Measurement of bending angle of induction bending for oil and gas transportation

What is Induction Bending?

Induction Bending is a controlled means of bending pipes through the application of local heating using high frequency induced electrical power.

Originally used for the purpose of surface hardening steels, induction technology when used in pipe bending consists basically of an induction coil placed around the pipe to be bent. The induction coil heats a narrow, circumferential section of the pipe to a temperature of between 850 and 1100 degrees Celsius (dependant on the material to be formed). As the correct bending temperature range is reached, the pipe is moved slowly through the induction coil whilst the bending force is applied by a fixed radius arm arrangement.

Manufacture of Induction Bends

Induction bends are formed in a factory by passing a length of straight pipe through an induction bending machine. This machine uses an induction coil to heat a narrow band of the pipe material. The leading end of the pipe is clamped to a pivot arm.

As the pipe is pushed through the machine, a bend with the desired radius of curvature is produced. The heated material just beyond the induction coil is quenched with a water spray on the outside surface of the pipe. Thermal expansion of the narrow heated section of pipe is restrained due to the unheated pipe on either side, which causes diameter shrinkage upon cooling.
The induction bending process also causes wall thickening on the intrados and thinning on the extrados. The severity of thickening/thinning is dependant on the bending temperature, the speed at which the pipe is pushed through the induction coil, the placement of the induction coil relative to the pipe (closer to the intrados or extrados), and other factors.
Most induction bends are manufactured with tangent ends (straight sections) that are not affected by the induction bending process. Field welds are made or pipe pup sections are attached to the unaffected tangent ends, allowing for fitup similar to that found when welding straight sections of pipe together.
Induction bends come in standard bend angles (e.g. 45°, 90°, etc.) or can be custom made to specific bend angles. Compound bends (out-of-plane) bends in a single joint of pipe can also be produced. The bend radius is specified as a function of the diameter. For example, common bend radii for induction bends are 3D, 5D and 7D, where D is the nominal pipe diameter.

ibending - Measurement of bending angle of induction bending for oil and gas transportation

Hot induction bending of 48 inch line pipe at Mannesmann   View on induction coil and heated zone during bending

From: Werner Sölken

Benefits of Induction Bends

  • Large radii for smooth flow of fluid.
  • Cost efficiency, straight material is less costly than standard components (e.g. bends) and bends can be produced faster than standard components can be welded.
  • bends can be replaced by larger radius bends where applicable and subsequently friction, wear and pump energy can be reduced.
  • Induction bending reduces the number of welds in a system. It removes welds at the critical points (the tangents) and improves the ability to absorb pressure and stress.
  • Induction bends are stronger than bends with uniform wall thickness.
  • Less non-destructive testing of welds, such as X-ray examination will save cost.
  • Stock of bends and standard bends can be greatly reduced.
  • Faster access to base materials. Straight pipes are more readily available than bends or standard components and bends can almost always be produced cheaper and faster.
  • A limited amount of tools is needed (no use of thorns or mandrels as required in cold bending).
  • Induction bending is a clean process. No lubrication is needed for the process and water needed for the cooling is recycled.

ASME B16.49

ASME B16.49 Standard covers design, material, manufacturing, testing, marking, and inspection requirements for factory-made pipeline bends of carbon steel materials having controlled chemistry and mechanical properties, produced by the induction bending process, with or without tangents.
This standard covers induction bends for transportation and distribution piping applications (e.g., ASME B31.4, B31.8, and B31.11). Process and power piping have differing requirements and materials that may not be appropriate for the restrictions and examinations described herein, and therefore are not included in this Standard.

Characteristics of Induction bends

  • ① The medium frequency induction ring is used for heating, and the heating is even, and no harmful pollutants such as carbon and sulfur are produced;
  • ② The bending quality is good, the roundness of the nozzle, the overall flatness and the bending angle are good, the inner side of the bending section is smooth, and there is no wavy fold;
  • ③ After bending, the residual stress is small, the bending angle range is large, the adaptability is strong, and the production efficiency is high. In order to ensure the best quality of bends, it is necessary to evaluate the process of steel pipes with different pipe diameters and wall thicknesses before the formal production, which can be carried out according to the requirements of nb-4212 forming and bending process. According to the PFI pipe prefabrication standard, the size of the bend after forming: bending angle, ovality, waviness and bending radius shall be evaluated to meet the tolerance requirements in PFI es-24.

