CHAPTER
18
18.1
INTRODUCTION
To help the reader understand and use the ASME Boiler and Pressure Vessel (B&PV) Code, this chapter for ASME Code Section IV2 (2007 edition) is presented in a simplified manner with the understanding that it is not a Codebook and was not writ-ten to replace the Codebook published by the American Society of Mechanical Engineers (ASME).
Although the rules of the ASME B&PV Code, Section IV con-stitute the minimum requirements for the safe design, construction, installation, and inspection of low-pressure-steam boilers and hot-water boilers (which are directly fired with oil, gas, electricity, or other solid or liquid fuels), they do not cover the operation, repair, alteration, rerating, and maintenance of such boilers. By definition, a low-pressure-steam boiler is a steam boiler in which the operat-ing pressure does not exceed 15 psi (103 kPa), whereas a hot-water boiler is defined as a boiler used for an operating pressure not exceeding 160 psi (1,100 kPa) and/or for a temperature not exceeding 250F (121C). Hot-water boilers include hot-water-heating boilers and hot-water-supply boilers. Also covered by the rules of Section IV are potable-water heaters and water-storage tanks for operation at pressures not exceeding 160 psi (1,100 kPa) and water temperatures not exceeding 210F (99C).
18.1.1 History of ASME Section IV
On September 15, 1911, the ASME appointed a seven-member Boiler Code Committee to formulate standard specifications for the construction of steam boilers and other pressure vessels.3The first chairperson was J. A. Stevens, a consulting engineer. In November 1914, an eighteen-member Advisory Committee was appointed, with C. W. Obert as secretary.
In its progress report during the sixth and final session on December 4, 1914, the Boiler Code Committee discussed the necessity of cooperation from users, industries, and states. Finally, on December 15, 1914, both the Boiler Code Committee and the Advisory Committee began to prepare the final draft of the Code.
The first ASME Code, Rules for the Construction of Stationary Boilers and Allowable Working Pressures, was finally adopted in the spring of 1915. Known as the 1914 edition, it consisted of 114 pages and a complete index of 28 additional pages. It was divided into the following parts:
• Part I, New Installation (87 pages) • Section I, Power Boilers (80 pages) • Section 2, Heating Boilers (7 pages) • Part II, Existing Installation (5 pages) • Appendix (20 pages)
The 1914 Code edition was revised in 1918; the Committee adopted the 1918 Code edition on December 3 of that year. At that time, revision of Part I, Section 2 was incomplete. A four-teen- member Subcommittee on Heating Boilers was appointed on September 26, 1919, with S. F. Jeter as the chairperson, to revise the section of the Code that discussed heating boilers. The subcommittee presented its final report to the Boiler Code Committee after holding a series of meetings and public hearings.
The 1923 edition of the Code for Low Pressure Heating Boilers, also known as the ASME Code Section IV, was approved by the ASME Council on May 23, 1923. It contained 113 pages and consisted of the following parts:
• Part I, Steel Plate Boilers (paragraphs H-1 to H-83) • Part II, Cast Iron Boilers (paragraphs H-84 to H-120)
In Part I, paragraph H-1, the Code was limited to steam boilers with pressures of 15 psi and hot-water boilers to 160 psi and 250F. The method of computing the maximum allowable work-ing pressure was stated in paragraph H-4, and the specifications of the materials to be used followed the rules for power boilers. Paragraphs H-15 to H-26 addressed the requirements for joints, braces, and stayed surfaces. Paragraphs H-27 to H-64 addressed the requirements for boiler openings, supports, settings, installations,
ASME S
ECTION
IV: R
ULES FOR THE
C
ONSTRUCTION OF
H
EATING
B
OILERS
Geoffrey M. Halley and Edwin A. Nordstrom
1
1M. A. Malek and John I. Woodworth were the authors of this chapter for the first edition.
2Copies of the ASME B&PV Code, Section IV may be obtained by writing to the American Society of Mechanical Engineers, 3 Park Avenue,
New York, NY 10016. Copies may also be obtained via the Society’s web site:www.asme.org.
3 History of the ASME Boiler Code, by Dr. A. M. Greene, Jr., is a collection of articles published in the journal Mechanical Engineering in July, August,
September, November, and December of 1952, and also in February, March, July, and August of 1953 (two chapters previously unpublished are also included). Dr. Greene was a member of the Boiler Code Committee from 1915 to 1943.
and fittings and appliances (including safety valves). Paragraphs H-65 to H-68 addressed hydrostatic tests with shop inspection, stamping, and marking. The remaining paragraphs, H-76 to H-83, addressed welded boilers. In Part II, paragraphs H-84 to H-86, the pressures and temperatures to be used with cast-iron boilers were specified. Paragraphs H-87 to H-89 contained the requirements for boiler openings, flanged connections, and threaded openings. Six paragraphs addressed boiler installation, and twelve para-graphs addressed both safety valves and water-relief valves.
The 1927 edition of Section IV (Low Pressure Heating Boilers) incorporated a number of revisions to the 1923 edition. The important changes were the following:
• The inclusion of specifications for flange-quality steel plates used for forged welding
• The inclusion of specifications for base metal used in autoge-nous welding
• The basing of safety valve–relieving capacity on the grate area rather than the radiating surface
• The inclusion of formulas for copper-tube thickness • The inclusion of an index of seven pages.
The 1931 edition of Section IV was prepared from addenda to clarify statements of the 1927 edition. Minor changes were made to paragraphs H-40, H-44, H-55, H-56, H-64, H-93, H-108, H-117, and H-120, and the portion of the Code entitled “Autogenous Welded Boilers” was changed to “Fusion Welded Boilers.”
The 1932 edition of Section IV contained only minor changes and rewordings, including some made to paragraphs H-35 and H-66. The Material Specifications that appeared in the 1931 edition were transferred to Section II (Material Specifications).
The 1935 edition of Section IV had few important changes. Paragraphs H-64 and H-117, both entitled “Automatic Low-Water Fuel Cut-Off and/or Waterfeeding Device,” were expanded to cover construction, location, attachment, operation, flushing, and training. Changes were also made to paragraphs H-1 to H-83, and H-84 to H-118.
The 1937 edition of Section IV was produced under the direc-tion of the same committee that produced the 1935 edidirec-tion. In this edition, the minimum allowable thickness of the tubesheet or heads given in Table H-1 was increased by in.; for shells or other areas, the thickness was reduced by in. Changes were made to para-graphs H-12, H-21, H-38, H-40, H-44, H-64, H-65, H-70, H-93, H-117, and H-118. An index of slightly more than four pages was included in the Code.
The 1940 edition of Section IV had major changes to para-graphs H-43 to H-54 of Part I and parapara-graphs H-96 to H-107 of Part 2. These paragraphs on both safety valves and water-relief valves addressed the markings of such valves with data and a symbol. Other major changes were made to Tables H-6 and H-7 and to paragraphs H-12, H-65, and H-118. A new Form No. H-1 (Manufacturer’s Test Report of Safety Valves) was added before the appendix.
The 1941 edition of Section IV was a revision of safety-valve and water relief-valve requirements. These changes expanded the first section on safety valves (paragraphs H-43 and H-96) and water-relief valves (paragraphs H-44 and H-97) describing the features of the valve. New methods for determining the required capacity of safety valves and water-relief valves, such as by divid-ing by 1,000 the maximum output at the boiler nozzle (Btu/hr) obtained from the fuel and also by multiplying the boiler heating surface by 5, simplified the computation and deleted paragraphs
1 16
1 16
H-52 and H-105 and the former Tables H-6, H-7, H-9 and H-10 from the 1940 edition.
