Safety valve

On the Stockton and Darlington Railway, the safety valve tended to go off when the engine hit a bump in the track.

The pivoting arm was commonly extended into a handle shape and fed back into the locomotive cab, allowing crews to 'rock' both valves off their seats to confirm that they were set and operating correctly.

Safety valves also evolved to protect equipment such as pressure vessels (fired or not) and heat exchangers.

The term safety valve should be limited to compressible fluid applications (gas, vapour, or steam).

They are generally sized for use in situations where significant quantities of gas or high volumes of liquid must be quickly discharged in order to protect the integrity of the vessel or pipeline.

In most countries, industries are legally required to protect pressure vessels and other equipment by using relief valves.

[3][4] Today, the food, drinks, cosmetics, pharmaceuticals and fine chemicals industries call for hygienic safety valves, fully drainable and Cleanable-In-Place.

The first safety valve was invented by Denis Papin for his steam digester, an early pressure cooker rather than an engine.

[6][7] Early safety valves were regarded as one of the engineman's controls and required continuous attention, according to the load on the engine.

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.

To avoid tampering, they were often shrouded in tall brass casings which also vented the steam away from the locomotive crew.

Once again by using the lever mechanism, such a spring balance could be applied to the considerable force of a boiler safety valve.

[18][19] It was later common with Salter valves for them to be fitted in pairs, one adjustable and often calibrated for use as a gauge, the other sealed inside a locked cover to prevent tampering.

[21] In 1855, John Ramsbottom, later locomotive superintendent of the LNWR, described a new form of safety valve intended to improve reliability and especially to be tamper-resistant.

A pair of plug valves were used, held down by a common spring-loaded lever between them with a single central spring.

A bellcrank arrangement reduced the strain (percentage extension) of the spring, thus maintaining a more constant force.

[27] All of the preceding safety valve designs opened gradually and had a tendency to leak a "feather" of steam as they approached "blowing-off", even though this was below the pressure.

Their construction was simple: the existing circular plug valve was changed to an inverted "top hat" shape, with an enlarged upper diameter.

When closed, the steam pressure acted only on the crown of the top hat, and was balanced by the spring force.

Once the valve opened a little, steam could pass the lower seat and began to act on the larger brim.

This greater area overwhelmed the spring force and the valve flew completely open with a "pop".

Instead of letting steam reach a feather at the valve, firemen (stokers) now tried to avoid noisy blowing off, especially at platforms or under the large roof of a major station.

This was mostly at the behest of stationmasters, but also because firemen realised that any blowing off through a pop valve wasted boiler pressure.

[30] The Cockburn Improved High Lift design has similar features to the Ross pop type.

Safety valves are required on water heaters, where they prevent disaster in certain configurations in the event that a thermostat should fail.