The Structure of DBB Plug Valves

1. Overview
In pipeline engineering, two isolation valves such as gate valves or ball valves and a discharge valve installed between the two isolation valves are often used to achieve the function of double block and bleed, as shown in Figure 1. This kind of structure is complicated and has high costs, complicated operations, and easy maintenance. The forced sealing double block and bleed plug valve is a high-performance valve designed for applications that require good sealing (zero leakage) and double block and bleed. It uses a unique telescopic seal, which eliminates friction and wear between the sealing surfaces during the opening and closing process; the upstream and downstream independent directly vulcanized rubber soft seats and metal hard seat are combined to meet the requirements for good sealing and fire resistance for the upstream and downstream; a variety of mid-chamber pressure relief devices are available to meet different applications, and the relief system can also be used to verify the sealing of the closing section.
 

Figure 1 Isolation valves
 
The double block and bleed plug valve is mainly used in occasions where the medium is cut off and good sealing is required. It is suitable for places where it is flammable, explosive and volatile, which can meet the requirement for being waterproof and explosion-proof and can be widely used in systems that require good sealing, such as storage tank isolation, multi-product manifolds, oil station terminals, offshore platforms, precision mixing devices, airport oil supply pipelines, etc. At present, many manufacturers manufacture double block and bleed plug valves in China, and some manufacturers with good R&D capabilities have a complete series. The product diameter has covered up to 36 inches (DN900). The technical level of the valves has reached the international advanced level, and some of them have even surpassed imported valves, involving many fields such as petroleum, chemical industry, natural gas, electric power, etc. The use of double block and bleed valves is introduced in this article, combined with the experience of on-site use of this type of valve. The structure of this valve is described. Its installation and usage methods and precautions are introduced, reducing the misoperation of this type of valve during on-site use, and at the same time providing certain technical guidance for equipment maintenance personnel.
 

2. An introduction to product structure

The double block and bleed plug valve is mainly composed of three parts: the main valve, the operator and the actuator, as shown in Figure 2. The actuator provides drive power. The operator converts the driving force into the thrust and torque of the main valve to realize the opening and closing of the main valve.

 
Figure 2 Double block and bleed plug valves
 
2.1 Main valve structure
The main valve of the double block and bleed plug valve mainly consists of the valve body, valve bonnet, plug, sealing slide and relief device, as shown in Figure 3.
 

Figure 3 Main valve structure
 
The valve body and end cover are the pressure-bearing parts of the main valve, and the plug and sealing slide are installed in the valve body and end cover. The plug and the sealing slide are connected using a dovetail groove connection structure. The profile is a tapered surface, which can ensure that the sealing slide and the plug are effectively disengaged in the opening and closing process, and realize friction-free movement between the sealing slide and the valve body. The sealing of the main valve is mainly achieved through the compression deformation of the rubber directly vulcanized on the sealing slide.
 
2.2 Operators
The operator of the double block and bleed plug valve mainly consists of the operator shell, the driving screw, the driving screw sleeve, the driving roller, the worm gear set, and the valve position indicator, as shown in Figure 4.
 

Figure 4 The structure of the operator
 
When the plug valve is opened and closed, the plug needs to perform lifting and rotating movements. The function of the operator is to convert the multi-turn motion of the actuator into a 90° rotational motion and up and down linear motion. The driving screw accepts the input torque of the actuator, and cooperates with the driving screw sleeve and the driving roller to realize the rising, falling, forward and reverse rotation of the driving screw sleeve. The driving screw sleeve of the operator is connected to the main valve plug through a cylindrical pin. The screw drive has requirements for a great lead and high strength, so multi-head trapezoidal thread drive is generally used. The function of worm gear transmission is torque amplification.
 
2.3 Relief devices
The relief device of the double block and bleed plug valve mainly consists of a manual relief valve, an isolation valve, an automatic relief valve, a pressure gauge and related pipeline connecting parts. The manual relief valve is used to manually relieve the pressure in the intermediate chamber and can also be used to check the sealing of the main valve when it is closed. The automatic relief valve is a one-way valve. When the pressure in the middle chamber is higher than the inlet pressure, the one-way valve opens and releases the pressure in the middle chamber to the main valve inlet.