What are the types of plug valves and what are the anti-static devices?

In order to ensure sealing, force must be applied along the axis of the plug body to make the sealing surface in close contact and form a certain sealing specific pressure. According to the different ways of applying force, plug valves can be divided into the following structural forms.

a. Tight fixed form. Tighten the nut at the small end of the plug body so that the plug is in close contact with the sealing surface of the valve body. This kind of plug valve has a simple structure and is generally used in situations no larger than PN6, as shown in Figure 1-1.

Figure 1-1 X13 type tight plug valve

b. Self-sealing. The pressure of the medium itself causes the plug to be in close contact with the sealing surface of the valve body. The medium enters the cavity at the lower end of the big head of the inverted cock body from the small hole in the cock body, and resists the cock body to seal. The greater the medium pressure, the better the sealing performance.

The spring plays a preloading role (Figure 1-2). It is suitable for plug valves with higher pressure and larger diameter.

Figure 1-2 X13 self-sealing plug valve

  c. Filling type. By tightening the nut on the packing gland, the packing pressing plug body is in close contact with the sealing surface of the valve body to prevent internal and external leakage of the medium.

  d. Oil seal type. Inject sealing grease into the valve through the oil injection hole to form a thin oil film between the plug body and the valve body to lubricate and increase sealing (Figure 1-3). This structure saves effort in opening and closing, has reliable sealing performance and long service life. Its operating temperature is determined by the performance of the sealing grease, and is suitable for higher pressure occasions.

Figure 1-3 X47 oil-sealed plug valve

e. Pressure balance type. The flow channel is straight-through, the cock is inverted, the lower part is larger and the upper part is smaller. There is a through hole in the water hole of the cock that leads to the big end or small end of the cock. There is a check valve in the through hole leading to the small end. When closed, the pressure of the medium in the large and small ends is equal. Because the working surface of the big end is large, the total force pushes the cock upward, making the valve easy to seal. At the moment of opening, the big end To relieve pressure, there is a check valve at the small end and the pressure cannot be released. At this time, the total force of the small end is greater than that of the big end, pushing the cock downward to reduce the opening torque and make it easier to open. The valve body and valve cover are connected by bolts and have adjustment pads to ensure the sealing of the valve body and valve cover as well as the sealing between the cock and the valve body.

There is an oil groove between the valve body and the plug sealing surface, which can be injected with sealing grease to enhance the sealing performance. It can withstand very high pressure (PN16~320) and is often used in trunk bypass and sewage and venting systems of natural gas pipelines. Since the diameter-reducing feature of its plug opening cannot meet the requirements for the passage of pigs and internal detectors during pipeline trunk cleaning and internal inspection, it cannot be used for trunk line truncation (Figure 1-4).

Figure 1-4 X47W pressure balanced plug valve

f. Lift type. The top of the plug valve is designed with a lifting mechanism. Before turning the plug, first rotate the handwheel counterclockwise to lift the plug away from the sealing surface of the valve body, and then use the handle to turn the plug counterclockwise 90° to connect the plug channel and the pipeline channel. To open the valve, on the contrary, use the handle to turn the cock 90° clockwise to make the cock channel and the pipeline channel vertical, and then rotate the handwheel clockwise to make the cock drop and engage with the sealing surface of the valve body to close the valve. The plug valve with this kind of structure has small opening and closing torque, small sealing surface wear and long service life. It can be used in high temperature working conditions. It can be used for isolation, extraction, oil and gas isolation, furnace outlet and venting in delayed coking devices in the refining and chemical industry. Position is used a lot (Figure 1-5).

Figure 1-5 X43 type lifting plug valve

Bushing or lined plug valves, the plugs are cylindrical or conical, fully or partially lined with soft materials (plastic, fluoroplastic), this type of lined valve is described in detail in the "Valve Manual - Selection".

Since the valve seat and packing are made of polymer material such as PTFE, the valve plug, valve stem and valve body are insulated from the circuit and cannot conduct electricity. In this case, the friction of the valve opening and closing process can generate static electricity in the valve stem of the valve plug that is high enough to cause sparks, which is more likely with two-phase flow. If the medium flowing through the valve is flammable, the valve must be equipped with an anti-static device, that is, the plug and valve stem can be used to communicate with the valve body circuit.

If the plug valve is used with flammable media, then all valve seals made of polymers are still required to basically seal the media in the event of fire even after complete peeling. The fire resistance quality requirements of plug valves are the same as those of ball valves, which have been explained in the section of ball valves.