Notices for Connection Modes of Pipeline Engineering

Pipeline connection is inevitable in every pipeline project, and it is also one of the most important factors that directly determine the quality of pipeline project. We summarize the major and minor matters needing attention in various pipeline connection modes, hoping to provide you with reference and help you improve the quality of pipeline engineering.

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Screw connection

  • (1) If it is necessary to break the pipe on the spot according to the surveying and mapping sketch, attention should be paid to cleaning the iron film and burrs on the pipe mouth section after breaking the pipe.
  • (2) Before installing pipe fittings, the appearance quality of the fittings should be carefully checked, and the surface defects such as sand holes and cracks should not be used.
  • (3) The threads shall be sleeved in different sizes according to the diameter of the pipe. Generally, the threads shall be sleeved twice in the diameter of 15-32 mm, three times in the diameter of 40-50 mm, and threads shall be sleeved 3-4 times in the case of 70 mm or more.
  • (4) All threads straightened by heating shall be marked before subsequent operation.
  • (5) It is necessary to ensure that there are no short wires and rotten teeth in the pipe sleeve, and install “horizontal and vertical”. When the galvanized layer on the surface of the pipe thread is damaged, the pipe thread should be coated with anti-rust paint.

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Flange connection

  • (1) Standard flanges shall be selected according to engineering design requirements and data.
  • (2) The diameter and length of the connecting bolt shall meet the requirements of the code. After the bolt is tightened, the exposed thread buckle shall be 2-3 buckles, and the diameter of the bolt shall not be greater than 1/2.
  • (3) For the flange connection gasket, attention should be paid to whether the rubber gasket is concentric with the pipe diameter and not offset; whether the flange groove coincides with the metal gasket, it should be coated with red Dan for matching research to ensure that its sealing condition is good.
  • (4) When using hydraulic and pneumatic tools to fasten bolts, attention should be paid not to exceed the prescribed moment.
  • (5) In order to avoid bonding between washer and thread when fastening bolts and nuts at high temperature, anti-sticking agent should be applied to the thread during operation.
  • (6) For high temperature valves above 300°C, flange connecting bolts, cover fastening bolts, pressure seals and packing gland bolts shall be tightened again after the temperature rises.
  • (7) In order to prevent leakage, when installing valves for low temperature and fastening bolts at high temperature, the shrinkage and moment produced by flanges, gaskets, bolts and nuts with different expansion coefficients at low temperature should be considered.

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  • (1) Weather disadvantage such as rain, snow and gale should be avoided when welding pipelines. Effective protective measures such as building windproof awning should be taken when wind speed exceeds 8 m/s or when rain and snow weather conditions are required. Weld joints should be preheated when ambient temperature of welding area is below – 20°C, and residual heat temperature is 100-200°C, and length is 200-250 mm.
  • (2) When the wall thickness of pipe is more than 5mm, shovel grooves at the weld joint of pipe end, such as gas welding for processing pipe grooves, remove the oxide layer on the groove surface, and smooth the bumps and bumps.
  • (3) Before welding, the groove and its inner and outer surfaces should be cleaned up, including dirt, oil, paint, rust and haircut within 100 mm of the pipe edge. Temporary blind plates should be installed at both ends of the pipe before closing.
  • (4) forcible alignment is forbidden during welding to reduce internal stress and prevent cracks.
  • (5) The coating of electrodes should be dried at 350-400°C without falling off and obvious cracks, and should not be dried more than twice.
  • (6) When pipe and flange are welded, the flange shall be welded on both sides, and the inner weld shall not protrude the flange sealing surface.
  • (7) In multi-layer welding, the root of the first layer of weld should be penetrated and not burned through; the slag spatters of the previous layer should be removed when welding the subsequent layers; the thickness of each layer of weld should be 0.8-1.2 times the diameter of the electrode, and the arc-initiating and arc-extinguishing points of each layer should be staggered; the arc-initiating and arc-extinguishing points of each layer should not be on the pipe wall, but should be welded between adjacent layers; The starting point of the road should be staggered by 20-30 mm.
  • (8) Unqualified welds shall not be repaired more than three times.
  • (9) Strength and tightness tests should be carried out for primary and secondary networks. Strength test pressure should be 1.5 times the design pressure, tightness test pressure should be 1.25 times the design pressure, and not less than 0.16 MPa. When the pressure rises to the test pressure and tends to be stable, it is necessary to check in detail whether the pipeline, welds, pipeline accessories and equipment have leakage, whether the fixed support has obvious deformation, etc.

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Socket connection

  • (1) Attention should be paid to the downstream flow of the pipe socket and the reverse flow of the socket.
  • (2) It is necessary to deal with the extruded outburst of mortar when the two pipe joints are installed in time. The extruded outburst of mortar can be smoothed by pulling straw bags, or smaller workers can be sent into the pipe to smoothen the mortar, fill the imprecise joint with mortar, and remove the impurities in the pipe.

