What is the function of a rupture disk in a pressure vessel, and under what circumstances is it preferred over a pressure relief valve?

The function of a rupture disk in a pressure vessel is to provide overpressure protection by bursting at a predetermined pressure, relieving the excess pressure and preventing vessel failure. A rupture disk is a non-reclosing pressure relief device, meaning it is designed to burst and remain open once activated. It is a thin, usually metallic, diaphragm that is designed to fail at a specific pressure. When the pressure inside the vessel exceeds the disk's burst pressure, the disk ruptures, creating a full, unrestricted opening for the fluid to escape. A rupture disk is preferred over a pressure relief valve in several circumstances. First, if the process fluid is corrosive or could foul a relief valve, a rupture disk provides a more reliable solution because it isolates the relief device from the process until an overpressure event occurs. Second, rupture disks offer a faster response time than relief valves, making them suitable for situations where rapid pressure relief is needed. Third, a rupture disk provides a full and unrestricted opening upon activation, which can be advantageous in cases where maximum flow capacity is required. Fourth, for extremely high pressure applications, rupture disks may be more practical or cost-effective than relief valves. Finally, rupture disks provide a positive, leak-tight seal up to their burst pressure, whereas relief valves may exhibit some weeping or leakage near their set pressure. However, unlike relief valves, rupture disks must be replaced after each activation. An example would be a reactor containing a highly corrosive chemical; a rupture disk made of a compatible material would protect the vessel more reliably than a spring-operated relief valve that could corrode or become blocked.