0 Preface
In chemical, refining, petrochemical, and nuclear power plants, a large number of flanged connections are used for pressure vessels and pressure piping, and bolted flange connections are also widely used in aerospace and aerospace equipment. Due to the large number of bolted flange connections and complicated working conditions, the leakage accidents of the flange connection system are also increasing, and the direct economic loss caused by the leakage of the flange joints reaches 100 million yuan per year [1]. The bolted flange connection seems to be simple, but the actual is very complicated. In addition to the nonlinear and permanent deformation of the mechanical properties of the gasket material, it also shows the deflection of the flange and the change of the gasket stress under the action of internal pressure [2] and the joint. The short-term relaxation [3], these are lurking the risk of leakage; when the flange is subjected to complex external loads, it is more prone to leakage [4 - 5]. The lack of effective management of the flange installation is one of the most important reasons for the leakage of the flange during use [6]. Some flanges have failure modes such as gasket crushing or flange overload deformation even during installation [ 1], some flanges leak after the pressure test [7].
In view of the importance of flange installation, ASME first published the “Guide to Installation of Pressure Boundary Flange Connections†[8] in 2000 and revised version [9] in 2010. Considering the lack of corresponding standards in China, the paper focuses on the introduction and analysis of the characteristics of the new edition of the guide.
1 PCC-1-2010 Flange Installation Guide
ASME established the Ad Hoc Task Group on Post Construction in 1993 to address the drafting of engineering standards for in-service equipment inspection and maintenance. In 1995, ASME's Department of Pressure Technical Specifications and Standards established the Post Construction Committee (PCC) to provide inspection and management related regulations and standards for in-service pressure equipment and pipelines. In 2000, PCC first released the ASME PCC-1 "Pressure Boundary Flange Connection Installation Guide" (hereinafter referred to as the "Guide"; in addition, PCC-2 is the Repair of Pressure Equipment and Piping Standard, PCC - 3 is the Inspection Planning Using Risk - Based Methods Standard; a revised version was issued in 2010. It is worth noting that the guide was approved by the American National Standards Institute (ANSI) as the US country. standard. The PCC-1-2010 version of the guide includes 16 positive article sections and 16 appendices.
1.1 Text
1 Scope; 2 Introduction; 3 Installation personnel's training, qualification and certification; 4 Flange and fastener contact surface cleaning and inspection; 5 flange joint alignment; 6 gasket installation; 7 working surface lubrication 8 bolt installation; 9 use a single tool to tighten the bolt number; 10 bolt fastening; 11 use a single tool to tighten the bolt sequence; 12 torque determination; 13 joint pressure and leakage test; 14 record; 15 joint Disassembly; 16 references.
1.2 Appendix
A description of flange joint installer qualification; B flange joint installation method recommendations; C recommended contact surface roughness for various gaskets; D gasket contact surface allow for flatness and defect depth guide; E flange joint Correction guide; F replaces traditional tightening sequence and mode; G professional contractor using bolt installation service; H bolt root and tensile stress area; I bolt fastening interaction; J mounting bolt torque calculation; K installation Nut coefficient calculation for bolt torque; L ASME B16. 5 Flange bolt information; M Overall hardened bolt washer guide and purchase technical requirements; N Reusable bolt definitions, notes and guidelines; O Mounting bolt stress determination; P Flange joint leak fault inspection and troubleshooting guide.
Obviously, the new appendix is ​​very powerful compared to the first version. These appendices fully reflect the latest technological advances and installation experience, especially in the training of flange installers, gasket and flange surface quality requirements, flange alignment requirements. The bolt fastening methods have been extensively revised. In addition, several new appendices have been added to the new version. The new version is nearly double the number of the 2000 version.
2 revised basis for the new guide
2.1 Learning from past leaks
The 2010 edition of the guide reflects the latest developments in theoretical research, design calculations and production practices for gaskets, bolts and flange joints. Practical experience in industrial installations has shown that leaks in bolted flange connections can sometimes be avoided by making minor improvements to the flange installation process. The new edition draws on these practical practices, such as the provisions in Chapter 7, “Lubrication of Working Facesâ€: The nut should be free to rotate before applying lubricant to the bolts and nuts; it also specifies that the surface of the nut should be removed before installation. Learned from the many accidents that caused the use of temporary gaskets that caused the gasket material to “blow out†and injure the installer. The use of undesigned temporary gaskets as final seals for joints is specified in Chapter 13, “Joint Pressure and Leak Testsâ€.
