Basic Knowledge of Safety Valves
What is a Safety Valve?
The primary purpose of a safety valve is the protection of life, property, and the 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.
There is several 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 the causes of overpressure. The most common are:
- Blocked discharge
- Exposure to external fire, often referred to as a “Fire Case”
- Thermal expansion
- Chemical reaction
- Heat exchanger tube rupture
- Cooling system failure
Each of the above-listed events may occur individually and separately from the other. They may also take place 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 a worst-case scenario for the sizing and selection of a suitable pressure relief device.
Types of Safety Valves
In the context of pressure relief devices, a fundamental distinction can be made between reclosing and non-reclosing devices. Safety valves are part of the reclosing devices and are differentiated based on their loading principle or controllability.
In addition, a distinction can be made based on the function of the valves. Here, a distinction is made between relief valves (full lift), normal safety valves, and proportional safety relief valves.
There are two main types of safety valves: Spring-loaded safety valves and pilot-operated safety valves. Also relevant are so-called change-over valves, which ensure continuous system availability (not included in the illustration)
How does a Safety Valve work?
The function and design of safety valves differ depending on the main type or loading principle. Use the following buttons to navigate to the respective main type pages. There you will find the corresponding LESER safety valves as well as further information, for example on the function and design of the main types.
Sizing of Safety Valves
To ensure the greatest possible plant safety, the analysis of the conditions and requirements of an application and the corresponding dimensioning of a safety valve is crucial. Determining the necessary dimensions and flow cross-section of a safety valve is referred to as “sizing the safety valve”. Numerous parameters are taken into account in this calculation.
Examples of relevant substance data:
- Type and aggregate state of the medium
- Design standard (DIN, API, AD, ASME)
Examples of relevant operating data:
- Set pressure
- The operating temperature of the medium
- Required mass flow (e.g. 10,000 kg/hour)
Safety Valve Symbols
The main types of safety valves are identified by symbols. Therefore, there is not only one but several safety valve and safety valve symbols. They are often shown on safety valves or their packaging. In addition, they are used in technical drawings and schematics as well as system sketches – for example, in the description of DIN standards.
The following symbols are related with the main types of safety valves:
Spring-loaded Safety Valves and Pilot-operated Safety Valves