DESIGN
PFT-8 GENERAL
The rules in the following paragraphs apply specifically to the design of firetube boilers and parts thereof and shall be used in conjunction with the general requirements for design in Part PG as well as with the specific requirements for design in the applicable Parts of this Section that apply to the method of fabrication used.
PFT-9 THICKNESS REQUIREMENTS
PFT-9.1 Shell and Dome. The thickness after forming shall be as determined in accordance with the rules in Part PG.
PFT-9.2 Tubesheet
PFT-9.2.1 The thickness shall be as determined in accordance with Part PG and Part PFT.
PFT-9.2.2 When buttwelded to the shell of a firetube boiler, a formed tubesheet with a straight flange longer than 1 ½
times the tubesheet thickness shall have a straight flange thickness not less than that specified in the table in PFT-9.2.1, but in no case less than 0.75 times the thickness of the shell to which it is attached.
PFT-10 SHELL JOINTS
Longitudinal and circumferential welded joints of a shell or drum shall comply with the rules in Part PW.
PFT-11 ATTACHMENT OF HEADS AND
TUBESHEETS
Flat heads and tubesheets of firetube boilers shall be attached by one of the following methods.
PFT-11.2 By flanging and butt welding in accordance with Parts PG and PW.
PFT-11.3 By attaching an outwardly or inwardly flanged tubesheet to the shell by fillet welding provided the following requirements are met.
PFT-11.3.1 The tubesheet is supported by tubes, or stays, or both.
PFT-11.3.2 The joint attaching an outwardly flanged tubesheet is wholly within the shell and forms no part thereof.
PFT-11.3.3 Inwardly flanged tubesheets are full fillet welded inside and outside.
PFT-11.3.4 The throat dimension of the full fillet weld is equal to not less than 0.7 of the thickness of the head.
PFT-11.3.5 The shell at the welds is not in contact with primary furnace gases.
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PFT-11.3.6 The construction conforms in all other respects to the requirements of this Section, including welding and postweld heat treating, except that volumetric examination is not required.
PFT-11.3.7 This construction shall not be used on the rear head of a horizontal-return tubular boiler and inwardly flanged tubesheets shall not be used on a boiler with an extended shell.
PFT-11.3.8 On inwardly flanged tubesheets, the length of flange shall conform to the requirements of PW-13 and the distance of the outside fillet weld to the point of tangency of the knuckle radius shall be not less than ¼ in. (6 mm).
PFT-11.4 By attaching an unflanged tubesheet to the shell by welding, provided the requirements of PFT-11.4.1 through PFT-11.4.7 are met.
PFT-11.4.1 The tubesheet is supported by tubes, or stays, or both.
PFT-11.4.2 The welded joint may be made through the tubesheet or shell thickness. When the weld joint is made through the shell, a minimum of 80% of the pressure load shall be carried by the tubes, stays, or both.
PFT-11.4.3 The weld is a full penetration weld equal at least to the full thickness of the base metal applied from either or both sides. When the full penetration weld is made through the shell, an external fillet weld with a minimum throat of ¼ in. (6mm) shall be provided, and no weld prep machining shall be performed on the flat tubesheet. The distance from the edge of the completed weld to the peripheral edge of the tubesheet shall not be less than the thickness of the tubesheet.
PFT-11.4.4 The shell or wrapper sheet, where exposed to primary furnace gases
14 and not water cooled, does not extend more than 1/8 in. (3 mm) beyond the outside face of the tubesheet.
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PFT-11.4.5 The weld attaching a furnace or a lower tubesheet of a vertical firetube boiler to the furnace sheet is wholly within the furnace sheet and is ground flush with the upper or water side of the tubesheet.
PFT-11.4.6 The construction conforms in all other aspects to the requirements of this Section including welding, and postweld heat treatment, except that volumetric examination is not required.
PFT-11.4.7 This construction shall not be used on the rear head of a horizontal-return tubular boiler.
PFT-12 TUBES
PFT-12.1 Allowable Working Pressure
PFT-12.1.1 The maximum allowable working pressure of tubes or flues of firetube boilers shall be as given in PFT-50 and PFT-51.
PFT-12.1.2 The maximum allowable working pressure for copper tubes or nipples subjected to internal or external pressure shall not exceed 250 psi (1.7 MPa). The maximum temperature shall not exceed 406° F (208° C).
The maximum allowable working pressure for copper clad tubes subjected to external pressure shall be determined by the formula in PFT-51, in which t may be increased by one-half the thickness of the cladding.