2.2 Reflecting the latest theoretical research results
Based on the unremitting and in-depth study of bolted flange connections, the new edition incorporates new research results in the past ten years in terms of concepts and methods. These results are reflected in many of the appendices of the new edition of the guide. For example, Appendix D has been greatly modified. The flange surface quality requirements in the initial edition are based only on the general manufacturing standards of the flange, while the new edition considers both the type of gasket and the installation. Experience; Appendix E expands the requirements for practical and effective flange alignment; adds the following new appendix: Appendix M gives the use and requirements for integral hardened gaskets; Appendix N gives the requirements for bolt reuse; Appendix O adds a new method for determining the installation bolt load, in addition to retaining the original method; Appendix P adds analysis of flange leakage causes and suggested improvements.
In addition, in the new version of the bolt fastening method, based on extensive theoretical research, under the requirements of the same integrity as the traditional bolt fastening method, five bolt fastening methods are replaced instead of the traditional method, including Multiple tools simultaneously tighten the bolts, reflecting the idea of ​​improving installation quality, reducing installation work and improving work efficiency [10].
3 Technical features of the new guide
The new edition of the guide highlights the modern concept and technical features of the guide to ensure the overall performance of the bolted flange connection in the following updates and additions.
3.1 Training, qualification and certification of the installer (Appendix A)
Lack of training for installers and lack of standardized quality control of flange connections are the main causes of flange joint leakage [1]. To this end, the guide emphasizes that no one can engage in flange installation work and requires a qualification certificate. In fact, any pressure system has two main connections: welded and flanged. From the comparison of the quality control of the two joints in Table 1, it can be seen that only the welded joints have perfect quality control requirements, and there is no complete quality control requirement for the flange joints.
The perfect quality control measures for welded joints are due to frequent failures and major accidents of welded joints before these measures are taken; while the leakage of flange joints is more frequent, on the one hand, it is believed that the leakage will not cause catastrophic accidents, on the other hand It is considered that the flange joint allows a small amount of leakage as long as the control is within the range of the specified leak rate. As a result, reports of flange leakage and the resulting losses are missing. In fact, the leakage of flange joints can also cause major explosions, poisoning and casualties, and the existing technology can completely avoid these serious leaks and catastrophic consequences.
Due to the lack of strict installation procedures, it is difficult to find the real cause even if a flange joint leak occurs. Modern refining, petrochemical, power generation and other devices are expanding in size, the number of flanges used in pressure vessels and pipelines is increasing, and the rapid development of enterprises has led to a large increase in temporary employment, and these people generally lack basic flange installation knowledge. Appendix A of the 2000 Guide has recommended training for flange installers and obtaining qualification certificates, but no specific requirements have been given. To this end, Appendix A of the 2010 edition requires unified training and qualification certification for installers, stipulates training content and assessment procedures including theoretical knowledge and practical training, and requires full-time institutions to undertake certification work. The installer qualification is divided into 3 levels: installation engineer, senior engineer and training instructor. At the time of forensics, a theoretical test and an operational test are required for the flange installer. The operation test requires that each person install at least 2 flange joints. Since on-site operation is an important source of personal knowledge of installation, the appendix also suggests that the installer should have some field experience and require six months of work experience to qualify for the installation engineer. The qualification certificate is valid for 3 years.
It can be seen that if Appendix A is implemented and promoted, the flange installer will be expected to improve the business qualifications through training and forensics like the welding personnel, and bring the integrity and sealing of the bolt flange joint to a new height.
3.2 Allowable flatness of the gasket contact surface and allowable defect depth (Appendix D)
Appendix D has extensively rewritten the 2000 version [11]. The 2000 version of the flange and gasket contact surface (hereafter referred to as the "sealing surface") quality requirements only consider the quality requirements of the sealing surface processing, without considering the quality requirements of the gasket to the sealing surface. The 2010 edition of Appendix D states that gaskets can be divided into two types of concepts: “soft gaskets†and “hard gaskets†(soft gaskets and hard gaskets are produced by gaskets under the final installation bolt load). The amount of compression to distinguish). Soft gaskets have a mounting compression of more than 1 mm, and hard gaskets have a much smaller compression. This also means that the hard gasket is more sensitive to the flatness deviation of the flange seal surface than the soft gasket. Therefore, different types of gaskets have different requirements for the flatness of the flange sealing surface. Table 2 lists the tolerances of the sealing surface flatness required for soft and hard gaskets.
