SUBSECTION NF SUPPORTS

Transcription

1 ASME_Ch10_p qxd 8/13/08 4:14 PM Page 1 CHAPTER 10 SUBSECTION NF SUPPORTS Uma S. Bandyopadhyay EXECUTIVE SUMMARY On December 31, 1973, the American Society of Mechanical Engineers (ASME) published Subsection NF [1] of the ASME Boiler and Pressure Vessel Code Section III, Division 1 (hereafter addressed as Subsection NF) as part of the winter 1973 addenda to the 1971 Code edition [2]. This was a historic publication; prior to it, supports (also called pipe hangers and restraints) were not addressed as part of ASME Section III. Existing nuclear plants and plants under construction during that time were using ANSI B31.1 [3], ANSI B31.7 [4], MSS-SP58 [5], and the AISC Manual of Steel Construction [6] to design supports. ASME Section III, Subsection NF, provided a stabilizing position for future nuclear plant support design by designating a single source of rules for the design, construction, fabrication, and examination of supports. This criteria-and-commentary chapter provides information on the origins and evolution of design rules and is intended to allow designers, engineers, and fabricators to make better use of Subsection NF of the ASME Code. Topics of greatest interest are discussed and addressed from both a technical and historical viewpoint. It is not the intent, however, to address every detail or anticipate every question associated with the use of the subsection. However, there will be situations when engineering judgment and special considerations will be used in conjunction with Subsection NF to qualify supports. ASME Boiler and Pressure Vessel Code Section III, Division 1, Subsection NF, was developed in an attempt to provide rules for the estimated 10,000 piping and component supports existing in a typical nuclear power plant. These rules have evolved so dramatically that the existing support rules seldom resemble the original rules of This document follows the evolution of Subsection NF as the industry attempted to apply the subsection s rules. Commentary is provided to explain how the criteria are used, the source and technical basis for the equations and the rationale, and the reasons for change. It is anticipated that readers will develop a better understanding of Subsection NF to appreciate its complexities and usefulness NF-1000 INTRODUCTION Article NF-1000 provides readers with general information regarding component supports such as their scope and classification, and also regarding types of supports and attachments. When it was first published, Subsection NF [1] was titled Component Supports, since the term component was relevant to both supports for nuclear components (e.g., tanks, pumps, and vessels) and supports for piping, which is also defined as a component. In a major rewrite first published in the winter 1982 addenda to the 1980 edition [7], the generic term component supports was redefined as supports. This subtle change allowed supports to be separated into two distinct categories: component supports and piping supports, which resulted in a revised philosophy of Subsection NF. The changes and impact resulting from this revision are discussed in various sections of this chapter Scope Subsection NF contains rules for the materials, design, fabrication, examination, installation, and preparation of certification documents (certificate of compliance and NS-1 certificate of conformance for supports) for Classes 1, 2, 3, and MC construction. This statement appears at the beginning of Subsection NF and defines the scope in only one sentence. However, the interpretation of this scope by industry users and the Working Group has provided many inquiries and discussions over the past 25 years or so. Simply stated, the purpose of a support is to provide a path to transmit specified loads from the pressure boundary component to the building structure. It should be noted that supports are not pressure-retaining components but rather structural components. Until the appearance of Subsection NF in the ASME Section III Boiler and Pressure Vessel Code, all components were, by definition, pressure-retaining. Because many of the requirements that eventually were included in Subsection NF were taken from the pressure-retaining portions of Subsections NB, NC, ND, NE, and NG, implementation of these rules was sometimes difficult. Many users of Subsection NF regarded some of the rules as too stringent because their purpose was for use on pressure boundary applications, a view that became more apparent when the boundaries of jurisdiction were established for supports. (See Section of this chapter for a detailed discussion of this subject.) Types of Supports Since there are thousands of supports in a typical nuclear power plant, it was considered prudent to identify various types of supports based on their historical use in both fossil fuel and nuclear power plants. Initially, component supports were separated into three types: plate-and-shell supports, linear supports, and component standard supports. Subsection NF defined plate-and-shell supports as supports that are normally subjected to a biaxial stress 1 Robert J. Masterson was the author of this Chapter for the first edition but revised by Uma S. Bandyopadhyay for the previous edition and this edition.

