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Plug Valve

» Plug Valve

Plug Valve

Plug valves have a cylindrical or conical shape plug which can be rotated 90°. In cylindrical valve plugs, the passageway is generally rectangular; while the conical valve plugs, the passageway is trapezoidal. These shapes make the structure of the plug valve lighter, but at the same time it also produces a certain flow loss. The plug valve is most suitable as a cut-off and connection medium and bypass. But depending on the nature of the application and the erosion resistance of the sealing surface, it can sometimes be used for throttling.


 
Table of Content

1. Definition of Plug Valve

2. Plug Valve Working Principles

3. Plug Valve Types

4. Plug Valve Design Standard

5. Advantages of Plug Valve

6. Application of plug valve

7. Plug Valve Installation Guideline

8. Selection Guide of Plug Valve

 
1. Definition of Plug Valve

Plug valve is a quick-opening straight-through valve. Because the movement between the sealing surfaces has a wiping effect, and it can completely prevent contact with the flowing medium when it is fully opened, it can also be used for medium with floating particles. Another important feature of the plug valve is that it can be designed to multi-port pipeline, one valve can design to two, three, or even four ports. This will simplify the design of the piping system, reduce the amount of valves and some connection accessories required in the piping system.
 
2. Plug Valve Working Principles

Use a plug with a through hole as a component for the opening and closing. The plug rotates with the valve stem to achieve opening and closing actions. The plug of the plug valve is mostly a cone (also a cylinder), which combinations with the cone hole surface of the valve body to form sealing. The plug valve is the earliest used valve, with simple structure, fast switching and low fluid resistance. Ordinary plug valves are sealed by direct contact between the machined metal plug and the valve body, so the sealing is poor, the opening and closing force is large, and it is easy to wear.
 
3. Plug Valve Types

Based on application, type of plug valves includes Soft seal plug valve, oil lubricated hard seal plug valve, lift plug valve, 3 way plug valve and 4 way plug valve

A. Soft Seal Plug Valve

Soft-sealed plug valves are often used in harsh environments such as corrosive, highly toxic and high-hazardous medium, where leakage is strictly prohibited, and where the valve material does not contaminate the medium. The valve body can be built from carbon steel, alloy steel and stainless steel according to the working medium

Soft seal plug valve

Soft Seal Plug Valve

a. Low friction bushings material such as F4, F46, PVDF, PP, PO, PE can be used according to the operating temperature and working medium. The friction coefficient of the sealing part is small during the entire working process, hence the plug has smooth rotation and prolong the life span.

b. The design of the inlet and outlet ports of the valve body and the double-port groove seal ring structure, during the rotation of the plug, the pressure between its sealing gradually changes until it reaches the fully open or fully closed position, generating sufficient sealing pressure and reaches zero leakage.

c. The double-port groove seal ring can make the bushing stable in the valve body, but also absorb the slight deformation caused by the temperature difference. At the same time, the friction between the bushing and the plug is strong, and it has an automatic cleaning effect which will prolong the life span of the valve.

d. The unique reverse lip seal design on the top of the plug, combination of a soft sealing material diaphragm and a metal diaphragm and balanced the pressure between the sealing surfaces will make the plug rotate smoothly, and ensure the inlet and outlet ends properly sealed with the connecting flange.

e. Suitable for high-erosion gas/liquid medium. Such as PVC, urea, acetic anhydride, caustic soda production equipment, or external leakage prohibited requirements pipeline, such as HF, phosgene, etc.

f. The medium can flow in both directions, and the installation direction is not limited. It realizes online maintenance and user friendly.

g. The unique design is electric static and fire safe.

B. Oil lubricated metal seal plug valve

Lubricated plug valve can be divided into conventional lubricated plug valves and pressure balanced plug valves. Special grease is injected from the top of the plug between the cone hole and the plug to form an oil film to reduce the opening and closing torque and improve the sealing performance and service life. The working pressure can reach up to 420MPa, the maximum working temperature can reach up to 325℃, and the maximum diameter can reach up to 24”.

Lubricated Plug Valve

Oil Lubricated Plug Valve

① Normal oil lubricated plug valve

a. Normal hard-sealed plug valve, the installation method of the plug cone is standard. In order to reduce the friction between the valve body and the sealing surface of the plug, the designed is tight sealed seat with lubricated oil. High-pressure sealing oil is injected around the plug and form a high pressure seal ring, the layer of oil film between the valve body and the sealing surface of the plug cone will confirm the tight seal and smoothen the open/close operation.

b. In order to further reduce the opening and closing torque of the plug valve, the method of reducing the diameter of the plug is usually used, so the plug valve usually uses a rectangular flow port. This method increases the fluid resistance of the plug valve while reducing the opening and closing torque of the plug valve.

c. The sealing surface of the plug cone is welded with Stellite alloy material or surface hardening to enhance the wear resistance and corrosion resistance of the sealing surface and prolong the life span

d. The sealing surface of the plug cone is grind to achieved mirror finished, and lapping with the valve seat to form a tight sealing and reduce the operation torque.

