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• If pressure inside a tank rises due to any reason, it may burst the tank from its weakest part or cause leakage from where it is possible.
• The content thus coming out is a material loss and in addition, it may create fire, explosion or toxic hazard.
• Therefore to avoid such situation, pressure relief devices are necessary.
• A ‘Safety Valve’ is a common pressure relief device.
• It can be set to a desired pressure and when pressure exerted on it exceeds the pre-set value, it automatically opens and allows the pressure to release in atmosphere or in a catch pot or drowning tank if the content coming out is hazardous or hot.
• It automatically closes down also after release of excessive pressure.
•Safety valves are used to release gas or vapor but not liquid.
• A ‘Rupture disc’ is required for the fast release or more flow from a bigger size hole or if internal pressure is too high or too rapid or the material is sticky and chokes the safety valve.
• This disc is selected based on many parameters (like type of chemical, W.P, temp, reaction, viscosity, corrosivity, toxicity etc.)
• One disc can be used for one pressure i.e its set pressure can not be changed like safety valve, and after rupture the same disc can not be re used.
• Once opened, it can not reset at the lowered pressure like safety valve and will allow the whole mass to come out till the hole is closed or disc is replaced.
• This is its disadvantage.
• Therefore, it is inadvisable on tanks containing flammable gases or liquids.
• Rupture disc can be used in conjunction with safety or relief valve.
• Then the disc will burst first without affecting the valve.
• Pressure gauge is to be provided between the disc and the valve to indicate that the disc has opened and what is the bursting pressure.
• The rupture disc is used to release gas, vapor or liquid.
• ‘Relief Valves’ do not full open at set pressure like safety valve but open slightly and then further as the pressure increases.
• Relief valves are used for liquid discharge and not for gas or vapors.
• ‘Safety relief valves’ can be used either as a safety valve or relief valve, depending upon the application,.
• They are used for gases, vapor and liquid.
• ‘Fusible Plug’ is a fitting filled with an alloy that melts at a predetermined temperature (not pressure) and gives way to the materials to come out.
• Fusible plugs are used in boilers, domestic pressure cookers and compressed gas cylinders to prevent violent bursting.
• They are used for gas, vapor and liquids of high temperature.
• ‘Vacuum Breaker’ is also a type of pressure relief device and works like a spring loaded safety or relief valve but in reverse direction.
• The outside atmospheric air pressure being higher than that inside the vessel, opens it to break the vacuum.
• This is required when the vacuum may increase to collapse the vessel.
• This device is used for air only.
