Preface
When mankind learned how to boil water to create steam, safety devices like safety valves became apparent. China used hinged lids on cauldrons to produce steam more safely as far back as 2000 years ago. As for safety devices on pressurized vessels, chemists used conical plugs and compressed springs at the beginning of the 14th century.
Boiler explosions on ships and locomotives frequently occurred. Because of faulty safety devices early in the 19th century. Which led to the development of safety relief valves.
The accumulation chamber was invented by Charles Ritchie in 1848. Which increased the compression surface within the safety valve. Allowed it to open rapidly within a narrow overpressure margin.
As a result of local health and safety regulations, most steam users must incorporate safety devices. Precautions into their plants and processes in order to prevent dangerous situations.
Therefore, the primary purpose of a safety valve is to protect life and property.
In plants, the safety or safety relief valve is the main type of device used to prevent overpressure. Upon reaching a predetermined maximum pressure, the safety valve releases a volume of fluid from within the system. Thereby reducing the excess pressure in a safe manner. It is vital that any such device can operate under all conditions and at all times. Since the safety valve may be the only device left to prevent catastrophic failure under overpressure conditions.
Benefits and Working Criteria of Safety Valves
In systems or pressure vessels where the maximum allowable working pressure (MAWP) is likely to be exceeded, safety valves should be installed. Steam safety valves are typically used to protect boilers from overpressurization and for downstream pressure-reduction controls. In addition to preventing product damage. Due to excess pressure, safety valves can also be used in the process industry. Excess pressure can be generated in several different situations, including:
- It is possible for isolation valves on a process vessel to be inadvertently closed or opened, thus causing an imbalance of fluid flow rate.
- Failure of a cooling system that allows vapor or fluid to expand.
- Unable to control instrumentation with compressed air or electricity.
- Pressure spikes due to transient events.
- Plant fire exposure.
- Tube failure in heat exchangers.
- Chemical plants experience uncontrollable exothermic reactions.
- Changes in ambient temperature.
Safety valves and safety relief valves are terms used to describe a wide variety of pressure relief devices. Intended to prevent excessive internal fluid pressure buildup. For a wide range of applications and performance criteria, valve manufacturers offer a wide range of valves.
In addition, there are numerous national standards. That govern the use of safety valves that require different designs.
For safety and safety relief valves, specific definitions are provided in most national standards. The terminology used in the USA and Europe differs significantly. A safety valve that in Europe would be known as a ‘safety valve’ in the USA would be called a ‘safety relief valve’ or a ‘pressure relief valve‘. A safety valve is also commonly referred to as a full-lift valve in the United States.
The following terms are defined in the ASME/ANSI PTC25.3 standards applicable to the USA for Safety Valves:
- Pressure relief valve – An automatic pressure relief valve that opens under excess pressure and closes again once normal conditions have been restored. Rapid-opening ‘pop actions’ are characteristic, as well as proportional opening to the increase in internal pressure. Depending on the design, adjustment, or application, it can handle compressible or incompressible fluids.
- Safety valve – This type of valve opens quickly when actuated by static pressure at the inlet.
Steam and air are most commonly serviced by safety valves. Which are designed to handle compressible gases. Additionally, they are used in process-type applications in which they may be needed to protect the plant or to prevent spoilage of the product being processed.
- Relief valve – By increasing the opening pressure, a gradual lift is induced in a pressure relief device that is actuated by static pressure at the inlet.
Relief valves are common in liquid systems. Especially for lower capacities and thermal expansion. Pumped systems can also use them as pressure overflow devices.
- Safety relief valve – Pressure relief valves are characterized by rapid opening or popping, or by opening in proportion to the pressure increase over the opening pressure, depending on the application, and may either be liquid or compressible.
When used in a compressed gas system, safety relief valves will perform as safety valves. However, when used in liquid systems, they will operate as relief valves in proportion to the overpressure.
According to the European standard EN ISO 4126-1:
- Safety valve – It is designed to automatically discharge a quantity of fluid so that a predetermined safe pressure is not exceeded and to re-close and prevent further fluid flow after normal pressure conditions of service have returned.