The 1943 edition of Section IV was broadened by the addition of new paragraphs in the preamble, by the revision of paragraph H-43 (Safety Valves) as well as paragraph H-44 (Water Relief Valves), the revision of paragraphs H-46, H-47, H-48, H-49, H-51, H-52, H-53, H-61, H-68, and H-96 to H-117, and the revision of Tables H-6 and H-7. This edition made a book of forty-five pages, with an index of slightly more than four pages.
Since 1953, the ASME B&PV Code, Section IV has been revised every three years, and the latest edition always includes all previous addenda. The Code edition was published in the fol-lowing years: 1946, 1949, 1952, 1953, 1956, 1959, 1962, 1965, 1968, 1971, 1974, 1977, 1980, 1983, 1986, 1989, 1992, 1995, and 1998.
Colored-sheet addenda, which include additions and revisions to individual sections of the Code, are published annually. Code Interpretations, that is, written replies to inquiries, are published separately from the Code edition; they are issued semiannually (in July and December). The B&PV Committee also issues Code Cases to clarify the intent of existing requirements or to provide (when the need is urgent) rules for material or construction not covered by the existing Code rules. The Code Cases, when adopt-ed, appear in the Code Casebook for Boiler and Pressure Vessels; later, they are incorporated into the Code itself.
18.1.2 Organization of this Chapter
This chapter is divided into five parts and two subparts, in a similar manner to the format of Section IV of the ASME Boiler and Pressure Vessel Code. Necessary figures and tables are included in each part, or subpart. For easy identification para-graph numbers, figures, and tables from the Code book are used in the running text. This chapter also includes several appendices, one of which—Appendix 18.D—provides a glossary of boiler, design and welding-related terms. The other appendices—all reproductions from the Code book—are:
• Appendix B—Methods of checking safety valve and safety-relief valve capacity—Shown in this chapter as Appendix 18.A • Appendix C—Methods of calculating welded ring reinforced
furnaces—Shown in this chapter as Appendix 18.B
• Appendix D—Examples of computation methods for boiler shell openings—Shown in this chapter as Appendix 18.C • Appendix L—Examples of Manufacturers’ Data Report
Forms—Shown in this chapter as Appendix 18.E.1 and 18.E.2
A brief outline of the contents of each part and subpart is given in the following paragraphs.
Part HG: General Requirements for all Materials of Construction Part HG addresses material requirements, design,
pressure-relieving devices, tests, inspection, stamping, instru-ments, fittings and controls, and the installation of low-pressure-steam-heating boilers, hot-water-heating boilers, hot-water-supply boilers, and the boilers’ appurtenances, but it does not cover the requirements of potable-water heaters. The requirements of Part HG are used together with the specific requirements in Part HF and Part HC, whichever is applicable.
Part HF: Requirements for Boilers Constructed of Wrought Materials There are special requirements for heating boilers
part are applicable to steam-heating boilers, hot-water-heating boilers, hot-water-supply boilers, and the boiler parts construct-ed of wrought materials. Part HF has two subparts: HW and HB.
Part HF, Subpart HW: Requirements for Boilers Fabricated by Welding The requirements of this subpart are applicable to
heating boilers and their parts fabricated by welding. Because the construction method of the boiler is welding, all the applicable rules of ASME Code Section IX are followed as well.
Part HF, Subpart HB: Requirements for Boilers Fabricated by Brazing This subpart addresses the requirements for
steam-heating boilers, hot-water-steam-heating boilers, hot-water-supply boil-ers, and the boilers’ fabrication by brazing. The applicable rules of ASME Code Section IX are also used.
Part HC: Requirements of Boilers Constructed of Cast Iron
The requirements of Part HC apply to steam-heating boilers, hot-water-heating boilers, and hot-water-supply boilers and their parts, which are fabricated primarily of cast iron. In addition to these requirements, those of Part HG are used.
Part HA: Requirements for Boilers Constructed of Cast Aluminum The requirements of Part HA apply to hot water
boil-ers constructed primarily of cast aluminum.
Part HLW: Requirements for Potable-Water Heaters Part
HLW apply to water heaters in commercial or industrial settings for supplying potable water at pressures not exceeding 160 psig (1,100 kPa) and temperatures not exceeding 210F (99C). It is not applicable to residential water heaters.
All details of design and construction are not covered by the foregoing rules. The Manufacturer, responsible for the safe design and construction of boilers, provides details of the design and construction that are subject to acceptance by the Authorized Inspector. The Authorized Inspector is the person responsible for reviewing the boiler designs and inspecting of the boilers during their construction to ensure that all require-ments are met.
Moreover, the repair, alteration, rerating, and inservice inspection of the heating boilers are done in accordance with the National Board Inspection Code (NBIC),3published by the National Board of Boiler and Pressure Vessel Inspectors, a non-profit organization composed of chief boiler inspectors of vari-ous U.S. states and cities as well as varivari-ous Canadian provinces and cities. It is the NBIC’s purpose to maintain the integrity of pressure-retaining items after they have been placed into ser-vice by providing rules for inspection, repair, and alteration. The NBIC provides guidance to jurisdictional authorities, Inspectors, Users, and organizations performing repairs and alterations.
18.1.3 Adoption of Section IV
Section IV is composed of a set of rules for the construction of heating boilers. These rules are extracted from neither design
manuals nor textbooks. The objective of the Code is to establish safety rules for providing reasonable protection of life and property. The rules of the Code encompass requirements, prohibitions, and guidance for the design, fabrication, inspection, testing, and certification of heating boilers.
The Code rules are nonmandatory unless they are adopted into the laws of a governmental jurisdiction—a state, province, county, or city or in the absence of a jurisdiction, by contract. A jurisdic-tion may adopt these rules either partly or completely. Once adopted, the jurisdiction enforces these now-mandatory rules by using Authorized Inspectors under the supervision of a Chief Inspector. Currently, most states, provinces, large cities, and some counties have adopted Section IV. A Synopsis of Boiler and Pressure Vessel Laws for the United States and Canada is pub-lished by the National Board of Boiler and Pressure Vessel Inspectors.4
18.1.4 Design Considerations Beyond Section IV of the ASME Boiler and Pressure Vessel Code (Authors’ Opinion)
It should be noted that this Code, and typically many Codes, provide a minimum set of design requirements that must be adhered to. However provided that these requirements are fol-lowed, there is nothing wrong with the designer going beyond the scope of the Code in order to circumvent potential problems, caused by specific operating or environmental conditions, which may affect the longevity of the boiler. The following are offered for consideration:
• In situations where the boiler is swing loaded rather than base loaded and could experience a cyclic operating condition, consideration should be given to an evaluation of the fatigue effects of thermally induced operating stresses caused either by the fuel burning equipment being cycled on and off, the pressure and/or bulk water temperature swinging in response to load excursions, or the effects of electric load shedding in hot water boiler systems whereby large quantities of cold water may return to the boiler, as heating zones are brought back on line. It has been shown that under certain types of system operating conditions, fatigue failures in the high strain-low cycle regime have been experienced5. While these failures are seldom catastrophic in nature, they can be both troublesome and expensive.
• The current trend to smaller footprint boilers for a given capacity and high furnace heat releases coupled with the desire for low NOxemissions has produced a generation of
fuel burning equipment that while environmentally friendly, is more sensitive to setup conditions and local environmental changes. This may result in more episodes of unstable com-bustion than previously experienced. Incidents have been noted in which unstable combustion (pulsations) has affected such things as tube joint integrity, particularly in designs that have long tube bundles. Thus a vibration analysis of the boil-er structure may become necessary for cboil-ertain types and sizes of boiler, in order to avoid costly repairs.
4 Copies of the NBIC and the Synopsis of Boiler and Pressure Vessel Laws may be obtained by writing to the National Board of Boiler and Pressure
Vessel Inspectors, 1055 Crupper Avenue, Columbus, OH 43229. Copies may also be obtained via the National Board’s web site: www.nationalboard.org.
