The American Society of Mechanical Engineers (ASME) defines a rupture disc as a ‘non-reclosing pressure relief device actuated by inlet static pressure and designed to function by the bursting of a pressure containing disc’.
Rupture discs can be used as the sole form of overpressure protection on a vessel (primary relief), in parallel with pressure relief valves (secondary relief) or in series with a pressure relief valve (relief valve protection).
Primary relief
As the primary form of pressure relief, rupture discs form an alternative to valves. Because the disc itself bursts, rupture discs are non-reclosing, single-use pressure relief devices that must be replaced following an overpressure event.
However, rupture discs offer superior leak-tightness compared to valves and contain no moving parts. This means that, unlike valves, they are not vulnerable to failure due to freezing, corrosion or damage from the process media.
Discs are also capable of venting excess pressure extremely quickly and may be preferred in processes where there is a risk of large pressure rises occurring rapidly.
Secondary relief
As the secondary form of pressure relief, a bursting disc is installed in parallel with a primary relief device such as a relief valve or safety valve.
The disc provides a backup vent to the primary device, protecting against both vessel overpressure and primary device (valve) failure. The rupture disc will activate if the valve either fails to operate or fails to relieve the overpressure rapidly enough.
This can be useful in situations where abnormally high or uncontrollable pressure conditions may develop, for example as a result of an exothermic reaction, or if there is a major event in which the pressure levels exceed the capacity of the primary relief device.
Relief valve protection
When used for relief valve protection, the rupture disc is generally installed upstream of the valve.
This isolates the valve from the process media and protects it from damage or contamination from any corrosive, flammable, carcinogenic or toxic substances used in the process. The rupture disc also acts as a leak-tight seal, eliminating leakage through the valve.
Using a rupture disc in combination with a valve therefore creates an efficient, leak-tight and cost-effective pressure relief solution that saves on valve maintenance.
Note: The space between the rupture disc and pressure relief valve must be vented and there must be a way of monitoring and notifying of any backpressure build-up. This is because an accumulation of pressure will affect the performance of both the valve and the rupture disc. A frequent combination is an excess flow-valve with a Tel-Tale pressure gauge.