What are plug valves?

What are plug valves?

The plug valve is a kind of valve with closing parts or plunger-shaped rotary valve. By rotating 90 degrees, the passage on the plug is connected or separated from the passage on the valve body to open or close.

20190413112753 94979 - What are plug valves?

Characteristics of plug valves
Working Principle of plug valve
Classification of plug valves
Advantages of plug valve
Installation of plug valve
Selection Principle of plug valve
Maintenance of plug valves

The shape of the valve plug can be cylindrical or conical. In a cylindrical plug, the passage is usually rectangular; in a conical plug, the passage is trapezoidal. These shapes make the structure of the plug valve lighter. It is most suitable for use as cutting and connecting medium and shunting, but sometimes it can also be used for throttling according to the applicable nature and erosion resistance of sealing surface.
The plug valve is a kind of through valve with fast switching. It can also be used in the medium with suspended particles because of the wiping effect of the movement between the rotary sealing surfaces and completely preventing contact with the flowing media when it is fully opened. Another important feature is that it is easy to adapt to multi-channel structure, so that a valve can obtain two, three, or even four different channels. This can simplify the design of the pipeline system, reduce the use of valves and some of the connecting parts needed in the equipment.

Characteristics of plug valves

  • 1) Fluid passes in straight way, small resistance, easy to open and close quickly.
  • 2) Plug valve is mainly used in low pressure, and medium temperature is not high.
  • 3) Plug valve in the pipeline is mainly used to cut off, lift and change the direction of medium flow. It is easily adaptable to multi-channel configurations so that one valve can have two, three, or even four different flow paths, simplifying the design of the piping system and reducing the number of valves and fittings required in the equipment.

Working Principle of plug valve

Plug valves are one of the most widely used valves for both on/off and throttling services. Their design is fairly simple. Plug valve is comprised of three main parts: body, cover and plug. The plug is a cylindrical, tapered, or generally cone-shaped device that can be rotated inside the valve body to control flow through the valve. The plugs have one or more hollow passageways going sideways through the plug, so that fluid can flow through the plug when the valve is open. The valve is opened by rotation, with the plug itself being the only element that is capable of movement. Plug Valve is simple and often economical. Early models of plug valve used metal-to-metal seals that were nonlubricated. This design is still used, but problems with galling and sticking limit their usefulness.
In order to expand the application range of plug valve, many new structures have been developed. lubricated plug valve is the most important type. The use of lubricant between the plug face and the seat eliminates most of these problems. The lubricant helps to control leakage around the plug, and reduces wear between the valve contact surfaces, and slightly lifts the plug to reduce the operating torque required to operate the valve.
A plug with a through hole is used as a valve for opening and closing parts. The plug body rotates with the valve stem to realize the opening and closing action. Small plug valves without packing are also called “Coke”. The stoppers of the plug valve are mostly conical (or cylindrical) and are combined with the conical orifice of the valve body to form a sealing pair. plug valve is the earliest valve, which has simple structure, fast switching and low fluid resistance. Ordinary plug valves are sealed by the direct contact between the finely machined metal plug and the valve body, so they have poor sealing performance, large opening and closing force and are easy to wear and tear. They can only be used in low pressure (not higher than 1 MPa) and small caliber (less than 100 mm).

Classification of plug valves

According to the structure, it can be divided into four types: tightening plug valve, self-sealing plug valve, plug valve and oil injection plug valve. According to the channel form, it can be divided into three types: through plug valve, three-way plug valve and four-way plug valve. There are also jacket plug valves.
Plug valves are classified by use as soft-sealed plug valves, oil-lubricated hard-sealed plug valves, lifting plug valves, three-way and four-way plug valves.
Soft seal
Soft-sealed plug valves are often used in harsh environments such as corrosive, highly toxic and hazardous media, where leakage is strictly prohibited, and where valve materials do not pollute the media. The valve body can be made of carbon steel, alloy steel and stainless steel according to the working medium.
Oil Lubricated Hard Seal
Oil lubricated hard seal plug valves can be divided into conventional oil lubricated plug valves and pressure balanced plug valves. A special grease is injected from the top of the plug body between the cone hole and the plug body to form an oil film to reduce the opening and closing moment of the valve and improve its sealing and service life. The working pressure can reach 64 MPa, the maximum working temperature can reach 325 degrees, and the maximum caliber can reach 600 mm.
Lifting type
There are many kinds of lifting plug valves. The lifting plug valves are divided into soft seal and hard seal according to the material of sealing surface. Its basic principle is to make the cock rise when opening, and then turn the cock 90 degrees to the full opening of the valve to reduce friction with the sealing surface of the valve body; when closing the valve, make the cock rotate 90 degrees to the closing position and then descend to contact with the sealing surface of the valve body to achieve sealing.
Three-way and four-way formulas
Three-way and four-way plug valves are suitable for changing the direction of medium flow or medium distribution in Shanghai industrial equipment. According to the requirements of service conditions, soft sealing bushing or soft sealing, hard sealing lifting plug valve can be selected.