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  • (1) The operating environment temperature of pipeline bonding should be 0 ~43 C and the humidity should not be too high. If the temperature is too low, the adhesive should be prevented from freezing, but not heated by open fire or electric furnace. If the temperature is too high, the surface of the adhesive should be cooled in a cool place or wet cloth and bonded after drying. Attention should be paid to ensuring that the operating area is away from the fire source.
  • (2) Before bonding pipe fittings, the inner side and outer side of socket should be wiped clean. If there is oil stain on the surface, cotton yarn can be dipped in acetone and other detergents. During operation, attention should be paid to keeping the bonding surface clean.
  • (3) When applying the adhesive with a brush, the width of the brush should be 1/3 to 1/2 pipe diameter, so as to facilitate the application of glue. Two times of glue is applied on both sides. First, the inner side of the socket is coated, then the outer side of the socket is coated. When applying the socket, it should be uniformly and moderately coated from the inside to the outside along the axis. No leakage or excessive thickness should be allowed.
  • (4) The adhesive should not be exposed to the air for a long time after smearing. It should be ensured that the bonding surface is wet and smooth.

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Sleeve connection

  • (1) Pipe orifices should be checked before connection. If burrs, irregularities or non-vertical pipe axes are found at the orifices, they should be corrected in time. It can be polished by grinding wheel and other tools, and then cleaned by high-pressure air blowing before it can be used.
  • (2) The depth of tapered holes on various types of joints is generally different. In order to avoid leakage, the types of joints at both ends of the pipeline should be kept the same.
  • (3) In pre-assembly, the pipeline and joint should be kept in coaxiality to ensure the sealing effect.
  • (4) When connecting pipelines, sufficient deformation allowance should be ensured to prevent damage caused by tensile stress, and multiple disassemblies should be avoided to ensure the sealing effect of pipelines.
  • (5) When connecting pipelines, excessive lateral force will lead to poor sealing, so it should be avoided to bear lateral force.
  • (6) Sealant and other fillers should not be added to improve the sealing performance. If the sealant is washed into the hydraulic system, there will be problems such as blockage of damper holes in hydraulic components.

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Hot melt connection

  • (1) Clean, dry and oil-free end of pipe and fittings connection shall be ensured.
  • (2) Operators must wear protective equipment, such as protective gloves.
  • (3) When welding elbows or tees, attention should be paid to the requirements of design drawings, especially the direction; in the prescribed processing time, the joints that have just been welded can be corrected, but the adjustment angle must not be greater than 5 degrees, and rotation is strictly prohibited.
  • (4) After the connection is completed, the pipe and fittings shall be firmly held by hand, and sufficient time shall be maintained for cooling.

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Groove connection
For the groove connection of DN < 250mm pipeline, the construction according to manuals provided by the manufacturer can ensure high quality in general. Therefore, the following points for attention are mainly directed at pipelines with DN < 300.

  • (1) Carefully check the outside diameter, wall thickness uniformity and non-roundness of the inlet pipes, and timely dispose of the unqualified pipes; when hoisting and loading and unloading, attention should be paid to protecting the grooves of the inlet pipes to avoid deformation.
  • (2) Before grooving, check whether the end face of the orifice is perpendicular to the central line of the pipe, whether the non-roundness of the orifice exceeds the standard, whether the burrs around the orifice are removed and polished, and if the non-roundness of the orifice exceeds the standard or is deformed, the orifice must be removed.
  • (3) When pressing groove for long steel pipe, support bracket and ball bearing support point should be installed on the bracket to reduce the fluctuation of pipe rotation during grooving, and grooving depth must meet the requirements of product specifications.
  • (4) After grooving, the stripped galvanized sheet in the orifice of the pipe should be removed and treated with anticorrosion.
  • (5) The appearance of large caliber clamps should be sampled to ensure their accuracy. The inspection contents include: 1) whether the diameter of the circular assembly on the flexible clamp plate is too large or too small, whether the gap between the two clamps is about 2 mm, and whether the roundness is beyond the standard; 2) whether the height and thickness of the flange in the pressure groove of each clamp meet the requirements; 3) whether the radial deformation of each clamp exceeds the standard.
  • (6) When installing clamps, attention should be paid to the material quality, shape size, deformation size of pipe orifice, groove quality and clamp quality, size overshoot, and each clamp must be guaranteed to be grooved.
  • (7) Flexible clamp installation should pay attention to check the material and appearance of rubber sealing ring, its appearance should be free from damage and deformation, and there should be no air bubbles, impurities, cracks or uneven defects on the sealing surface; the construction unit should ask the production unit for its product quality commitment and physical and mechanical performance test report of rubber sealing ring.

Source: China Steel Pipelines Manufacturer – Yaang Pipe Industry Co., Limited (

(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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Please notice that you might be interested in the other technical articles we’ve published:

  • Types of pipe connection

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