CASE:
Use of Metric Units
Boiler and Pressure Vessel Code Sections I, III, IV, V, VIII, Divisions 1, 2 & 3, X, IX and XII
Inquiry: Under what conditions may a Manufacturer use a mixed system of units (e.g. a combination of U.S. Customary units, SI units and / or local customary units) to demonstrate compliance with all of the requirements of Sections I, III, IV, V, VIII Divisions 1, 2 & 3, X, IX and XII of the Boiler and Pressure Vessel Code?
Reply: It is the opinion of the Committee that a Manufacturer may use a mixed system of units (e.g. a combination of U.S. Customary units, SI units and / or local customary units) to demonstrate compliance with all of the requirements of Sections I, III, IV, V, VIII Divisions 1, 2 & 3, X, IX and XII of the Boiler and Pressure Vessel Code under the following conditions:
(a) For any single equation, all variables must be expressed in a single system of units.
(b) When separate equations are provided for US Customary and SI units, those equations must be executed using variables with the specific associated units system. Data expressed in other units shall be converted to U.S. Customary or SI units for use in these equations. The result obtained from execution of these equations may be converted to other units.
(c) Production measurement and test equipment, drawings, welding procedure specifications, welding procedure and performance qualifications, and other fabrication documents may be in U.S. Customary, SI or local customary units in accordance with the fabricator’s practice. When values shown in calculations and analysis, fabrication documents or measurement and test equipment are in different units, one or the other value shall be converted in accordance with paragraphs (g) and (h) to provide consistent units when necessary for verification of Code compliance.
(d) Material that has been manufactured and certified to either the U.S. Customary or SI material specification (e.g. SA-516 or SA-516M) may be used regardless of the unit system used in design.
(e) Standard fittings (e.g., flanges, elbows, etc.) that have been certified to either US Customary units or SI units may be used regardless of the units system used in design.
(f) Soft conversion of units, using the precision specified in paragraphs (g) and (h) shall be performed to assure that dimensional consistency is maintained. Soft conversion factors between US Customary and SI units may be found in the Nonmandatory Appendix, Guidance for the Use of US Customary and SI Units in the ASME Boiler and Pressure Vessel Code. Whenever local customary units are used the Manufacturer shall provide the source of the conversion factors which shall be subject to verification and acceptance by the Authorized Inspector or Certified Individual.
(g) Conversion factors shall be accurate to at least four significant figures.
(h) The results of conversions of units shall be to a minimum of three significant figures.
(i) All entries on a Manufacturer’s Data Report shall be in units consistent with the fabrication drawings for the component using U. S. Customary, SI or local customary units. It is acceptable to show alternate units parenthetically.
(j) U.S. Customary, SI or local customary units may be used for the Code required nameplate marking consistent with the units used on the Manufacturer’s Data Report. It is acceptable to show alternate units parenthetically.
(k) This Case shall be shown on the Manufacturer’s Data Report.
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