The Working Principles and Classification of Plug Valves

The Working Principles and Classification of Plug Valves

Plug valve is a type of quick switch direct valve. Due to the wiping effect between the rotating sealing faces, it can completely prevent contact with the flowing medium when fully open, making it suitable for media with suspended particles. Another important feature is the easy adaptation to multi-channel structures, so that one valve can achieve two, three, or even four different flow paths. This can simplify pipeline system design, reduce the amount of valves needed, and eliminate some connection fittings required in equipment.

Working principle of plug valve

A valve with a plug body with a through-hole as the opening and closing component. The plug body rotates with the valve stem to achieve the opening and closing action. Small non-filled plug valves are also called "cock valves". The plug bodies used in the plug vavle oil and gas industry are mostly conical (cylindrical), which match the conical hole surface of the valve body to form a sealing pair. Plug valves are one of the earliest types of valves, with a simple structure, quick switching, and low fluid resistance. Ordinary plug valves rely on the direct contact between the finely processed metal plug and the valve body for sealing, so the sealing performance is poor, the opening and closing force is large, and it is prone to wear. Therefore, they can only be used in low-pressure (not exceeding 1 megapascal) and small-caliber (less than 100 millimeters) occasions.

Classification of plug valves

According to the structural form, plug valves can be divided into four types: fixed plug valves, self-sealing plug valves, plug valves with oil injection, and clamp sleeve plug valves. According to the channel form, they can be divided into straight-through plug valves, three-way plug valves, and four-way plug valves. There are also clamp sleeve plug valves. Plug valves are classified according to their uses, including soft-sealing plug valves, oil-lubricated hard-sealing plug valves, lift plug valves, three- and four-way plug valves.

1. Soft-sealing

Soft-sealing plug valves are commonly used in harsh environments with corrosive, highly toxic, and highly hazardous media, where leakage is strictly prohibited, and in situations where valve materials will not contaminate the media. The valve body can be made of carbon steel, alloy steel, and stainless steel materials according to the working medium.

2. Oil-lubricated hard-sealing

Oil-lubricated hard-sealing plug valves can be divided into conventional oil-lubricated plug valves and pressure-balanced plug valves. Specially made lubricating grease is injected from the top of the plug body into the conical hole between the valve body and the plug body to form an oil film, reducing the opening and closing torque of the valve, improving the sealing performance and service life. Its working pressure can reach 64MPa, the highest working temperature can reach 325℃, and the maximum diameter can reach 600mm.

3. Lift

Lift plug valves have multiple structural forms and can be divided into soft-sealing and hard-sealing types according to the material of the sealing surface. The basic principle is to make the plug rise when opening the valve, and then rotate the plug 90 degrees to reduce the frictional force with the sealing surface of the valve body during the full opening process. When closing the valve, rotate the plug 90 degrees to the closed position and then lower it to contact the sealing surface of the valve body to achieve sealing.

4. Three-way and four-way

Three-way and four-way plug valves are suitable for changing the flow direction of the medium or distributing the medium in chemical equipment. Soft-sealing lining or soft-sealing, hard-sealing lift plug valves can be selected according to the requirements of operating conditions.