Safety relief valves protect from excess pressure by targeted release of vapours and gases. David Breil, product specialist at the German manufacturer WITT, talks about these passive protective devices. Excerpts...

Mr Breil, who benefits mostly from safety relief valves?

Anyone who operates pressurised systems, components or plants. These valves are typically fitted onto pressurised gas tanks or loop line systems which, for instance, work with an operating pressure of 16 or 25 bar. Safety relief valves prevent serious accidents by relieving pressure before excess pressure can build up and become dangerous to systems and people.

What are the dangers?

Process safety issues can exist, through design or operation. Excess pressure can build up rapidly and exceed the material's burst limit. Safety relief valves are a last line of defence to prevent damage.

So, does this mean that safety relief valves are only required for high pressures?

No, not at all. Excess pressure can also have undesired consequences when working with low pressures, for instance, below 500 mbar. In these cases, it is less about life and limb but rather about its effects on the quality of the products or the devices used. I am referring to applications such as transport receivers for highly sensitive high-tech products, isolator and glovebox systems or industrial furnaces under a protective atmosphere. Even minor pressure fluctuations can put the process at risk. This area in particular is where the sensitive Witt valves of the AV series with opening pressures of 5 mbar can prove their strengths.

How do these safety relief valves work?

Our high quality valves are spring-loaded and are direct acting. This means that as soon as the opening pressure is reached, a spring-loaded component gives in, opens the valve and releases the excess pressure. It then closes automatically after equalising the pressure. A renewed increase in pressure will open it again.

For which pressure range can these valves be used?

Witt safety relief valves can cover an extremely wide operating range. The opening pressure can be set from 5 mbar to 45 bar, depending on the model. Thus, even minimal increases in pressure can be detected. As previously mentioned, this is important when it comes to protecting highly sensitive systems or housings. At the same time, they have a high venting output of up to 970 m3/h at temperatures ranging from -60 to +270 °C. And because they are small, they can be installed just about anywhere, and in any orientation.

However the orientation needs to be known in advance for safety valves in millibar range, so that Witt can set the opening pressure to suit the system's operating pressure.

You also offer special TÜV approved valves – why is this?

Witt always checks the entire production line before delivery, piece by piece. Each device’s set pressure testing is witnessed and signed-off by a TÜV Inspector. Consistent with our certification to the Pressure Equipment Directive, this individual approval is prescribed for the higher pressure models. However, we also offer this service for all models in the lower pressure range. In this way, our customers receive an external certificate that guarantees extra peace of mind with regard to safety.

What other selection criteria are there for safety relief valves?

The type of gas plays an important role. We use brass as a standard housing material for technical gases. In most cases, aggressive or corrosive media require stainless steel equipment, which is used for securing biogas plants, for instance. Aluminium is another possibility. Besides the housing, even the seals must be adapted. We have different materials available depending on the type of gas.