Similarly, soft and hard gaskets have different allowable ranges for local defects (points, pits, grooves, dents, scratches, corrosion, etc.) on the flange seal surface. Table 3 lists the requirements for the depth and width (radial projection length) of the local defects allowed on the flange seal face (see Figure 1 for the definition of the symbols in the table).
3.3 Bolt fastening sequence and method (Appendix F)
Appendix F of the 2010 edition, in addition to the traditional bolt fastening method in version 2000, provides an alternative to five fastening bolts, including three single tool fastening options and two multiple tools for simultaneous fastening. Program. Figure 2 shows the traditional fastening method for 24 bolts. The new alternatives are not based on advanced installation equipment and technology, but rather the results of years of research on bolt fastening methods that reduce the complexity of bolt tightening and achieve bolt loading at faster speeds. The efficiency of tightening the bolts makes the gasket more uniform. In some cases, the alternative method has a better fastening effect. However, for specific construction, these alternative methods should be carefully selected. To evaluate the alternative method of choice, carefully analyze the installed flange connection system to see if the alternative method is more suitable for the flanged system installed. At the same time, when choosing an alternative method, the following possible problems should be considered: Partial overload of the gasket, uneven deformation leads to deformation of the flange, uneven force on the gasket, excessive loading and unloading of the gasket during installation, and flange after installation The faces are not parallel.
The new edition also states that the traditional star fastening method is still the standard "optimal" method, which has been successfully applied to the flange connection of various types of gaskets and has been shown to prevent in various industrial applications. The important role of gasket damage and leakage reduction. In the new alternative method, in addition to the single tool or multiple tool fastening, based on the tightening load step and the bolt fastening method, there are actually two types: the improvement of the traditional fastening method and the so-called "four bolts" Solid method. Three alternatives to installing 24 bolts for a single tool are shown below.
3.3.1 Improvements to traditional bolt fastening methods (PCC - 1 Alternatives 1 and 2)
Both of these solutions are improvements to the traditional method, in which the bolting method of the alternative 1 is still in the traditional star mode (see Figure 2 for the tightening sequence), but first tightened at 20% to 30% of the mounting bolt torque. Fix 4 bolts, then raise to the next 50% to 70% of the mounting bolt torque level to tighten the next set of 4 bolts, and finally tighten all bolts with 100% mounting bolt torque. Therefore, the bolt load of Option 1 is increased faster, reducing the number of tightening and the amount of work. This method has been successfully applied to all types of gaskets and flange connections in some applications.
(2) Alternative 2
The fastening procedure for Alternative 2 is similar to Option 1, except that the fastening method is different from the traditional star method, but by the bolt sequence number in a crosswise manner (see Figure 3), so that the assembler is not required to number the bolts on the flange. .
3.3.2 Four bolt fastening method (PCC - 1 Alternative 3)
This method is actually to use the 100% mounting bolt torque at the end, and before tightening the bolts in the circumferential direction, tighten the four bolts in three steps, as shown in Figure 4. This method is simpler, and does not require the assembler to number the bolts on the flange, and to reduce the workload by reducing the movement from one side of the flange to the other. This method has been used in flange joints for some refinery hard gaskets.
3.3.3 Multi-bolt synchronous fastening method (replacement methods 4 and 5)
A multi-bolt synchronous fastening method using multiple fastening tools can reduce damage to the gasket. The multiple bolt synchronous fastening method is simpler than the single tool cross fastening method and has a better fastening effect. This method has been successfully applied to all types of gaskets and flange joints in some applications, and is commonly used in the refining and petrochemical sectors.
3.3.4 Review of alternative methods
In the traditional bolt fastening method, it is required to tighten once every 4 hours, and this step is eliminated in all five alternative methods. For most types of gaskets, the gasket joints will be slack at the beginning of the operation after driving, and the tightening will not be effective after 4 hours of installation. In the alternative method, the final continuous tightening is specified until the nut is no longer loose.
The number of times the bolts are tightened is also related to the gasket used. For soft gaskets with a large amount of compression, more tightening times are required. Some shims can be tightened more than 10 times in the circumferential direction (until the nut is no longer loose), because the nut is slightly loosened every time it is tightened.
3.4 Technical requirements for the use and procurement of integral hardened bolt washers (Appendix M)
Appendix M is new to the 2010 release. It is not mandatory to use washers in ASME Boiler and Pressure Vessel Code, ASME "B31. 1 Power Pipeline" and "B31.3 Chemical Pipeline". Although ASME PCC - 1 stipulates that the gasket can be optional, PCC - 1 also shows that the use of an integrally hardened gasket provides a smooth and low-friction bearing surface for the nut, which improves the fastening efficiency of the torque wrench and reduces the flange. And the nut carries damage to the surface. However, some gaskets only harden the surface, plastic deformation of the softer core causes the gasket to be bucked and thinned [12], and the bolt tightening load is lost, so it is not recommended.
