PRESSURE RELIEF VALVES
• Fluids containing suspended particles;
• Cases in which ice may form on the sealing surfaces after the valve releases pressure, such as during gas discharge;
• Corrosive fluids;
• Stresses from heavy, poorly supported discharge piping that may cause the valve internals to misalign.
Note: The resilient seat allows for a tight seal even in installations subject to vibration. However, this same resilient seat, in these same applications, does not prevent a reduction in the valve’ s set pressure when exposed to excessive vibration.
Figure 1. Sealing discs with metal seat and resilient seat.
photo of a safety valve with a flexible disc, where the“ thermal lip” and the integral deflector can be seen: This design offers another advantage over the solid disc design, in which the lower tip of the stem, which transmits the spring force to the disc, ends at a value equal to or below the horizontal line of the nozzle sealing surface, providing a distribution of forces concentric to the disc, keeping it centered with the nozzle sealing surfaces, thus preventing vibrations in the horizontal direction. l are minimized, allowing greater repeatability of the set pressure value, in addition to providing better sealing and ensuring repeatability of the valve closing pressure value.
Disc with resilient seat seal
The seal in conventional disc designs can be metal-to-metal or resilient. Metalto-metal is most commonly used in the case of vapours because the temperature cannot be tolerated by elastomer sealing rings, which are typically made of Viton, Silicone, Kalrez, Buna-N, or thermoplastics such as PTFE. Valves with metal-to-metal seals have lapped contact surfaces to achieve the highest degree of tightness with a small force differential( between the operating pressure of the protected equipment and the valve opening pressure), acting between the internal sealing area of the nozzle and the force exerted by the spring. In valves with a resilient seat on the disc, the nozzle sealing surface must also be lapped. Depending on each manufacturer’ s design, part of the force exerted by the valve is absorbed by the metal sealing surfaces between the disc and nozzle, with a small portion of that force being absorbed by the resilient seat. Safety and relief valves with a soft seat are not recommended for use with steam. Those made of PTFE are recommended only for corrosive fluids and the valve set pressure must be at least 100 psig( 7.03 kgf / cm 2). This minimum pressure is due to the hardness of PTFE. A resilient seal is used when maximum valve tightness is desired, such as in the following cases:
• Fluids that are difficult to confine, such as hydrogen, helium, ammonia, compressed air, and toxic gases, among others;
• When the operating pressure fluctuates significantly and approaches the valve set pressure;
• In installations subject to excessive vibration;
• Installations subject to vacuum;
Seat selection
The resilient seat must be selected according to its chemical compatibility with the process fluid, as well as the pressure limits( according to the nozzle passage area) and temperature supported by the materials. Therefore, the larger the valve size, the lower the set pressure limit. Therefore, if, according to the process flow capacity required, the user needed to select a 6” x 8” valve, with an“ Q” orifice( 11.05 in 2) adjusted to open at 45 kgf / cm 2, they would need to specify and install two 4” x 6” valves, with an“ P” orifice( 6.38 in 2), with staggered adjustments.
Nozzle
The nozzle in safety and relief valves, like the disc, is the part that is in direct contact with the fluid, whether the valve is closed or open and discharging. There are two types of nozzles used: the full nozzle and the half nozzle.
Full Nozzle
This type of nozzle is a rigid stainless steel part that prevents the process fluid from contacting the valve body while the valve is closed and sealing. They are generally internally tapered to increase fluid flow velocity at the outlet, helping to reduce pressure more quickly and
Figure 2. Set of main internal components of a pressure relief valve( PRV).
36 Valve World November 2025 www. valve-world. net