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How to check the quality of elbow

How to check the quality of elbow?

The construction acceptance specification and quality evaluation standard for industrial pipeline installation are the guidelines and rules for engineering operators. The relevant construction acceptance regulations shall be strictly implemented for each material and link used in construction. Elbow is the most widely used pipe fitting in industrial pipeline system, accounting for 80% of the pipe fittings used in the project. Its indicators shall comply with the current relevant national or industrial technical standards. Seamless elbow (full name: seamless steel pipe elbow) has the advantages of good pressure resistance, good heat resistance, good corrosion resistance, good impact resistance and long service life. Seamless elbows are generally used in industrial pipelines in petroleum, natural gas, chemical industry and other industries. To conduct detailed inspection on seamless elbows at the construction site, seamless elbows should first be classified and the causes of defects should be scientifically analyzed To determine the appropriate on-site accurate detection method.

Classification of seamless elbows

  • 1) According to the radius of curvature: short radius elbow and long radius elbow.
  • 2) According to manufacturing materials: carbon steel, cast steel, alloy steel, stainless steel, non-ferrous metals, etc.
  • 3) According to the forming method, it is divided into stamping method, forging method, rolling method, bulging method and bending method.
  • 4) According to the angle: 45 ° elbow, 60 ° elbow, 90 ° elbow, 180 ° elbow, special angle elbow.
  • 5) According to the nominal diameter: equal diameter elbow and reducing elbow.

Causes of elbow defects

The manufacturer does not operate in strict accordance with the production process in the elbow production process, such as the chemical composition does not reach the design ratio, and the hardness and brittleness of the elbow body are still large after heat treatment, resulting in the unqualified mechanical and process performance of the elbow. Backward manufacturing equipment and facilities and collision during transportation cause defects such as unequal half cutting length of elbow, large ovality of middle section or port of elbow, uneven surface of elbow, included angle and unequal thickness of two ports. The chemical and physical property defects of the elbow are very likely to cause that the elbow can not meet the requirements of the engineering design documents, and the geometric dimension deviation will bring some difficulty to the assembly, resulting in the excessive misalignment of the welded junction and the excessive local weld gap, which makes it impossible to weld normally, and the weld is prone to defects such as incomplete penetration, slag inclusion and porosity, because the working section of the weld is reduced, The mechanical strength and impact toughness of the joint are reduced and the stress is concentrated. If the pipes and equipment are forcibly assembled, the torque and tension of the pipes and equipment will be increased, so that they will be subjected to external stress for a long time, and the service life of the pipes and equipment will be shortened. The welds may also crack due to long-term stress, which will have a great impact on production safety.

Inspection of elbow

The elbow shall be subject to chemical composition analysis, mechanical property test and appearance and dimension inspection before delivery. For some elbow fittings with special requirements, low-temperature toughness impact testing, nondestructive testing, magnetic particle testing, etc. are also required. In order to avoid unnecessary rework or hidden danger to the operation of the device due to unqualified elbow quality, resulting in waste of human and material resources, loss of construction quality and extension of project construction period, the construction unit shall conduct on-site inspection before using elbow and other pipe fittings. This paper focuses on the detection method of elbow on the construction site.

Check of elbow

First, check the material grade of elbow, product code of elbow, standard number of elbow, standard size and wall thickness grade of elbow, and professionals use testing equipment to test and analyze the material of elbow. Generally, the chemical composition of the elbow shall be the same or similar to that of the welded pipe section, and shall be of the same steel grade, and the physical properties will not change or produce cracks and other defects due to reheating.

Appearance inspection of elbow

The inner and outer surfaces of the elbow shall be smooth and regular, and shall be free of defects such as crack, delamination, stutter, thinning, depression, sharp scratch, notch, folding, etc.