Appendix M lists the Class 4 gasket materials used for different operating temperatures (see Table 4). The hardness of the gasket is between 38 and 45 HRC. Carbon steel washers are available at lower temperatures and low alloy steels (UNS G41300 or G41400) at slightly higher temperatures. For gaskets at higher operating temperatures, the ideal material is austenitic stainless steel, but the hardness of austenitic stainless steel is lower, for which martensitic stainless steel ( UNS S4100) is used. Because the corrosion resistance of martensitic stainless steel is lower than that of austenitic stainless steel, precipitation hardening stainless steel (S17400) has been added in the appendix. This material has high corrosion resistance, but can not reach the hardness of martensitic steel. This specifies that the material has a hardness between 33 and 42 HRC.
3.5 Determination of the installation bolt load (Appendix O)
Appendix O is new to the 2010 release, and the 2000 release does not provide a way to determine the mounting bolt load. Under the premise of considering the overall performance of the flange connection, the new version proposes two methods to determine the mounting bolt load: (1) Simple method. This method is easy to apply, but the individual components of the flange connection may be damaged; (2) The more complicated method based on the integrity of the connected components. For the first method, the mounting gasket stress (obtained from the gasket manufacturer) is determined based on the minimum leak rate requirement, and the mounting bolt load is determined by:
The second method is based on the first method, which further considers the ultimate load carrying capacity of each component in the flange joint, the gasket stress required to ensure the seal, all operating loads and the slack of the gasket. Appendix O Based on the elastic analysis of the flanges in the ASME specification and the elastoplastic finite element analysis of the flanges in recent years, the calculation results of the flange strength and stiffness are listed in ASTM SA105 and SA-182 F304 steel ASME B16. 5 and B16. Mounting bolt stress for standard flanges of Series 47 A. These mounting bolt stresses are determined by limiting flange stress and flange ring deflection angle. Excessive installation bolt stress causes the flange to undergo severe plastic deformation (GPD), so that the flange ring produces permanent deflection deformation, which causes the local stress of the gasket to increase, and finally the joint is leaked due to the gasket crushing. However, if the excessively conservative limit on the strength of the flange is used, it will not help to increase the mounting bolt load, resulting in a decrease in the overall performance of the flange joint. When the installation bolt load exceeds 5% to 10% of the calculated bolt load, it will not cause a big accident in the flange, and a small number of flanges will be permanently deflected and deformed, and will not affect the overall performance of the flange joint. In fact, the bolt tightening process is significantly affected by the elastic deformation of the flange. The bolt load after tightening is reduced to the original 95% [14], so the bolt load needs to be properly increased. Appendix O gives the upper and lower limits for the mounting bolt stress. The highest stress to limit bolt installation is to avoid damage to bolts or flanges and gaskets. The maximum installation bolt stress is generally controlled at 40% to 70% of the room temperature yield strength of the bolt; and the minimum stress for limiting the bolt installation is to avoid the installation bolt stress being too low due to the inaccuracy of the calculation of the installation bolt load, thereby reducing the flange joint. Sealing. The lower limit of the bolt stress is generally controlled at 20% to 30% of the bolt's room temperature yield strength.
Appendix O also gives eight steps and examples for determining the installation bolt load according to this method, as well as a part of the construction of ASME B16. 5 Mounting bolt torque table with neck flange (flange material ASTM SA10, bolt material SA193 - B7 and wound gasket with inner ring).
The second method fully considers the requirements for ensuring the integrity of the flange structure and the sealing of the flange joint. However, the complete information required for the above calculations needs to be obtained from the industrial test or manufacturer before calculating the bolt load.
3.6 Guide to Checking and Troubleshooting Flange Joint Leakage (Appendix P)
Appendix P is also a new version of the new content, designed to help users understand the causes of flange joint leakage, make correct judgments and choices, and deepen the understanding of the overall performance of flange joints [13].
Appendix P contains three parts: survey and diagnostic guides; flange design and practical inspection items; and leak diagnosis and solutions for service time. The Survey and Diagnostic Guide section lists the survey content, and the accuracy of the survey depends on the integrity of the installation record, the level of understanding of the operation and maintenance. For example, Chapter 14 of the Guide gives 14 information on the flange installation record. The leakage of the flange joint is very important; the second part includes inspection items such as load (such as external load, temperature difference stress caused by temperature change), bolt material, bolt spacing and flange joint type. Finally, a series of causes of leakage under various operating conditions and the measures to be taken are given.
