The safety relief valve is one of the most important devices on any boiler. It is the boiler’s last measure of protection against overpressure. It must be adequately sized and of the correct pressure rating for the boiler. But getting a safe installation is only the beginning.
The safety relief valve also must be inspected and tested regularly. Mud and scale from the boiler can interfere with the operation of the relief valve. Plugged discharge lines can prevent proper operation or allow discharged water and steam to come in contact with equipment or operating personnel.
Lifting the test lever while the boiler is operating will confirm its proper operation. At no time should technicians test the valve by increasing the pressure of the boiler to a level higher than the safety-valve setting.
They should exercise caution when testing relief valves, as steam or hot water will be discharged through the valve at the operating pressure of the boiler. Valves should be tested every time a boiler is started and at the interval recommended by the manufacturer.
This section provides you with the knowledge you need to inspect and test relief valves and safety valves. Specifically, this section covers:
- How relief valves and safety valves are categorized and characterized
- How relief valves and safety valves are constructed and how these valves operate
- Data that is marked on relief valves and safety valves to permit accurate identification and assessment for proper application
- Relief and safety valve terminology
The table below provides terminology associated with relief and safety valves.
| TERM | DEFINITION |
|---|---|
| Fluid | Any substance that will flow and assume the shape of the container it is held in is known as a fluid. This term includes both liquids and gasses. |
| Liquid | A liquid is a substance with no definite shape but a definite volume. A liquid will take the shape of its container. Pressure is exerted on all surfaces that contact the liquid. |
| Gas | A substance that has no definite volume or shape. A gas must be enclosed in a container to prevent escape to the atmosphere. A gas exerts pressure on all sides of a container; it can also be compressed. |
| Vapor | A fluid that is a combination of gas with liquid mixed in it. The liquid is said to be entrained in the gas. |
| Blowdown | This term relates to both safety and relief valves. It is defined as the difference in pressure between the valve’s set pressure, and the pressure at which a valve reseats. It is expressed as a percentage of the valve’s set pressure. |
| Accumulation | This term relates to both safety and relief valves. It is defined as the difference in pressure between the pressure at which the valve is fully open, and the valve’s set pressure. It is expressed as a percentage of the valve’s set pressure. |
| Set Pressure | The pressure at which a safety or relief valve first begins to open in response to an overpressure condition. This is also known as a valve’s setpoint. |