A relief valve set to open at a pressure safely below the bursting pressure of a container, such as a boiler or compressed air receiver. Typically, a disk is held against a seat by a spring; excessive pressure forces the disk open (see illus.). Construction is such that when the valve opens slightly, the opening force builds up to open it fully and to hold the valve open until the pressure drops a predetermined amount, such as 2–4% of the opening pressure. This differential or blow-down pressure and the initial relieving pressure are adjustable. Adjustments must be set by licensed operators, and settings must be tamperproof. The ASME Boiler Construction Code gives typical requirements for safety valves.