Safety valves operating under high pressures( both in boilers and processes) above 560 ° F( 293 ° C) can also utilise a special disc design called a flexible disc. In this disc, the fluid’ s own pressure and temperature assist in sealing, allowing the valve to operate with a smaller pressure differential between the operating pressure and the set pressure, increasing the efficiency of the boiler or process in which it is installed. The sealing surface of this type of disc has a certain flexibility, called a thermal lip, caused by the pressure and temperature of the process fluid. This flexibility increases the valve’ s sealing performance, as in addition to thermal flexibility caused by temperature, it also has mechanical flexibility, caused by fluid pressure acting in favor of the spring’ s closing force. Unlike conventional designs( solid discs), in which process pressure acts against this force. Thermal distortions on the sealing surface are also eliminated in this design, as the temperature differential is quickly equalized, ensuring a permanent seal. This type of disc is typically constructed of either Inconel X 718, AISI 347, or quenched and tempered AISI 422. The figure to the side shows a π www. valve-world. net Valve World November 2025
PRESSURE RELIEF VALVES
Disc and nozzle: The main internal components of pressure relief valves
The main internal components of a safety relief valve are: disc, nozzle, adjusting ring( s), spring and its supports, guide, disc support, spring adjusting screw, stem, bellows, and manual actuation lever, in addition to the ring lock screw( s).
By Artur Cardozo Mathias
Sealing Disc
The disc is the movable obturator of a safety relief valve, in direct contact with the nozzle sealing surface when the valve is still closed. This part is housed within the disc support and is exposed to the pressure, temperature, and corrosiveness of the process fluid on its underside and ambient temperature, or to the backpressure and corrosiveness of the environment on its opposite side, inside the valve body. This exposure to varying temperatures can cause thermal distortions on the sealing face in contact with the nozzle in solid disc designs. These distortions are minimised by the minimal thickness design, allowing for temperature equalisation across the entire disc as quickly as possible. The disc and nozzle are often made of the same material, but they can also be made of different materials. In this case, the disc material must be harder because it is fully exposed to the flowing fluid. The fluid flow velocity at the nozzle sealing surface is virtually unchanged compared to the velocity at the disc sealing face, due to the disc’ s movement during valve opening and closing. The closer the disc is to the nozzle sealing surface, the higher the fluid flow velocity, especially during closing. The most commonly used materials for this type of disc are: AISI 304( older models), AISI 316, AISI 347, AISI 422, Bronze, Inconel, 19.9 DL( forged stainless steel), or AISI 17.4 PH stainless steel. The disc is designed to float freely within the disc support, ensuring proper alignment with the guide system and the nozzle sealing surface. An excellent seal and proper seating are possible after the relief process has occurred. The sealing discs of safety valves operating in boilers or some industrial processes have an integral deflector that has the following functions:
• Directing the flow during the valve opening and closing cycle;
• Protecting the disc and nozzle sealing area from erosion due to the high fluid flow velocity at this point;
• Increase fluid flow velocity, thereby helping to reduce pressure, as occurs with nozzle taper;
• Prevent flow turbulence at the nozzle outlet during the valve’ s operating cycle, resulting in rotation of the disc and disc support, which would cause wear on the disc and nozzle sealing surfaces and on the guide surfaces between the disc support and the guide.
Safety valves operating under high pressures( both in boilers and processes) above 560 ° F( 293 ° C) can also utilise a special disc design called a flexible disc. In this disc, the fluid’ s own pressure and temperature assist in sealing, allowing the valve to operate with a smaller pressure differential between the operating pressure and the set pressure, increasing the efficiency of the boiler or process in which it is installed. The sealing surface of this type of disc has a certain flexibility, called a thermal lip, caused by the pressure and temperature of the process fluid. This flexibility increases the valve’ s sealing performance, as in addition to thermal flexibility caused by temperature, it also has mechanical flexibility, caused by fluid pressure acting in favor of the spring’ s closing force. Unlike conventional designs( solid discs), in which process pressure acts against this force. Thermal distortions on the sealing surface are also eliminated in this design, as the temperature differential is quickly equalized, ensuring a permanent seal. This type of disc is typically constructed of either Inconel X 718, AISI 347, or quenched and tempered AISI 422. The figure to the side shows a π www. valve-world. net Valve World November 2025
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