What type of seal utilizes graphite impregnated semi-metallic rings?

The correct answer is compression packing. This type of seal uses a combination of materials, including graphite-impregnated semi-metallic rings, to provide a reliable sealing solution in various applications, particularly in situations involving moving parts such as valves and pumps.

Graphite is chosen for its excellent dry lubrication properties and high thermal resistance, while the semi-metallic nature of the rings offers added strength and durability. When used in compression packing, these materials can effectively seal against pressures and temperatures that would typically cause failure in other types of seals.

Compression packing works by being tightly compressed between the shaft and the gland housing, which forms a barrier that prevents leaks of fluids or gases. The unique combination of graphite and metal helps in maintaining a consistent seal even as the packing wears, as it can self-adjust to some degree under operating conditions.

Other sealing types, while useful in their own right, do not utilize the specific combination of materials or mechanism that defines compression packing. This makes it particularly suited for industrial applications where robust and reliable sealing is essential.