Inspection of Pipe Fittings for Oil and Gas Transportation

Pipe fittings, as the parts that play the roles of direction change, connection, diversion, sealing, control and support in pipeline system engineering, play an important role in pipeline engineering construction. Among them, pipe fittings used for pipeline connection include elbows, tees, big and small heads, pipe caps, elbows, branch pipes, flanges, blind flanges and short joints, etc. In order to verify the coordination between the new pipe fitting production process and the production line, as well as the conformity between the new products and the ordering requirements, and avoid the waste of resources caused by unqualified pipe fitting products in batches, it is necessary to conduct a manufacturing process evaluation, small batch trial production, first batch inspection and batch test during the pipe fitting production. At present, the requirements of relevant standards and technical agreements for pipe fitting inspection are inconsistent, so it is difficult to implement them. According to the production process of pipe fittings, the existing problems in the inspection of pipe fittings for oil and gas transportation are discussed in detail, and some suggestions are put forward.
 
Process and key inspection contents
 
The production process of pipe fittings is as follows: material inspection, blanking, coil pipe welding, forming plate head, rounding, heat treatment, surface treatment, end face machining, dimension inspection, nondestructive testing, mechanical property inspection, marking, packaging and warehousing, and product shipment. Among them, material inspection, heat treatment, dimension inspection, nondestructive inspection and mechanical property inspection are the key technologies to ensure the quality of pipe fittings, which should be paid more attention to in actual inspection. Material inspection: Check the conformity of the furnace number, pipe number, appearance size, marking, specification, quality guarantee, and incoming physical and chemical re-inspection results of the mother pipe or steel plate with the requirements of the technical agreement. Blanking: Usually, different shapes of pipe fittings correspond to different kinds of raw materials, and the raw materials are steel plates or steel pipes. According to different forms of raw materials, check the consistency between the blanking dimensions of steel pipes/steel plates and the drawings. Coil welding: check the steel plate size, welding method, current,
Compliance with voltage process parameters and process documents. Molding: Check the conformity of the heating temperature, heating time, molding times, the interval between heating and molding, etc. with the process documents according to the selected molding method of specific pipe fittings. Head and rounding: check the conformity of the head, roundness and appearance of the head with the process documents. Heat treatment: check the conformity of heat treatment parameters, such as heating temperature, holding temperature, heating speed, cooling speed and holding time, with the heat treatment process. Sandblasting: check whether the external surface after sandblasting meets the requirements of Sa2.5. Machining of pipe ends: check the conformity of appearance and size of machined pipe fittings with standard requirements. Inspection: Check the conformity of appearance, wall thickness, outer diameter, groove size and other parameters with the standard requirements. Non-destructive testing: Non-destructive testing of pipe fittings includes magnetic particle/penetrant testing (one of them is selected) and ultrasonic testing, and inspection instruments, testing methods, sensitivity, inspection parts, non-destructive reports, etc. are checked for compliance with process documents. Mechanical properties: check the compliance of sampling and physical and chemical test results with the standard requirements. Signs: check the conformity of the contents and forms of signs with the standard requirements. Storage: Check the packing method and storage environment. Shipment of products: check the completeness and completeness of the ex-factory data, and check whether there are bumps in the pipe fittings during shipment and hoisting.
 
Objections of different standards to inspection
 
Generally, the national standards adopted for pipe fittings are GB/T 12459—2017 Types and Parameters of Steel Butt Welded Pipe Fittings and GB/T 13401—2017 Technical Specification for Steel Butt Welded Pipe Fittings, which are used together; by China National Petroleum Corporation (hereinafter referred to as PetroChina) generally follows CDP-S-OGP-PL-010-2014-3 Technical Specification for Pipe Fittings of DN350 and Below for Oil and Gas Pipeline Engineering and CDP-S-OGP-OGPPL-011-2014-3 Pipe Fittings of DN400 and Above for Oil and Gas Pipeline Engineering.

