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Pressure relief valves  from LESER

Pressure Relief Valves for Industrial Applications

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High Performance 441 XXL - casted design

LESER Type 441 XXL

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What is a Pressure Relief Valve?

The primary purpose of a pressure relief valve is the protection of life, property and environment. A pressure relief 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.

Safety Notice

A pressure relief valve is a safety device and in many cases the last line of defence. It is important to ensure that the pressure relief valve is capable to operate at all times and under all circumstances. A pressure relief valve is no process valve or pressure regulator and should not be misused as such. It should have to operate for one purpose only: overpressure protection.

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

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.

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Types of Pressure Relief Valves

In the context of pressure relief devices, a fundamental distinction can be made between reclosing and non-reclosing devices. Pressure relief valves are part of the reclosing devices and are differentiated on the basis of 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.

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There are two main types of pressure relief valves / safety valves: Spring-loaded pressure relief valves and pilot-operated pressure relief valves. Also relevant are so-called change-over valves, which ensure continuous system availability (not included in the illustration).

How does a Pressure Relief Valve work?

The function und design of pressure relief valves differs depending on the main type or loading principle. Use the following icons to navigate to the respective main type page. There you will find the corresponding LESER pressure relief valves as well as further information, for example on the function and design of the main types.

Pressure relief valves from LESER

LESER Digital ID

The LESER Digital ID provides access to all digital information of a safety valve, quickly and directly on the valve.

Digital ID

Sizing of Pressure Relief Valves

In order to ensure the greatest possible plant safety, the analysis of the conditions and requirements of an application and the corresponding dimensioning of a pressure relief valve is crucial. Determining the necessary dimensions and flow cross-section of a pressure relief valve is referred to as "sizing the pressure relief 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
  • Operating temperature of the medium
  • Required mass flow (e.g. 10,000 kg/hour)


For the sizing of pressure relief valves LESER has developed its own free software VALVESTAR®. You can access the tool via the following link.

Valvestar from LESER


Sizing of your safety valve.

LESER Engineering - The technical online manual


The comprehensive technical handbook on everything you would like to know about safety valves.


Pressure Relief Valve Symbols

The main types of pressure relief valves are identified by symbols. Therefore, there is not only one, but several pressure relief valve and safety valve symbols. They are often shown on pressure relief 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 pressure relief valves:
        safety-valve-symbol             Spring-loaded Pressure Relief Valves and Pilot-operated Pressure Relief Valves

 change-over-valve-symbol      Change-over valves


What is a pressure relief valve used for?

The primary purpose of a pressure relief valve is to protect life, property and the environment. Pressure relief valves are designed to open and release excess pressure from vessels or equipment and then close again.

How does a pressure relief valve work?

The function of pressure relief valves differs depending on the main type or loading principle of the valve. The main types of pressure relief valves are spring-loaded, weight-loaded and controlled pressure relief valves.

What is the set pressure of a pressure relief valve?

Regardless of the type or load, pressure relief valves are set to a specific set pressure at which the medium is discharged in a controlled manner, thus preventing overpressure of the equipment. In dependence of several parameters such as the contained medium, the set pressure is individual for each safety application.
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