Five common connection modes of pressure pipes

Pressure piping commonly used connection methods are welded connections, flange connections, threaded connections, hot fusion connections, groove connections, a variety of connection methods for different occasions, a reasonable choice of connection methods to ensure the quality of the connection is an important guarantee of safe operation of the pipeline, to avoid accidents caused by leakage.

Welding connection

Welded connection is the most widely used and reliable connection in metal pressure piping engineering, and therefore is also the most important connection method. Its main advantages are: the interface is solid and durable, not easy to leak, high strength and tightness of the joint. It is suitable for high sealing requirements, such as the transport medium is highly toxic, strong permeability, explosive, high-pressure occasions and other connection methods can not meet the occasion.

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Welding method

The main welding methods of metal pipes are electrode arc welding (SMAW), tungsten gas shielded welding (GTAW), submerged arc welding (SAW), plasma arc welding (PAW), flux cored wire arc welding (FCAW). The choice of welding method should be determined according to the installation unit equipment, personnel, technical level, and the need to meet the requirements of regulations, standards and design documents. Due to the limited construction conditions on site, the most commonly used welding methods are electrode arc welding and tungsten gas shielded welding. Welding rod arc welding seam strength, equipment requirements and simple operation, low cost, should be preferred; but when the tube thickness is relatively small (less than 3mm) and regulations, standards or design documents, the use of tungsten gas shielded welding, stainless steel piping connection is also more tungsten gas shielded welding. Tungsten gas shielded welding spatter, no slag, metal oxidation is small, better quality than electrode arc welding forming, but the cost is slightly higher.

Welding joint inspection and test

Welding joint inspection is mainly visual inspection and nondestructive testing. Visual inspection is mainly to check whether the welded joints have obvious defects, biting depth, weld residual height to meet the regulations, standards; metal pipeline welded joints should also be non-destructive testing (such as GC2 grade austenitic stainless steel pipeline butt joints should be 10% of the ray or ultrasonic detection), the test results should meet the relevant regulations, standards.
Pressure piping system should be tested for strength to ensure the pressure strength and sealing. Usually hydraulic test or pneumatic test; when conveying extremely hazardous, highly hazardous fluids and combustible fluids, leakage test should also be conducted, leakage test pressure should be the design pressure of the pipeline.

Flange connection

Flange connection is the need to connect the two sections of pipe or pipe components welded flange, set the gasket between the two matching flanges, and then use bolts and other fasteners to make it combined with the seal of the removable joint. Flange connection is rigid, easy to connect, high strength and sealing. Is the most commonly used connection in the removable connection, some flange connection (butt welding flange) can also meet the high sealing requirements of the occasion, such as high pressure, explosive media and other occasions.

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Pipe flange

Flange type welding flange, threaded flange, loose flange; flange sealing surface with concave (FM), convex surface (M), protruding surface (RF), tongue and groove surface (T), groove surface (G). Welded flange is the most commonly used flange type in the pipeline connection, welded flange is divided into plate flat welding flange (PL), slip on flange (SO), and welding neck flange (WN). Pipeline flange priority according to HG/T20592-20635-2009 selection.
Flammable, explosive media and toxicity degree of medium, mildly hazardous media pipeline, flange pressure level should not be less than 1.6MPa; toxicity degree of extreme, highly hazardous media and strong permeability of the pipeline, flange pressure level should not be less than 2.0MPa.
The pipeline with extreme toxicity, high hazard, medium hazard with strong permeability and pipeline conveying liquefied petroleum gas should be used with a butt-welding flange with a belt neck.

Gasket

Commonly used flange gaskets are non-metallic flexible gaskets, flexible graphite composite gaskets, metal covered gaskets, metal wound gaskets, wave tooth composite gaskets and metal gaskets. They should be selected according to the nature of the transported medium and its working conditions and flange form.
Non-metallic flexible gaskets and metal clad gaskets are suitable for low pressure and temperature; metal gaskets and metal composite gaskets or metal wound gaskets are suitable for high pressure and temperature; gaskets with good resilience and certain self-tightening effect are suitable for pressure and temperature fluctuation.

Bolts (pillars) and nuts

Bolts and nuts are preferred according to HG/T20613-2009 and HG/T20634-2009.
The degree of toxicity is extreme, high hazard, strong permeability medium hazard and pipeline transporting liquefied petroleum gas, should use special grade high-strength stud combination.

Threaded connection

Threaded connection structure is commonly used in the non-toxic non-combustible fluid transport pipeline system with working pressure less than 2.5MPa. Pipe thread is divided into conical pipe thread and cylindrical pipe thread. The threads machined by the installation site with a stranded plate or sleeve machine are conical pipe threads.

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There are two types of threaded connection structure fits:

  • (1) Cylindrical female thread with conical male thread: for low pressure (<1.6MPa) pipelines, static load environment;
  • (2) Conical female thread with conical male thread: used for medium pressure (<2.5MPa) pipeline, static load environment.

The first type is often used to seal the external threads with packing such as Teflon raw material tape and lead oil twist.

