The Code includes three Categories of fluid service, which provides a means of discriminating among possible degrees of hazard. Less stringent design, examination, and testing are permitted for fluid service of lower hazard (Category D) and more stringent requirements are applied for more hazardous fluid service (Category M). All fluid services are considered Normal unless the owner designates them as Category D or Category M.
It is the owner’s responsibility to select the fluid service category. Selections of Category D or Category M cannot be made without the owner’s permission. The fluid service is assumed to be Normal Fluid Service unless a different selection is made. Because some owners may not be familiar with this responsibility, the piping designer should inform the owner of the responsibility and may offer advice with respect to the selection process.
Category D Fluid Service is the less hazardous service. It includes fluids that are nontoxic, nonflammable, and not dangerous to human tissue and are at a pressure less than 1035 kPa (150 psi) and temperature from -29°C through 186°C (-20°F through 366°F). These criteria can be found in the definitions in Chapter I of the Code, under "fluid service.” Water piping is an obvious candidate for Category D Fluid Service. An important additional distinction is that it is the condition of the fluid on leakage, not in the pipe, that must be considered. For example, 150# steam can be classified as Category D Fluid Service even though an individual in a container filled with 150# steam would obviously suffer tissue damage. Rules are provided in the Code that basically permit less expensive construction for these less hazardous services.
Category M Fluid Service is reserved for extremely hazardous fluid services. Examples of fluid services that would usually be designated as Category M include systems containing methyl isocyanate, phosgene, and nerve gas. Systems containing fluids such as H2S and hydrogen cyanide are not typically designated as being in Category M Fluid Service. However, it is not possible to create a list of Category M fluids, because the conditions of the installation must be considered in making the classification. For a fluid service to be Category M, the potential for personnel exposure must be judged to be significant. If a piping is double-contained, for example, it could be judged that even highly toxic fluids such as phosgene do not make the system Category M, because the potential for personnel exposure is not significant. For Category M fluid service, the rules for Normal Fluid Service are not applicable. Instead, additional rules that lead to more costly construction, with provisions designed to enhance piping system tightness, are provided in Chapter VIII.
The definition of Category M fluid service is as follows, based on the definition of fluid service in Chapter I of ASME B31.3. Note that, for purposes of emphasis, it has been broken into subparts, all of which must be satisfied for the service to meet the definition of Category M:
“a fluid service in which the potential for personnel exposure is judged to be significant and in which a single exposure of a very small quantity
of a toxic fluid, caused by leakage, can produce serious irreversible harm to persons on breathing or bodily contact, even when prompt restorative measures are taken.”
Note that the Code considers many very hazardous fluid services to be normal fluid service. The design and construction rules for normal fluid service arc suitable for hazardous services. Category M provides a higher level. If higher integrity piping is desired by the owner, even though the fluid service does not meet
the definition of Category M, the owner can still specify the additional design, construction, examination, and testing requirements that are provided in Chapter VIII. Hydrofluoric acid is one example of a fluid for which many owners specify more stringent requirements than are provided in the Code for Normal Fluid Service, although it actually would be considered Normal Fluid Service.
In addition, it may be desirable to use more stringent rules than those provided in Chapter VIII for Category M Fluid Service. For example, it may be considered appropriate to perform 100% radiographic examination rather than 20% (the requirement of Chapter VIII) or to double-contain a fluid that would otherwise be considered Category M.
A flowchart is provided in ASME B31.3, Appendix M, Guide to Classifying Fluid Services. This Appendix is considered by the Code to provide guidance, not Code requirements. The actual Code requirements for fluid service classification are the definitions in Chapter 1. #Little_PEng