Geometric dimension inspection of elbow

The geometric dimension inspection is the work carried out by the pipeline installer after checking the elbow identification and elbow materials, and the appearance of the elbow has been visually inspected. It is the continuation of the appearance inspection. Since most construction sites do not have professional testing equipment, installation workers can only use ordinary tools such as level ruler, angle ruler, ruler, caliper, powder line and thickness gauge to carry out a series of inspections on bending, flatness, ovality, end groove, port perpendicularity, bending radius and thinning rate, so as to check whether the pipe fittings meet the requirements of the document and determine the blanking size of the pipe section, And eliminate the defects found, which requires pipeline installers to have high operation skills. Select flat ground or steel plate and place the elbow horizontally. Check repeatedly with a level ruler, adjust the height of different parts of the elbow, and plug gaskets under it until the middle and both ends of the elbow are horizontal. Use a square, level and steel ruler to find out the top, middle and bottom four bisection points of the two ends of the elbow, and connect the corresponding points respectively to obtain the inner arc, outer arc and two middle arcs. As shown in Figure 1.

20211215063857 37992 - How to check the quality of elbow


  • (1) Ovality detection: ovality, also known as out of roundness, is usually the diameter measurement of the same section in the vertical and horizontal directions. If the diameter is large, the measurement points must be added. During the measurement, the values of the two ends and the middle three sections shall be recorded respectively, and the two values of the same section shall be subtracted to ovality.
  • (2) Detection of bending radius: take the outer arc point and inner arc point at both ends of the elbow as the reference point, lead two chalk lines to intersect at a point, and measure the distance between the point and the inner arc with a ruler or tape. The distance value plus the bending radius of the elbow ½ The outer diameter is the bending radius of the elbow. According to the specification, the bending radius measured at both ends of the elbow shall be equal, but the value may be different due to machining deviation. If any, the two halves of the elbow shall be calculated separately to ensure the accurate blanking of the pipe section. If there is a steel plate, you can also draw the standard inner arc of the elbow on the steel plate with a scribe gauge, and place the elbow on the steel plate so that the inner arc of the elbow coincides with the projection of the scribed arc, so that the required value can be measured on the steel plate. The measured values of the above two detection methods are easy to deviate due to the roundness of the elbow port, so the influence of Ovality on the measurement should be considered in advance.
  • (3) Detection of thinning rate: elbow thinning is generally local thinning, which mostly occurs at the outer arc. At present, the detection method of thinning rate is relatively simple. Use a portable thickness gauge to measure the wall thickness of the elbow at multiple points, and the thinning rate can be obtained by comparing the measured value with the standard value. There is little research on the detection method of elbow local thinning in the industry, and the current detection method is the same The possibility of missing inspection cannot be completely avoided. If the wall thickness difference between the elbow and the pipe section exceeds 2mm, the inner wall shall be thinned at 15 °.
  • (4) Detection of bending degree: the detection method is basically similar to the detection method of bending radius. Use two long steel ruler or powder line to lead out and intersect at one point based on the inner and outer arc of the end face, and measure the bending degree with an angle ruler or protractor.
  • (5) Detection of port perpendicularity: it is best to detect the perpendicularity on the steel plate. If there is no suitable steel plate due to conditions, you can use a ruler or find a straight pipe section flat on the top of the elbow as the reference line. One end of the square rests against the reference line and the other end is close to the elbow port for perpendicularity detection.
  • (6) Groove and flatness detection: most seamless elbows are connected by welding. In order to ensure the welding quality, there are machine-made grooves at both ends, and the angle and the thickness of the blunt edge are strictly regulated, which can be measured with caliper. When checking the groove, the flatness of the surfaces at both ends must also be checked, and the flatness can be determined by clinging a flat wooden board or steel plate to the end face and checking the gap Whether the flatness deviation exceeds the standard.


When inspecting the elbow on site, records must be made to provide data for subsequent work. Due to the lack of special inspection equipment for elbows on the construction site, it is generally not recommended to weld elbows to repair defects. If defects beyond the allowable range of technical documents are found, the nonconforming products shall be returned in time to prevent the use of pipe fittings with out of gauge defects in the project. Some deficiencies found can be corrected and eliminated in advance. The correction operation should be scientific and reasonable to avoid new deficiencies. After the operation is completed, the correction must be tested again to ensure that the elbow quality meets the specifications.

Source: Network Arrangement – China Elbow Manufacturer – Yaang Pipe Industry (

(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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