Hot-melt connection

The hot melt connection is used in the connection of pipes made of non-metallic materials such as polyethylene (PE). Polyethylene pipe is widely used in gas transmission, water supply, sewage, agricultural irrigation, mining fine particle solids transmission, as well as oil fields, chemical industry and post and telecommunications, etc., especially in gas transmission has been commonly used.

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Polyethylene pipe hot melt connection method is mainly socket type, butt type, polyethylene pipe can generally be melted in the range of 190-240 ℃ (melting temperature of different raw material grades is slightly different), at this time, if the melted part of the pipe fully contact, and keep the appropriate pressure (their own thermal expansion pressure), the correct connection position, after cooling can be firmly fused together.
According to the different welding methods, generally can be divided into hot fusion welding and electric fusion welding. Hot fusion welding has a socket connection and butt connection.

Socket connection

Heating parts to 275 ℃, the pipe and pipe fittings from both sides slowly pushed to the heating bush and heating pad, apply the appropriate pressure to make the pipe and pipe fittings accurately in place, and maintain the axial level of the heating bush. When the heated parts are removed, quickly push the tube into the tubing, paying attention to the positioning marks and keeping the concentricity of the tube and tubing during the push. Push the joint axially with proper pressure to obtain sufficient fusion, then ensure the same cooling time as the heating time.

Butt jointing

In the PE pipe butt welding, should first set up the pipe butt machine, respectively, placed around the butt machine four level pipe, adjust the height of the butt machine stand, so that the butt machine in a horizontal state. When the pipe is long, in order to make the center of the two sections of the pipe in the same line, should be set in two-thirds of the two pipe bracket, the use of bracket lift adjuster, so that the two pivot points and butt joints in the same level. In order to make the joint heated evenly, the pipe butt must first cut the pipe mouth flat.
Heating plate heating to 215 ℃, in the two pipe ends smoothly and forcefully push the two welded parts to the heating plate, until the end of each welded parts close to the heating plate, to be turned out on the two pipe ends of fresh hot melt layer that stops hot melt, remove the hot melt machine, uniform force to make the two pipe ends butt fusion as one. To be welded to a height of 0.5-1.5mm, pipe end hot melt butt that is entering the cooling phase, cooling must maintain pressure, to be cooled to ambient temperature before stopping the external force.

Quality control

  • (1) Strictly control the straightness of the pipe, baking bending with open flame is strictly prohibited to ensure the quality of polyethylene pipeline construction.
  • (2) Hot melt machine temperature must reach the specified temperature in the process of use before it can be used for hot melt butt construction, forbidden not to the specified temperature that hot melt. When the hot melt to strictly control the time, for the pipe diameter model, accurate grasp of the hot melt machine continuous melting time, forbidden to lift the hot melt machine in the middle of interrupting the hot melt, hot melt process must maintain a constant pressure.
  • (3) Immediately after the melting of the mouth of the pipe butt holding pressure natural cooling, to ensure that the corresponding cooling time, to be naturally cooled to produce sufficient strength before the next process. Hot melt joint natural cooling process is prohibited to use cold water cooling, to prevent the joint from sudden cold brittle, to avoid leaving safety hazards.
  • (4) Polyethylene pipe is less hard, less tough and less rigid than metal pipe, so it should be protected during the construction process to avoid imposing improper external force and causing mechanical damage.
  • (5) After the pipeline connection is completed, the quality inspection should be carried out according to regulations and standard requirements, and those that do not meet the requirements must be reworked.
  • (6) Pipeline construction interruption, intermittent, the mouth of the pipe should be promptly sealed to avoid blockage and pollution in the pipe.
  • (7) Should try to avoid welding of materials with different melt flow rates, such as PE80 and PE100 welding compatibility problems, need to pay attention to.

Groove connection

Groove connection is in the pipe, pipe fittings and other pipe joints processed into a circular groove, with pipe clamps, rubber seals and fasteners and other components of the sleeve type quick coupling. When installed, the adjacent end of the pipe set on the shaped rubber seal, with a combination of clamps connected. This connection method has the advantages of not destroying the anti-corrosion layer of the steel pipe, quick and simple construction, good sealing, easy disassembly and so on. It can be used for building water supply, fire water supply, production water supply and other pipeline projects.

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Groove connection pipe fittings include two types: rigid joints, flexible joints, mechanical tees and groove flanges to connect the role of sealing; to connect the role of transition pipe fittings elbow, tee, cross, reducer, blind flange, etc..
The role of the connection sealing groove connection fittings have three main components: sealing rubber ring, clamps and locking bolts. The rubber seal ring located in the inner layer is placed on the outside of the connected pipe and coincides with the pre-rolled groove, then the rubber ring is fastened on the outside of the clamp, and then fastened with two bolts. Due to the unique sealable structure design of the rubber seal and clamp, the groove joint has good sealing, and the sealing is enhanced with the increase of fluid pressure inside the pipe.

Source:  China Pressure Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading flange 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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