PFT-12.2 Attachment of Tubes
PFT-12.2.1 Figure PFT-12.1 illustrates some of the acceptable types of tube attachments. Such connections shall be
(a) expanded and beaded as in illustrations (a), (b), and (d)
(b) expanded and beaded and seal welded as in illustration (c)
(c) expanded and seal welded as in illustration (e)
(d) welded, as in illustrations (f) and (g)
Tube ends attached by expanding and welding are subject to the provisions specified in PFT-12.2.1.1 through PFT-12.2.1.3.
PFT-12.2.1.1 Where no bevel or recess is employed, the tube shall extend beyond the tubesheet not less than a distance equal to the tube thickness or 1/8 in. (3 mm), whichever is the greater, nor more than twice the tube thickness or ¼ in. (6 mm), whichever is the lesser [see Fig. PFT-12.1, illustration (e)].
PFT-12.2.1.2 The tubesheet hole may be beveled or recessed. The depth of any bevel or recess shall not be less than the tube thickness or 1/8 in. (3 mm), whichever is greater, nor more than one-third of the tubesheet thickness, except that when tube thicknesses are equal to or greater than 0.150 in. (4 mm), the bevel or recess may exceed T/3. Where the hole is beveled or recessed, the projection of the tube beyond the tubesheet shall not exceed a distance equal to the tube wall thickness [see Fig. PFT-12.1, illustrations (f) and (g)].
PFT-12.2.1.3 On types of welded attachment shown in Fig. PFT-12.1, illustrations (c) and (e), the tubes shall be expanded before and after welding. On types shown in illustrations (f) and (g), the tubes may be expanded.
PFT-12.2.2 Expanding of tubes by the Prosser method may be employed in combination with any beaded or seal welded attachment method [see Fig. PFT-12.1, illustration (b)].
PFT-12.2.3 After seal welding as shown by Fig. PFT-12.1, illustrations (c) and (e), a single hydrostatic test of the boiler shall suffice.
PFT-12.2.4 The inner surface of the tube hole in any form of attachment may be grooved or chamfered.
PFT-12.2.5 The sharp edges of tube holes shall be taken off on both sides of the plate with a file or other tool.
DOORS AND OPENINGS
PFT-40 WELDED DOOR OPENINGS
Arc or gas welding may be used in the fabrication of door holes provided the sheets are stayed around the opening in accordance with the requirements of PFT-27.6 and PFT-27.7.
No calculations need be made to determine the availability of compensation for door openings spanning between the plates of waterlegs. The required thickness of circular access openings shall be determined in accordance with PFT-51. The required thickness of door openings of other than circular shape shall be calculated using eq. (1) of PG-46, using 2.1 or 2.2 for the value of C, depending on the plate thickness, and a value of p equal to the waterleg inside width. Volumetric examination of the joining welds is not required.
PFT-41 OPENINGS IN WRAPPER SHEETS
Openings located in the curved portion of the wrapper sheet of a locomotive type boiler shall be designed in accordance with the rules in PG-32.
PFT-42 FIRESIDE ACCESS OPENINGS
The minimum size of an access or fire door opening, in which the minimum furnace dimension is 24 in. (600 mm), shall be not less than 12 in. x 16 in. (300 mm x 400 mm) or equivalent area, 11 in. (280 mm) to be the least dimension in any case. A circular opening shall be not less than 15 in. (380 mm) in diameter.
For furnace dimensions less than 24 in. (600 mm), the opening should be 2 ¾ in. x 3 ½ in. (70 mm x 89 mm) or larger where possible. In cases where the size or shape of the boiler prohibits an opening of that size, two openings with a minimum size of 1 in. (25 mm) may be used, preferably opposite each other, to permit inspection and cleaning of the furnace. If the burner is removable so as to permit inspection and cleaning through the burner opening, a separate access opening need not be provided.
The bonnet or smoke hood of a vertical flue or tubular boiler shall be provided with an access opening at least 6 in. x 8 in. (150 mm x 200 mm) for the purpose of inspection and cleaning the top head of the boiler.
PFT-43 REQUIREMENTS FOR INSPECTION
OPENINGS
All firetube boilers shall have sufficient inspection openings, handholes, or washout plugs with a minimum of four openings to permit inspection of the waterside of the tubesheets, furnaces, and tubes and to permit flushing of loose scale and sediment from the boiler. Except where space restrictions would prohibit entry to the boiler, a manhole shall be provided in the upper portion of the shell. All openings shall meet the requirements of PG-32 through PG-44. Where washout plugs are used, the minimum size shall be NPS 1½ (DN 40), except for boilers 16 in. (400 mm) or less in inside diameter, the minimum size shall be NPS 1 (DN 25).