2 ASME_Ch10_p qxd 8/13/08 4:14 PM Page 2 2 Chapter 10 field and are fabricated from plate-and-shell elements. Examples were given as vessel skirts and saddles. It was apparent from this definition that plate-and-shell supports are closely related to pressure boundary items; these types of supports essentially represent a method of making the transition from the pressure boundary of the vessel or piping into the support load path. It was common for plateand-shell supports to be integrally attached (i.e., welded) to the pressure boundary component and designed and analyzed as part of the component. An example of this was a vessel skirt, which normally would be provided with the vessel. It was initially believed that supplying plate-and-shell supports with the components would be a normal procedure (this was often the case as Subsection NF was implemented). Also, the number of plate-and-shell supports turned out to be very low relative to the other two types of supports. Subsection NF defined linear supports as acting under a single component of direct stress that also may be subjected to shear stress. Examples given included tension and compression struts and beams and columns subjected to bending stresses. Linear supports were meant to be structural steel members used to connect the supports and complete the load path to the building structure. Linear supports were also intended to form unique piping and component restraints when these components were subjected to loads other than simple deadweight. These loads were dynamic (both seismic and hydrodynamic); transient, such as water and steam hammer; and thermal operation. Based on the number of supports in a typical plant, linear supports constituted the majority. Finally, Subsection NF created a new category of supports called standard supports, which were defined as one or more generally mass produced units usually referred to as catalog items. For design, fabrication, and examination purposes, standard supports were further classified as being of the linear or plate-and-shell type; this classification was required to maintain consistency with the other two support types. Based on the sheer numbers of standard supports, the linear type again constituted the vast majority. The concept of the standard support was unique considering its sheer numbers. Standard supports were created to take advantage of the historically good manufacturing record of catalog supports. At one of the early Subsection NF meetings (circa 1973 or 1974), a Working Group member who was an employee of a support manufacturer, described standard supports as having a time-tested history of success, a statement that was proven correct because very few if any failures of catalog supports were documented under normal operating conditions on older fossil fuel and nuclear power plants. The design factor of safety and the quality assurance used in the manufacturing process of these catalog supports served as powerful arguments for establishing this category of supports. Figure 10.1 provides pictorial representations of typical standard supports. As we will see, the advantage of this type of support is manifested in the relaxation (i.e., the less stringent) of requirements for materials, design, fabrication, and examination. With the publication of the winter 1982 addenda to the 1980 edition [7], component supports were redefined to more closely represent their use in service. The term component support initially was used on the cover of Subsection NF and was descriptive of the entire family of supports. The winter 1982 addenda separated supports into two groups: component supports and piping supports, a Code revision that was the outcome of years of debate within the Code Committee on how to make Subsection NF more useful for the engineering community. It was apparent from the many guests attending the Working Group s meetings that a more descriptive categorization of supports was required to make implementation more efficient. The term component support ceased to be the term used to describe the entire family of supports and also to describe the type of supports used to support components; it was redefined as the group that supported nuclear components, and piping support was defined as the group that supported nuclear piping. The types of supports remained the same because there could be plate-and-shell, linear, and standard (previously termed component standard) supports in both groups of component and piping supports. This descriptive revision was most profoundly beneficial in Article NF-3000 Design Intervening Elements As more nuclear power plants were designed to Subsection NF, it became apparent that each type of support had its place in the overall design of piping and components. Many support assemblies consisted of standard supports, such as clamps, that attached to the pressure boundary, and also of linear supports, such as steel beams, that attached to the building structure. In some cases, standard supports composed the entire assembly; similarly, in other cases, linear supports composed the entire assembly. There was, however, another species of support that was about to make an appearance. After four years of Subsection NF implementation, a Code revision was needed to address the concept of a non-code item in the support load path between the pressure boundary and the building structure. Such items as diesel engines, electric motors, coolers, valve operators, and access structures were bearing on supports or were welded to, bolted to, pinned to, or clamped to them. By definition, supports extending from the pressure boundary to the building structure were within the support load path. Guests attending the Working Group s meetings submitted inquiries concerning what should be done with these intervening elements that were within the load path. After many debates, Subsubarticle NF-1110(c) was revised and paragraph NF-1111 and subparagraph NF were added in the summer 1978 addenda to the 1977 edition [8] to introduce the concept of intervening elements. In addition, Fig. NF was revised to add sketches (g), (h), (i), (j), and (k) to illustrate the many ways that an intervening element may be used in the support load path (see Figs and 10.3). Essentially, intervening elements were outside the Subsection NF jurisdiction; however, paragraph NF-1111 provided the clear requirement that the owner s Design Specification shall furnish specific information to the designer of the intervening elements regarding loads, materials, temperature, environmental effects, design, fabrication, examination, testing, and installation. Addressing the concept of intervening elements was a challenge that the Working Group was likely to encounter given the nature of the support load path, that is, between two existing boundaries of jurisdiction. With so many components and other equipment vying for the space between the piping and the building structure, it was inevitable that the concept of intervening elements would eventually manifest itself. What was ironic, however, was that the concept of intervening elements was a jurisdictional boundary issue, but not the most difficult one to address and solve. Working Group members found that addressing the basic concept of the boundary on each side of the support load path, the piping, or component and the building structure, became a monumental task Boundaries of Jurisdiction From the initial issue of Subsection NF in the winter 1973 addenda to the 1971 edition of ASME Section III [1], it eventually became clear that the most challenging task facing the Working Group would be to explain and defend the requirements for boundaries of jurisdiction. At first glance, it seemed to be a

3 ASME_Ch10_p qxd 8/13/08 4:14 PM Page 3 COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE 3 FIG TYPICAL STANDARD SUPPORTS (Source: Fig. NF , Subsection NF of the ASME B&PV Code) straightforward task to establish the boundaries of jurisdiction for supports at the pressure boundary and building structure ends of the support. In fact, the Working Group was convinced that Fig. NF provided a simplified illustration to define the boundaries and to specify under which subsection the connection responsibilities rested. For the pressure boundary side of the support load path, it appeared that the figure did indeed provide such an illustration. No doubt existed about when the pressure boundary ended and when the support began. Even for supports welded to the pressure boundary, it was clear that the weld was in accordance to the pressure-retaining portion of the Code (NB, NC, ND, or NE) and the support jurisdiction began with the item that was welded to the component. However, the building structure end of the support was another matter. Because many supports could contain structural steel elements in their design, and because the building

