Process Piping Design Guide to ASME B31.3 Category M Fluid Service

ASME B31.3 Code from American Society of Mechanical Engineers contains various fluid service categories. For day-to-day engineering initiatives, piping engineers typically follow rules of standard fluid services. The category M fluid service and category D service are different from the standard fluid service.

The writeup aims to provide an overview of category M fluid services in process piping design.

The fluid services are usually assumed to be Normal unless the owner defines a different selection. So what makes category M fluids service different from the standard fluid services? It is the owner’s responsibility to categorize what liquids and gases constitute category M fluids. The classification is dependent on their understanding and experience with the chemicals running through the pipelines.

Category M Fluid Service:

The fluid is so highly toxic (poisonous) in which a single exposure to a minimal quantity, caused by leakage can produce highly irreversible harm to affected persons on breathing or physical contact, even when prompt therapeutic measures are taken. It is an extremely hazardous service. The potential for personnel exposure is believed to be highly significant in M fluid service. An example of Category M fluid services could be Hydrogen Sulphide, Methyl Isocyanate, or Hydroflouric Acid.

After careful consideration of piping design, service conditions, location, and experience, the owner concludes that the requirements for Normal Fluid Service do not adequately provide the leak tightness needed to protect personnel from the potential exposure.

The rules of Normal Fluid service are not applicable for Category M. Instead, ASME B31.1 code provides additional practices that lead to more costly construction to the piping designated owner in chapter VIII. These rules are aiming to provide added measures to enhance leak tightness. For instance, special precautions to prevent leakage past valve stems and a sensitive leak test to finding even tiny leaks are included in the system.

It is vital to register that it is Category M Fluid Service and not Category M fluids, as it is not merely the fluids considered in making the designation but also the conditions of installation. Chapter I of ASME B31.3 contains a guide for the owner to classify the piping system by defining Category M Fluid Service. Moreover, Appendix M includes a guide to applying these rules in a flow chart to assist the owner.

Top tips for Category M Fluid Service Piping Design

• Design, layout, and operation of a piping system shall be conducted to minimize impact, vibration and shock loads.
• Heat transfer calculations must substantiate the use of any temperature different than the fluid temperature confirmed through tests or experimental measurements
• Suitable dynamic analysis such as computer simulation should be applied to minimize conditions leading to pulsation, detrimental vibration, or resonance effects in the piping system
• Allowances for variations in pressure and temperature are not permitted in piping design
• Valve design features must be given special consideration to prevent stem leakage
• Prohibit the use of caulked joints
• Avoid usage of unknown specification materials
• Relief set pressure and maximum relieving pressure must be following ASME BPVC, Section VIII, Division 1
• Avoid using creased or corrugate types of bends.
• It is advisable not to use socket welded joints higher than DN 50 (NPS 2)

Hence, there are various factors associated with process piping engineering and design based on the requirement of the project.