18.2
PART HG: GENERAL
REQUIREMENTS FOR ALL
MATERIALS OF CONSTRUCTION
18.2.1 Article 118.2.1.1 Scope The scope of Part HG addresses material
require-ments, design, pressure-relieving devices, tests, inspection, stamping, instruments, fittings and controls, and installation requirements of low-pressure-steam-heating boilers, water-heating boilers, hot-water-supply boilers, and the boilers’ appurtenances, but it does not cover potable-water heaters. The rules of Part HG are required to be used with the specific requirements of Part HF (Boilers of Wrought Materials) and Part HC (Cast-Iron Boilers), whichever is applicable.
Provisions are made in Part HG for mandatory requirements, specific prohibitions, and nonmandatory guidance for minimum construction requirements for the design, fabrication, installation, and inspection of steam-heating, heating, and hot-water-supply boilers. Enforcement or regulatory authorities having juris-diction at the location of an installation may establish applicability of these rules in whole or in part. Therefore, the rules become mandatory only if they are adopted by a jurisdiction.
18.2.1.2 Service Restrictions Section IV, Part HG rules are
restricted to steam-heating boilers for operation at pressures not exceeding 15 psi (Saturation temperature 250F) (103 kPa) and to hot-water-heating and hot-water-supply boilers at pressures not exceeding 160 psi (1,100 kPa) and/or temperatures not exceeding 250F (121C).6If the service requirements exceed these limits, the rules of Section I will be applicable.
18.2.2 Article 2
18.2.2.1 Material Specifications and Properties are found in
Section II Parts A, B, and C. Material requirements in terms of max-imum allowable design stress levels are found in Section IV while other material requirements are found in Section II D of the Code.
18.2.2.2 General Requirements Material subject to stress from
pressure shall be selected from the Material Specifications of Section II (Materials), Parts A, B, C, and D. If the material is not list-ed in Section II, it can be uslist-ed only after approval is obtainlist-ed from the ASME Boiler and Pressure Vessel Committee.7Section II, Part D, Appendix 5 (Guideline on the Approval of New Materials under the ASME Boiler and Pressure Vessel Code) is to be followed for adoption of new materials. Material Specifications in Section II are not limited by the production method if the product complies with the requirements of the specification. If it complies with the other
requirements of the specification and with the thickness require-ments of this Code, materials exceeding the thickness limit of Section II may be used. Materials not identified by mill test reports may be used for nonpressure parts, provided failure of such parts do not affect the pressure parts to which they are attached.
18.2.2.3 Specific Materials In addition to the wrought
materi-als described previously, Sections HF (wrought materimateri-als) and HC (cast iron) provide specific material requirements beyond the gen-eral requirements of HG-200. Wrought materials are those that have been “worked” by such manufacturing processes as rolling.
18.2.3 Article 3: Design
The design of a heating boiler involves calculations, drawings, and specifications to satisfy the service conditions of the plants or facilities. During the process of designing a boiler, two terms are used frequently: design pressure, used for calculating the minimum thickness requirement for the boiler parts, and maximum allowable working pressure, which is the maximum gage pressure or the pres-sure above atmospheric that is permitted in the boiler and which is based on the lowest design pressure of any boiler part.
18.2.3.1 Design Pressure A heating boiler is designed for a
minimum pressure of 30 psi (kPa). The required thickness or design pressure is calculated by formulas provided in paragraphs HG-301 to HG-345.
The maximum allowable working pressure (MAWP) is stamped on a boiler as specified in paragraph HG-530 (Stamping of Boilers) and must not exceed the design pressure of any parts. No boiler shall be operated at a pressure exceeding the MAWP except when the safety devices are discharging, at which time the MAWP shall not be exceeded by more than the amount provided in paragraph HG-400 and Table HG-400.1
18.2.3.1.1 Vacuum Boilers A vacuum boiler is a factory-sealed
steam boiler that is operated below atmospheric pressure. It is designed for a MAWP of 15 psi vacuum (103 kPa vacuum) and a maximum temperature of 210F (99C). Rules for vacuum boilers are given in Appendix 5 (Mandatory Vacuum Boilers).
Example What is the Design Code for a vacuum boiler with an
MAWP of 15 psi and a temperature of 200F?
The vacuum boiler must be designed in accordance with ASME Section IV, Appendix 5 (Mandatory Vacuum Boilers). (See paragraph 5-200 of that appendix for details.)
FIG. 18.1 CYLINDER UNDER INTERANAL PRESSURE
6 Similar international boiler codes are the British Standard BS 2790, Specifications for Design and Manufacture of Shell Boilers of Welded
Construction, and the Canadian Standard CSA B51, Part 1, Boiler, Pressure Vessel, and Pressure Piping Code.
18.2.3.2 Cylindrical Parts under Internal Pressure Consider
a cylinder of radius, R, under internal pressure, P. For the balanc-ing of forces, it is easy to understand that PR F. The stress in the wall is F/A, or, for a cylinder of unit length, F/t. Thus S Ft PRt. The equation S PRt is the simplest form of the circumfer-ential stress in a cylinder, commonly known as the hoop stress. It assumes a uniform stress distribution across the thickness. In real-ity, the stress varies from a maximum at the inside surface to a minimum at the outside. (See Fig. 18.1.)
In 1852, Gabriel Lamé published a complex set of equations describing the actual variation of stress in the cylinder wall. Primarily because of the complexity and difficulty of use, Lamé equations have been replaced with an approximate simplified for-mula that accurately represents the maximum circumferential stress at the inside surface. This formula [1], known as the modified Lamé formula, was used in the preceding paragraph. This formula is identical to the equation for circumferential stress given in paragraph UG-27(C)(1)8 of Section VIII, Division 1, and although it is also similar to the equation in paragraph PG-27.2.2 of Section I, it lacks the y factor that accounts for creep relaxation and stress redistribution. Elevated temperatures do not occur with Section IV construction.
18.2.3.2.1 Cylindical Parts The following formula is used to
calculate the required thickness and design pressure of any cylin-drical part under internal pressure:
Where
P the design pressure, psi (not less than 30 psi or 207 kPa) S the maximum allowable stress value from Tables
HF-300.1 and HF-300.2, psi t the required wall thickness, in. R the inside radius of cylinder, in.
E the efficiency of the longitudinal joint or ligament between tube holes, whichever is less.
For seamless shells, E 1, but for welded joints, the efficiency in paragraph HW-702 should be used.
18.2.3.2.2 Tubes The foregoing formula, modified as shown in
the following equation, is used when determining the required P = SEt
R + 0.6t t = PR
SE - 0.6P
thickness of tubes and of pipes used as tubes, neither one of which is strength-welded to the tubesheet, header, or drum.
where the rolling and structural stability allowance 0.04 in. (1 mm). This factor is based on Section I’s use of 0.04 in the factor e in para-graph PG-27.2, Note 4. Rolling and structural stability allowance 0 for tubes strength-welded to tubesheets, headers, or drums. Sealed-welded tubes would still require the addition of 0.4 in. The mini-mum thickness of a tube or of a pipe used as a tube shall not be less than 0.061 in. (1.5 mm), although there is no minimum thickness requirement for nonferrous tubes installed by brazing.
Example What is the required thickness of a cylinder, welded
by a full-penetration double-butt joint, if the plates are of SA-285, Grade B; the inside diameter is 42 in.; and the working pressure is 150 lb?
Find the drum thickness, given the following: P 150 psi
S 10,000 psi for SA-285, Grade B (Table HF-300.1) R 21 in. (half the inside diameter of 42 in.)
