The ASME B31.3 Code provides minimum requirements for safety. It is not a design handbook. Furthermore, it is for new piping. Although the intent of ASME B31.3 may be considered in evaluating existing piping, piping that has been in service is not within the scope of the Code. Other standards, such as API 570, Piping Inspection Code, should be considered for use with piping after it has been placed in service. The ASME B31.3 Code does not address operation or maintenance of piping systems.
The scope of the Code is new piping; it does not include repair. However, the issue of replacement is less clear. While an earlier interpretation indicated that replacement was covered by ASME B31.3, a recent interpretation stated that the subject of replacement was not addressed. The wording of the Code scope had been changed between those two interpretations, and the committee is perhaps more rigorous now in issuing interpretations that are clearly supported by specific requirements in the Code.
From a practical standpoint, if an entire piping system is to be replaced, it should be constructed to the current Code and if a small portion of a piping system is to be replaced, it should be replaced in kind (as a repair). Where to draw the line between these two extremes is a matter of judgment. However, in the opinion of the author, it is prudent in any case to assess the nature of the Code changes that would impact the design of a portion of a piping system being replaced as a repair.
When a new piping system is being attached to an existing piping system, the demarcation is at the connection to the existing system. The new piping, exclusive of the attachment to the existing system, is governed by ASME B31.3. The connection to the existing system is not considered new construction, but is rather subject to the requirements of a post construction code, such as API 570. It is for this reason that inquiries with respect to hot taps have been have received responses that ASME B31.3 does not apply (see, for example, Interpretation 13-04). With respect to leak testing, the new piping is required to be pressure tested; it can be leak tested prior to tying it in to the existing system. For the connection to the existing system, alternatives to leak testing are provided in API 570.
The following paragraphs, quoted from para. 300(c) at the beginning of the Code, provide important insights into its basic intent:
"(3) Engineering requirements of this Code, while considered necessary and adequate for safe design, generally employ a simplified approach to the subject. A designer capable of applying a more rigorous analysis shall have the latitude to do so, however, the approach must be documented in the engineering design and its validity accepted by the owner. The approach used shall provide details of design, construction, examination, inspection, and testing for the design conditions of para. 301, with calculations consistent with the design criteria of this Code."
"(4) Piping elements should, insofar as practicable, conform to the specifications and standards listed in this Code. Piping elements neither specifically approved nor specifically prohibited by this Code may be used provided they are qualified for use as set forth in applicable Chapters of this Code."
"(5) The engineering design shall specify any unusual requirements for a particular service. Where service requirements necessitate measures beyond those required by this Code, such measures shall be specified by the engineering design. Where so specified, the Code requires that they be accomplished."
These statements should help in understanding the philosophy of the ASME B31.3 Code. The Code is not intended to rigorously set forth every procedure, approve every component, and approve every material. Instead, procedures are set forth for evaluating the use of unlisted components and unlisted materials. This differs, for example, from the ASME Boiler and Pressure Vessel Code and Power Piping Code requirements that Code Cases be prepared to approve any material for use that is not listed by the Code.
Paragraph 300(c)3 states that more rigorous analysis methods may be used. Finite-element analysis is an example of one such method. However, the designer must be able to demonstrate (to the owner) the validity of the more rigorous analysis method. The paragraph was revised in the 2000 Addendum of the Code. It was clarified to specifically state that the validity must be accepted by the owner and the approach be documented in the engineering design. Further, the last sentence was added. This change was made to address concerns that 300(c)3 could be interpreted too liberally, and to more specifically state the intent.
The same approach can be found throughout the Code. For example, heat treatments other than those specified may be used, components in listed standards may be rerated, and the temperatures for which allowable stresses are provided may be exceeded. There is a great deal of freedom for good engineering practice and much responsibility for owners. #Little_PEng