must be able to discharge all the generated steam without allowing the pressure to rise six percent (6%) above the maximum allowable working pressure.
SV and SRV Installation Guidelines
The ASME Standards state that all SVs and SRVs are to be installed in locations that are accessible for inspection and repair. Steps must also be taken to ensure that the valves cannot be made inoperative. A vessel which operates completely filled with liquid must have a safety relief device which is suitable for liquid relief.
SVs and SRVs must be installed properly. Safety relief devices are to be connected to the vapor space of the vessel to be protected. For a boiler, the safety relief device should be connected independently of any other connection. For high temperature water boilers, of the water tube, forced circulation type, the SRV should be located at the boiler outlet. The safety device should be located as close to the boiler as possible without any unnecessary piping or fittings. The opening in the vessel should be designed to provide an unobstructed flow to the pressure relief device. Upstream piping, kept to a minimum, should be of such size that the pressure drop will not affect the operation of the safety relief device.
Safety devices for air and gas systems are preferably installed inverted to collect moisture and particles to create a better seal at the seating surface.
When a safety device is exposed to the outdoor elements, it should be shielded. This shield or cover is not to hinder the operation of the valve.
Stop valves cannot be installed between the vessel and the safety relief device, nor in the relief piping from the safety relief device.
Relief piping should have proper drainage of content to prevent buildup at the safety relief device. The piping should discharge its contents into a safe place. Specific guidelines for the piping may be required through local building codes.
When mounting an SV or SRV, install the valve vertically with the discharge holes either open or piped to a drain. Avoid overtightening the valve or allowing the weight of the discharge piping to be carried by the valve. These mistakes may distort the valve seat. A drip pan elbow on the discharge side of the SV will prevent the weight problem. Use a moderate amount of pipe thread lubricant. Install clean flange connections with new gaskets, properly aligned and parallel, and bolt with even torque to prevent distortion. Wire cable or chain pulls attached to the test levers should allow for a vertical pull. The weight of wire cable or chain pulls should not be carried by the valve.
When two or more SRVs are required, they may be mounted either individually or as sets of twin valves. Twin valves are in two forms: individual valves on a Y base, or a duplex valve that has two valves in the same body. The twin valves are of equal capacities.
SV and SRV Testing
SVs and SRVs should be tested on a regular basis. Safety devices with a working pressure under 400 psig should be tested manually once per month, and pressure tested once per year. For higher pressures, testing should be based on operating experience. If a steam safety device is in need of repairs, adjustments, or a set pressure change, only the manufacturer or approved personnel having an ASME V, UV, or VR stamp may perform the work. If a valve is rated to ASME Section IV Standard, only the manufacturer is allowed to repair it.