A safety valve is an accessory which protects pressurised equipments from the overtaking of an admitted pressure limit, being therefore, a device for the emergency discharge of pressurised fluids, designed to act automatically when a set pressure is reached (valve set pressure).
How a safety valve works
- The pressure applied by process medium inside the valve body acts on the disc surface, generating a force F.
- When F reaches the same intensity of the spring’s force (spring is fitted inside the valve and previously adjusted by compression to a set value) the disc begins to lift from the seat/nozzle tightness area and the process medium starts flowing (this is not anyway, the valve’s maximum flow rate).
- At this point normally, the upstream pressure keeps raising, causing with an increase of about 10%-(named overpressure) over the set pressure , the sudden and complete disc lift which lets the process fluid pass through the valve’s minimum section.
- When the safety valve capacity is equal to the flow rate to be discharged, the pressure inside protected equipment, is kept constant. Otherwise if safety valve is capacity is higher than the flow rate to pass, the pressure inside the equipment tends to decrease. When this occurs, the disc, on which the spring force keeps acting, begins to reduce its lift (i.e the distance between seat and disc) until the valve’s flowing section would be closed (generally a decrease – named blowdown- equal to 10 % less than set pressure) and the process fluid stops blowing.