A safety valve protects people, the environment, and equipment from overpressures that may cause explosions and severe damages. A safety valve opens automatically anytime a set pressure is exceeded. Due to their critical function, safety valves are extremely important devices and intervene when any other protection device has failed. Safety valves may be spring or pilot operated.
This type of valves is therefore installed onboard pressure equipment such as tanks and vessels in petrochemical plants, standard boilers, electric power boilers, and gas storage tanks or in oil and gas pipelines.
A safety valve opens automatically, releasing any overpressure, anytime the pressure of the fluid passing through the valve increases above a predefined pressure level (in bars or psi). When this happens, the disc of the valve opens so the valve can discharge the fluid and reduce the overpressure happening within the equipment (steam, gas, and other conveyed fluids). On the other hand, as the pressure of the fluid decreases below a set value, the disc closes again.
It has to be noted that safety valves generally open immediately, releasing the fluid at once, as the set overpressure is detected by the device (whereas the release of the fluid is progressive and proportional to the overpressure for pressure relief valves).
Safety valves control the pressure and discharge fluid overpressures by mechanical actuation, without external electric forces.
Safety valves are available either in the API 526 and ASME construction.
SAFETY RELIEF VALVE VS. PRESSURE RELIEF VALVE
Sometimes, the terms safety valve and pressure relief valve are used as synonyms. However, some differences exist between these two terms:
- A Pressure Relief Valve (called also, “PRV”) is a pressure release device mounted on a vessel filled with a fluid. For PRV valves, the opening of the disc is proportional to the increase in the vessel pressure (gradual) and not sudden. These types of valves, as the name suggests, are therefore used to relieve the pressure existing in a vessel and prevent an overpressure condition in the equipment. Pressure relief valves generally operate through the combination of a pressure controller and an actuating mechanism (the operator opens the PRV disc as a specific control signal is received).
- Safety Valve refers to relief devices for compressible filled vessels. Safety valves open fully and suddenly as the set overpressure limit is reached. Their scope is to protect the equipment (and the personnel operating within the range of the equipment) from dangerous explosions that may threaten the integrity of the system and human health. Safety valves relieve the pressure without the assistance of a controller and an operator. A safety valve on a boiler or other types of fired pressure vessels must be capable of removing more energy that is possible to be put into the vessel (1,1 of boiler rated capacity may be acceptable).
VESSEL OVERPRESSURE ROOT CAUSES
The pressure in a vessel or system can exceed a predetermined limit for a high number of different reasons. The API 521 specification describes such reasons, which are:
- Blocked discharge
- Chemical reaction
- Tubes and other devices rupture
- Fire case (the equipment is exposed to external fire)
- Thermal expansion
- Cooling system failure
Each of these adverse events may occur separately or simultaneously. Each cause of overpressure also will create a different mass or volume flow to be discharged, e.g. small mass flow for thermal expansion and large mass flow in case of a chemical reaction. It is the user’s responsibility to determine the worst-case scenario for the sizing and selection of a suitable pressure relief device.
SAFETY VALVES TYPES
The three main types of safety valves are listed below
SPRING LOADED SAFETY VALVES
Spring loaded safety valves are the most common type. The load of the spring is designed to press the “Disc” against the inlet pressure. As the pressure exceeds a set value, the spring opens releasing the disc and the fluid flowing through the valve.
Depending on the fluid type, such as steam, gas or liquid, bellows can be added to the spring to clear the back pressure effect.
The image shows the action of the spring on keeping the disc on the seat and have a close or open position for the valve.
For more information, consult the Leser’s guide to spring operated safety valves.
DEAD-WEIGHT PRESSURE SAFETY VALVE
These are safety valves without spring, used when the design pressure of the vessel is set at a very low value. They adjust the pressure of the equipment only by the action of the disc weight (when the pressure increases, the disc is lifted and the valve discharges the overpressure).
PILOT OPERATED SAFETY VALVES
Pilot-operated safety and pressure relief valves are composed of a “pilot valve” and a “main valve”. Differently from spring operated valves, in this type of safety valve the pressure pick up and adjustment is operated by the pilot valve and not by the main valve. This arrangement is used for larger sized safety valves and is used in cases of very high-pressure equipment.
Pressure relief valves are available in most material grades from carbon steel to stainless steel and can manage overpressure of any fluid (hydrocarbons, gas, steam or water). For each type of valve, hundreds of possible configurations are actually available (based on the combinations of parameters as valve type, sub-type, bore size, pressure rating, manufacturing norm, body and trim materials, valve operation, etc).
For more information, consult the Leser’s guide to pilot operated safety valves.
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