E 0.85 (paragraph HW-702)
18.2.3.3 Formed Heads In a manner similar to the derivation of
the simple hoop stress formula, the underlying basis for Section IV head formulas can be derived. By considering the balance of forces, the following equations can be obtained (see also Fig. 18.2).
or
or
Again, as with the hoop stress, this relationship assumes that the stress is uniformly distributed across the cross section. The actual maximum stress accurately represented by the approximate formula in paragraph HG-305.4 is stated as
18.2.3.3.1 Heads The three types of heads used for construction
of a boiler are ellipsoidal, torispherical, and hemispherical. The pressure may be on either the concave or convex side. The design pressure on the convex side shall be equal to 60% of that for heads of the same dimensions having pressure on the concave side.
The following symbols are used in the formulas to calculate required thickness of different types of heads under pressure on the concave side:
t = PL SE - 0.2P t = PR/2S S = Pr/2t PpR2 = 2pRS t = 0.37 in. t = 3,150 8,410 t = 150 * 21 10,000 * 0.85 - 0.6 * 150 t = PR SE0.6P t = PR SE - 0.6P + 0.4
t the required wall thickness after forming, in.
P the design pressure, psi (not less than 30 psi or 207 kPa) D the inside diameter of the head skirt, or the inside length
of the major axis of an ellipsoidal head, or the inside diameter of a cone head at the point under consideration and measured perpendicular to the longitudinal axis, in. S the maximum allowable stress value from Tables HF-300.1
and HF-300.2, psi.
L the inside spherical or crown radius, in.
E the lowest efficiency of any joint in the head (for seamless head, E 1; for welded joints, the efficiency provided in paragraph HW-702 should be used)
18.2.3.3.2 Ellipsoidal Heads Formulas for ellipsoidal heads are
basically modifications of the equation for spherical heads to account for greater stress as the head becomes shallow. For a dished head with a depth of half the radius (2:1 elliptical head), the maximum stress is twice that for a spherical head (consult ref. [2] for a discussion of dished and flanged heads); for the spherical head, 2L D. The 2:1 elliptical head formula of paragraph HG-305.2 is the same as the spherical head formula except that D is substituted for L.
The following formulas shall be used for calculating the required thickness and design pressure of a dished head of ellip-forging, in. soidal form. Use of this formula is limited to 2:1 elliptical heads.
or
18.2.3.3.3 Torispherical Heads Tests by Hohn and others [3]
were used to establish the empirical formula used by Section VIII, Division 1 in equation (3) of Appendix I-4. With the proportions specified in paragraph HG-305.3, M becomes 0.885 and provides the equation for a torispherical head.
The following formulas shall be used for calculating the required thickness and the design pressure of a dished head of torispherical form. Use of this formula is limited to heads that meet dimensional requirements of paragraph HG-305.
or
18.2.3.3.4 Hemispherical Heads The following formulas shall
be used for calculating the required thickness and the design pres-sure of a dished head of hemispherical form:
or
A formed head of a thickness less than that of the required thickness calculated by the foregoing formulas may be used if it is stayed as a flat surface in accordance with paragraph HG-340.
P = 2SEt L + 0.2t t = PL 2SE - 0.2P P = SEt 0.885L + 0.1t t = 0.885PL SE - 0.1P P = 2SEt D + 02t t = PD 2SE - 0.2P
18.2.3.4 Flat Heads Flat heads and covers are used for the
con-struction of boilers. The thickness of such unstayed heads, cover plates, and blind flanges shall be calculated in accordance with the formulas specified in paragraph HG-307. The following symbols are used for acceptable types of unstayed flat heads and covers shown in the Fig. HG-307 (given here as Fig. 18.3.)
C the factor depending on the method of attachment (val-ues may be found in paragraph HG-307.4)
D the long span of noncircular heads or covers measured perpendicular to short span, in.
d the diameter or short span, in. HG the gasket moment arm, in.
L perimeter of the noncircular bolted head, measured along the centers of the bolt holes, in.
l the length of flange or flanged heads, measured from tangent line of knuckle, in.
m the ratio tr/ts
P the design pressure, psi
r the inside corner radius on the head firmed by flanging or forging, in.
S the maximum allowable working stress value, psi (from Tables HF-300.1 and HF-300.2)
t the minimum required thickness of the head or cover, in. te the minimum distance from the beveled end of drum,
pipe, or header (before welding) to the outer face of the head, in.
tf the actual thickness of the flange on the forged head (at
the large end), in.
th the actual thickness of the flat head or cover, in. tr the required thickness of the seamless shell, pipe, or
header for pressure, in.
ts the actual thickness of the shell, pipe or header, in. tw the thickness through the weld joining the edge of the
head to the inside of the drum, pipe, or header, in t1 the throat dimension of the closure weld, in. W the total bolt load, 1b
Z the factor or noncircular heads and covers that depends on the ratio of short span to long span.
18.2.3.4.1 Circular Flat Plates The maximum stress in
flat-circular plates with fixed and simply supported edge [4] is shown by the following equations:
These equation take the following general form:
where C varies with the degree of edge restraint. (See Fig. 18.4.) Solving for t, the equation takes the following form:
except when the head, cover, or blind flange is attached by bolts. t = dACPS S = CPR 2 t2 Smax = 3(3 + p) 8 * PR2 t2 Smax = 3 4 * PR2 t2
FIG. 18.3 SOME ACCEPTABLE TYPES OF UNSTAYED FLAT HEADS AND COVERS (THESE ILLUSTRATIONS ARE DIA-GRAMMATIC ONLY; OTHER DESIGNS THAT MEET THE REQUIREMENTS OF HG-307 ARE ACCEPTABLE)
This equation is the same as paragraph PG-31.2.2(1) of Section I and paragraph UG-34(c)(2)(1) of Section VIII, Division 1 with the various c factors, depending on the method of attachment of the head to the shell.
For the head, cover, or blind flange attached by bolts:
The thickness shall be determined for both operating conditions and gasket setting; the value greater of these two shall be applica-ble. Note: The terms Wand HGare not well-defined in Section IV; they are the same as in the flanged design rules of Section VIII, Division 1, Appendix 2, where a complete definition can be found.
18.2.3.4.2 Noncircular Flat Heads The following formula
shall be used to calculate the required thickness for noncircular, flat, unstayed heads, covers, or blind flanges:
where
or
when the noncircular heads, covers, or blind flanges are attached by bolts causing a bolt-edge moment. The thickness shall be deter-mined for both the operating conditions and the gasket seating, from which the value greater of these two shall be applicable.
18.2.3.5 Spherically Dished Covers (Bolted Heads) Circular,
spherically dished heads with bolting flanges are used for the con-struction of boilers and are shown in Fig. HG-309. The following symbols are used in the formulas for thickness calculations:
A the outside diameter of flange, in. B the inside diameter of flange, in. C the bolt circle diameter, in.
t the minimum required thickness of head plate after forming, in.
L the inside spherical or crown radius, in. t = d 2ZCP/S + 6WHG/SLd2
Z = 3.4 - 2.4d
D (Z should not exceed 2.5) t = d 2ZCP/S
t = d 2CP/S + 1.9WHG/Sd3
r the inside knuckle radius, in. P the design pressure, psi
S the maximum allowable stress value, psi (from Tables HF-300.1 and HF-300.2)
T the flange thickness, in.
Mo the total moment, in./lb determined as in Section VIII,
Division 1, Appendix 2
The following formulas shall be used for calculating the mini-mum thickness requirement of heads conforming to Fig. HG-309(a).
or
or
The following formula shall be used for calculating the mini-mum thickness requirement of heads conforming Fig. HG-309(b). For head thickness:
For flange thickness of the ring gasket:
For flange thickness of the full-face gasket:
The following formula shall be used for calculating the mini-mum thickness requirement of heads shown in Fig. HG-309(c). For head thickness:
Flange thickness of the ring gasket shall be calculated as follows for heads with round bolting holes:
where
The required flange thickness shall be calculated as above, but the value shall not be less than that of the head thickness calculated above.
