Safety Relief Valves: Purpose & Function Guide

The primary function of Safety Relief Valves is the protection of life, property and environment. A Safety Relief Valve is a (safety) device designed to protect a pressurised vessel or system against over-pressure should all other safety systems fail.

An overpressure incident is defined by any situation/condition which would cause the pressure in a vessel or system to increase beyond its specified design pressure or maximum allowable working pressure (MAWP).

A Safety Relief Valve is designed to open and relieve excess pressure from vessels or equipment …and then to re-close and prevent the further release of fluid, gas, or steam once normal conditions have been restored.

There are a number of reasons why the pressure in a vessel or system can exceed a predetermined limit.

A PI Standard 521 | I SO 23251 Sect. 4 provides a detailed guideline about causes of overpressure.

The most common causes of safety relief valve overpressure are:​

• Blocked discharge
• Exposure to external fire, often referred to as “Fire Case”
• Thermal expansion
• Chemical reaction
• Heat exchanger tube rupture
• A Safety Relief Valve is in many cases the last line of defence for a system or operation. It is vitally important that the Safety Relief Valve is certified to, and capable of, operating at ALL times and under ALL circumstances.

A typical example of this would be the event of power failure, when system controls are non-functional …in this situation the sole source of power for the Safety Relief Valve must be the process fluid or substance itself. The Safety Relief Valve must open at a predetermined set pressure, flow a rated capacity at a specified overpressure, and close when the system pressure has returned to a safe level.

Since reliability is directly related to the functionality and performance of the device, it is important that the design of the Safety Relief Valves is as uncomplicated and efficient as possible.

NOTE: A Safety Valve has a singular purpose: Overpressure Protection and should NEVER be used as a process valve or pressure regulator).​

Safety Relief Valves: Testing​

One of the critical quality characteristics of a Safety Relief Valve is its functional tightness, i.e. the tightness between the seat and the disc.

On gas­tight safety valves, tightness is measured using the internationally accepted air bubble count test (also known as “Kellogg Test”) as per the API 527 standard (American Petroleum Institute ­ Seat Tightness of Pressure Relief Valves).

Seetru have developed an automatic, electronic system for recording the Kellogg Bubble Count; our trademarked Compu­testTM fast operating system automatically measures and calculates the bubble count / leak rate in a manner compliant with the requirements of API 527.​

The informative and comprehensive display:­​

• ­ Manages the test sequence
• ­ Shows the bubble leak rate
• ­ Provides a Pass / Fail Indication

Safety Relief Valve Design: Selection​

Safety Valves are sophisticated instruments, to ensure their safe operation it is important that they are always dealt with by competent specialist engineers, this applies to their selection, installation and maintenance.

The use of Safety Relief Valves across a very broad spectrum of industries and applications means that considerations with regards to design must be correctly evaluated. Valves need to be available with a large variety of options for ­ construction materials, sealing materials, connections …and must be able to tolerate wide variances in both pressure and temperature.

We have been designing and manufacturing Safety Valves for over 65 years and have an extensive range of devices which have many National and International approvals such as US National Board (ASME Section VIII Division I) and of course the European Pressure Equipment Directive 97/23/EC, update soon to be to introduced under PED 2014/68 enabling European CE mark; and, Russian EAC (TR CU) Customs Union certification and declaration.

This allows Seetru to offer their products into the global marketplace.

The extensive ranges of products include Safety Valves in the key materials of construction including bronze, brass and stainless steels, Safety Valve Pressures: 0.2 bar g. ­ 500 bar g. | Safety Valve Temperatures: ­196° C +250°C.

Safety Relief Valve Design: Technology​

The generation of compressed gas has considerable cost and therefore reliability of safety valve performance is essential to the optimisation of running costs. Where possible, the safety valve should provide reliable and repeatable leak tight performance up to the Safety Valve’s set pressure: elastomeric seals are particularly suited for this demand.

It is also essential that the flow capacity of the Safety Valve matches that of the application: if the valve capacity is too low it may not protect the system and personnel adequately against overpressure; if the flow capacity of the valve is too large it may chatter (open and close quickly and repeatedly) in operation – this may cause damage and subsequently result in valve failure.

In the early 1960s, Seetru pioneered the use of elastomeric seals in safety valves for use in the compressor industry across Europe. The key benefit of elastomeric seals in compressor applications is that they will seal fully, reliably and repeatably.

It is, of course, necessary to select a sealing technology that is suitable for the duty: here, the seal material must be suitable for the temperature and medium, i.e. the compressed fluid and any other substances that may be contained in the fluid, such as lubricating oils or gas contaminants. 

While modern elastomeric seals have very extensive performance envelopes (temperature and chemical compatibility etc.), it may still be necessary to use plastic or metal sealing technology in order to deal with aggressive or difficult applications.

Safety Valves have to be of the highest quality and provide a long term reliable operation solution. The company’s range of elastomeric safety valves for compressed air and gas applications incorporate proprietary compact design technology which is highly efficient and cost effective, and incorporates the exclusive Tutchtite® seal technology for repeatable bubble­tight sealing performance.

Another example of Safety Valve innovation is a range of specialist ultra­low emission safety valves for refrigeration applications: these world leading safety valves are tested during assembly using gas spectrometry leak detection, to ensure leakage rates of less than 3 grams of gas per year.

Safety Relief Valve Design: Evolution​

Although generally speaking the design and manufacture of Safety Relief Valves is varied and remains driven by the individual requirements of specific applications, some manufacturers have developed multi­purpose devices which aim to provide a more complete and practical solution.

In keeping with this, we have developed our own range (The LGS™ Safety Valve range) which incorporates all of our technological advances into a more versatile system.

This Safety Valve range has been designed with a robust and reliable construction so as to be suitable for the widest possible range of industrial applications, while also achieving a broad variation in flow characteristics …coping with both low volume and high relief operations. A ‘single trim’ design ensures that all components are common across Liquid, Gas & Steam applications.

Safety Relief Valve Training​

It is worth mentioning that Seetru offer a range of training courses through its online division BEI Services (Blupax Engineering and Industrial Services). BLUPAX­Training (BEI Services) are able to offer a wide spectrum of specialist safety valve training courses.

We offer personal development training at locations throughout the UK, subjects include: Safety Relief Valves, Safety Pressure Devices, Practical Workshop instruction.