4 Conclusion
Bolted flange connection is an important component of pressure vessels and pipelines. High-quality bolted flange connections have an important impact on the safe operation of pressure vessels and pressure piping. There are many bolting methods at the annual ASME academic conference in the United States. Lan connects related papers for communication. These papers reflect the long-standing research work of many foreign research institutions, and are skilled in ensuring the strength and sealing of flange joints. These research results have also entered the design specifications and standards, and also reflected in the installation inspection and maintenance specifications after construction, reflecting the significant progress in the structural integrity of the flange joints and the connection and sealing technology.
The PCC-1 developed by ASME is an example, and Japan and the European Union have or are developing corresponding standards. There is still a big gap between the research on bolt flange connection in China and abroad, especially in the aspect of bolt flange installation, which should attract the attention and research of colleges, enterprises and engineering circles, and should also formulate corresponding standards.
references:
[1] Warren Brown. Background on the New ASME PCC -1—2010 Appendix A “Training, Qualification and Certification of Joint Assembly Personnel†[A]. Proceedings of ASME of Pressure Vessels and Piping Division [C]. Bellevue, Washington, 2010, PVP2010 -25769.
[2] Murali Krishna M, Shunmugam MS, Siva Prasad N. A Study on the Sealing Performance of Bolted Flange Joints with Gaskets Using Finite Element Analysis [J]. International Journal of Pressure Vessels and Piping, 2007, 84: 349 - 357.
[3] ABID M, NASH DH, Pakistan. Joint Relaxation Behavior of Gasketed Bolted Flanged Pipe Joint during Assembly [A]. Proceedings of the 2nd WSEAS Int. Conference on Applied and Theoretical Mechanics [C]. Venice, Italy, 2006, 20 - 22: 319 - 326.
[4] Allen C Smith. Evaluation of Torque vs. Closure Bolt Preload for a Typical Containment Vessel under Service Conditions [A]. Proceedings of ASME of Pressure Vessels and Piping Division [C]. Bellevue, Washington, 2010, PVP2010 - 25135.
[5] Abid M. "Determination of Gasketed and Non Gasketed Flanged Pipe Joint s Capacity Subjected to Combined Loading: An Experimental Approach [J]. International Journal of Mechanics and Materials in Design, 2005: 35 - 47.
[6] Payne JR. PVRC Flanged Joint User Survey[Z]. WRC Bulletin No. 3064.
[7] Akli Nechache, Abdel - Hakim Bouzid. Creep Analysis of Bolted Flange Joints [J]. International Journal of Pressure Vessels and Piping, 2007, 84( 3) : 185 - 190.
[8] ASME PCC - 1 - 2000, Guidelines for Pressure Boundary Bolted Flange Joint Assembly [S].
[9] ASME PCC - 1—2010, Guidelines for Pressure Boundary Bolted Flange Joint Assembly [S].
[10] Clyde Neely, Clay Rodery. Whats New in the PCC- 1—2011 Update “Guidelines for Pressure Boundary Bolted Joint Assembly†[A]. Proceedings of ASME of Pressure Vessels and Piping Division [C]. Bellevue, Washington, PVP 2010 - 25738.
[11] Warren Brown. Background on the New ASME PCC- 1—2010 Appendices D & O “Guidelines for Allowable Gasket Contact Flatness and Defect Depth†& Assembly Bolted Load Selection†[A]. Proceedings of ASME of Pressure Vessels and Piping Division [C]. Bellevue, Washington, PVP2010 - 25766.
[12] Joseph Barron. Background on the New ASME PCC- 1—2010 Allendix M: “Washer Usage Guidance and Purchase Specification for Though - hardened Washers†[A]. Proceedings of ASME of Pressure Vessels and Piping Division [C]. Bellevue, Washington, PVP 2010 - 25774.
[13] James R Payne, Clyde C Neely, Edward W Hayman. Background on the New ASME PCC - 1—2010 Appendix P “Guidance on Troubleshooting Flanged Joint Leakage Incidents†[A]. Proceedings of ASME of Pressure Vessels and Piping Division [C]. Bellevue, Washington, PVP 2010 - 25784.
[14] Finn Kirkemo. Design of Compact Flange Joints[A]. Proceedings of ASME of Pressure Vessels and Piping Division [C]. Vancover, British Colombia, Canada, 2002, PVP 2002 - 1087.
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