Hard-Seated Valves
The term “hard-seated” or metal-to-metal seated is exactly as it sounds. The valve seat and sealing components, whether it is a plunger, disc, or cone, are constructed of metal. This metal-on-metal seal is used where some leak-by can be tolerated. Positive shutoff, or zero leakage, can be achieved through a lapping process that conforms both sealing edges of the mating components. However, this positive shutoff can be compromised quickly in service as foreign debris and high velocity media flow can negatively alter these surfaces.
Hard-seated valves are very rugged and will provide a long service life even after foreign debris passes through the seat bore. Typically, both the seat and seal are constructed of very hard and tough materials that can hold up to high temperature and high pressure applications. It is this hardness that makes a continuous positive shutoff difficult. Those who choose hard-seated valves are accepting the lack of positive shutoff in exchange for durability. In many applications, a small amount of leak-by can be tolerated and may go unnoticed.
Soft-Seated Valves
The term “soft-seated” indicates that one or both of the sealing components are constructed from an elastomeric or polymer material. Soft-seated designs are preferred when a repeatable positive shutoff is required. Unlike a hard-seated design, no lapping process is needed to achieve positive shutoff. The sealing object is soft enough to deform to the sealing edge of the seat, creating a complete contact ring and providing a positive shutoff.
Because of this softness, a soft-seated valve can tolerate moderate amounts of foreign debris damage and will continue to conform to the seat edge with positive shutoff. However, the softer plastic or elastomer can be more quickly damaged if large pieces of debris pass through. Temperature limitations must also be considered. Although many soft-seated valves are rated to 450°F, the seal material becomes softer as the application approaches these temperatures and is more susceptible to extrusion and creep. Many elastomeric seal types have lower temperature ratings and must be used within the limits defined by the vendor.
In applications where positive shutoff is required, soft-seated valves are the preferred choice. Even in high temperature applications, soft-seated valves will often outperform a hard-seated counterpart when comparing positive shutoff.
Key Takeaways for Selecting a Valve Seat Type
Both hard-seated and soft-seated valves have clear strengths and limitations. Hard-seated valves offer durability in demanding environments where some leak-by can be tolerated. Soft-seated valves provide reliable, repeatable shutoff where sealing performance is critical. Understanding these differences helps ensure that the correct seat design is selected for the application.