4 ASME_Ch10_p qxd 8/13/08 4:14 PM Page 4 4 Chapter 10 FIG ILLUSTRATIONS OF JURISDICTIONAL BOUNDARIES [Source: Fig. NF (a) (f), Subsection NF of the ASME B&PV Code]

5 ASME_Ch10_p qxd 8/13/08 4:14 PM Page 5 COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE 5 FIG ILLUSTRATIONS OF JURISDICTIONAL BOUNDARIES [Source: Fig. NF (h) (k), Subsection NF of the ASME B&PV Code]

6 ASME_Ch10_p qxd 8/13/08 4:14 PM Page 6 6 Chapter 10 structure boundary was generally composed of structural steel, the actual boundary of jurisdiction could become difficult to identify accurately. One school of thought, implemented initially to create Fig. NF , defined the building structure as the surface of concrete or steel as shown on civil/structural drawings. Any additional steel framed between existing steel or concrete (also called supplementary steel) and needed to support piping or components would be under Subsection NF jurisdiction. The debate on boundaries of jurisdiction began soon after the publication of Subsection NF in December 1973 [1]. Many potential users had difficulty identifying the boundary at the building structure, and guests at the Working Group meetings began to have jurisdictional boundary questions. One typical case, which would eventually change philosophy in a future Code revision, concerned baseplates and concrete anchor bolts. Initially, with the first edition of Subsection NF, it was clear that baseplates and anchor bolts were within the jurisdiction of Subsection NF. Figure 10.2 [Fig. NF (e) and (f)] clearly states that an integral or nonintegral support connected to the building structure has a connection in accordance with Subsection NF, which means that a baseplate secured to the concrete by anchor bolts (e.g., Hilti Quik bolts and Phillips Red Heads) would conform exactly to the Fig. 10.2(f) sketch. The nonintegral support (the baseplate) was connected to the building structure (the concrete) by means of the anchor bolts; the connection was in accordance with Subsection NF. However, as clear as the boundary to the building structure may appeared to have been, a large number of users questioned its location. On March 30, 1978, Interpretation III [9] was published in an attempt to answer one of the jurisdictional boundary dilemmas. The following is a verbatim presentation of the question and reply: Question: How are the jurisdictional boundaries between structural members fabricated and installed with the building structure and supports for Section III components to be determined? Reply: It is the responsibility of the Owner to define the jurisdictional boundaries of component supports in the Design Specification (NCA-3254) [10]. Items furnished as part of the building structure are normally constructed to the appropriate portion of the building code used for the design and construction of the building structure. The Owner is responsible for designating whether or not metallic supports for Section III components, which are attached to the items defined as part of the building structure, are required to be constructed with the provisions of Section III, Subsection NF. The Owner is also responsible for the compatibility of the boundaries and corresponding loads between the building structure and the component supports constructed in accordance with Section III. This interpretation clearly put the responsibility of determining the boundary of jurisdiction with the owner and the Design Specification where, in actuality, the responsibility belonged. Subsection NCA clearly stipulates that one responsibility of the owner s Design Specification was the identification of the boundaries of jurisdiction. Therefore, the interpretation answered the inquiry by using existing Code words. The jurisdictional boundary questions continued for many Working Group meetings. A Task Group was established to resolve the questions and produce the appropriate Code revision or Code Case. The 1986 edition of Subsection NF published a revision to subsubarticle NF-1130 [11], which defined the boundaries of jurisdiction applicable to Subsection NF. The old Fig. NF (Figs and 10.3), a generic presentation of the boundaries between both the component and the building structure, was replaced with Fig. NF (Figs. 10.4, 10.5, and 10.6). This figure was applicable only to the boundary between the piping support and the building structure. The jurisdictional boundary between supports and the component was deferred to paragraphs NB-1132 [12], NC-1132 [13], ND-1132 [14], or NE [15] as applicable. A review of this figure shows that Fig (d), (g), and (i) now specifies that the baseplate and anchor bolts are part of the building structure. This is a complete reversal from the original Code edition for identifying jurisdictional boundaries between baseplates and anchor bolts and the building structure. This new revision, however, more realistically separates normal building structure items from support items; historically, baseplates and anchor bolts were normally regarded as building structure components. Since the initial jurisdictional boundary questions were eventually resolved, Subsection NF has evolved without any additional substantial jurisdictional boundary issues NF-2000 MATERIALS Permitted Material With the initial publication of Subsection NF in 1973, ASME Section III Boiler Pressure Vessel Code Division 1 [1] was providing rules for non pressure-retaining components. Prior to this, ASME Section III was a pressure-retaining code, concerned primarily with pressure-retaining components such as pumps, valves, piping, vessels, and tanks. Subsection NF, however, brought a new concept on a large scale to ASME Section III because supports were non pressure-retaining structural elements, of which there were thousands in a typical nuclear plant. To accommodate support design and material requirements, materials specifically designated for supports were needed. Mandatory Appendices Tables I-11.1 (Table 10.1), I-12.1 (Table 10.2), I-13.1 (Table 10.3), and I-13.3 (Table 10.4) were included to provide design stress intensities, allowable stresses, and yield strength values for Class 1, 2, 3, and MC plate-and-shell type and linear type supports. With the publication of the 1992 edition, the Code requires that material for supports shall conform to the requirements of the specifications for materials listed in the tables of Section II, Part D [35]. Initially, these tables accounted for a very limited number of permitted material specifications (less than 20 support materials and 20 bolting material specifications). It soon became evident that additional material specifications were needed to address the numerous materials used by different support manufacturers. A Materials Task Group was formed consisting of all occupations from the Working Group including the Nuclear Regulatory Commission (NRC). The purpose of the Task Group was to develop a Code Case to permit additional materials and to provide the design stress intensities, allowable stresses, and yield strength values for these materials. After several Task Group meetings, Code Case 1644 [16] was issued to permit the use of numerous additional structural material specifications. The intent of this Code Case was to expedite the publication of structural materials needed for the design and construction of Subsection NF supports. It was planned that a Code Revision would eventually be published to complete the action.