PFT-44 OPENING BETWEEN BOILER AND SAFETY VALVE
The opening or connection between the boiler and the safety valve shall have at least the area of the valve inlet.
After the boiler Manufacturer provides for the opening required by the Code, a bushing may be inserted in the opening in the shell to suit a pressure relief valve that will have the capacity to relieve all the steam that can be generated in the boiler and which will meet the Code requirements. The minimum size of the connection and opening for the pressure relief valve shall be not less than NPS ½ (DN 15).
No valve of any description shall be placed between the required pressure relief valve or valves and the boiler, or on the discharge pipe between the pressure relief valve and the atmosphere. When a discharge pipe is used, the cross-sectional area shall be not less than the full area of the valve outlet or of the total of the areas of the valve outlets discharging thereinto and shall be as short and straight as possible and so arranged as to avoid undue stresses on the valve or valves.
DOMES
PFT-45 REQUIREMENTS FOR DOMES
PFT-45.1 The longitudinal joint of a dome may be butt welded or the dome may be made without a seam of one piece of steel pressed into shape. The dome flange may be double full fillet lap-welded to the shell if all welding complies fully with the requirements for welding in Part PW. Volumetric examination of the fillet welds may be omitted. The opening shall be reinforced in accordance with PG-32 through PG-44.
PFT-45.3 When a dome is located on the barrel of a locomotive-type boiler or on the shell of a horizontal-return tubular boiler, the outside diameter of the dome shall not exceed six-tenths the inside diameter of the shell or barrel of the boiler unless the portion of the barrel or shell under the dome (the neutral sheet) is stayed to the head or shell of the dome by stays which conform in spacing and size to the requirements given in PG-46 and Table 1A of Section II, Part D. With such stayed construction the outside diameter of a dome located on the barrel or shell of a boiler is limited to eight-tenths of the barrel or shell inside diameter.
PFT-45.4 All domes shall be so arranged that any water can drain back into the boiler.
PFT-45.5 Flanges of domes shall be formed with a corner radius, measured on the inside, of at least twice the thickness of the plate for plates 1 in. (25 mm) in thickness or less, and at least three times the thickness of the plate for plates over 1 in. (25 mm) in thickness.
PFT-45.6 Domes and manhole frames attached to shells or heads of boilers shall be designed in accordance with PG-32 through PG-44.
PART PEB
REQUIREMENTS FOR ELECTRIC BOILERS
GENERAL
PEB-1 GENERAL
The rules in Part PEB are applicable to electric boilers and parts thereof and shall be used in conjunction with the general requirements in Part PG as well as with the special requirements in the applicable Parts of this Section that apply to the method of fabrication used.
PEB-2 SCOPE
PEB-2.1 This part contains special rules for construction of electric boilers, both of the electrode and immersion resistance element type. This Part does not include electric boilers where the heat is applied externally to the boiler pressure vessel by electric resistance heating elements, induction coils, or other electrical means. These types of electric boilers shall be constructed in accordance with other applicable Parts of this Section.
PEB-2.2 Electric boilers and parts thereof that do not exceed the diameter, volume, or pressure limits of PMB-2 may be constructed using the applicable paragraphs of Part PMB in conjunction with this Part.
PEB-2.3 An electrode type boiler is defined as an electric boiler in which heat is generated by the passage of an electric current using water as the conductor.
PEB-2.4 An immersion resistance element type boiler is defined as an electric boiler in which heat is generated by the passage of an electric current through a resistance heating element directly immersed in water, or enclosed in a pipe immersed in water.
PEB-2.5 Electric boilers may be field assembled provided the boiler is manufactured and assembled in compliance with the provisions and requirements of Part PEB and other applicable Parts of this Section.
PEB-3 OPTIONAL REQUIREMENTS FOR THE BOILER PRESSURE VESSEL
The boiler pressure vessel may be constructed in compliance with the ASME Pressure Vessel Code Section VIII, Division 1, rules for unfired steam boilers [UW-2(c)] subject to the conditions specified in PEB-3.1 through PEB-3.4.
PEB-3.1 The Manufacturer who certifies and stamps the completed boiler shall specify to the “U" stamp holder all additional requirements of Part PEB, which are not requirements of Section VIII, Division 1, and shall ensure that these requirements are satisfied.