The following formulas shall be used for calculating the mini-mum thickness requirement of heads shown in Fig. HG-309(d). For head thickness:
For flange thickness [5]:
T = F + 2F2 + J t = 5PL 6S Q = PL 4Sc C + B 7C - 5Bd T = Q + A1.875MSB(7C - 5B)o (C + B) t = 5PL 6S
T = 0.6A
P
S
c
B(A + B)(C - B)
A - B
d
T = AMSBo c A + B A - Bd t = 5PL 6S t = PL 2SE - 0.2P t = 0.88PL SE - 0.1P t = PD 2SE - 0.2Pwhere
and
18.2.3.6 Cylindrical Parts under External Pressure Cylindrical
pressure parts—furnaces, tubes, and so on—are subjected to exter-nal pressure. Section II, Part D, Appendix 3 (Basis for Establishing External Pressure Charts) is used as the basis of these rules. The external pressure is equal to the compressive stresses, and buckling can occur below the elastic limit. (See Fig. 18.5.) Equations devel-oped for this activity are similar to those develdevel-oped for column theory, where different relationships exist for the critical load depending on the length of the column. In the ASME approach, the two equations for critical buckling pressure [6] are the following:
and
For generalizing and simplifying the design methodology, graphi-cal charts were developed by the Pressure Vessel Research Council (PVRC)8; they appear in Section II, Part D. Figure G, given here as Fig. 18.6, represents the geometric properties of the cylinder, and Figs. CS-1 to CS-6 represent material properties for carbon steels.
18.2.3.6.1 Furnaces Plain-type furnaces, tubes, ring
rein-forced-type furnaces, corrugated-type furnaces, combination-type furnaces, and semicircular-type furnaces are all examples of items having cylindrical parts used for their construction. Formulas and procedures for the calculation of wall thickness for the cylindrical parts of these items can be found in paragraph HG-312. The fol-lowing symbols shown in Fig. HG-312.3 are used in the formulas:
Pc = 2.6Eat db2.5 aLdb
P
c=
2E
1 - p
2a
t
d
b
3 J = aMo SBb a A + B A - Bb F = PB 24L 2 - B2 8S(A - B)A the factor determined from Fig. G in Subpart 3 of Section II, Part D
B the factor determined from the applicable material in Sub-part 3 of Section II, Part D for maximum design and metal temperature, psi.
Do the outside diameter of the furnace, in.
L the design length of the plain furnace taken as the dis-tance from center to center of weld attachment, in. P the design pressure, psi
t the minimum required wall thickness of furnaces, in. (a) Plain-Type Furnaces These furnaces are the most common-ly used because of their simplicity and low cost. The design tem-perature of the furnace is specified to be 500F (260C), but no design temperature is specified for other components, as this task is left to the designer. Furnace thickness shall not be less than in. (6 mm). The following procedures shall be followed to deter-mine the required minimum wall thickness of the furnace:
(1) Assume a value for t; then determine the ratios L/Doand Do/ t.
(2) Enter Fig. G (Subpart 3 of Section II, Part D) at the value of L/Do. If L/Dois less than 0.05, enter the chart at a value of L/Do 0.05; if L/Dois more than 50, enter the chart at a
value of L/Do 50.
(3) Move horizontally to the line for the value of Do/t, as
deter-mined is step (2). From this point of intersection, move ver-tically downward to determine the value A.
(4) With value A, move vertically to an intersection with the mate-rial-temperature line for the design temperature. From this point of intersection, move horizontally to read the value B. (5) Calculate the value of the maximum allowable external
working pressure, Pa, by using the following formula:
(6) Compare Pawith P. If it is less than P, then a different value
of t, a different value of L, or a combination of both must be selected for Pato be equal to or greater than P.
(b) Tubes The procedure outlined in the preceding list shall be used to calculate the wall thickness of ferrous tubes under exter-nal pressure. Additioexter-nal thickness allowance of 0.04 in. (1 mm) shall be added as additional allowance for rolling and structural
Pa = B Do>t
1 4
FIG. 18.5 STREES-STRAIN CURVE
stability. This additional thickness is not required for tubes that are strength-welded to tubesheets, headers, or drums.
(c) Ring Reinforced–Type Furnaces Ring-reinforced furnaces are not typically used for Section IV construction because of the low pressure involved and the cost of fabrication. The ring-reinforced furnace is a plain furnace with a circular stiffening ring welded to it. This stiffening ring is cross-sectionally rectangular, and its thickness shall not be less than in. (8 mm) or more than in. (21 mm)—in no case times thicker than the furnace wall.
The ratio of ring height to its thickness (Hr/Ht) shall be neither
less than 3n nor more than 8. Both the furnace- and tube-wall thickness and the stiffening-ring design are determined by the procedures that are applied to the plain furnace.
The moment of inertia of the stiffening ring shall be calculated by the following formula:
where
Is the required moment of inertia about its neutral axis
par-allel to the axis of the furnace, in.4
As the cross-sectional area of the stiffening ring, in.2
The procedure for determining the moment of inertia is as follows: (1) Assume that Do, L, and t are known. Select a rectangular
ring and determine its area, As. Is = Do2Lat + As LbA 14 114 13 16 5 18
(2) Calculate moment of inertia I by using preceding the for-mula. Use the following formula to calculate B:
(3) Enter the righthand side of the chart in Fig. G (Subpart 3 of Section II, Part D) for the material at the value B, calculat-ed in the preccalculat-eding formula.
(4) Follow horizontally to the material line; then move down vertically to read A.
(5) With the value of all the symbols now known, calculate Is. If
it is greater than I, select a new section and calculate a new Is.
The ring section is satisfactory if Isis smaller than I. (d) Corrugated-Type Furnaces The rules, developed in the late 1800s, are based on tests performed at the Leeds Foundry in England. Corrugated furnaces welded to plain furnaces are some-times used in construction for easy expansion and cleaning. As with ring-reinforced furnaces, corrugated furnaces are not typical-ly used because of their low pressure and high cost of fabrication. Different types of corrugated furnaces exist, depending on the type of construction used: for example, Leeds suspension bulb, Morison, Fox, Purves, and Brown.
The design pressure of a corrugated furnace with plain portions at the ends not exceeding 9 in. (229 mm) long shall be determined by the following formula:
P = Ct/D B = PDo t + As
L
FIG. 18.6 GEOMETRIC CHART FOR COMPONENTS UNDER EXTERNAL OR COMPRESSIVE LOADINGS (FOR ALL MATERIALS) (Source: Fig. G, Section II, Part D of the ASME B&PV Code)
Where
P the design pressure, psi
t the thickness, in.-minimum in. (8 mm) for Leeds sus-pension bulb, Morrison, Fox, and Brown types and mini-mum in. (11 mm) for Purves and other furnace types corrugated by sections not exceeding 18 in. (45 mm) long D the mean diameter, in.—take the inside diameter plus 2
in. as the mean diameter for the Morison furnace C 17,300 for Leeds suspension bulb furnaces; 15,600 for
Morison furnaces; and 14,000 for Fox, Purves, and Brown furnaces
Example A corrugated furnace (Brown type, 42 in. mean diameter) is found in a boiler. The plain ends of the furnace are in. long, the desired working pressure is 125 psi, and the corruga-tions are in. from center to center and in. deep, with sections in. long. Given the following, what is the required thickness? P 125 D 42 C 14, 000 t = PD C P = Ct D 1712 158 834 812 7 16 5 16
(e) Combination-Type Furnaces Combination furnaces are widely used. They are designed so that each type of furnace used in combination is self-supporting—that is, not requiring support from other furnaces at the connecting points. For plain-section furnaces, the formulas in paragraphs HG-312.1 and HG-312.3 shall be used; for corrugated-section furnaces, the formula of paragraph HG-312.6 shall be used. Full-penetration welding must be used to connect a plain self-supporting section to a corrugated self-supporting section, as shown in Fig. HG-312.6.