When the best bending effect is obtained by the process evaluation, the process parameters are also selected, including heating temperature, propulsion speed and cooling speed. Heating temperature: the selected heating temperature should be higher than the sensitization temperature of stainless steel, and the solution temperature is the best. Heating in this temperature range can effectively avoid the intergranular corrosion tendency of stainless steel. From the point of view of the balance of strength and toughness, the heating temperature is not easy to be too high, which may lead to grain growth and affect the mechanical properties of the material. The heating temperature can be reduced with the increase of alloy element content. Heating should be fast, and the speed is generally greater than 20 ℃ / s. Propulsion speed: to determine the feeding speed, not only the production efficiency should be considered, but also the different heating temperature, cooling speed and heating speed on the inside and outside of the raw material steel pipe should be paid attention to.

Cooling speed: the cooling speed is an important parameter of bending process. The cooling speed depends on the wall thickness, bending radius and feeding speed of the raw steel pipe. Since the cooling process also needs to go through the sensitization temperature range, the residence time in this temperature range should be minimized. In addition, when selecting cooling and heating speed, the internal stress produced in the bending process should also be considered.

After the bend is installed, the appearance and size of the bend shall be checked. The appearance of the bend shall be free of surface cracks, cracks, folds or other phenomena that may damage the quality. The size inspection of bends mainly includes: wall thickness, bending angle, bending radius, waviness, flatness, ovality, etc. When checking the wall thickness of the bend, it shall be carried out according to the requirements of process evaluation. Due to the influence of material structure and wall thickness inequality, the bending part is prone to wave phenomenon in actual bending. For the measurement of waviness, please refer to the requirements of PFI standard es-24: the difference between the average value of the two adjacent maximum wave crest diameters of the pipe after bending and the minimum wave trough diameter of the pipe between the two peaks shall not exceed 3% of the nominal outer diameter of the pipe. When the waviness exceeds the tolerance requirements, it shall be trimmed in an appropriate way and finally meet the tolerance range required by the standard. For the measurement of bending radius, bending angle and other dimensions, please refer to the tolerance requirements recommended in PFI standard es-24. Data shall be recorded after dimensional measurement. In summary, the bend size directly affects the installation quality of the pipeline. Reasonable bend technology and strict process control are the guarantee of manufacturing qualified bend. Only by strengthening the improvement and quality control of bend technology, can nuclear power safety be effectively guaranteed.
The quality control of large-diameter bends is as follows:

  • 1. The influence of lubrication on the product quality in the forming process of large-diameter bend: in the traditional bend production process, in order to get better surface quality of bend, the enterprise will take many methods, including: using more advanced and high-grade pipe bender, using higher strength mold, or using lubricating products.
  • 2. The position of the neutral axis is different from that of the bending method. The neutral axis is about 1 / 2 away from the outer wall in the top bending (compression bending) operation and 2 / 2 away from the outer wall in the rotary bending (back bending) operation. Therefore, it is beneficial to use rotary bending method for thin-walled pipe bending.
  • 3. The accuracy of the bending tire is also one of the factors affecting the quality of the bend. In addition to the specification and size required to be controlled within a certain tolerance range, we also require the user to select the corresponding bending tire according to the bending pipe diameter in use.
  • 4. The bendability and surface corrosion of pipes may also affect the quality of bends. During the field construction, the operator also needs to know the material, processing performance and production judgment of the surface corrosion of the processed pipeline.
  • 5. In pure bending, under the action of external force distance m, the outer arm wall of the sex layer is thinned and the inner arm wall is compressed. The resultant force N1 and N2 make the cross section of the pipe change.

Measurement of bending angle of induction heating bend for oil and gas transportation

With the rapid development of China’s economy, as one of the most economical, safe and uninterrupted long-distance transportation tools for oil and natural gas, long-distance pipeline has made great progress in the past decade, and this development momentum will continue in the future. With the continuous growth of the laying mileage of long-distance pipeline, the internal performance requirements of steel pipes for long-distance pipeline in the industry are constantly improved. At the same time, the dimensional accuracy of steel pipes is also put forward higher requirements.
Induction bending is an important part of long-distance pipeline construction, which is widely used in mountainous areas, hills, rivers and other relief areas. Its functions are mainly in two aspects:

  • In order to meet the design requirements of elastic laying of long-distance pipeline and prevent the pipeline from being damaged by thermal expansion and contraction;
  • In order to meet the requirements of terrain change, change the direction of the pipeline.

In the geometric dimension of the bend, the bending angle is the most important parameter that directly reflects the use function of the bend. If the bending angle error is too large, it will directly affect the quality of the site construction, resulting in the difficulty of butt joint, the increase of the stress at the weld joint, and even lead to the scrap of the bend, which seriously affects the construction quality and progress.