Advantages of plug valve

  1. The plug valve is used for regular operation, quick and portable opening and closing.
  2. The fluid resistance of the plug valve is small.
  3. The plug valve has the advantages of simple structure, small relative volume, light weight and easy maintenance.
  4. Good sealing performance.
  5. Without restriction of installation direction, the flow direction of medium can be arbitrary.
  6. No vibration and low noise.

The passage of the plug valve

There are many forms. Common through type is mainly used to cut off fluid. Three-way plug valve and four-way plug valve are suitable for the opening and closing parts of fluid reversing plug valve, which is a cylinder with holes and rotates around the axis perpendicular to the channel to achieve the purpose of opening and closing the channel.

Installation of plug valve

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 paid attention to:

  1. Make sure the valve is open. First heat the pipe. Transfer as much heat as possible from the pipe to the plug valve. Avoid prolonging the heating time of the plug valve itself.
  2. Use gauze or wire brush to remove pipes and cutting parts to make the metal surface shine. It is recommended not to use steel wool.
  3. Cut the pipe along the vertical direction, trim and remove burrs, and measure the diameter of the pipe.
  4. Coating flux on the outside of the pipe and the inside of the welding cover, the flux must cover the welding surface completely. Please use flux sparingly.

Selection Principle of plug valve

According to the structural characteristics and design functions of the plug valve, it can be selected according to the following principles.
(1) For distributing and changing the flow direction of medium, the working temperature is not higher than 300 C, the nominal pressure PN is less than 1.6 MPa and the nominal diameter is not more than 300 mm. It is suggested that the multi-channel plug valve be selected.
(2) Austenitic stainless steel conical plug valves are recommended for the equipment and pipes of food enterprises such as milk, juice, beer and pharmaceutical factories.
The nominal pressure level is not greater than Class300 and the nominal diameter is not more than 300 mm in the pipeline, refining and cleaning equipment of oil field exploitation, natural gas field exploitation and pipeline transportation. It is suggested that oil-sealed conical plug valve be selected.
The nominal pressure level is not greater than Class7500, nominal diameter is not greater than 900mm and working temperature is not higher than 340 C in the pipeline, refining and cleaning equipment of oil field exploitation, natural gas exploitation and pipeline transportation. It is suggested that oil-sealed conical plug valve be selected.
In large chemical industry, in pipelines and equipment containing corrosive media, when the opening or closing speed is relatively fast, PTFE sleeve sealing conical plug valve can be used for nitric acid-based medium, and Crl8Nil 2M02Ti stainless steel PTFE sleeve sealing conical plug valve for acetic acid-based medium.
In gas, natural gas, heating and ventilation system pipelines and equipment, nominal diameter is not more than 200 nm, it is appropriate to choose filling conical plug valve.

Maintenance of plug valves

  • 1) Check the plug valve every month, and maintain the plug valve quarterly in January, April, July and October every year on the basis of monthly inspection.
  • 2) Check the operating flexibility of the plug valve. The plug valve should ensure that one person can operate without adding the break bar.
  • 3) Check all sealing points of plug valve body and flange for leakage prevention.
  • 4) Check plug valve for internal leakage prevention.
  • 5) Check plug valve actuator for water collection and lack of lubricant.
  • 6) For the maintenance of the plug valve actuator, if it is found that the lubricant in the actuator does not cover all the contact surfaces of the transmission mechanism, the old lubricant shall be removed completely and then sufficient new lubricant shall be applied on all the contact surfaces.

Source: China Plug Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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