Types of Safety Relief Devices and Valves
Types of Safety Relief Devices Valves
A pressure relief device is actuated by inlet static pressure. It is designed to open during emergency or abnormal conditions to prevent a rise of internal fluid pressure over a specified value or set pressure.
The devices are used on all types of pressurized equipment from water heaters to power generation equipment, and even in spacecrafts. The purpose of these devices is to protect a vessel against overpressure, though they also may be designed to prevent excessive internal vacuum.
Many types of pressure relief devices are available in the market today. The main types of pressure relief devices are: reclosing and non-reclosing pressure relief devices.
Nonreclosing–Type Pressure Relief Devices
A nonreclosing-type pressure relief device is designed to remain open after operation. Generally, a manual means of resetting is provided. If it’s not, the device is replaceable.
- Rupture Disks : A rupture disk is designed to rupture at a predetermined pressure and temperature. Rupture disks are used where instantaneous and full opening of a pressure relief device is required. These devices protect vessels, piping and other pressurized systems from excessive pressure and/or vacuum.
- Pin Devices : A second type of non-reclosing pressure relief device is a pin device, which functions similar to rupture disks. A pin device is actuated by static differential pressure or static inlet pressure. They are designed to function by the activation of a load bearing section of a pin that supports a pressure containing member.
Reclosing Pressure Relief Devices
The primary purpose of a pressure relief valve is to open to relieve excess pressure, reclose and prevent further flow of fluid after normal conditions have been restored. A secondary purpose is to minimize damage to other system components through operation of the pressure relief valve itself.
The many types of pressure relief valves that exist are based on different designs and construction. Generally, they’re classified based on functions as: safety relief valves, relief valves and safety valves.
A safety relief valve can be used for either a relief valve or a safety valve, depending on the application. Safety relief valves are classified as:
- Conventional Safety Relief Valve
- Balanced bellows safety relief
- Pilot-operated safety relief valves
- Power-actuated safety relief valves
Conventional Safety Relief Valve : A conventional safety relief valve is a spring-loaded pressure relief valve characterized by a rapid-opening pop action. Conventional safety relief valves are used for applications where excessive variable or built-up back pressure is not present in the system. The operational characteristics of these valves are directly affected by changes in the back pressure on the valve.
Balanced bellows safety relief : When back pressure is variable and exceeds 10% of the set pressure, a balanced bellows safety relief valve is recommended.
The advantages of balanced bellows safety relief valves are:
- The relieving pressure is not affected by the back pressure.
- They can handle higher built-up back pressure.
- They protect springs from¬corrosion.
- They have good chemical and high-temperature capabilities.
Pilot-operated safety relief valves : The primary difference between a pilot-operated safety relief valve and a spring-loaded pressure relief valve is that the pilot-operated valve uses process pressure to keep the valve closed instead of a spring. A pilot is used to sense process pressure and to pressurize or vent the dome pressure chamber, which controls the valve opening or closing.
A pilot-operated safety relief valve consists of the main valve, a floating, unbalanced piston assembly, and an external pilot. The pilot controls the pressure on the top side of the main valve’s unbalanced moving chamber. A resilient seat is normally attached to the lower end.
Power-actuated safety relief valves : Valve movement to open or close is fully controlled by a source of power such as electricity, steam or water (hydraulic). The valve may discharge to the atmosphere or to a container that is at lower pressure. The discharge capacity can be affected by downstream conditions.
Power-actuated safety relief valves are used mostly for forced-flow steam generators with no fixed steam or waterline. They are also used in nuclear power plants.