(f) Semicircular-Type Furnaces or Crown Sheets The thickness of the semicircular furnace or crown sheet shall be a minimum of in. (8 mm); the allowable working pressure shall not exceed 70% of Pa, as calculated by the formula as outlined in Paragraph
18.2.3.6.1(a) and by using the applicable chart.
18.2.3.7 Openings in Boilers
18.2.3.7.1 Shape of Openings The shape of the openings shall be
circular, elliptical, or obround for cylindrical, spherical, and conical portions of boilers or in formed heads. An obround opening denotes one that is formed by two parallel sides and semicircular ends.
5 16 t = 0.375 or 3 8 in. t = 125 * 42 14,000 FIG. 18.6 (CONTINUED)
18.2.3.7.2 Size of Openings With adequate reinforcement,
openings in cylindrical and spherical shells are not limited to any size. The rules are intended to apply to the openings of the follow-ing dimensions:
(1) Half of the boiler diameter, but not exceeding 20 in. (508 mm) for boilers up to 60 in. (1,520 mm) in diameter.
(2) One-third of the boiler diameter, but not exceeding 40 in. (1,000 mm) for boilers over 60 in. (1,520 mm) in diameter.
18.2.3.7.3 Design of Finished Openings Finished openings
shall be designed as specified in paragraph HG-320.3 The follow-ing symbols are used in Fig. HG-320 (given here as Fig. 18.7):
P design pressure, psi
FIG. 18.7 CHART SHOWING LIMITS OF SIZES OF OPENINGS WITH INHERENT COMPENSATION IN CYLINDRICL SHELLS (MAXIMUM PERMISSIBLE DIAMETER OF OPENING IS 8 IN.) (Source: Fig. HG-320, Section IV of the ASME D&PV Code)
d maximum allowable diameter of opening, in. D outer diameter of the shell, in.
t nominal thickness of the shell, in.
S maximum allowable stress value, psi (from Table HF-300) K PD/2St (K represents the ration of hoop stress to allowable
stress)
For any K value 50%, the opening will be inherently com-pensated by the excess thickness in the shell (i.e., A1 Ar).
Calculate K from the formula; then calculate Dt. Locate the
inter-secting point of K and Dt, go left horizontally from this intersecting
point, and determine the maximum diameter of the opening.
18.2.3.8 Reinforcement Requirement for Openings
18.2.3.8.1 Theory of Reinforced Openings When a hole for a
nozzle is cut in a shell, the vessel is weakened. Extra metal is pro-vided to compensate for this weakness and strengthen the vessel. A certain amount of metal in the shell and in the nozzle wall is required to support the internal pressure. Any metal in excess of that required to support the internal pressure in the shell and nozzle is available as compensation for the nozzle opening.
Figures 6.17 and 6.20, given here as Figs. 18.8 and 18.9, respec-tively, explain the basic theory of reinforced openings. The general principle is one of direct area replacement. The common rules are as follows:
(1) Add enough reinforcement to compensate for weakening yet preserve the general dilation or strain pattern.
(2) Place material adjacent to the opening for optimum distrib-ution.
18.2.3.8.2 Reinforcement This is required for all openings
except those in definite patterns and those that are small. This
reinforcement shall be provided in such amount and distribution that the requirements are satisfied for all planes through the center of the opening and normal to the boiler surface. The total cross-sectional area of reinforcement A required shall be determined by the following formula by using the symbols as shown in Figs HG-326.1 and HG-326.2, the latter given here as Fig. 18.10.
where
d the diameter in the given plane of the finished opening, in.
For the correction factor, a value 1.00 may be used. The F fac-tor is based on Mohr’s circle for principal stress.
tr the required thickness of a seamless shell or head, in. A1 the area in excess of thickness in the boiler shell
avail-able for reinforcement, in.2
A2 the area in excess of thickness in the nozzle wall avail-able for reinforcement, in.2
E1 1.0 when an opening is in the solid plate or when the opening passes through a circumferential joint in a shell or cone
the joint efficiency obtained when any part of the open-ing passes through any other welded joint
Dp the outside diameter of the reinforcing element, in. Rn the inside radius of the nozzle under consideration, in.
S the maximum allowable stress value, psi (from Table HF-300)
Sn the allowable stress in nozzle, psi F = 1
4 cos2u + 0.75 A = dtrF + 2tntrF(1 - fr1)
FIG. 18.8 VARIATION IN STRESS IN REGION OF A CIRCULAR HOLE IN: (A) CYLINDER; (B) SPHERE SUBJECTED TO INTERNAL PRESSURE (Source: Ref. [6])
FIG. 18.9 DIAGRAMMATIC LOCATION OF NOZZLE OPENING REINFORCEMENT: (A) UNBALANCED INSIDE; (B) UNBAL-ANCED OUTSIDE; (C) BALUNBAL-ANCED (Source: Ref. [6])
Sv the allowable stress in vessel, psi
Sp the allowable stress in reinforcing element (plate) fr the strength-reduction factor (not more than 1.0) fr1 Sn/Svfor nozzle inserted through the vessel wall
1.0 for nozzle wall abutting the vessel wall fr2 Sn/Sv
fr3 (lesser of Snor Sp) /Sv
fr4 Sp/ Sv
h the distance that the nozzle projects beyond the inner surface of the vessel wall, in.
te the thickness of attached reinforcing pad, in. t the nominal thickness of the boiler shell, in.
tr the required thickness of the seamless shell or head, in. tn the nominal thickness of the nozzle wall, in.
FIG. 18.10 NOMENCLATURE AND FORMULAS FOR REINFORCED OPENINGS (THIS FIGURE ILLUSTRATES A COMMON NOZZLE CONFIGURATION AND IS NOT INTENDED TO PROHIBIT OHER CONFIGURATIONS PERMITTED BY THE CODE)
trn the required thickness of the seamless nozzle wall, in. d the diameter in the plane of the finished opening, in.
18.2.3.9 Inspection and Access Openings A boiler is equipped
with a manhole or a combination of handholes, inspection open-ings, and washout plug openings to allow for the inspection and removal of accumulated deposits. An electric boiler with an internal gross volume of more than 5 ft3(exclusive of casing and insula-tion) shall have an opening for inspection. Also, an electric boiler equipped with immersion-type resistance elements that lacks a manhole shall have an inspection opening not less than 3 in. pipe size (DN 75) located in the lower portion of the shell or head; an electric boiler for steam service shall have an inspection opening or manhole at or near the normal waterline. In addition, access doors shall be provided for the furnaces of internally fired boilers.
18.2.3.9.1 Manholes A manhole is required in the front head
below the tubes of a horizontal-return-tubular boiler that is at least 60 in. (1,520 mm) in diameter. Except in a vertical-firetube boiler, a manhole shall be provided in the upper part of the shell or in the head of a firetube boiler that is at least 60 in. (1,520 mm) in diameter. An elliptical manhole size shall be a minimum of 12 in. 16 in. (305 mm 406 mm); a circular manhole size shall be a minimum of 15 in. (381 mm) in diameter. Moreover, the width of the bearing sur-face for a gasket of a manhole shall not be less than in. (18 mm).
18.2.3.9.2 Handholes, Inspection Openings, and Washout Plug Openings In the absence of a manhole, each boiler shall be
provided with a combination of handholes, inspection openings, and washout plug openings. The locations of these openings are given in paragraph HG-330.4. Locomotive- or firebox-type boilers must have one handhole or washout plug opening in the lower part of the waterleg and at least one opening near the line of crown sheet. In addition, the boilers for steam service shall have at least one inspec-tion opening above the top row of tubes. The minimum size of the inspection opening shall be NPS 3 (DN 75); of the washout plug opening, in. pipe size (DN 40); and of the handhole opening,
in. in. (70 mm 89 mm).