7 ASME_Ch10_p qxd 8/13/08 4:14 PM Page 7 COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE 7 FIG TYPICAL EXAMPLES OF JURIDICTIONAL BOUNDARIES BETWEEN PIPING SUPPORTS AND THE BUILDING STRUCTURE [Source: Fig. NF (a) and (d), Subsection NF of the ASME B&PV Code] In November 1976, as part of the 1974 edition, Code Case 1644 Revision 6 [17] was issued as Code Case N-71 [18] under the revised Code Case numbering system. Subsequently, Code Case N-71 has been revised numerous times to add and delete material specifications as needed. At one point between March 1978 and March 1982, Code Case N-71 was revised to remove the material specifications that did not permit welding. These materials were placed in the new Code Case N-249 [19]. The most current revision to each Code Case is N [20] and N [21]. As mentioned previously, the Working Group s intention was to eventually incorporate Code Cases N-71 and N-249 into the body of Subsection NF, an action currently that is being prepared as Mandatory Appendix NF-1 [22] Exempt Material Since some Subsection NF supports were designed with nonmetallic and/or bearing materials, the concept of exempt materials needed to be addressed by the Working Group. Subparagraph

8 ASME_Ch10_p qxd 8/13/08 4:15 PM Page 8 8 Chapter 10 FIG TYPICAL EXAMPLES OF JURIDICTIONAL BOUNDARIES BETWEEN PIPING SUPPORTS AND THE BUILDING STRUCTURE [Source: Fig. NF (e) and (g), Subsection NF of the ASME B&PV Code] NF-2121(b), Permitted Material Specifications, provides guidance for those materials for which the requirements of Article NF-2000, Materials, do not apply. Items such as gaskets, seals, springs, compression spring end plates, bearings, retaining rings, washers, wear shoes, and hydraulic fluids, are exempt from the requirements of Article NF Initially, some users made the assumption that since a material is exempt from Subsection NF-2000, the material must also be exempt from the remaining articles of Subsection NF. This belief was incorrect because it was clear from subparagraph NF-2121(b) that the exemption applied only to materials, not to design, fabrication, and examination. However, a recent Subsection NF action has

9 ASME_Ch10_p qxd 8/13/08 4:15 PM Page 9 COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE 9 FIG TYPICAL EXAMPLES OF JURISDICTIONAL BOUNDARIES BETWEEN PIPING SUPPORTS AND THE BUILDING STRUCTURE [Source: Fig. NF (h) and (i), Subsection NF of the ASME B&PV Code] reversed this stand, and the requirements of all Subsection NF articles no longer apply to exempt materials, except for a minor list of caveats. Additionally, NF-2121 (b) states that (1) exempt material requirements, if any, shall be stated in the Design Specification; (2) the material shall not be affected by fluid, temperature, or irradiation conditions; (3) the materials do not require the material manufacturer s Certificate of Compliance (COC); and (4) the support manufacturer shall provide the owner with a list of exempt materials. These additional provisions, with the exception of the COC, are intended to provide the owner with the assurance that exempt materials meet the most basic ASME Section III

10 10 TABLE 10.1 ASME SECTION III MANDATORY APPENDIX I, DESIGN STRESS INTENSITY VALUES, S m, FOR FERRITIC STEELS FOR CLASS 1 PLATE-AND-SHELL-TYPE COMPONENT SUPPORTS (Source: Table I-11.1, Section III, Appendix 1 of the ASME B&PV Code) ASME_Ch10_p qxd 8/13/08 4:15 PM Page 10

11 TABLE 10.2 ASME SECTION III MANDATORY APPENDIX I, ALLOWABLE STRESS VALUES, S, FOR FERRITIC STEELS FOR CLASS 2, 3, AND MC PLATE-AND-SHELL-TYPE COMPONENT SUPPORTS (Source: Table I-12.1, Section III, Appendix 1 of the ASME B&PV Code) ASME_Ch10_p qxd 8/13/08 4:15 PM Page 11 11

12 12 TABLE 10.3 ASME SECTION III MANDATORY APPENDIX I, YIELD STRESS VALUES, S Y, FOR FERRITIC STEELS FOR CLASS 1, 2, 3, AND MC LINEAR-TYPE COMPONENT SUPPORTS (Source: Table I-13.1, Section III, Appendix 1 of the ASME B&PV Code) ASME_Ch10_p qxd 8/13/08 4:15 PM Page 12

13 TABLE 10.4 ASME SECTION III MANDATORY APPENDIX I, YIELD STRENGTH VALUES S Y, FOR BOLTING MATERIALS FOR CLASS 1, 2, 3, AND MC COMPONENT SUPPORTS (Source: Table I-13.3, Section III, Appendix 1 of the ASME B&PV Code) ASME_Ch10_p qxd 8/13/08 4:15 PM Page 13 13