The technical specification is "to purchase pipe fittings, and select standards according to the specifications of purchased pipe fittings. At present, the above two kinds of standards are basically adopted in the procurement standards of pipe fittings. Now, the different requirements of national standards and PetroChina's technical specifications for pipe fittings inspection are elaborated in detail.
 
1. Process evaluation
 
(1) Technical specification of PetroChina pipe fittings.
According to the 5.2.3.1 clause of CNPC CDP-S-OGP-PL-011-2014-3 technical specification, before the official production of pipe fittings with DN400 mm (nominal diameter of 400 mm) and above, the supplier shall conduct the process qualification test of a single pipe fitting according to the regulations, and then carry out small batch trial production after the process qualification is passed, and the first batch production can only be carried out after the sampling inspection is qualified. The 5.2.3.2 clause of the technical specification stipulates: if the supplier has verifiable equivalent or higher steel grade and larger straightness within one year before signing the contract.

The supply performance of the product and the perfect process evaluation documents prove that it is not necessary to repeat the trial production of a single root and small batch. However, according to the technical specification of CNPC CDP-S-OGP-PL-010-2014-3, the pipe fittings with a diameter of DN350 mm and below are not required to undergo process evaluation. Appendix of CDP-S-OGP-PL-011-2014-3 Technical Specification A: The butt welding of process evaluation pieces should be 100% ultrasonic testing and +100% radiographic testing; the Ultrasonic flaw detection method is adopted to inspect the longitudinal/transverse cracks of the base material of the pipe body of the process evaluation piece; Magnetic particle or penetrant inspection shall be adopted to inspect the surface of the process evaluation piece, and tensile, impact, guided bending test, Vickers hardness and metallographic examination shall be carried out according to the requirements of sampling position and sampling direction.
 
(2) National standards.
National standards do not require process evaluation.
 
(3) Comparative analysis.
If the pipe fittings are produced according to the national standard, process evaluation is not required. However, PetroChina's technical specification requires that the specifications of DN400 mm and above should be produced in the order of "process evaluation test, small batch trial production and first batch inspection". Process evaluation tests and small batch trial production can be exempted. The process evaluation test is to verify the feasibility of the process, and small batch trial production and first batch inspection are to verify the feasibility of process batch production. Pipe fittings with DN350 mm and below do not need process evaluation and small batch trial production. This is mainly due to the consideration of mature technology and low steel grade of small diameter pipe fittings. On the whole, the technical specification of PetroChina is stricter than the national standard, and the requirements of large-diameter pipe fittings are stricter than those of small-diameter pipe fittings.
 
2. First inspection
 
(1) Technical specification of PetroChina pipe fittings
According to the 5.2.3.3 clause of the technical specification of CNPC CDP-S-OGP-PL-011-2014-3, the first batch of pipe fittings shall be inspected according to the requirements of Appendix B, and the first batch of pipe fittings shall be qualified before the formal production. Within three years before the signing of the contract, the supplier may not repeat the hydraulic blasting test if it has verifiable documents and materials. Clause 5.2.3 of technical specification CDP-S-OGP-PL-010-2014-3 stipulates that the first batch of pipe fittings shall be inspected according to the requirements of Appendix B, and the first batch of pipe fittings shall be qualified before formal production. However, the technical specification does not mention the replacement requirement of hydraulic blasting.

The first batch of inspection requirements of Appendix B of CDP-S-OGP-PL-011-2014-3 and CDP-S-OGPPL-010-2014-3 technical specifications are as follows: five pipe fittings produced for the first time in the same manufacturing process are selected for Richter hardness inspection, appearance quality and size inspection and nondestructive inspection. Among them, one pipe fitting is randomly selected for chemical analysis, tensile test, Charpy impact test (a series of temperature impact tests should also be carried out in the transverse direction of the tee and elbow pipes), guided bending test, Vickers hardness and metallographic examination, and then one pipe fitting is selected from the remaining four pieces for water pressure blasting test.
 
(2) National standards
No first inspection is required.
 
