As defined in ASME B31.3, fluid service is a general term concerning the application of a piping system, considering the combination of fluid properties, operating conditions, and other factors, which establish the basis for design of the piping system.1.Category D Fluid Service: A fluid service in which all the following apply: (a) the fluid handled is nonflammable, nontoxic, and not damaging to human tissues (see definition below). (b) The design gage pressure does not exceed 1035 kPA (150 psi). (c) The design temperature is from -29ºC (-20ºF) through 186ºC (366ºF).2.Category M Fluid Service: A fluid service in which the potential for personnel exposure is judged to be significant and in which a single exposure to a very small quantity of a toxic fluid caused by leakage can produce serious irreversible harm to persons upon breathing or bodily contact, even when prompt restorative measures are taken.3.High Pressure Fluid Service: A fluid service for which the owner specifies the use of chapter IX (High Pressure Piping) for piping design and construction.4.Normal Fluid Service: A fluid service pertaining to most piping covered by ASME B31.3, i.e., not subject to the rules for Category D, Category M, or High Pressure Fluid Service, and not subject to severe cyclic conditions.Damaging to human tissues for the purpose of the Code, describes a fluid service in which exposure to the fluid, caused by leakage under expected operating conditions, can harm skin, eyes, or exposed mucous membranes so that irreversible damage may result unless prompt restorative measures are taken. These measures may include, flushing with water, administration of antidotes, or medication.