Safety and Relief Valves

Safety of personnel is the most important issue in an industrial plant. To enhance the personnel safety most of the industrial plants are equipped with safety and relief valves.

In a boiler, there is always a danger of explosion due to excessive build up of pressure inside the boiler. To protect against such situations, boilers are provided with safety valves.

Before we go any further, let’s define the different between a safety valve and a relief valve. Both types of these devices are used to allow reduction in pressure when an overpressure condition occurs. Safety valves are generally used for vapor and gases, while relief valves are generally used for liquids.

American Society of Mechanical Engineers (AMSE) has developed many safety codes for pressure vessels. One of their codes requires that all boiler systems must be equipped with enough safety valves to release all steam which the system is capable of generating. This provision is designed to limit the internal pressure. Under over-pressure conditions, at a set pressure value, the safety valve opens up and releases steam, thereby reducing the internal pressure. When the internal pressure drops to a pre-determined value, the safety valve automatically reseats itself. For example consider a steam drum operating at a normal pressure of 1000 psig. It is possible to equip a steam drum like this with a safety valve which will open at 1100 psig and reseat itself if the pressure dropped to 1030 psig.

Usually large boilers are equipped with a number of safety valves, some having slightly different opening pressures.