② Pressure balanced plug valve

In order to reduce the torque of normal hard seal plug valves, pressure balanced plug valves are often used. In addition to the characteristics of normal oil-lubricated plug valves, pressure balanced plug valves have the following features.

Pressure Balance plug valve

Pressure Balanced Plug Valve

a. The installation method of the plug cone of the pressure balanced plug valve is from down to top. There is a check valve on the upper part of the plug cone.

When the valve is closed, due to the difference in cross-sectional area of ​​the plug cone, the injected high-pressure sealing oil causes the plug to receive upward lifting force, and resulting a good sealing.

b. When the valve is opened, the pressure in the lower chamber of the valve body is balanced with the medium pressure in the pipeline. The high-pressure sealing oil in the upper chamber causes the plug to be pushed downward, and a slight gap appears between the plug cone and the sealing surface of the valve body. The torque when turning the plug is reduced and protect the sealing surface.

c. Under high temperature conditions, the thermal expansion of the plug can be absorbed by its rise and fall, to avoid the sealing from jamming.

Oil-lubricated hard-sealed plug valves, although the use of oil lubrication can appropriately reduce the opening and closing torque, but it may contaminate the medium. Therefore, the selection of the sealing lubricant shall be based on actual working conditions.

C. Lift Plug Valve

Lift plug valves have many design. Lifting plug valves are divided into soft seals and hard seals according to the material of the sealing surface. The basic principle is to raise the plug when it is opened, and then rotate the plug 90 degrees to fully open the valve to reduce the friction with the sealing surface of the valve body; when closing the valve, turn the plug 90 degrees to the closed position and then lower it to seal with the valve body seat surface to seal.

Lift Plug Valve

Lift Plug Valve

① Double seal Lift Plug Valve
The double-seal lift plug valve has a soft sealing structure. The plug is a cylindrical plug composed of three plates, and the rubber sealing surfaces are inlaid on the plates on both sides, and the cylindrical wedge plug is in the middle.

When the valve is opened, the transmission mechanism raises the plug, drives the two sides of the plate together, and then detaches the combined plug body and the valve sealing surface. Then drives the plug to rotate 90 degrees to the fully open position of the valve

When the valve is closed, the transmission mechanism rotates the combined plug 90° to the closed position, and then pushes the plug down. After the two side plates contact the bottom of the valve body, it does not move down again. The inlet and outlet ends move, so that the soft sealing surface of the side plate contacts the sealing surface of the valve body and is compressed to achieve the seal

The double-sealed lift plug valve has double blocking and bleed functions. The valve is equipped with a pressure balancing device to prevent abnormal pressure rise of the system and reduce the resistance at the moment of valve opening and closing. Because the metal surfaces contact each other after the sealing ring is compressed, the valve has a fireproof function. The maintenance of the valve is very convenient. Without removing the valve body from the pipeline, the two side plates can be removed and replaced by removing the end cover at the bottom of the valve body. Therefore, it is also widely used in various industries such as petroleum, natural gas, metallurgical mines, gas engineering, light industrial textiles, ship terminals, food and environmental water treatment for regulating and opening and closing pipeline fluids.

② Metal Seated Lift Plug Valve

Metal Seated lift plug valves can be sealed on both sides of the seat sealing surface, the use of the valve is not restricted by the flow direction of the medium. When the metal seated lifting plug valve is closed, the sealing surface of the valve seat can be pressurized by the hand wheel and the stem. By increasing the specific sealing pressure, the plug valve can have better sealing performance. The sealing surface of the valve seat is convex, which is convenient for cleaning the sealing surface, and the sealing surface is not easy to accumulate crystalline medium or solid particles. It is often used in high temperature applications in chemical plants. There are many types of metal seal lift plug valves, and there are two common types.

a. Handwheel and handle lift plug valve, working principle: ascending, make the plug and valve fully open. When closing, first turn the handle to turn the cock 90 degrees to the closed position, and then turn the hand wheel to lower the cock to reach the seal.

