a time. Pilot-operated relief valve (POSRV, PORV, POPRV): A device that relieves by remote command from a pilot valve which is connected to the upstream system pressure. Low-pressure safety valve (LPSV): An automatic system that relieves by the static pressure of a gas. The relieving pressure is small and near the atmospheric pressure. Vacuum pressure safety valve (VPSV): An automatic system that relieves by the static pressure of a gas. The relieving pressure is small, negative, and near the atmospheric pressure. Low and vacuum pressure safety valve (LVPSV): An automatic system that relieves by the static pressure of a gas. The relieving pressure is small, negative, or positive, and near the atmospheric pressure. Pressure vacuum release valve (PVRV): A combination of vacuum pressure and a relief valve in one housing. Used on storage tanks for liquids to prevent implosion or overpressure. Snap acting: The opposite of modulating, refers to a valve that "pops" open. It snaps into a full lift in milliseconds. Usually accomplished with a skirt on the disc so that the fluid passing the seat suddenly affects a larger area and creates more lifting force. Modulating: Opens in proportion to the overpressure. Legal and code requirements in industry In most countries, industries are legally required to protect pressure vessels and other equipment by using relief valves. Also in most countries, equipment design codes such as those provided by the American Society of Mechanical Engineers (ASME), American Petroleum Institute (API) and other organizations like ISO (ISO 4126) must be complied with and those codes include design standards for relief valves. The main standards, laws, or directives are: AD Merkblatt (German) American Petroleum Institute (API); Standards 520, 521, 526, and 2000 American Society of Mechanical Engineers (ASME); Boiler & Pressure Vessel Code, Section VIII Division 1 and Section I American Water Works Association (AWWA), storage tanks EN 764-7; European Standard based on pressure Equipment Directive 97/23/EC Eurocode EN 1993-4-2, storage tanks. International Organization for Standardization; ISO 4126 Pressure Systems Safety Regulations 2000 (PSSR); UK Design Institute for Emergency Relief Systems (DIERS) Formed in 1977, the Design Institute for Emergency Relief Systems was a consortium of 29 companies under the auspices of the American Institute of Chemical Engineers (AIChE) that developed methods for the design of emergency relief systems to handle runaway reactions. Its purpose was to develop the technology and methods needed for sizing pressure relief systems for chemical reactors,