APPENDIX DGUIDELINES FOR ALLOWABLE GASKET CONTACT SURFACE

FLATNESS AND DEFECT DEPTH

D-1ð19Þ FLANGE FACE FLATNESS TOLERANCES

Existing industry flatness tolerance limits1 do notinclude an assessment of the ability of the gasket totolerate imperfections. The tolerances in Table D-1M/Table D-1 are dependent on the type of gasket employedand are categorized based on the initial axial compressionof the gasket to the final assembled load.The imperfectionsand flange flatness limits listed in this

Appendix are intended as inspection guidance. Ifexceeded, then engineering judgment should be usedto determine whether the particular defect is acceptable.Such determinations should consider factors such as theactual gasket construction, flange flexibility, bolt spacing,joint leakage history, and the risk associatedwith leakage.Soft gaskets (see Appendix B) are more tolerant of

flange flatness imperfections but are typically more diffi-cult to assemble. Hard gaskets (see Appendix B) have lesscompression than soft gaskets and, while this can helpwith improved assembly due to less bolt interaction(cross-talk), it generally means that hard gaskets aremore sensitive to flange flatness out-of-tolerance. It issuggested that load-compression test results for thegasket being used be obtained from the gasket manufac-turer to determine which of the listed flatness tolerancelimits should be employed. Some types of gaskets, such asexpanded or microcellular PTFE, elastomers, and flexiblegraphite, with sufficient initial thickness and applied load,may provide suitable sealing performance to justify theuse of larger tolerances.The highest and lowest measurements around the

entire circumference of the gasket seating surface maybe recorded and the differences between the twocompared to the sum of the radical and circumferentiallimits stated in Table D-1M/Table D-1. Mating flangesthat have only one possible alignment configurationmay also be gauged to determine that any waviness of

the flange faces is complimentary, such that theseating surfaces follow the same pattern. This is foundin multipass exchanger joints and is often caused bythermal distortion. In this case, it is conservative to calcu-late the overall gaps between the flanges at points aroundthe circumference and use the single-flange tolerances asshown in Table D-1M/Table D-1 to determine accept-ability of the gap.

D-2 FLANGE FACE IMPERFECTION TOLERANCES

The tolerances shown in Table D-2M/Table D-2 areseparated into two categories, depending on the gasketbeing employed in the joint (see Appendix B). Careshould be taken to ensure the correct tolerances areemployed for the gasket being installed. It is importantto note that the tolerances apply to the gasket seatingsurface (area where the gasket seats both initially andfinally after assembly).

D-3 RTJ GASKETS

Flanges forRTJgaskets are typically inspected for flangeflatness and seating surface imperfections in a differentmanner than that for raised-face flanges. The flange flat-ness and groove dimensions are examined prior to jointdisassembly by inspection of the gap between the outeredges of the raised faces. If the gap at any location aroundthe joint circumference is less than 1.5 mm (0.0625 in.),then consideration should be given to repair or rema-chining of the groove at the next opportunity. This elim-inates the risk of the flange faces touching duringassembly, which can lead to joint leakage. Once thejoint is disassembled, the gasket seating surface (seeFigureD-5) should be inspected for damage in accordancewith the requirements listed for hard gaskets in TableD-2M/Table D-2.

1 For example: PIP VESV1002, Fabrication Specification for Vessels:ASME Boiler and Pressure Vessel Code Section VIII, Divisions 1 and2 (March 2012), para. 4.4.3.13; and API 660, 9th ed., Table 5.

ASME PCC-1–2019

38

ASME PCC-1–2013

APPENDIX D GUIDELINES FOR ALLOWABLE GASKET CONTACT SURFACE

FLATNESS AND DEFECT DEPTH

D-1 FLANGE FACE FLATNESS TOLERANCES

Existing industry flatness tolerance limits1 do not include an assessment of the ability of the gasket to tolerate imperfections. The tolerances in Table D-1M/ D-1 are dependent on the type of gasket employed and are categorized based on the initial axial compression of the gasket to the final assembled load.

Soft gaskets (see Appendix B) are more tolerant of flange flatness imperfections but are typically more diffi- cult to assemble. Hard gaskets (see Appendix B) have less compression than soft gaskets and, while this can help with improved assembly due to less bolt interaction (cross-talk), it generally means that hard gaskets are more sensitive to flange flatness out-of-tolerance. It is suggested that load-compression test results for the gas- ket being used be obtained from the gasket manufacturer to determine which of the listed flatness tolerance limits should be employed.

The highest and lowest measurements around the entire circumference of the gasket seating surface may be recorded and the differences between the two compared to the sum of the radial and circumferential limits stated in Table D-1M/D-1. It is acceptable to gauge Mmating flanges that have only one possible alignment configuration may also be gauged to and determine that any waviness of the flange faces is complimentary, such that the seating surfaces follow the same pattern. This is found in multipass exchanger joints and is often caused by thermal distortion. In this case, it is conservative to calculate the overall gaps

1 For example: PIP VESV1002, Design and Fabricat ion Specification for Vessels, ASME Boiler and Pressure Vessel Code Section VIII, Divisions 1 and 2 (March 2012), para. 4.4.3.13; and API 660, 98th ed., Table 53.

between the flanges at points around the circumference and utilize the single- flange tolerances as shown in Table D-1M/D-1 to determine acceptability of the gap. D-2 FLANGE FACE IMPERFECTION TOLERANCES

The tolerances shown in Table D-2M/D-2 a re sepa- rated into two categories, depending on the gasket being employed in the joint (see Appendix B). Care should be taken to ensure the correct tolerances are employed for the gasket being installed. It is important to note that the tolerances apply to the gasket seating surface (area where the gasket seats both initially and finally after assembly). D-3 RTJ GASKETS

Flanges for RTJ gaskets are typically inspected for flange flatness and seating surface imperfections in a different manner than that for raised-face flanges. The flange flatness and groove dimensions are examined prior to joint disassembly by inspect ion of the gap between the outer edges of the raised faces. If the gap at any location around the joint circumference is less than 1.5 mm (0.0625 in.), then consideration should be given to repair or remachining of the groove at the next opportunity. This eliminates the risk of the flange faces touching during assembly, which can lead to joint leak- age. Once the joint is disassembled, the gasket seating surface (see Fig. D-5) should be inspected for damage in accordance with the requirements listed for hard gas- kets in Table D-2M/D-2.

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Copyright c 2013 by the American Society of Mechanical Engineers. No reproduction may be made of this material without written consent of ASME.

17-2255 27 Sep 2017

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