18.2.3.9.3 Fire or Access Doors A fire door or access door shall
be provided for a furnace that has at least a 28 in. diameter within an internally fired boiler. The size of the door shall be a minimum of 11 in. 15 in. (280 mm 381 mm), 10 in. 16 in. (254 mm 406 mm), or 15 in. (381 mm) in diameter; the access door for use in a boiler setting shall be 12 in. 16 in. (305 mm 406 mm) or an equivalent size.
18.2.3.10 Stayed Surfaces Flat plates with cross-sectionally
uniform stays or staybolts are used for boiler construction. Figures HG-340.1 and HG-340.2 show acceptable methods of staying. The equation for flat-stayed surfaces is an adoption of the flat-head equation, with the diameter replaced with the distance stays or pitch. In this case, the C factor represents the degree of restraint to rotation that the stay attachment provides.
The thickness and design pressure for the stayed plates are cal-culated by the following formulas:
Where
t the required thickness of plate, in. P the design pressure, psi
P = t2SC/P2 t = p 2P/SC 312 234 112 11 16
S the maximum allowable stress value from Tables HF-300.1 and HF-300.2
P the maximum pitch measured between straight lines passing through the centers of the stays in the different rows, in.
C a factor that depends on the type of stays given in para-graph HG-340.1 While connecting two plates by staying and one of them requires staying, the value of C shall be governed by the thickness requiring staying.
C 2.7 for stays welded to plates C 3.1 for stays screwed through plates
r the radius of the firebox corner, in.
Two considerations in the design stays are the loading on the plate and the loading on the stay. The plate loading is addressed by the foregoing formula, and the stay load is addressed by the following:
stay load the pressure (the full pitch area – the area occupied by the stay)
Stay load/allowable stress the required area of the stay The SCI requires the stay area to be increased by an additional 10%.
Two flat-stayed surfaces may intersect at an angle as shown in Fig. HG-340.1, in which case the pitch shall be calculated by the following formula:
The maximum pitch shall not exceed in. (216 mm) except for welded-in stays, in which case it shall not exceed 15 times the stay diameter.
18.2.3.11 Staybolts
18.2.3.11.1 Threaded Staybolts The ends of the threaded
stay-bolts should extend to a minimum of two threads beyond the plate, after which point they are riveted over or fitted with extend-ed nuts. The outside ends of solid staybolts up to 8 in. (203 mm) long should be drilled with telltale holes at least in. (4.8 mm) in diameter to a depth extending at least in. (13 mm) beyond the inside of the plate. However, solid staybolts over 8 in. (203 mm) long should not be drilled, for hollow staybolts may be used instead of solid staybolts with drilled ends. Moreover, telltale holes are not required for staybolts attached by welding. Annealing should also be done after upsetting on the ends of the threaded stays upset for threading. It is important to not expose the ends of the nut-fitted staybolts to radiant heat.
18.2.3.11.2 Welded-In Staybolts Installation requirements of
welded in staybolts are given in paragraph HW-710.
18.2.3.12 Dimensions of Stays
18.2.3.12.1 Area of Stays The required area of a stay is
deter-mined by the minimum cross section at the root of the thread. Corrosion allowance is not considered. This required area is cal-culated by dividing the load on the stay by the allowable stress value for the material.
18.2.3.12.2 Load Carried by Stays The area supported by a
stay is based on the full pitch dimensions minus the area occupied by the stay. The load carried by a stay is calculated by multiplying the area supported by the design pressure.
1 2 3 16 812 P = 90t b A CS p
TABLE 18.1 COPPER PLATES AND COPPER STAYBOLTS
Copper Plate Thickness (In.) Minimum Staybolt Diameter (In.)
but not
18.2.3.12.3 Stays Fabricated by Welding Normally, stays of
one-piece construction are used; indeed, those made of individual parts may be attached by welding. A joint efficiency of 60% is used for calculating the strength. Also, ferrous stays welded in by fusion have a minimum cross–sectional area of 0.44 in.2(284 mm2).
18.2.3.12.4 Nonferrous Stays There are minimum diameter
requirements for nonferrous stays such as those composed of cop-per and copcop-pernickel stays. Tables 18.1 and 18.2 show these min-imum diameters.
18.2.3.13 Diagonal Stays
18.2.3.13.1 Area of Diagonal Stays Diagonal stays are used to
support the tubesheet and shell diagonally. All details of the instal-lation of diagonal stays are shown in the Fig. HG-343. The required area of a diagonal stay is calculated by the following formula:
where
A the sectional area of the diagonal stay, in.2 a the sectional area of the direct stay, in.2 L the length of the diagonal stay, in.
l the length of the line (drawn perpendicular to the support-ed boiler head or surface) to the center of the palm of the diagonal stay, in.
18.2.3.14 Staying of Heads Any portions of the heads that
require staying are stayed as flat heads. In boilers with equal to or less than 30 psi (207 kPa) pressure that contain unflanged heads, staying is not required if the greatest distance measured along a radial line from the inner surface of the shell to a point does not exceed 1.25p. In boilers with over 30 psi (207 kPa) pressure that contain unflanged heads or in boilers of any pressure that contain flanged heads, staying is not required when the greatest distance measured does not exceed 1.5p. The maximum distance between the inner surface of the shell and the centers for unflanged heads should not be more than the allowable pitch as calculated in para-graph HG-340, in which the value of C is given for thickness of the plate and the type of stay. The greatest distance between the inner surface of the supporting flange and the lines parallel to the shell
A = aL l 3 4 3 16 5 8 3 16 1 8 1 2 1 8
surface of a flanged head welded to the shell should be p (as calculated in paragraph HG-340) plus the inside radius of the sup-porting flange, in which the value of C is given for the thickness of the plate and the type of stay. Also, the greatest distance between the edges of the tube holes and the center of the first row of stays is calculated in paragraph HG-340, in which the value of C is given for the thickness of the plate and the type of stay.
Horizontal-firetube boilers with manholes on the heads are shown in Fig. HG-345.1(a) and Fig. HG-345.1(b). The area to be stayed may be reduced by 100 in.2(645 cm2) if an unflanged man-hole ring that meets the requirements of paragraph HG-321 is pre-sent in a flat-stayed head under the following conditions:
(1) The distance between the manhole and the inner surface of the supporting flange does not exceed half the maximum allowable allowable pitch for an unflanged manhole or half the maximum allowable pitch plus the inside radius of the supporting flange for a flanged-in manhole in a flanged head. (2) The distance between the centers of the first row of stays or
the edges of the tube holes and the manhole does not exceed half the maximum allowable pitch as determined in para-graph HG-340.
18.2.3.15 Tubesheets Firetubes in a firetube boiler may be used
as stays. The required thickness, maximum pitch, and design pres-sure for tubesheets with firetubes used as stays may be calculated by using the following formulas:
where
t the required plate thickness in.
p the maximum pitch measured between the centers of tubes in different rows, in.
C 2.7 for firetubes welded to plates not over in. (11 mm) thick
C 2.8 for firetubes welded to plates over in. (11 mm) thick
S the maximum allowable stress values given in Tables HF-300.1 and HF-300.2
P the design pressure, psi
D the outside diameter of the tubes, in.
The pitch of firetubes used as stays does not exceed 15 times the diameter of the tubes. Firetubes welded to tubesheets and used as stays meet the requirements of HW-713.
18.2.3.16 Ligaments A ligament—sometimes referred to as a
webbing—is the area of metal between the holes in a tubesheet. The three types of ligaments are as follows:
(1) longitudinal, which are located between the front and lengthwise holes along the drum.
(2) circumferential, which are located between the holes and which encircle the drum.
7 16 7 16 P = CSt 2 P - a pD2 4 b P = A aCSt 2 P b + a pD2 4 b t = A aCSP b + ap 2 - pD2 4 b
TABLE 18.2 COPPER–NICKEL PLATE AND COPPER–NICKEL STAYBOLTS
Copper–Nickel Plate Thickness Minimum Staybolt Diameter
(In.) (In.) but not 1 2 3 16 7 16 3 16 1 8 3 8 1 8
(3) diagonal, which constitute a special case because they are located between the holes and are offset at an angle to each other.
