It is well known in the chemical engineering and pharmaceutical industries to provide pressure relief devices for protecting pressure systems from over-pressurisation. One such pressure relief device is a bursting disc, for example as disclosed in WO 03/031853. When the pressure at one side of the disc rises above a predetermined burst strength, the disc ruptures thereby releasing pressure from the system. Typical applications are on reaction vessels and chambers, in the chemical, pharmaceutical and food industries.
It is also known in the art to provide a bursting disc with a rupture detector. WO 2005/054731 discloses a bursting disc type over-pressure detector having a bursting disc clamped at its flange between inlet and outlet pipe members. At the vent side of the device is mounted a magnet, its movement sensed by a non-invasive sensor. When the pressure at one side of the disc rises above a predetermined burst strength, the disc ruptures thereby releasing pressure from the system and moving the magnet relative to the sensor. A signal is produced by the sensor, signalling that the bursting disc has ruptured.
Bursting discs are usually classified into two types, forward acting discs and reverse acting discs. Forward acting discs have a domed shape with a concave side facing the high pressure. Reverse acting discs have a convex side facing the high pressure.
Forward acting discs are under tension in use, which gives them a tendency to stretch under pressure cycling and they are generally less durable and stable than reverse acting discs. For most applications, reverse acting discs are preferred. Reverse acting discs are in compression when under pressure and can operate closer to the burst strength during normal use and rupture more reliably than forward acting discs.
Bursting discs are often provided with a line of weakness (usually a score) which facilitates bursting as a disc begins to buckle at the predetermined burst strength. However, forming a score line in the domed part of a bursting disc can cause accidental damage, which reduces the reliability and durability of the disc. It is known in the art that these problems can be reduced by forming a score line in a frustoconical transitional portion between a domed portion and an annular flange, as disclosed in WO 03/031853.
It is difficult to produce a bursting disc that ruptures reliably at low pressures. A thin disc is fragile and difficult to handle. Materials that are malleable when thin are expensive and/or not easily available. Sometimes discs are scored or pre-damaged to give a lower burst strength, but this reduces the reliability and durability of the bursting disc. Additionally, pre-damaged discs tend to deform in a wave motion from one side to the other, when they buckle.
There is an additional problem in using reverse acting bursting discs at low pressures in that the disc must reverse through in order to burst. This reversal increases the volume of the system on the inlet side and may reduce the pressure of the system. Since the low pressure fluid cannot immediately fill the space behind the dome portion, the kinetic energy/momentum of the buckling dome portion is reduced and this can prevent the disc from bursting at the intended pressure. In an attempt to overcome this problem, one or more projections may be provided in the assembly on the vent side of the disc to facilitate tearing of the disc, as shown in EP 0395318.