Safety valves are an arrangement or mechanism to release a substance from the concerned system in the event of pressure or temperature exceeding a particular preset limit. The systems in the context may be boilers, steam boilers, pressure vessels or other related systems. As per the mechanical arrangement, the safety valves get fitted into the bigger picture (part of the bigger arrangement) called as PSV or PRV that is pressure safety or pressure relief valves.
This type of safety mechanism was largely implemented to counter the problem of accidental explosion of steam boilers. Initiated in the working of a steam digester, there were many methodologies which were then accommodated during the phase of industrial revolution. And since then this safety mechanism has come a long way and now accommodates various other aspects.
These aspects like applications, performance criteria, ranges, nation based standards (countries like United States, European Union, Japan, South Korea provide different standards) etc. manage to differentiate or categorize this safety valve segment. So, there can be many different ways in which these safety valves get differentiated but a common range of bifurcation is as follows:
Lift safety valve
Flow controlled valve
Power operated valve
Full lift solid line valve
Direct safety loaded valves
The American Society of Mechanical Engineers (ASME) I tap is a type of safety valve which opens with respect to 3% and 4% of pressure (ASME code for pressure vessel applications) while ASME VIII valve opens at 10% over pressure and closes at 7 %. Lift safety valves get further classified as low-lift safety valves and full lift safety valves. The flow control valves regulate the pressure or flow of a fluid whereas a balanced valve is used to minimize the effects induced by pressure on operating characteristics of the valve in context.
A power operated valve is a type of pressure relief valve is which an external power source is also used to relieve the pressure. A proportional-relief valve gets opened in a relatively stable manner as compared to increasing pressure. There are 2 types of direct-loaded safety valves, first being safety valve diaphragms and second: bellows valves. Safety valve diaphragms are valves which spring for the protection of effects of the liquid membrane while bellows valves provide an arrangement where the parts of rotating elements and sources get protected from the effects of the liquid via bellows.
In a master valve, the operation and even the initiation is controlled by the fluid which gets discharged via a pilot valve. Now coming to the bigger picture, the pressure safety valves based segment gets classified as follows:
Thermal pressure valves (used for the provision of protection incase of excess pressure if the device experiences increase of temperature).
Flow valves (come into picture where larger quantities of gases or liquids need to be released quickly to relive the growing pressure).
Shut off valves (used to close a fluid or gas based line and to stop the flow of materials).
So all in all, pressure safety valves, pressure relief valves, relief valves, safety relief valves, pilot-operated relief valves, low pressure safety valves, vacuum pressure safety valves etc. complete the range of safety measures in boilers and related devices.