The rules of ligaments are applicable to groups of openings in cylindrical-pressure parts that form a definite pattern. These rules also apply to openings not spaced to exceed two diameters center to center. The following symbols are used in the formulas for cal-culation of ligament efficiency:
P the longitudinal pitch of adjacent openings, in. the diagonal pitch of adjacent openings, in. the transverse pitch of adjacent openings, in.
P1 the pitch between corresponding openings in a series of symmetrical groups of openings, in.
d the diameter of openings, in. n the number of openings in length, p1 E the ligament efficiency
18.2.3.16.1 Openings Parallel to Shell Axis These openings
may have equal pitch in every row or unequal pitch in symmetri-cal groups. The ligament efficiency is determined by the follow-ing formulas.
For equal pitch of openings in every row (see Fig. HG-350.1, given here as Fig. 18.11):
(18.1)
For unequal pitch in symmetrical groups of openings (see Figs. HG-350.2 and HG-350.3):
(18.2)
If the openings are not in symmetrical groups, the efficiency is determined as follows for the group of openings that gives the lowest efficiency:
(1) efficiency as calculated by equation (18.2), using p1equal to the inside diameter of the shell or 60 in. (1,520 mm), whichever is less; or
(2) .25 times the efficiency calculated by equation (18.2), using P1equal to the inside radius of the shell or 30 in.(760 mm), whichever is less.
18.2.3.16.2 Opening Transverse to Shell Axis The ligament
efficiency of openings spaced at right angles to the axis is equal to two times the efficiency of similarly spaced holes parallel to the shell axis, as calculated by equations (18.1) and (18.2).
E = p1 - nd p1 E = p - d P P– P¿
18.2.3.16.3 Holes Along a Diagonal The ligament efficiency of
openings that are equally spaced diagonally (see Fig. HG-350.4, given here as Fig. 18.12) is calculated by the following formula:
where F is the factor shown in Fig. HG-321 (given here as Fig. 18.13) for the angle of the longitudinal axis through which the diagonal makes a plane. Using the F factor for diagonal ligaments is an approach that differs from that of Section I and Section VIII, Division 1, as it gives slightly higher efficiencies.
ExampleThe shell of a vessel is drilled for tube holes of in. diameter spaced longitudinally in groups of two tubes pitched in. with 6 in. of spacing between the groups. Circumferentially, the tube holes are symmetrical. Given the following equations, what is the efficiency of the longitudinal tube ligaments?
P1 10.875 in. n 2
d 3.03125
E 0.4425 or 44.25%
18.2.3.17 Tube Holes and Tube Attachments
18.2.3.17.1 Tube Holes Tube holes are drilled to full size from
the solid plate. They may also be punched in. (13 mm) smaller in diameter than full size if the plate thickness exceeds in. (10 mm) or punched in. (3.2 mm) smaller in diameter than full size when the plate thickness is in. (10 mm) or less. After punching, the holes are drilled, reamed, or finished full size with rotating cutters; then, the sharp edges of the tube holes are removed with a file or other suitable tool.
18.2.3.17.2 Tube Attachments The ends of firetubes may be
attached to tubesheets by expanding and flaring, by expanding and beading, by expanding and welding, or simply by expanding
3 8 1 2 3 8 1 2 E = 4.813 10.875 E = 10.875 - (2)(3.03125) 10.875 E = P1 - nd p1 478 3321 E = P¿ - d p¿F
FIG. 18.11 EXAMPLE OF TUBE SPACING WITH PITCH OF HOLES EQUAL IN EVERY ROW (Source: Fig. HG-350.1,
Section IV of the ASME B&PV Code)
FIG. 18.12 EXAMPLE OF TUBE SPACING WITH TUBE HOLES ON DIAGONAL LINES (Source: Fig. HG-350.4, Section IV of the ASME B&PV Code)
or welding alone. Firetubes attached by expanding and welding or by welding only should comply with the requirements of para-graph HW-713. However, those that are attached by flaring and expanding or by expanding only should comply with the follow-ing requirements:
(1) If the fire tube ends are in contact with primary furnace gases, they should extend not less than the tube thickness or in. (3.2 mm), whichever is greater, nor should they extend more than in. (6 mm) or the tube thickness, whichever is greater.
(2) If the firetube ends are not in contact with primary gases, they should extend not less than the tube thickness or in. (3.2 mm), whichever is greater, nor should they extend more than in. (10 mm) or the tube thickness, whichever is greater.
Watertubes may be attached to the drums by expanding and flaring, by expanding and beading, by expanding and welding, or by expanding or welding only. Watertubes attached by other than expanding and beading should extend neither less than in. (6 mm) nor more than in. (13 mm). Nevertheless, watertubes in hot-water boilers may be installed into headers by using O-ring seals instead of expanding, welding, or brazing under the condi-tions of paragraph HG-360.2(e) of the Code.
1 2 114 3 8 1 8 1 4 1 8 18.2.3.18 External Piping
18.2.3.18.1 Threaded Connections Threaded pipe connections
should be tapped into material with a minimum thickness as shown in Table HG-370, given here as Table 18.3. The minimum thickness of a tapped, curved surface should be sufficient to per-mit at least four full threads.
18.2.3.18.2 Flanged Connections Flanged connections to
external piping should meet the requirements of ANSI B16.5 (Steel Pipe Flanges and Flanged Fitttings). Steel flanges, which do not meet the requirements of ANSI B16.5, should be designed in accordance with Appendix II of Section VIII, Division 1.
18.2.4 Article 4: Pressure-Relieving Devices
18.2.4.1 Pressure Relieving–Valve Requirements
18.2.4.1.1 Safety–Valve Requirements for Steam Boilers The
safety valve shall relieve all the steam generated by a steam-heating boiler. Each boiler shall have at least one spring pop–type safety valve to discharge all the steam at a pressure not exceeding 15 psi (103 kPa). In addition, the size of the safety valve shall be a mini-mum of NPS (DN 15) and a maximini-mum of NPS 4 (DN 115).
The minimum capacity required by the safety valve can be determined by either of the following methods:
(1) Determine the maximum Btu output at the boiler nozzle and divide that output by 1,000. (Doing so is applicable for a boiler that uses any type of fuel.)
(2) Determine the minimum amount of steam generated per hour per square foot of boiler heating surface as shown in Table HG-400.1, given here as Table 18.4.
18.2.4.1.2 Safety Relief–Valve Requirements for Hot-Water Boilers For each water-heating or -supply boiler, there shall be at
least one safety-relief valve of the automatic-reseating type. This valve shall be identified with the ASME Code Symbol V or HV and shall be set at or below the maximum allowable working pressure. The size of the safety-relief valve shall not be less than NPS (DN 20) nor more than NPS 4 (DN 115). A safety-relief valve of NPS (DN 15) size may be used for a boiler with a heat input of not more than 15,000 Btu /hr (4.4 kW).
The relieving capacity of the pressure-relieving device on a hot water boiler shall be determined by the same methods used for a steam boiler. For a cast-iron boiler, the minimum relieving capaci-ty shall be determined by the maximum output method.
Example 1 A 72 in.-diameter stoker-fired horizontal-return-tubular (HRT) boiler has 1,450 ft2 of heating surface and a MAWP of 15 psi. What safety valve–relieving capacity is required?
1 2 1 2 3 4 1 2 1 2
FIG. 18.13 CHART FOR DETERMINING VALUES OF F
(Source: Fig. HG-321, Section IV of the ASME B&PV Code)
TABLE 18.3 MINIMUM THICKNESS OF MATERIAL FOR THREADED CONNECTIONS TO BOILERS