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 which conforms both sealing edges of the mating components together. However, very quickly this positive shutoff can be compromised 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 very 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 both 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 for the most part would go unnoticed.
The term "soft-seated" is stating that one or both of the sealing components are constructed from an elastomeric or polymer (plastic) material. Soft-seated designs are preferred when a repeatable positive shutoff is required. Unlike its hard-seated counterpart, no lapping process is needed to achieve a positive shutoff. Rather, the sealing object is soft enough to deform the sealing edge of the seal to the sealing edge of the seat, creating a complete contact ring and thus providing a positive shutoff.
Because of this softness, a soft-seated valve can tolerate moderate amounts of foreign debris damage and will conform to the seat edge time and time again with positive shutoff. However, unlike the hard seated valve, the softer plastic (or elastomer in some valve types) can be more quickly damaged if large pieces of debris pass through. In addition, special consideration must be taken when choosing soft-seated over hard-seated. Although most 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. In addition, many elastomeric seal type valves have temperature ratings much lower and must be used within the vendor defined limits.
In cases where positive shutoff is required, soft-seated valves are the preferred choice. Even in high-temperature applications, the soft-seated valves will outperform a hard-seated counterpart when comparing positive shutoff.
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