The primary purpose of a safety valve is the protection of life, property and environment. A safety valve provides a layer of protection and in many cases the last protection device. It is important to ensure that the safety valve is capable to operate at all times and under all circumstances. A safety valve should not be considered as a process valve or pressure regulator and should not be misused as such. Safety valves have to be durable and reliable, and stringently maintained and tested, including recertification where applicable for different industries.
A safety valve is an automatic valve, safety valves are used in domestic and industrial use.
There is a wide range of safety valves available to meet the many different applications and performance requirements demanded by different industries. Importantly it should be noted national and industry standards define many varying types of safety valve. The following is not intended to be an exhaustive list but highlight the large range of safety valve types.
• Conventional safety relief valve - The spring housing is vented to the discharge side, hence operational features are directly affected by changes in the backpressure to the valve.
• 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 satisfactory for the minimum area at any section, at or below the seat, to become the controlling orifice.
• Balanced safety relief valve -A balanced valve encompassing a means of minimising the effect of backpressure on the operational characteristics of the valve.
• Pilot operated pressure relief valve -The main relieving device is combined with, and is controlled by, a self-actuated secondary pressure relief device.
• Power-actuated safety relief valve -A pressure relief valve in which the main pressure relieving device is combined with, and controlled by, a device requiring an external source of energy.
• Standard safety valve -A valve which, following opening, reaches the degree of lift necessary for the mass flowrate to be discharged within a pressure rise of not more than 10%.
• Full lift safety valve -A safety valve which, after commencement of lift, opens rapidly within a 5% pressure rise up to the full lift as limited by the design. The amount of lift up to the rapid opening (proportional range) shall not be more than 20%.
• Direct loaded safety valve -A safety valve in which the opening force underneath the valve disc is opposed by a closing force such as a spring or a weight.
• Proportional safety valve -A safety valve which opens more or less steadily in relation to the increase in pressure. Unexpected opening within a 10% lift range will not occur without pressure increase. Following opening within a pressure of not more than 10%, these safety valves achieve the lift necessary for the mass flow to be discharged.
• Diaphragm safety valve -A direct loaded safety valve where linear moving and rotating elements and springs are protected against the effects of the fluid by a diaphragm.
• Bellows safety valve -A direct loaded safety valve where sliding and (partially or fully) rotating elements and springs are protected against the effects of the fluids by a bellows.
• Controlled safety valve -Consists of a main valve and a control device. It also includes direct acting safety valves with additional loading in which, until the set pressure is reached, an additional force increases the closing force.
• Emergency Shutdown valve - ESD/ESV valves are the final defence against process upsets. These valves have a function which requires much more reliable performance than standard remotely operated on-off valves. Still today ESD valves are ranked often as ordinary shut-off valves in specifications. Valves as such may still remain the same but being a part of Safety Instrumented Function (SIF) the performance expectations are much higher. ESD/ESV's used in a SIF, must meet and maintain low failure rates which require them to be tested to ensure they meet and maintain low probability of failure on demand. Testing is the key to ensuring a SIF meets or exceeds its SIL (safety integrity level) value.
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