API RP 576:2009 pdf download
API RP 576:2009 pdf download.Inspection of Pressure-relieving Devices.
Common examples include direct spring-loaded pressure-relief valves, pilot-operated pressure-relief valves, rupture disks, weight-loaded devices, and pressure- and/or vacuum-vent valves.
4.2 Pressure-relief Valve
A pressure-relief valve is designed to open for the relief of excess pressure and reclose thereby preventing further flow of fluid after normal conditions have been restored. A pressure-relief valve opens when its upstream pressure reaches the opening pressure. It then allows fluid to flow until its upstream pressure falls to the closing pressure. It then closes, preventing further flow. Examples of specific types of pressure-relief valves include: safety valve, relief valve, conventional safety-relief valve, balanced safety-relief valve, and pilot-operated pressure-relief valve.
4.3 Safety Valve
4.3.1 General
A safety valve is a direct spring-loaded pressure-relief valve that is actuated by the static pressure upstream of the
valve and characterized by rapid opening or pop action.
When the static inlet pressure reaches the set pressure, it will increase the pressure upstream of the disk and overcome the spring force on the disk. Fluid will then enter the huddling chamber, providing additional opening force. This will cause the disk to lift and provide full opening at minimal overpressure. The closing pressure will be less than the set pressure and will be reached after the blowdown phase is completed.
The spring of a safety valve is usually fully exposed outside of the valve bonnet to protect it from degradation due to the temperature of the relieving medium. A typical safety valve has a lifting lever for manual opening to ensure the freedom of the working parts. Open bonnet safety valves are not pressure tight on the downstream side. Figure 1 illustrates a full-nozzle, top-guided safety valve.
4.3.2 Applications
A safety valve is normally used with compressible fluids. Safety valves are used on steam boiler drums and super- heaters. They are also used for general air and steam services in refinery and petrochemical plants. Safety valve discharge piping may contain a vented drip pan elbow or a short piping stack routed to the atmosphere.
4.3.3 Limitations
Safety valves should not be used:
— in corrosive services (unless isolated from the process by a rupture disk),
— in installations that impose any backpressure unless the effects of the backpressure have been accounted for in the installation,
— where the escape of process fluid around blowing valves is not desirable,
— in liquid service,
— as pressure control or bypass valves.
A relief valve is a direct spring-loaded pressure-relief valve actuated by the static pressure upstream of the valve. Thevalve opens normally in proportion to the pressure increase over the opening pressure.
A relie valve begins to open when the static inlet pressure reaches its set pressure. When the static inlet pressureovercomes the spring force, the disk begins to lit off the seat, allowing flow of the liquid.The value of the closingpressure is lower than the set pressure and will be reached after the blowdown phase is complete.Relief valvesusually reach full lift at either 10 % or 25 % overpressure, depending on the type of valve and trim.
These valves have closed bonnets to prevent the release of corrosive, toxic, flammable, or expensive fluids. They canbe supplied with lifting levers, balancing bellows, and soft seats as needed.Figure 2 illustrates one type of relief valve.The ASME BPVC requires that liquid service relief valves installed after January 1,1986 have their capacity certifiedand stamped on the nameplate.
Some relief valves are manufactured with resilient O-rings or other types of soft seats to supplement or replace theconventional metal-to-metal valve seating surfaces. Usually, the valves are similar in most respects to the otherpressure-relief valves, with the exception that the disks are designed to accommodate some type of resilient seal ringto promote a degree of tightness exceeding that of the usual commercial tightness of conventional metal seats.Figure 3 illustrates one type of O-ring seat seal as installed in a safety-relief valve.