(3) Comparative analysis.
When pipe fittings are produced according to the national standard, the first batch inspection is not required; however, when pipe fittings are produced according to the technical specification of PetroChina, the first batch inspection is required regardless of the diameter, and the first batch inspection quantity, inspection items, etc. are clearly specified, and the exemption of water pressure blasting test for pipe fittings of DN400 mm and above is clearly defined, and the exemption of water pressure blasting for pipe fittings of DN350 mm and below requires water pressure blasting test. For pipe fittings with different specifications, this provision in PetroChina's technical specification is mainly due to the consideration of higher risk and higher cost of large diameter pipe fittings.
 
3. Batch test
 
(1) Technical specification of PetroChina pipe fittings.
Clause 8.1.2 of the technical specification of CNPC CDP-S-OGP-PL-011-2014-3 stipulates that one pipe fitting product shall be sampled for mechanical properties and metallographic examination during trial production and small batch production. In the formal production process, no more than 50 similar pipe fittings with the same melting batch, the same heat treatment process and the blank nominal wall thickness difference of less than 6 mm shall be a batch, and one piece from each batch shall be selected for mechanical properties and metallographic examination. Clause 5.2.3 of technical specification CDP-S-OGP-PL-010-2014-3 stipulates that in the formal production process, in principle, the same melting batch, the same heat treatment process and the same heat treatment furnace batch should be used. And the same kind of pipe fittings are in one batch, and one piece from each batch shall be taken for mechanical properties and metallographic examination.
 
The inspection items of technical specifications CDP-S-OGP-PL-011-2014-3 and CDP-S-OGPPL-010-2014-3 are basically the same, including tensile test, Charpy impact test, weld-guided bending test, hardness test and metallographic test for the specified pipe fittings. 2 Yuan Yuan national standard. The 7.1.1.1 clause of GB/T 13401—2017 standard stipulates that one batch of pipe fittings manufactured with the same raw material brand, the same smelting furnace number, and similar raw material specifications (the wall thickness difference is less than 6 mm) and the same heat treatment process. The inspection items are the same as those in PetroChina's technical specification, including the tensile test of the specified pipe fittings, Inspection, Charpy impact test, weld guide bending test, hardness test and metallographic test.
 
(2) National standards.
The 7.1.1.1 clause of GB/T 13401—2017 standard stipulates that one batch of pipe fittings manufactured with the same raw material brand, the same smelting furnace number, similar raw material specifications (the wall thickness difference is less than 6 mm) and the same heat treatment process. The inspection items are the same as those in PetroChina's technical specification, including tensile test, Charpy impact test, weld-guided bending test, hardness test and metallographic examination for the specified pipe fittings.
 
(3) Comparative analysis.
There are slight differences in the principle of assembling and approving pipe fittings with different diameters in PetroChina's technical specification. The specification of DN400 mm and above means that the nominal wall thickness difference of the blank is no more than 6 mm, and the specification of DN350mm and below means the same heat treatment batch. This is mainly because no heat treatment of small diameter pipe fittings with the same specification can be carried out in the same heat treatment batch, which reduces the cost of batch tests. Usually, the heat treatment process of different specifications of large diameter pipe fittings (especially when the wall thickness difference is large) is definitely different, so the range of wall thickness difference is specified to ensure the performance of pipe fittings after heat treatment. The technical specification of PetroChina is completely consistent with the inspection items of the national standard, but the principles of the inspection are quite different. For pipe fittings with DN400 mm and above, PetroChina's technical specification requires similar pipe fittings, while the national standard requires the same raw material brand, and other principles are the same. Therefore, PetroChina's technical specifications are stricter than national standards.
 
Significance of stricter technical specifications of PetroChina
 
In recent years, there have been many pipe leakage accidents in PetroChina stations and oil and gas pipelines. Due to the particularity of the petroleum industry, the utilization rate of pipe fittings is high, and the harsh environment of pipe fittings in the petroleum industry is superimposed. The probability of accidents is much higher than that of other industries. For example, a crack in the repair welding seam of the parent metal of the head shoulder of a three-way pipe fitting of a gas transmission pipeline caused the whole pipeline to burst, and a three-way pipe fitting at the bypass of the wellhead metering skid of a certain engineering station caused personal accidents. These accidents caused extremely serious personal and property injuries and economic losses to society. These factors lead the petroleum industry to formulate technical specifications that are stricter than the national standards and more suitable for the industry, so the technical specifications of PetroChina were born.
 
