SAFETY IN THE PLANT Pressure relief for cryogenic containers

In the field of occupational safety the pressure relief of cryogenic (transport) containers is an essential topic. The protection of man and machine is particularly important if an unexpected evaporation occurs due to a collapse of the insulation vacuum, uncontrolled heat rise or an excessive pressure rise in the inner container.

As a special form of a pressure vessel, cryostats have special requirements for the leak-tightness of the attached protective systems.

The basic layout of a cryostat consists of 2 containers - a container with the cryogenic medium and an insulation container.

The inner container, also known as the product container, is often secured with a safety valve and a rupture disc. The additional secondary protection with a rupture disc ensures a sufficiently fast pressure relief even during extremely high / fast pressure increases. Depending on the requirements regarding leak-tightness or corrosion resistance, various types of rupture discs are used, e.g. reverse or forward-acting rupture discs, as well as “plugs” - rupture discs with a thread connector.

The outer containers of cryostats are often designed for a pressure of less than 0.5 bar so that they do not fall under the Pressure Equipment

Compact rupture disc UKB-LS

Directive. Nevertheless, high demands are placed on pressure relief here: The combination of high vacuum, low burst pressure and high leak- tightness requirements (low leakage rate) is not feasible for standard pressure relief devices. Therefore, these outer cryostat containers are protected by so-called compact rupture discs. While most operators have a relatively good knowledge of safety valves, there is still a need for further information regarding compact rupture discs. Read more about this type of rupture disc and what you should look out for when performing the selection.

Customized compact rupture discs as a pressure relief device

Rupture discs are typically made-to-order because the material type, thickness, disc size and other properties are selected based on process parameters such as burst pressure, burst temperature, working pressure, operating temperature, process medium, etc.

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They are divided into various categories, one of which is known as compact rupture discs. While, for example, the term “reverse-acting rupture disc” (see below) actually only refers to the rupture disc, the term / product compact rupture disc usually includes both the actual rupture disc and the housing in which the rupture disc is enclosed.

Due to their small dimensions, these units are referred to as compact. High-quality compact bursting discs are characterised by extraordinary leak-tightness (leakage rates < 10-9

mbar l/s) and are also available in

corresponding versions for sterile applications and containers.

An example of such a compact rupture disc is the UKB-LS from the German rupture disc manufacturer REMBE. They consist of a housing, a burst membrane and an opening aid, similar to a three-part knife. If the working pressure exceeds the defined maximum and reaches the bursting pressure, the rupture disc presses against the three-part knife welded into the outlet part of the housing and cuts itself open through this - the largest possible vent area will become available immediately. Through the integrated three-part knife, predetermined breaking points on the rupture disc itself can be omitted, which makes the rupture disc itself more resilient. The leak- tightness is also increased by the weld connection of the rupture disc and housing, whereby both elements become one unit. At the same time, this unit is particularly easy to handle during installation and removal, thus preventing damage to the rupture disc during installation and normal operation. This ensures a reliable protection of the system for operators, and guarantees the defined burst pressure

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