Plug Valve: Principle, Classification & Advantages

• 2024-12-19

A plug valve is a straight-through valve with quick opening and closing functions. Thanks to the wiping effect generated by the movement between its sealing surfaces, and the complete isolation from the flowing medium when fully open, it is also suitable for media containing suspended particles.

A plug valve is a straight-through valve with quick opening and closing functions. Thanks to the wiping effect generated by the movement between its sealing surfaces, and the complete isolation from the flowing medium when fully open, it is also suitable for media containing suspended particles. Another important feature of this valve is its easy adaptability to multi-channel structures, enabling a single valve to achieve two, three, or even four different flow paths. This helps simplify pipeline system design, reduce the number of valves used, and cut down on connecting fittings required in the equipment.

Working Principle

It is a type of valve that uses a plug with a through-hole as its opening and closing element. The plug rotates along with the valve stem to realize the opening and closing action. A small-sized plug valve without packing is also called a "cock". The plug of a plug valve is mostly conical (or cylindrical) and forms a sealing pair by matching with the conical bore surface of the valve body. As one of the earliest valves to be used, the plug valve features simple structure, quick opening and closing, and low fluid resistance. Conventional plug valves achieve sealing through direct contact between the precision-machined metal plug and the valve body, resulting in poor sealing performance, high opening and closing torque, and easy wear. Therefore, they are generally only suitable for low-pressure (not exceeding 1 MPa) and small-diameter (less than 100 mm) applications.

Classification

According to the structural form, it can be divided into four types: screwed plug valve, self-sealing plug valve, lubricated plug valve, and grease-injection plug valve. According to the flow path form, it can be classified into three types: straight-through plug valve, three-way plug valve, and four-way plug valve. There is also the compression-type plug valve.

According to application scenarios, plug valves include: soft-sealed plug valve, oil-lubricated hard-sealed plug valve, lift-type plug valve, and three-way & four-way plug valve.

Soft-sealed Type

Soft-sealed plug valves are often used in harsh environments involving corrosive, highly toxic, and high-hazard media, as well as occasions requiring strict zero leakage and where valve materials will not contaminate the medium. The valve body can be made of carbon steel, alloy steel, or stainless steel according to the working medium.

Oil-lubricated Hard-sealed Type

Oil-lubricated hard-sealed plug valves can be divided into conventional oil-lubricated plug valves and pressure-balanced plug valves. Special grease is injected from the top of the plug into the gap between the valve body's tapered bore and the plug, forming an oil film to reduce the valve's opening and closing torque, enhance sealing performance, and extend service life. Its maximum working pressure can reach 64 MPa, the maximum operating temperature can reach 325 °C, and the maximum diameter can reach 600 mm.

Lift-type

Lift-type plug valves come in various structural forms, and can be divided into soft-sealed and hard-sealed types based on the material of the sealing surface. Its basic principle is that during opening, the plug is lifted first, and then rotated 90 degrees to the fully open position, which can reduce friction with the valve body's sealing surface; during closing, the plug is rotated 90 degrees to the closed position and then lowered to contact the valve body's sealing surface to achieve sealing.

Three-way & Four-way Type

Three-way and four-way plug valves are suitable for occasions in industrial installations where the direction of medium flow needs to be changed or medium distribution is required. According to the requirements of the operating conditions, soft-sealed bushings, soft-sealed, or hard-sealed lift-type plug valves can be selected.

Advantages

1. Plug valves are suitable for frequent operation, with quick and easy opening and closing.

2. Plug valves feature low fluid resistance.

3. Plug valves have a simple structure, relatively small volume, light weight, and are easy to maintain.

4. Plug valves offer excellent sealing performance.

5. Plug valves are not restricted by installation direction, and the medium can flow in any direction.

6. Plug valves operate without vibration and produce low noise.