Question: In the valve industry, what do the terms single-seat and double-seat refer to?

Single-seat valves have one valve seat, such as globe valves, while double-seat valves have two valve seats, such as gate valves

What about butterfly valves? How to distinguish

A single-seat valve contains only one valve core and one valve seat within its body, which makes it easy to ensure sealing and results in low leakage; however, it cannot completely shut off the flow. Main features: (1) With only one valve core, it is easy to ensure sealing, resulting in low leakage; it is suitable for applications where low leakage levels are required. With further design, it can be used as a shut-off valve. (2) Since there is only one valve spool, the unbalanced thrust exerted by the medium on the spool is large, which results in a low allowable pressure difference ΔP for the valve. Therefore, straight-through single-seat control valves are suitable only for applications with low ΔP; otherwise, an actuator with greater thrust or a valve positioner must be used. However, when the diameter is small, the area over which ΔP acts is also small, allowing such valves to be used in applications with high pressure differences as well. (3) Since there is only one valve orifice, the flow coefficient is not as large as that of a double-seat valve. (4) Under the dual-guidance mechanism, the balance holes in the guidance parts are prone to blockage, resulting in unreliability ； (5) The flow path of the valve body is complex, resulting in poor anti-clogging performance. (6) S-shaped flow path; complex flow structure, large external dimensions, and bulky ； (7) Peripheral throttling: the throttling area is small, making it difficult for particulate media to pass through, which can lead to blockages ； (8) Unbalanced structure with large unbalanced forces, allowing only a small pressure difference. The body of a two-seat valve contains a valve spool and two valve seats. Main features: (1) The valve core and valve seat cannot ensure simultaneous contact, resulting in a leakage rate that is dozens to hundreds of times higher than that of a single-seat valve ； The leakage rate increases when the temperature changes. (2) Force-balanced structure, with low unbalanced force and high allowable pressure difference. (3) The flow path is a double-S shape, making it more complex and prone to blockages; it is also larger in size and more cumbersome. (4) The balance holes in the guiding portion of the dual-guidance mechanism are prone to clogging, resulting in reliability issues. (5) Double-circumference throttling results in a smaller throttling area, making it more difficult for particulate media to pass through and increasing the risk of blockages. (6) The sleeve is pressed against the valve body by the upper valve cover; it has no threaded structure, making maintenance and disassembly easy.

Butterfly valves are of single-seat type, including centerline butterfly valves and eccentric butterfly valves; ball valves are of double-seat type, although there is a type of hemispherical valve that is single-seat and has flow direction requirements

Among ball valve types, there are also those with a single seat, such as the VTI