As soon as mankind was able to boil water to create steam, the necessity of the safety device became evident. As long as 2000 years ago, the Chinese were using cauldrons with hinged lids to allow (relatively) safer production of steam. At the beginning of the 14th century, chemists used conical plugs and later, compressed springs to act as safety devices on pressurized vessels. Early in the 19th century, boiler explosions on ships and locomotives frequently resulted from faulty safety devices, which led to the development of the first safety relief valves. In 1848, Charles Retchie invented the accumulation chamber, which increases the compression surface within the safety valve allowing it to open rapidly within a narrow overpressure margin. Today, most steam users are compelled by local health and safety regulations to ensure that their plant and processes incorporate safety devices and precautions, which ensure that dangerous conditions are prevented. The primary function of a safety valve is therefore to protect life and property. A safety valve is a valve mechanism for the automatic release of a gas from a boiler, pressure vessel, or other system when the pressure or temperature exceeds preset limits. It is part of a bigger set named Pressure Safety Valves (PSV) or Pressure Relief Valves (PRV). The other parts of the set are named relief valves, safety relief valves, pilot-operated safety relief valves, low pressure safety valves, vacuum pressure safety valves. The basic spring loaded safety valve, referred to as 'standard' or 'conventional' is a simple, reliable self-acting device that provides overpressure protection. The basic elements of the design consist of a right angle pattern valve body with the valve inlet connection, or nozzle, mounted on the pressure-containing system. The outlet connection may be screwed or flanged for connection to a piped discharge system. However, in some applications, such as compressed air systems, the safety valve will not have an outlet connection, and the fluid is vented directly to the atmosphere. The safety valve operates by releasing a volume of fluid from within the plant when a predetermined maximum pressure is reached, thereby reducing the excess pressure in a safe manner. As the safety valve may be the only remaining device to prevent catastrophic failure under overpressure conditions, it is important that any such device is capable of operating at all times and under all possible conditions. Safety valves are precision items of safety equipment; they are set to close tolerances and have accurately machined internal parts. They are susceptible to misalignment and damage if mishandled or incorrectly installed. Valves should be transported upright if possible and they should never be carried or lifted by the easing lever. In addition, the protective plugs and flange protectors should not be removed until actual installation. Care should also be taken during movement of the valve to avoid subjecting it to excessive shock as this can result in considerable internal damage or misalignment. There is a wide range of safety valves available to meet the many different applications and performance criteria demanded by different industries. Furthermore, national standards define many varying types of safety valve. Now we take the American standard as example. The ASME standard I and ASME standard VIII for boiler and pressure vessel applications and the ASME / ANSI PTC 25.3 standard for safety valves and relief valves provide the following definition. These standards set performance characteristics as well as defining the different types of safety valves that are used:• ASME I valve - A safety relief valve conforming to the requirements of Section I of the ASME pressure vessel code for boiler applications which will open within 3% overpressure and close within 4%. It will usually feature two blow down rings, and is identified by a National Board 'V' stamp. • ASME VIII valve - A safety relief valve conforming to the requirements of Section VIII of the ASME pressure vessel code for pressure vessel applications which will open within 10% overpressure and close within 7%. Identified by a National Board 'UV' stamp. • Low lift safety valve - The actual position of the disc determines the discharge area of the valve. • Full lift safety valve - The discharge area is not determined by the position of the disc. • Full bore safety valve - A safety valve having no protrusions in the bore, and wherein the valve lifts to an extent sufficient for the minimum area at any section, at or below the seat, to become the controlling orifice. • Conventional safety relief valve - The spring housing is vented to the discharge side, hence operational characteristics are directly affected by changes in the backpressure to the valve. • Balanced safety relief valve - A balanced valve incorporates a means of minimising the effect of backpressure on the operational characteristics of the valve. • Pilot operated pressure relief valve - The major relieving device is combined with, and is controlled by, a self-actuated auxiliary pressure relief device. • Power-actuated safety relief valve - A pressure relief valve in which the major pressure relieving device is combined with, and controlled by, a device requiring an external source of energy. The safety valves are widely used. In the petroleum refining, petrochemical and chemical manufacturing, natural gas processing and power generation industries, the term safety valve is associated with the terms pressure relief valve (PRV), pressure safety valve (PSV) and relief valve. They are also required on water heaters, where they prevent disaster in certain configurations in the event a thermostat should fail. Even you can find it in our life, for example, pressure cookers. Pressure cookers usually have two safety valves. One in a hole upon which a weight sits. The other is a sealed rubber grommet which is ejected in a controlled explosion if the first valve gets blocked .