What are bursting discs? – Bursting disc glossary
Bursting discs are pressure relief components that remain open after triggering. This “disadvantage” compared to standard safety valves is offset by many specific advantages: Bursting discs are designed to be space-saving with no dead space. They are particularly suitable for use in hot and corrosive environments. They can be produced cost-effectively and to customer specifications, particularly in high-volume production. They are, however, components that demand a certain degree of consideration and understanding from the user. The specific characteristics stem from the fact that they are components that are designed to fail and that their main property – the burst pressure – can only be verified in destructive tests.
In the following we have compiled a short “Bursting Disc Glossary” to facilitate this understanding:
The pressure at which the bursting disc ruptures – determined in bursting tests under laboratory conditions. Bursting under realistic operating conditions can deviate from the bursting test due to different temperatures, pressure increase rates or varying loads.
Decisive quality test of a bursting disc: A selection of test pieces is taken from a production batch of bursting discs and tested under controlled conditions until reaching the bursting point. Important factors include the temperature, the pressure increase rate (e.g. up to burst pressure within 1 minute) and the compressive force (e.g. torque). The results are noted down in the certificate and apply to this production batch.
The bursting tolerance specifies the range in which the burst pressures, defined in the bursting test, must be. This may be a simple upper and lower limit (e.g. 10 bar +/-10 % means: all results must be between 9 and 11 bar). But it is also possible to determine that the mean value of the burst results has to be within this range (according to US standard). According to this definition, freak values do not necessarily lead to the rejection of the production batch. The permissible scatter can be specifically defined in a so called “manufacturing range”.
The operating pressure refers to the normal load a bursting disc should withstand without rupturing. This pressure must, of course, be lower than the burst pressure. It must be sufficiently lower than the burst pressure in order to ensure an adequately long service life of the bursting disc. The range between operating pressure and burst pressure depends on the type of bursting disc (reverse buckling bursting discs are the most resistant), the operating temperature and on the varying loads. The main priority in the design of bursting discs must always be to guarantee the burst pressure.
Bursting discs are often made from various materials. Decisive are the material of the bursting foil and the material of the parts that come into contact with the medium. The materials must be specified in the certificate and on the type plate. Moreover, it is of vital importance that the manufacturer uses material only from one batch and that complete traceability is guaranteed. Commonly used materials are stainless steel such as SS316L or 1.4435/1.4571 etc., as well as nickel materials such as Inconel or Hastelloy. Materials such as titanium, tantalum or aluminium are also used in special cases. PTFE or PFA are often used as sealing foil in composite bursting discs.
Rupture discs mainly rupture from the centre and form an irregular opening pattern. Fragmenting is also very common for rupture discs, whereas scored or slit bursting discs are different: The predetermined breaking points define the opening pattern – circular or in segments. The opening behaviour is included in the calculation of the net flow area and thus influence the volumetric flow. A non-fragmenting opening is often required for safety reasons.