14 ASME_Ch10_p qxd 8/13/08 4:15 PM Page Chapter 10 requirements. It is important that users be aware that materials exempt from Article NF-2000 requirements must still meet some basic concerns. Currenly, guidance for exempt materials is shown in Subparagraph NF 1110 (e) Certification of Material Many nuclear plant components require the highest level of material certification the Certified Material Test Report (CMTR). The NF Working Group determined that because of the large number of supports in a nuclear power plant, requiring a CMTR for all supports was impractical and unnecessary because of the structural rather than pressure-retaining nature of supports. Subsubarticle NF-2130 requires CMTRs for Class 1 plate-and-shell and linear supports, as well as for material of other types and classes of supports where impact testing is required. Certificates of Compliance with the material specification, grade, class, and heat-treatment condition may be provided for all other supports, which means that the majority of supports could be supplied with a COC rather than a CMTR because of the relatively few Class 1 plate-and-shell or linear supports. Support manufacturers would benefit greatly because maintaining full traceability throughout the manufacturing process for supports with COCs is not required. Also, many simple supports such as rods, clamps, and clevises could be manufactured and shipped with a single COC. However, material for small items would need to be controlled during the manufacturing process so that it is identifiable as acceptable material until the material is actually consumed in the final product. To meet this requirement, many support manufacturers would transfer a color-coding system to material after cutting so that the material identification remained on both items of the cut material Impact Testing and Fracture Toughness Because most support material is structural, the Working Group did not consider it necessary to require support materials to be impact tested when Subsection NF was first published. As previously mentioned, if impact testing were required, material for any class or type of support would need to be supplied with CMTRs. Doing so would require support manufacturing facilities to initiate a comprehensive material separation and control system for the full line of products. The Working Group concluded that impact testing would be required only when specifically stated in the owner s Design Specification. Based on service conditions, most support Design Specifications would specify impact testing for all support materials being used in cold environments, such as below 40 F. When impact testing was required, however, several exemptions were in effect especially for small products. Impact testing would not be required for such items as material thickness 5 8 in. and less; bolting nominal size 1 in. and less; austenitic stainless steel; nonferrous materials; bars of 1 sq. in. in area; supports with a maximum stress not exceeding 6000 psi; and other material dimensional limits. Paragraph NF-2311 was revised in the winter 1982 addenda to the 1980 edition [7] to require impact testing for all classes of Component supports. Piping and Standard supports would still need the Design Specification to state whether impact testing was required Quality System Program Material organizations were required to have quality system programs that met the requirements of ASME Section III, Subarticle NCA-3800 [23]. However, except for paragraph NCA-3862 [24], the other requirements of subarticle NCA-3800 did not need to be met for small products that were defined as pipe, tubing, bolting material including nuts, and structural material meeting specific dimensional limits. Similar to the NPT stamp required for welded products, material organizations needed to successfully complete an ASME Quality Systems survey to obtain a Quality Systems Certificate for support material NF-3000 DESIGN When Subsection NF was first published in the winter 1973 addenda of the 1971 edition of ASME Section III [1], the Code was considered complete because it now addressed all major nuclear plant components. Supports were different from many other components because they were non pressure-retaining structural components rather than pressure-retaining components. In fact, much of the structural steel required for support design and construction closely resembles the building structural steel. This critical difference, non pressure-retaining components, required the Working Group to establish new rules for structural elements of supports. Design rules for supports were initially established for two types of supports: plate-and-shell and linear. Although both procedures were structural, the plate-and-shell techniques were more consistent with those from other Code subsections. Because Subsections NB, NC, ND, and NE contained rules for pressure-retaining components such as pumps, valves, piping vessels, and tanks, ASME concluded that component supports with a design exhibiting a biaxial stress field should follow similar design rules. This conclusion was a reasonable one and allowed rules for plate-and-shell supports to be established that were familiar to owners and Architectural Engineers (AEs). Similarly, the rules for linear supports were patterned after an accepted and recognized structural Code found in the seventh edition of the Manual of Steel Construction [25], published by the American Institute of Steel Construction (AISC). The AISC Steel Manual was well recognized by support designers because it was extensively used to design steel supports and other structures in fossil fuel and pre Subsection NF nuclear power plants. Allowable stresses were based on the material minimum specified yield strength rather than on allowable stresses and stress intensities. The AISC specification was incorporated in its entirety with some enhancement of design criteria such as temperature and buckling requirements. This section discusses the design rules for supports both plate-and-shell and linear. All aspects of design are considered including stress theory, loadings, welding, bolting, load testing, and functional requirements Design Loadings and Service Conditions Because design loadings and service conditions are established as a requirement of the Design Specification (NA/NCA-3252) [26], Subsection NF is also governed by these requirements. Design loadings are defined as design temperature and design mechanical loads. Because supports are subjected to non pressure-retaining loads, temperature-generated loads are transmitted to supports by the movement of piping and equipment. Structural loads are transmitted to supports through the deadweight of piping and its contents and also of piping components such as valves, flanges, flowmeters, and in-line pumps. Design mechanical loads also include dynamic loads caused by earthquakes, flow-induced loads such as water and steam hammer, and hydrodynamic loads. The assortments of design loadings are combined based on identified service conditions with specified service limits. When