Problems and suggestions in the current inspection
 
1. Importance and applicability of selected standards.
The national standard and PetroChina's technical specification have different provisions on whether to carry out inspection, inspection items and alternatives. The purchaser shall reasonably adopt relevant standards according to its own requirements, and it is better to formulate appropriate technical agreements according to its own requirements. On the whole, PetroChina's technical specifications are stricter than the national standards, and the requirements for large-diameter pipe fittings are stricter than those for small-diameter pipe fittings. The application scope of PetroChina's technical specification is steel butt welded pipe fittings for oil and gas transmission pipeline engineering, and the manufactured pipe fittings are suitable for pipeline engineering trunk lines and stations for transporting natural gas, crude oil, refined oil and liquid liquefied petroleum gas. The national standard is applicable to steel butt welded seamless pipe fittings and welded pipe fittings. From the standard applicability, PetroChina's technical specification is only applicable to pipe fittings for oil and gas transmission pipeline engineering in the petroleum industry, while the national standard is applicable to pipe fittings in various industries. To sum up, the regulations in PetroChina's technical specifications have taken into account the actual situation of oil and gas pipeline projects and added many corresponding clauses in view of the situation of the petroleum industry. On the whole, the national standard is the basis for the formulation of PetroChina's technical specification, which is the national standard.
Application in the quasi-petroleum industry, both of them complement each other and complement each other. Therefore, the revision of PetroChina's technical specification cannot be separated from the development and improvement of national standards. PetroChina's technical specification should be revised and added in time according to the characteristics of pipe fittings used in this industry, taking full account of the revision of national standards.
 
2. In some cases, single trial production, small batch production, first batch inspection and batch test cannot be carried out separately. At present, the quantity of pipe fittings purchased by some purchasers in a single time is very small, and many of them are less than 10 pieces. If the national standard is adopted, it is more reasonable to carry out group inspection instead of single trial production, small batch production and the first batch inspection. However, according to the requirements of PetroChina's technical specification, it is necessary to strictly follow the sequence of single trial production, small batch production and first batch inspection and batch test, which will bring difficulties to manufacturers and increase the procurement cost. If the procurement ratio is small, it is suggested that single trial production, small batch production, first batch inspection and batch test can be carried out in a "four-in-one" way, and all pipe fittings can be produced at one time, and then the batch test can be carried out according to the batch test principle.
 
3. Repeated inspection of products with mature technology is of little practical significance
It is completely unnecessary to carry out single trial production, small batch production, first batch inspection and batch test for mature products in strict accordance with the requirements. Therefore, PetroChina's technical specification has detailed provisions on the exemption of single trial production and small batch production, and general manufacturers of conventional products can meet the exemption requirements. PetroChina's technical specification also stipulates the exemption of the first batch of water pressure blasting tests, which fundamentally saves the manufacturing cost.
 
4. The inspection items are difficult to choose
Generally, the production process of pipe fittings is made by pushing, drawing, forging, extruding, stamping and other processes of seamless steel pipes or welded steel pipes. For some unconventional pipe fittings, such as 3D elbow, the factory usually chooses the hot-bending process for forming. At this time, special attention should be paid to the inspection items of 3D elbow according to the relevant standards of elbow, but not the standards of pipe fittings. This is because all standard test items are the inspection items summarized by experiments according to the forming process, which is conducive to detecting the weak links of various forming processes.
 
Conclusion
 
According to the technical requirements, there are many inspection procedures in the production process of pipe fittings, and different inspection items are caused by different standards adopted by the purchaser. It is suggested that the purchaser should consider the actual needs and economic benefits, comprehensively formulate the technical agreement of pipe fittings, and select the appropriate inspection contents and items according to the factors such as different specifications, different steel grades, different purchase quantities, the difficulty of pipe fittings production, and whether it is a mature product.