Materials processed in Chemical, Process, Petroleum or Petrochemical industries for converting raw materials into useful products are generally Fluidic in nature. Fluids flow through pipes from one equipment to another and it can be processed to be heated, cooled, evaporated, condensed, mixed, expanded, compressed or reacted.
All fluids whether at rest or flowing have pressures associated with them. Each and every equipment has a permissible value of pressure which it can withstand without breaking apart. Normally, this safety limit is provided in terms of Design Pressure.
The Operating Pressure associated with the fluid in any equipment whether the fluid is at rest or flowing should be smaller than the given Design Pressure. If the operating pressure becomes equal to or larger than design pressure then the system is said to be over-pressurized.
The principle type of device used to prevent overpressure in plant is the safety or safety relief valve. The safety valve operates by releasing a volume of fluid from within the plant when a predetermined maximum pressure is reached, thereby reducing the excess pressure in a safe manner. As the safety valve may be the only remaining device to prevent catastrophic failure under overpressure conditions, it is important that any such device is capable of operating at all times and under all possible conditions.
In steam systems, safety valves are typically used for boiler overpressure protection and other applications such as downstream of pressure reducing controls. Although their primary role is for safety, safety valves are also used in process operations to prevent product damage due to excess pressure. Pressure excess can be generated in a number of different situations, including:
An imbalance of fluid flowrate caused by inadvertently closed or opened isolation valves on a process vessel.
Failure of a cooling system, which allows vapour or fluid to expand.
Compressed air or electrical power failure to control instrumentation.
Transient pressure surges.
Exposure to plant fires.
Heat exchanger tube failure.
Uncontrollable exothermic reactions in chemical plants.
Ambient temperature changes.
The terms 'safety valve' and 'safety relief valve' are generic terms to describe many varieties of pressure relief devices that are designed to prevent excessive internal fluid pressure build-up. A wide range of different valves is available for many different applications and performance criteria. Furthermore, different designs are required to meet the numerous national standards that govern the use of safety valves.
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