15 ASME_Ch10_p qxd 8/13/08 4:15 PM Page 15 COMPANION GUIDE TO THE ASME BOILER & PRESSURE VESSEL CODE 15 Subsection NF was first published [1] in 1973, these service conditions were identified as normal, upset, emergency, faulted, and testing. The winter 1976 addenda to the 1974 edition of ASME Section III [27] changed the term service condition to service limit. Normal condition was changed to Level A service limit ; the term pertains to specified loadings to which supports may be subjected for its specified service function. Upset condition was changed to Level B service limit ; the term pertains to specified loadings on supports that must be withstood without damage requiring repair. Emergency condition was changed to Level C service limit ; the term pertains to specified loadings that permit large deformations in areas of structural discontinuities and that may require removal of the support from service for inspection or repair. Faulted condition was changed to Level D service limit ; the term pertains to specified loadings that permit gross general deformations and may require the removal of the support from service Code Class and Design Procedures Supports are grouped by class as all ASME Section III components. Support Classes 1, 2, 3, and MC (metal containment) follow the definitions of paragraph NA/NCA-2131 [28]. Because there are more supports in a nuclear power plant than there are any other components (the total number is in the thousands), only a few were anticipated as being Class 1 supports; indeed, only supports are usually classified as Class 1 supports, and the remaining 5,000 10,000 fall into Classes 2 and 3. This is significant because material, design, and examination rules are less stringent for Class 2 and 3 supports. As discussed later in this chapter, there is also a significant advantage for Class 2 and 3 standard supports, especially material certification, design certification, load capacity data sheets, and visual examination of welds. The Working Group s intent was to provide less stringent rules for standard supports to take advantage of the industry s exceptional manufacturing history. Many support manufacturers had developed an extensive line of pipe support products, many with designs that were based on physical and empirical testing. A safety factor of 5 was a common design factor for these catalog products, and failure of supports in field use was a rare occurrence. Associated with support class is the type of support and the design procedure to be used for Code qualification. Based on existing support designs and how they are used, the Working Group created three types of supports: plate-and-shell, linear, and standard. Plate-and-shell supports exhibit a biaxial stress field (in a flat plate this would be membrane and bending stresses in both of the plate s in-plane axes, that is, S x and S y ). Also, plate-andshell supports are more associated with pressure-retaining components and usually are vessel skirts and saddles. Very few supports in a nuclear plant will be of the plate-and-shell category. Linear supports are defined as supports that essentially act under a single component of direct stress such as a structural beam or column. Component standard supports (later redefined as standard supports) are defined as support assemblies composed of several catelog items and are generally mass produced. Three design procedures also were specified: design-by-analysis, experimental stress analysis, and load rating. The design-byanalysis procedure was established to allow a calculation stress analysis method for Code qualification similar to Subsections NB, NC, and ND. However, because supports can be both plate-andshell or linear in design, a different design-by-analysis procedure was provided for each support type. Plate-and-shell supports are required to be analyzed by elastic analysis based on the maximum shear stress theory for Class 1 construction and the maximum stress theory for Class 2, 3, and MC construction. Linear supports are required to be analyzed by elastic analysis based on the maximum stress theory for Class 1, 2, 3, and MC construction. The design-by-analysis procedure for standard supports either will be the maximum shear stress theory or the maximum stress theory depending on whether the standard support is constructed of plate-and-shell or linear elements. The maximum shear stress theory calculates principal stresses and transforms these into stress differences or stress intensities. At any point on the support, the stress components for each type of loading may be calculated: namely, x, y, and z,or l, r, and t. These loadings may result in general primary membrane stress, P m ; primary bending stress, P b ; expansion stress, P e ;or secondary stress, Q. (Definitions for these stresses are given in paragraph NF-3121 [29].) For each category of stress the algebraic sum of the j stresses for each loading is obtained and the l, r, and t stress components are translated into principal stresses, 1, 2, 3,. Finally, the stress differences, S 12, S 23, and S 31 are calculated, where S , S , and S The calculated stress intensity for each location on the support is the largest absolute value of S 12, S 23, and S 31. The maximum stress theory calculates membrane, bending, and shear stresses as direct, not principal, stresses. Membrane stress, 1, is the average stress across a solid section. It includes the effects of discontinuities but not local stress concentrations. Bending stress, 2, is the linearly varying portion of the stress across the solid section. It excludes the effects of discontinuities and concentrations. With the initial publication of Subsection NF in 1973 [1], a third direct stress was required to be evaluated. The maximum tensile stress, 3, at the contact surface of a weld producing a tensile load in a direction through the thickness of a plate or rolled shape, had a reduced allowable stress. This reduction in stress was intended to reduce the maximum load on the connection to prevent plates with the potential of laminations from experiencing the full allowable stress a behavior that applied to all classes and types of supports. The fact that this requirement was a design and manufacturing anomaly was eventually discovered because the intent to limit the load to address the lamination concern in effect exacerbated the condition. As noted in Fig. NF (c)-1 (Fig. 10.7), for any given joint, because the allowable stress was essentially limited to 50% of the normal allowable stress, the maximum applied load permitted was effectively reduced. To circumvent this low allowable stress, many designers simply made the weld contact surface larger to permit larger loads but still remain within the reduced allowable stress. It was quickly experienced that increasing the weld size increased the heat input to the joint, and for those plates that exhibited laminations, these conditions caused some joint designs to be compromised. The Working Group was made aware of this condition and quickly revised Subsection NF between the 1977 [30] and 1980 [31] editions to remove this requirement. It was felt that additional rules for fabrication and examination of these types of welded joints were needed as a better approach to the problem. Experimental stress analysis is the second design procedure permitted by Subsection NF. Designers are directed to ASME Section III, Division 1, Appendix II [32], which contains mandatory rules for employing experimental stress analysis. It was the intent of the Working Group to permit a design procedure that provided Code qualification by means of physical testing to determine

