The conventional plug type of the plug valve is in the shape of a reverse cone, and it can produce the closed sealing pressure by the pressure of the media.
 
Conventional plug valves can be divided into the plug valves with lubricant and those without lubricant. Plug valves with lubricant can have the metal-to-metal seal, and are suitable for high temperature and high pressure medium conditions. However, their operating temperature is limited by the thermal stability of the lubricant. Attention also should be paid to whether media are sensitive to pollution of the lubricant or not. The lubricant plays an important in making valves move smoothly and have good sealing performance.

Plug valves without lubricant usually have the non-metallic linings. They are often applied to the working conditions without high media pressure or high temperature, but with strong corrosion. However, their operating temperature is limited by the cold flow properties and thermal stability of the non-metallic linings.
 
Besides the above mentioned conventional plug valves, other plug valves such as the lift plug valves and the pressure balanced plug valves are also used in the practical engineering. All these valves are specially designed based on the principle of reducing the wear of the internal parts' sealing surfaces, or are the patented designs of the valve manufacturers. If these valves are needed, it is needed to consult the plug valve manufacturers.
 
The lift plug valves can reduce or avoid the wear of the sealing surfaces through making the valve stems equipped with the structure similar to the orbit ball valves' so that the valve clacks can be separated from the valve seats' sealing surface before they start to rotate. For lift plug valves, even if they have the metal-to-metal seal, there is no need to inject the lubricant into them. Thus, they will not be affected by the thermal stability of the lubricant and you do not have to worry about the pollution brought by the lubricant to the media.
 
The balanced pressure plug valves adopt the reverse cone plug which is set with the balanced pressure lead holes. The position of the plug can be adjusted through the pressure of the media. When the needed closed sealing pressure forms, the wear of the sealing surfaces is also reduced. However, in order to further decrease the wear of the plug valves' sealing surfaces, it is recommended to use the lubricant. The balanced pressure plug valves usually have the metal-to-metal hard sealed structure and are applicable to the high pressure medium conditions.
 
Under normal circumstances, plug valves with the reverse cone plugs are the default choices for valve manufacturers, while the lift plug valves and the balanced pressure plug valves are the special kinds of plug valves. Therefore, if the lift plug valves and the balanced pressure plug valves are used, it should be indicated in the valve data sheet. Besides indicating the plug type, whether it is needed to inject the lubricant and adopt the lining structure are needed to be indicated in the valve data sheet at the same time.