A safety valve is a valve that acts as a fail-safe. An example of safety valve is a pressure relief valve (PRV), which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds preset limits. Pilot-operated relief valves are a specialized type of pressure safety valve. A leak tight, lower cost, single emergency use option would be a rupture disk.
For liquid-packed vessels, thermal relief valves are generally characterized by the relatively small size of the valve necessary to provide protection from excess pressure caused by thermal expansion. In this case a small valve is adequate because most liquids are nearly incompressible, and so a relatively small amount of fluid discharged through the relief valve will produce a substantial reduction in pressure.
In the petroleum refining, petrochemical, chemical manufacturing, natural gas processing, power generation, food, drinks, cosmetics and pharmaceuticals industries, the term safety valve is associated with the terms pressure relief valve (PRV), pressure safety valve (PSV) and relief valve. The generic term is Pressure relief valve (PRV) or pressure safety valve (PSV). PRVs and PSVs are not the same thing, despite what many people think; the difference is that PSVs have a manual lever to open the valve in case of emergency.
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 ASME, API and other organizations like ISO (ISO 4126) must be complied with. These codes include design standards for relief valves and schedules for periodic inspection and testing after valves have been removed by the company engineer.
Although the lever safety valve was convenient, it was too sensitive to the motion of a steam locomotive. Early steam locomotives therefore used a simpler arrangement of weights stacked directly upon the valve. This required a smaller valve area, so as to keep the weight manageable, which sometimes proved inadequate to vent the pressure of an unattended boiler, leading to explosions. An even greater hazard was the ease with which such a valve could be tied down, so as to increase the pressure and thus power of the engine, at further risk of explosion.
The Salter coil spring spring balance for weighing, was first made in Britain by around 1770. This used the newly developed spring steels to make a powerful but compact spring in one piece. Once again by using the lever mechanism, such a spring balance could be applied to the considerable force of a boiler safety valve.
Paired valves were often adjusted to slightly different pressures too, a small valve as a control measure and the lockable valve made larger and permanently set to a higher pressure, as a safeguard. Some designs, such as one by Sinclair for the Eastern Counties Railway in 1859, had the valve spring with pressure scale behind the dome, facing the cab, and the locked valve ahead of the dome, out of reach of interference.
A drawback to the Ramsbottom type was its complexity. Poor maintenance or mis-assembly of the linkage between the spring and the valves could lead to a valve that no longer opened correctly under pressure. The valves could be held against their seats and fail to open or, even worse, to allow the valve to open but insufficiently to vent steam at an adequate rate and so not being an obvious and noticeable fault. Mis-assembly of just this nature led to a fatal boiler explosion in 1909 at Cardiff on the Rhymney Railway, even though the boiler was almost new, at only eight months old.
Developments in high-pressure water-tube boilers for marine use placed more demands on safety valves. Valves of greater capacity were required, to vent safely the high steam-generating capacity of these large boilers. As the force on their valves increased, the issue of the spring's increasing stiffness as its load increased (like the Naylor valve) became more critical. The need to reduced valve feathering became even more important with high-pressure boilers, as this represented both a loss of distilled feedwater and also a scouring of the valve seats, leading to wear.