16 ASME_Ch10_p qxd 8/13/08 4:15 PM Page Chapter 10 testing and test reports to be used in conjunction with additional testing and the load rating equations in Subsubarticle NF-3280 [33] to establish Code qualification, especially for standard supports. This topic is discussed in more detail in Section of this chapter. FIG ILLUSTRATION OF MAXIMUM DESIGN STRESS IN THROUGH-THICKNESS DIRECTION OF PLATES AND ELEMENTS OF ROLLED SHAPES [Source: Fig. NF (c)-1, Subsection NF of the ASME B&PV Code] stress levels within supports. The procedure uses strain gages to determine stresses within actual supports under load. Appendix II contains complete guidelines for performing the tests, obtaining results, and interpreting the results. A third design procedure, known as load rating, was created to provide support manufacturers with a method of establishing maximum load ratings for standard supports using techniques and test reports newly and/or previously developed by the manufacturer. Many manufacturers had established testing procedures and methods that allowed load ratings to be published with confidence. It was the Working Group s intent to permit existing Stress Intensities and Allowable Stresses Allowable stresses for all types and classes of supports are categorized in Table NF-2121 (a)-1 (Table 10.5). For Class 1 plateand-shell supports, design stress intensity values, S m, are used to limit the calculated stresses. Allowable stresses values, S, are used for Class 2, 3, and MC plate-and-shell supports, and yield strength values, S y, are used for all classes of linear supports and for all classes and types of bolting. Component standard supports are qualified using design stress intensity values, and allowable stress values or yield strength values are based on whether the standard support elements are composed of plate-and-shell or linear items. The specific values for S m, S, and S y for supports were provided in ASME Section III, Division 1, Appendix I Tables I-1.1, I-1.2, I-2.1, I-2.2, I-7.1, I-7.2, I-8.1, I-8.2, I-10.1, I-10.2, I-11.1 (given here as Table 10.1), I-12.1 (given here as Table 10.2), I-13.1 (given here as Table 10.3), and I-13.3 (given here as Table 10.4) until the publication of the 1992 edition of ASME Section II [34]. At that time, all material property tables were transferred from Section III, Appendix I, to Section II, Part D [35] for both ferrous and nonferrous materials. For supports, these are Tables 1A, 1B, 2A, 2B, 3, 4, U, and Y-1. These tables contain essentially the same data used in Tables Similarly, Section II, Part A contains the material specifications for all materials permitted for use in Section III construction. The basis for establishing the design stress intensity and allowable stress values are currently found in Section II Part D, Appendices 1 [36] and 2 [37]. These Appendices are very useful for determining stress values because they are essentially a function of yield strength, S y, and ultimate strength, S u, values at temperature and at room temperature. In many cases, the ultimate strength value at temperature of a particular material specification is not published in Section II, Part D. Appendix 1 and/or Appendix 2 can be used conservatively to determine the S u value if either the S or S m values are published. Because Appendix 1 stipulates that the design stress intensity 1 value, S m, can be established as 3S u at temperature (this choice is the most conservative of the ones given), the value of S m can be established as S u 3S m. Similarly, Appendix 2 can be used to establish S u 4S, and the smallest value of S u should then be used. This method was used by the Nuclear Regulatory Commission (NRC) in Regulatory Guide [38] in Section C2c, Regulatory Position Method 3. It was recognized early after the initial publication of Subsection NF in 1973 that the material specifications permitted in Appendix I, Section III (later Section II, Part D), were not of sufficient quantity to address the materials used by many support manufacturers. The Working Group acted quickly to establish a Task Group to identify and bring these additional materials into ASME Section III for use in Subsection NF. It was expected that additional structural materials would become necessary because most of the existing material specifications were required from the pressure-retaining nature of ASME Section III. This important Task Group comprised all Working Group professional disciplines: utilities, manufacturers, AEs, consultants, and regulatory members. The Task Group worked closely with the ASME Section III Subgroup on Materials; consequently, in 1975 Code

17 TABLE 10.5 MATERIALS TABLES REQUIRED FOR SUPPORTS [Source: Table I-NF-2121(a)-1, Subsection NF of the ASME B&PV Code] ASME_Ch10_p qxd 8/13/08 4:15 PM Page 17 17