Deficiency of setting out method

In the past, the bend design is 3 ° as the first grade, and the standard error is ± 1 ° (see 8.7.4 in Chinese Standard SY / T 5257-2012). However, with the improvement of construction quality requirements, the design of bends has been improved to 1 ° and the error requirements have been raised to ± 0.4 °.
At present, the measurement method of bend angle with straight pipe section usually adopts the measurement method (also known as setting out measurement method) given in SY / T 5257-2012. The angle ruler, platform, 2000 mm steel ruler and right angle ruler are used as measurement tools to place the bend on the platform, and then use the right angle ruler to find n on the straight pipe section at both ends of the bend Project points (n ≥ 6) onto the platform (as shown in Figure 1); then take the bend away from the platform, find out the center lines of straight pipe sections at both ends according to these points, intersect at point a, and measure the bending angle α of the bend by angle ruler or trigonometric function calculation method.
20200303001104 83338 - Measurement of bending angle of induction bending for oil and gas transportation

Figure.1 Schematic diagram of existing bending angle measurement method

The process of setting out to measure the angle of bend is shown in Figure 2. From Figure 2, it can be seen that there are many tools used in this measurement method, the measurement steps are complex, there are many human interference factors, time-consuming and laborious, and there are measurement errors. To solve this problem, a new measuring device is designed to measure the bending angle of the bend.

20200303001922 95070 - Measurement of bending angle of induction bending for oil and gas transportation

Fig.2 Process of setting out method for measuring the angle of bend

Design of bending angle measuring device for bend

Based on many years of working experience, the author designs a measuring device for bending angle of bend, whose structure is shown in Figure 3. The device is mainly composed of electronic level measuring instrument, base and power supply. The base consists of a rectangular platform and four legs equipped with a travel switch. The length of the rectangular platform is 203.2 mm, the height of the legs is 50.8 mm, and the width is 25.4 mm. The spacing between the two legs set along the width direction of the rectangular platform is 101.6 mm. One travel switch is set at the bottom of each leg, and four travel switches are connected in series at both ends of the power supply to form a measurement trigger circuit (as shown in Figure 4); the electronic level measuring instrument is set at the upper part of the rectangular platform, which is connected in series with the measurement trigger circuit.
Working principle of bending angle measuring device of bend: when the outrigger of datum base is in full contact with the straight pipe section of bend, all four travel switches are closed, the measuring trigger circuit is connected, and the electronic level measuring instrument starts to measure and display the measuring angle.
20200303002708 80499 - Measurement of bending angle of induction bending for oil and gas transportation

Fig.3 Structural diagram of bend angle measuring device

20200303002940 27796 - Measurement of bending angle of induction bending for oil and gas transportation

Figure.4 Working principle of measuring trigger circuit in measuring device

Measurement of bending angle of bend

The measurement process of angle of bend is shown in Figure 5, and the measurement steps are as follows:

  • (1) Use the crown block and sling to lift the bend to be measured. With the help of the gravity of the bend to be measured, make the plane formed by the centerline of the bend to be measured vertical to the horizontal plane, and make the bending section below;
  • (2) Select straight pipe section 1 and straight pipe section 2 respectively at the height of straight pipe section on both sides of bending section;
  • (3) Place the leg of the bending angle measuring device of the bend on the straight pipe section 1, make its bottom travel switch close to the pipe wall, close the travel switch, and read the angle value of the electronic level measuring instrument; measure the two positions of the highest point A1 and the lowest point A2 of the same circumference of the straight pipe section to obtain the angles α 1 and α 2;
  • (4) In the same step (3), measure the highest point B1 and the lowest point B2 of the same circumference of straight pipe section 2 to obtain the angles β 1 and β 2;
  • (5) According to the angle measured in step (3) and step (4), calculate the bending angle α of the bend according to formula (1).

20200303003122 52352 - Measurement of bending angle of induction bending for oil and gas transportation

20200303003329 43035 - Measurement of bending angle of induction bending for oil and gas transportation

Figure.5 Schematic diagram of bend angle measurement process


The setting out method is low in efficiency and greatly affected by human factors in measuring the angle of the bend. The measurement method, the accuracy of the measuring equipment and the error in angle calculation can cause the angle of the measured bend to deviate from the true angle, or even exceed the deviation range required by the standard, resulting in the outflow of the unqualified bend products, and ultimately affecting the construction quality of the long-distance pipeline laying site. Therefore, it is of great significance to control the outflow of bends whose angle is out of tolerance to improve the field construction quality of long-distance pipeline.
It can greatly improve the inspection efficiency and measurement accuracy of bend angle measurement, meet the requirements of equipment accuracy for long-distance pipeline under the new situation, and improve the construction quality of long-distance pipeline.

Source: China Pipe Bend 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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measurement of bending angle of induction heating bend for oil and gas transportation - Measurement of bending angle of induction bending for oil and gas transportation
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Measurement of bending angle of induction bending for oil and gas transportation
Induction bending is an important part of long-distance pipeline construction, which is widely used in mountainous areas, hills, rivers and other relief areas.
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