Bursting discs are thin membranes, which act like a blank flange under normal conditions. The membrane is very thin compared to the other pressure retaining parts, and when the pressure exceeds the rated pressure, the membrane ruptures and allows the fluid to escape. Unlike most relief valves, the bursting disc cannot close once the pressure has been relieved, and the disc must be replaced. However, they do have the advantage of being completely leak-free during normal working. When handling hazardous fluids, bursting discs can be used in conjunction with relief valves to provide protection, preferably with the bursting disc being fitted before the relief valve.