18 ASME_Ch10_p qxd 8/13/08 4:15 PM Page Chapter 10 Case 1644 [16] was published. This Code Case contained a considerable number of material specifications used by support and snubber (both hydraulic and mechanical) manufacturers. In 1976, Code Case 1644 was renamed Code Case N-71 [18]; it contained material specifications for both welded and nonwelded construction. The alternative rules for bolted joints eventually were removed from the Code Case and published as part of the NF rewrite in the winter 1982 addenda to the 1980 edition to ASME Section III [7]. In 1980 the nonwelded materials were removed from CCN-71 and placed in a new CCN-249 [19]. Since their original publication, both Code Cases have been revised many times and currently appear as CCN [20] and CCN [21]. All revisions to these Code Cases are addressed and approved for use with caveats in NRC Regulatory Guide 1.85 Code Case Acceptability ASME Section III Materials. Material specifications have been added and removed over the years; however, currently there is an effort to reduce the number of material specifications to only those that are in use at any given moment. Future plans are to move both Code Cases into Subsection NF as Appendices Plate-and-Shell Supports While the Working Group was preparing Subsection NF for its initial publication, it wrestled with the concept of writing rules for structural and non pressure-retaining components in what was essentially a pressure boundary Code. The Working Group finally agreed to create three types of supports: plate-and-shell, linear, and component standard supports. As defined previously, plateand-shell supports exhibit a biaxial stress field (in a flat plate, membrane and bending stresses would develop in both in-plane directions together with shear stress). When using the design-byanalysis procedure, calculations used to qualify Class 1 plate-andshell supports would essentially be the same as structural calculations for Class 1 pressure boundary components. Terms relating to design-by-analysis were presented in paragraph NF-3121 [29], which contained definitions for the following stresses: normal, shear, membrane, bending, primary, secondary, free-end displacement, expansion, and total. When Subsection NF was initially published in 1973 [1], Table NF (Table 10.6) contained a matrix that provided classification of stresses for some typical cases. Also associated with Class 1 plate-and-shell design-byanalysis are the design and operating conditions that allow varying allowable limits of stress intensity based on the type of stress (primary and secondary) and the conditions (design and operating) for the plate-and-shell support. This concept is presented in Fig. NF (Fig. 10.8), also known as the Hopper Chart, and is essentially identical to Figs. NB [39] and NB [40] except for the stresses in Class 1 pressure boundary components that are not required to be evaluated in Subsection NF. These stresses are Q stresses as well as secondary membrane and bending stresses (except expansion stress, P e ), an example of which is thermal stress within the support. Unlike piping and other pressure-retaining components, supports were not required to be evaluated for thermal stresses within the support, that is, thermal stress caused by a large differential in temperature in the through-thickness direction of a plate or shell. The only exception to this was expansion stress caused by the restraint of free-end displacement (of piping) and the effect of differential support or restraint motions. Initially, expansion stress, P e, was considered a secondary stress because it was self-limiting, or local yielding and minor distortions would satisfy the conditions that caused the stress to occur; failure from one application of stress would not be expected. However, the Task Group, during one of the meetings for the NF-3000 rewrite in the winter 1982 addenda [7], redefined expansion stresses in supports caused by free-end displacement of piping as primary stresses. It was concluded that the growth in the piping caused by thermal expansion was a true secondary stress in the piping; however, the support would see this as a primary load and thus a primary stress. Design-by-analysis of plate-and-shell supports for Class 2 and 3 construction is less complicated than Class 1 supports. Because the maximum stress theory is used rather than the maximum shear stress theory, true allowable stresses are employed to qualify supports rather than design stress intensities. Calculated membrane and bending stresses are compared directly to the allowable stress, S, and to factors of S to account for the differences in membrane and bending action and the different operating conditions (service limits). It should be noted that the total number of plate-and-shell supports for all Classes is relatively small when compared to the linear and standard supports. Based on the many pre-subsection NF nuclear power plants designed to ASME B-31.1 [3] and US AS B31.7 [4] Codes for Power Piping, the dramatic difference in quantities of linear and standard supports with plate-and-shell type supports was anticipated by the Working Group. It was this distinction in the quantity of linear and standard supports that prompted the Working Group to concentrate its efforts to establish new rules for supports that exhibited mainly structural behavior Linear Supports During the early days of the Subsection NF Working Group s mandate to write rules for supports applicable to ASME Section III, Division 1 philosophy, it was evident that the task of addressing linear and standard supports would be a challenge. The literally thousands of supports that were composed of various structural steel elements (viz., wide flanges, channels, structural angles, square tubing, structural pipe, and manufacturers standard catalog products) presented some difficulties for the Working Group. It eventually became apparent that in lieu of creating exclusively new design rules for linear and standard supports, the Working Group should take a more pragmatic approach by investigating rules for these supports already existing in other design Codes. For linear supports, this document was the seventh edition of AISC Manual for Steel Construction [41]. Originally used to design building structures, such as commercial steel buildings, the AISC Code was also already in use at nuclear power plants to design the building structures that housed the nuclear piping and other components. In fact, almost all linear and/or standard supports are attached to the building structure at one end of their load path. Therefore, it was a reasonable approach to extend the AISC rules to include the design of linear supports because many of the support elements were the same as those of the AISC Code [41]. Because the distinct differences between building structures and nuclear power plants, especially in areas of varying temperatures, environmental conditions, and multiple operating conditions, some additional considerations needed to be added to the design rules of AISC to adjust the rules for supports for use in nuclear plants. At this juncture, the Working Group considered it necessary to include the structural rules in ASME Section III as opposed to making reference to the AISC Manual of Steel Construction. When Subsection NF was first published in the winter 1973 addenda to the 1971 edition of ASME Section III, Division 1 [1], the design rules for linear supports were published in Mandatory Appendix XIII [42]. However, when the 1974 edition was