The differences between ball valves and plug valves

The working principle of a plug valve

A plug valve is a relatively old type of valve that relies on the rotation of a plug around the centerline of the valve body to achieve opening and closing. The main function of a plug valve is to shut off, distribute, and change the direction of fluid flow. Plug valves have a simple structure, small overall size, and low fluid resistance during operation, requiring only a 90-degree rotation. They are widely used, especially in low-pressure, small-diameter, and low-temperature applications.

The disadvantages of plug valves include the effort required to operate them, the tendency for the sealing surface to wear, the possibility of getting stuck under high temperature and pressure conditions, and their limited suitability for flow regulation.

Plug valves are also known as plug cocks or quarter-turn valves. There are various types of plug valves based on the flow passage, including straight-through, three-way, and four-way configurations. The latter two types are used for fluid distribution and changing flow direction, while the former is used for shut-off purposes.

Plug valves can be classified based on the sealing mechanism. There are three types: fixed type, which relies on tightening a nut under the plug to achieve sealing between the plug and the valve body; packed type, which uses compressed packing to force the plug and the valve body to seal; and self-sealing type, where the sealing between the plug and the valve body is achieved by the force of the medium itself. The medium enters the small hole on the plug located at the inlet, then flows into the lower part of the plug's larger end, pushing it upward and tightly sealing it. A spring underneath provides preloading. This structure is commonly used for air media.

To improve the friction and wear between the plug and the valve body, an oil-injected plug valve is sometimes used. In this type of valve, a layer of oil film separates the plug and the valve body, resulting in smoother opening and closing. However, different working media have varying temperatures and chemical characteristics, and the adaptability of the lubricating oil becomes a key issue for the widespread use of oil-injected plug valves. For some media, viscosity changes significantly with temperature. When the temperature is low, the flowability is poor, or even non-existent (e.g., heavy oil). To meet the needs of such media, factories often use jacketed plug valves, where the medium inside the jacket is heated and insulated by steam or hot water, ensuring smooth flow.

Differences between plug valves and ball valves

Plug valves and ball valves are both widely used and have similar functional characteristics, but there are several differences between them.

The working principles of plug valves and ball valves are different.

Ball valves evolved from plug valves. The closure element of a ball valve is a ball, while plug valves are mainly used for shut-off, distribution, and changing the direction of fluid flow.

In a plug valve, the main closing element is a rotatable conical plug (also known as a plug) with a tapered shape. It has a passage inside with a rectangular cross-section, and there is a hole in the middle. When the plug is rotated 90 degrees, the fluid passage is completely closed. Plug valves control the opening and closing of the valve using a conical plug with a hole. The conical plug matches with the conical sealing surface inside the valve body, and the gap between the plug and the valve body is sealed by packing in the upper part of the valve body. A special square hole wrench is used to rotate the plug. By rotating it a certain angle, the valve can be opened or closed. Types of plug valves include straight-through, three-way, and four-way configurations.

The characteristics of plug valves include a simple structure, rapid opening and closing, low resistance, but high torque required for rotation, and time-consuming plug grinding during maintenance.

The main difference between plug valves and ball valves is that plug valves have better regulating capabilities in terms of shut-off, although the advantage is not significant. The sealing surface of plug valves is much larger than that of ball valves, resulting in better sealing performance but higher torque and less flexibility in operation. It is difficult to achieve large sizes for plug valves.

Ball valves have slightly poorer sealing performance but can be made in larger sizes. With advancing technology, the sealing performance of ball valves has also improved.

In general, plug valves are commonly used in applications that require strict sealing but have relatively small sizes, while ball valves are used in applications where sealing requirements are not as strict but larger sizes are needed.