Safety valve is a valve that act as a protection of equipment from exploding or damaging and it is mainly installed in pressure vessels such as chemical plants, electric power boilers and gas storage tanks.
Safety Valve is a type of valve that automatically actuates when the pressure of inlet side of the valve increases to a predetermined pressure, to open the valve disc and discharge the fluid (steam or gas ) ; and when the pressure decreases to the prescribed value, to close the valve disc again. Safety valve is so-called a final safety device which controls the pressure and discharges certain amount of fluid by itself without any electric power support.
Safety valve support not only the safety of energy industry but also the safety and security of our life.
Classification of safety valve
1. Spring-loaded pressure-relief valves
Generally, the safety valveｓ are referred to the Spring-loaded Pressure-relief valves, the most common type ( See the left figure ).
The load of the spring is designed to press the “Disc” against the inlet pressure.
Depending on the fluid type, such as steam, gas or liquid, we are offering a Bellows model to clear the back pressure effect.
2. Pilot-operated pressure-relief valves
Pilot-operated Pressure-relief Valves are composed of Pilot Assy and Main Valve. Although Spring-loaded Pressure-relief valves adopts the force of the spring against the inlet pressure, the reliving pressure and reseating pressure of the Pilot-operated Pressure-relief Valves is controlled by Pilot Assy, which acts nearly as same as Spring-loaded Pressure-relief valves.
Pilot-operated Pressure-relief Valves have larger size variations compared to Spring-loaded type, which is applied in a severe condition such as high pressure.
3. Dead-weight pressure-relief valves
In case the design pressure of the pressure vessel is set at very low pressure, Dead-weight Pressure-relief valve adjusts relieving pressure only by the disc weight.
The Vacuum relief valve has been adopting this functional characteristic which absorbs the pressure when the inside of the pressure vessel falls into a negative pressure.
What Can It Do?
The primary purpose of a safety valve is the protection of life, property and environment. A safety valve is designed to open and relieve excess pressure from vessels or equipment and to reclose and prevent the further release of fluid after normal conditions have been restored.
A safety valve is a safety device and in many cases the last line of defence. It is important to ensure that the safety valve is capable to operate at all times and under all circumstances. A safety valve is not a process valve or pressure regulator and should not be misused as such. It should have to operate for one purpose only: overpressure protection.
Reasons for excess pressure in a vessel
There is a number of reasons why the pressure in a vessel or system can exceed a predetermined limit. API Standard 521/ISO 23251 Sect. 4 provides a detailed guideline about causes of overpressure. The most common are:
- Blocked discharge
- Exposure to external fire, often referred to as “Fire Case”
- Thermal expansion
- Chemical reaction
- Heat exchanger tube rupture
- Cooling system failure
1. “Nozzle” inside the Safety Valve starts to receive a higher pressure from the inlet side of the valve.
2. When the pressure becomes higher than the set pressure, “Disc” starts to lift and discharge the fluid.
3. When the pressure decreases until the predetermined pressure, the force of the spring closes “Disc”.
Difference between Safety valves & Relief valve:
The names “safety” and “relief” are often used interchangeably but are not supposed to be. For compressible fluids, safety valves are: steam and other gases.
Compressible fluids need quick pressure relief. So safety valves have pop seats and plugs which open rapidly on over pressure.
Relief valves are for the non-compressible fluids-liquids such as water and oil.. Immediate full-flow discharge is not required as a very tiny flow considerably decreases overpressure, thus opening and closing the socket and seat very slowly, discharging the liquid back to some low pressure point in the scheme.