ASME B31.3 incorporates the concept of safeguarding. Safeguarding involves consideration of factors beyond the simple design of the pipe in the overall safety of the piping installation. It brings in the concepts
of consequences of failure and probable sources of damage, which essentially considers risk.
Safeguarding is a concept that works well in the context of ASME B31.3, because the owner has over-all responsibility for all aspects of the piping system. This differs from the much more limited scope of responsibilities in ASME B31.1 and ASME BPVC, Section VIII, Division 1. Because the owner has complete responsibility for both design and operation, the owner also has the ability to effectively specify and implement safeguarding provisions.
ASME B31.3 permits the use of certain components, joining methods, and other procedures when appropriate safeguards are provided. For example, brazed joints are prohibited from use in piping systems
containing flammable or toxic fluids, unless safeguarded. Because the concern with brazed joints is failure of the joint on fire exposure due to melting of the brazing material, appropriate safeguards could involve protecting the joints from fire exposure.
A partial list of conditions where safeguarding is required includes the following:
Paragraph 305.2.2, use of ASTM A 134 pipe made from ASTM A 285 plate and A 139 pipe for other than Category D Fluid Service
Paragraph 308.2.4, use of flanges other than weld neck flanges meeting certain criteria for severe cyclic conditions
Paragraph 313, use of expanded joints when the fluid is toxic or damaging to human tissue
Table 314.2.1, use of threaded joints in sizes larger than DN 50 (NPS 2) when the fluid is flammable, toxic, or damaging to human tissue
Paragraph 314.2.2, use of straight-threaded joints under severe cyclic conditions when the joint is subject to external moment loadings
Paragraph 315.2, use of flared, flareless, or compression-type tubing fittings covered by listed standards, in normal fluid service, subject to severe cyclic conditions
Paragraph 317.2, use of brazed and braze-welded joints in fluid services that are flammable, toxic, or damaging to human tissue
Paragraph 318.2.3, use of bell and gland-type joints (other than caulked joints, which are subject to further limitations in para. 316) under severe cyclic conditions
Paragraph 323.2.2(d), exemption of safeguarded piping from impact testing based on the stress state from external loads
Paragraph 323.4.2, use of cast iron other than ductile iron permitted for specified conditions only when safeguarded against excessive heat and thermal shock and mechanical shock and abuse
Paragraph A323.4.1, safeguarding against excessive temperature, shock, vibration, pulsation, and mechanical abuse when nonmetallic materials are used in any fluid service
Paragraph A323.4.2(a), use of thermoplastics in other than Category D fluid service
Paragraph A323.4.2(b), use of reinforced plastic mortar (RPM) in other than Category D Fluid Service
Paragraph A323.4.2(c), use of reinforced thermosetting resin (RTR) piping in toxic or flammable fluid service
Paragraph A323.4.2(d), use of safeguarding against large, rapid temperature changes when borosilicate glass and porcelain are used in any fluid service; additionally, use of general safe-guarding when they are used in toxic or flammable fluid services
The following safeguards are also mentioned in the Code:
Paragraph M300(d) requires consideration be given to additional engineered safeguards for Category M fluid service.
Paragraph FA323.4(b) (Appendix F) recommends safeguards for thermoplastic piping in above ground compressed gas (including air) service.
Appendix G of ASME B31.3 discusses the concept of safeguarding and provides examples of safeguarding by plant layout and operation and engineered safeguards.
Safeguarding requires addressing the potential consequences of failure. Therefore, the hazardous properties of the fluid, the quantity of the fluid that could be released by a piping failure, the consequences of such a release with respect to personnel exposure and equipment damage (with potential additional consequences), and the conditions of the environment and their effect on the hazards caused by a possible piping failure are all considerations of safeguarding. These address the consequence aspect of risk. The safety inherent in the piping by virtue of materials of construction, methods of joining, and history of service reliability, also mentioned, addresses the probability-of-failure aspect of risk.
Safeguarding by plant layout and operation includes the following examples from Appendix G of ASME B31.3:
Plant layout features, such as open-air process equipment structures; spacing and isolation of hazardous areas; slope and drainage; buffer areas between plant operations and populated communities; and control over plant access.
Protective installations, such as fire protection systems; barricades or shields; ventilation to remove corrosive or flammable vapors; instruments for remote monitoring and control; and containment and/or recovery facilities or facilities (e.g., incinerators) for emergency disposal of hazardous materials.
Operating practices, such as restricted access to processing areas; work permit system for hazardous work; and special training for operating, maintenance, and emergency crews.
Means for safe discharge of fluids released during pressure relief device operation, blowdown, clean out, etc.
Procedures for startup, shutdown, and management of operating conditions, such as gradual pressurization or depressurization and gradual warmup or cooldown, to minimize the possibility of piping failure, e.g., brittle fracture.
Examples of engineered safeguards include the following:
Means to protect piping against possible failures, such as (1) thermal insulation, shields, or process controls to protect from excessively high or low temperature and thermal shock; (2) armor, guards, barricades, or other protection from mechanical abuse; (3) damping or stabilization of process or fluid flow dynamics to eliminate or to minimize or protect against destructive loads (e.g., severe vibration pulsations, cyclic operating conditions).
Means to protect people and property against harmful consequences of possible piping failure, such as confining and safely disposing of escaped fluid by shields for flanged joints, valve bonnets, gages, or sight glasses; or for the entire piping system if of frangible material; limiting the quantity or rate of fluid escaping by automatic shut-off or excess flow valves, additional block valves, flow-limiting orifices, or automatic shutdown of pressure source; limiting the quantity of fluid in process at any time, where feasible.
The above serve as examples of what is meant by safeguarding. They do not, however, represent all the possibilities of how piping systems can be safeguarded. #Little_PEng