The rupture disc or pressure safety element is not a valve because it cannot be opened or closed; however it is a fail-safe type of relief device. These devices are fitted to spur lines from certain pipelines and vessels. They are additional relief devices to the normal safety valve.
A rupture disc (a form of diaphragm) is made from materials such as aluminium, stainless steel or monel. It is fitted between two companion flanges called a holder, and is used to contain the fluid in a system but allow relief above a set pressure rating.
Discs of different thickness are available for most diameter pipes. While normal pressures exist in a process system the disc acts as a blind. If excessive pressure is not relieved quickly enough by the pressure safety valve, the disc ruptures to assist the safety valve. So a rupture disc is a designed weak point in a system that will self destruct (like a fuse in an electrical system) to protect the equipment.
Rupture discs are safety devices which protect process vessels, pipelines and tanks which would be damaged by excessive pressure. A ruptured disc allows the pressure to be relieved quickly and is the final safety device installed on vessels.
Rupture discs are generally made in two forms, tension loaded and re-buckling. Figure 21 shows one type of rupture disc. The downstream clamping flange includes several equally spaced knife blades arranged in a shallow pyramid with a sharp point at the apex. At predetermined system pressure, the disc will suddenly reverse bulge and be pierced by the knife point and segmented by the knife to provide full-flow opening. This type is called a knife edge re-buckling disc. A variation of this type is the jaws re-buckling disc. Tension loaded disc have no scored device but rely on system pressure to push through the metal.
FIGURE 21 RUPTURE DISCS
Rupture disc require protection from the environment to reduce corrosion of the metal surface which could downgrade the pressure rating. They need to be correctly positioned as incorrect positioning could raise the rupture pressure. Constant pressure change can cause flexing or movement of the disc. This could lead to metal fatigue so discs are pre-bulged to reduce this problem.
The rupture disc can be used as a primary relief device or for secondary relief in conjunction with a relief valve. When used with a safety relief valve, the rupture disc pressure is higher than the safety relief valve pressure but less than the maximum allowable working pressure. On rupturing the disc is simply replaced but a system of periodic inspection is usually carried out to check and/or replace discs.