During the opening and closing of the valve, the plug is separated from the valve seat, so the opening and closing of the valve is very easy. And the sealing surface is scratched proof during the opening or closing process. So the valve has longer life span. However, the driving torque of the handle is small, so the nominal size of the valve is usually not large. The handwheel handle lifting type plug valve has a compact structure, small volume and low fluid resistance.

b. Mechanical cross-drive lifting plug valve. The working principle of the mechanical cross-drive lifting plug valve is that when the valve is opened, the transmission device turns the thread of the valve stem to make the plug body rise. After the sealing surfaces of the plug and the valve body are separated, the plug is driven to rotate 90 degrees until the valve is fully opened. When closing, the transmission device causes the plug to rotate 90 degrees to the closed position first, and the plug no longer rotates. After the driving force rotates the thread, the plug is lowered and comes into contact with the sealing surface of the valve body to achieve sealing.

The mechanical cross transmission mechanism can be connected to pneumatic actuator, electric motor actuator, hydraulic actuator, etc. for remote control.

The mechanical cross drive lift type plug valve solves the limitation of insufficient input torque of the driving device, and makes the valve develop toward the direction of large diameter and high pressure.
The plug passageway of the mechanical cross drive lifting plug valve is the same as the diameter of the pipeline, and the fluid resistance is small, which is convenient for pigging. The development on oil and gas long-distance pipelines is getting bigger and bigger.

D. Three-way and four-way plug valve

Three-way and four-way plug valve are suitable for complex piping, where the medium flow direction is changed or the medium is distributed. According to the requirements of the working conditions, you can choose soft seal bushing or soft seal, hard seal lift plug valve
 
4. Plug Valve Design Standard

No Type Design Standard
1 Soft Seal Plug Valve BS 5353
2 3 Way Plug Valve JIS B2191/ MSS-SP-78/ BS 5158
3 Lubricated Plug Valve API 6D/ API 599
4 Pressure Balance Plug Valve API 6D
5 Lift Plug Valve ASME B16.34

 
5. Advantages of Plug Valve

 1)  For frequent operation, Rapid and light open close

2)  Small flow resistance

3)  Simple design, smaller size, light weight and maintenance friendly

4)  Tight sealed

5)  No limitation of flow direction

6)  No vibration and low noise
 
6. Application of plug valve

Plug valves are widely used in oilfield, gas field mining, transportation and refining equipment, but also widely used in petrochemical industry, chemical industry, coal gas, natural gas, liquefied petroleum gas, HVAC industry and general industry.
 
7. Plug Valve Installation Guideline

When installing the plug valve, in order to prevent damage to the plug valve and ensure the full performance of the plug valve, the following four points should be noted:

1) Make sure the valve is open. First heat up the pipeline and transfer as much heat as possible from the pipe to the valve. Avoid extending the heating time of the plug valve itself.

2) Use gauze or wire brush to remove the pipe and cutting parts to make the metal surface shine. The use of steel wool is not recommended.

3) First cut the pipe in the vertical direction, trim and remove burrs, and measure the pipe diameter

4) Apply flux to the outside of the pipe and the inside of the welding shield. The flux must completely cover the welding surface. Please use flux sparingly.
 
8. Selection Guide of Plug Valve

According to the structural features and functions, plug valve can be selected according to the following principles

1) If the application is distributing medium and changing the flow direction of medium, its working temperature is not higher than 300℃, nominal pressure PN≤1.6MPa, nominal diameter is not more than 300mm, it is recommended to choose multi-channel plug valve.

2)For the equipment and pipelines of food companies such as milk, juice, beer, and pharmaceutical plants, it is recommended to use austenitic stainless steel, tight-type conical plug valves.

3)In oil field mining, natural gas field mining, oil and gas transportation pipeline, refining and cleaning equipment, the nominal pressure level is not greater than Class300 and the nominal diameter is not greater than 300mm. It is recommended to use oil-sealed conical plug valves.

4)In oil field mining, natural gas mining, pipeline transportation branch pipes, refining and cleaning equipment, the nominal pressure level is not greater than Class1500, the nominal diameter is not greater than 900mm, and the operating temperature is not higher than 340℃. It is recommended to use oil-sealed conical plug valves.

5)In the large chemical industry, pipelines and equipment containing corrosive medium require fast opening or closing speeds. For nitric acid-based medium, a Teflon sleeve seal conical plug valve can be used; for acetic acid For the medium based, Crl8Nil 2M02Ti stainless steel inlaid PTFE sleeve sealed conical plug valve can be selected.

6)In the pipeline neutralization equipment of gas, natural gas, HVAC system, the nominal diameter is not more than 200nm, and the gland packing conical plug valve should be used.

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