The American Society for Mechanical Engineers (ASME) Code for Pressure Piping (B-31) sets forth engineering requirements deemed necessary for safe design and construction of piping systems. While safety is the basic consideration, this factor alone will not necessarily govern the final specifications for any piping system. The designer is cautioned that the Code is not a design handbook; it does not do away with the need for the engineer and competent engineering judgment.
To the greatest possible extent, Code requirements for design are stated in terms of basic design principles and formulas. These are supplemented as necessary with specific requirements to assure uniform application of principles and to guide selection and application of piping elements. The Code prohibits designs and practices known to be unsafe and contains warnings where caution, but not prohibition, is warranted.
The specific design requirements of the Code usually revolve around a simplified engineering approach to a subject. It is intended that a designer capable of applying more complete and rigorous analysis to special or unusual problems should have latitude in the development of such designs and the evaluation of complex or combined stresses. In such cases the designer is responsible for demonstrating the validity of approach which is taken.
The Code for Pressure Piping includes the following:
(1) Material specifications and component standards including dimensional requirements and pressure ratings which have been accepted for Code usage
(2)Requirements for design of components and assemblies, including pipe supports
(3) Requirements and data for evaluation and limitation of stresses, reactions, and movements associated with pressure, temperature changes and other forces
(4)Guidance and limitations on the selection and application of materials, components and joining methods
(5)Requirements for the fabrication, assembly and erection of piping
(6) Requirements for examination, inspection and testing of piping
The Code for Pressure Piping is organized and operated under the direction of ASME Committee B31, under procedures of the American Society of Mechanical Engineers (ASME), which has been accredited by the American National Standards Institute. The Committee is a continuing one, and keeps all Code Sections current with new developments in materials, construction and industrial practice. Addenda are issued periodically. New editions are published at intervals of three to five years.
The ASME B31 Code for Pressure Piping consists of seven sections. Each section prescribes the minimum requirements for the design, materials, fabrication, erection, testing and inspection of a particular type of piping system.
831.1, Power Piping. This section covers power and auxiliary service systems for electric generation stations; industrial and institutional plants; central and district heating plants; and district heating systems. This section excludes boiler external piping, which is defined by Section I of the ASME Boiler and Pressure Vessel Code. Boiler piping requires a quality control system and third party inspection similar to those required for boiler fabrication. Otherwise the materials, design, fabrication, installation and testing for
boiler external piping must meet the requirements of B3 1.1. A fabricator is not required to provide a quality control system and third party inspection for the other piping systems covered by B3 1.
B31.2, Fuel Gas Piping. This section covers piping systems for fuel gases including natural gas, manufactured gas, liquefied petroleum gas (LPG) and air mixtures above the upper combustible limits, LPG in the gaseous phase, or mixtures of these gases. These piping systems, both in and between buildings, extend from the outlet of the consumer’s meter set assembly (or point of delivery) to and including the first pressure-containing valve upstream of the gas utilization device.
831.3 Chemical Plant and Petroleum Refinery Piping. This section covers all piping within the property limits of facilities engaged in processing or handling of chemicals, petroleum or related products. Examples are chemical plants, petroleum refineries, loading terminals, natural gas processing plants (including liquefied natural gas facilities), bulk plants, compounding plants and tank farms. This section applies to piping
systems that handle all fluids, including fluidized solids and to all types of service including raw, intermediate and finished chemicals; oil and other petroleum products; gas, steam, air, water, and refrigerants, except as specifically excluded.
Piping for air and other gases, which is not within the scope of existing sections of this Code, may be designed, fabricated, inspected and tested in accordance with the requirements of this section of the Code. The piping must be in plants, buildings: and similar facilities that are not otherwise within the scope of this section.
831.4, Liquid Petroleum Transportation Piping System. This section covers piping for transporting liquid petroleum products between producers’ lease facilities, tank farms, natural gas processing plants, refineries, stations, terminals and other delivery and
receiving points. Examples of such products are crude oil, condensate, gasoline, natural gas liquids and liquefied petroleum gas.
831.5, Refrigeration Piping. This section applies to refrigerant and brine piping for use at temperatures as low as -196°C (-320°F) whether erected on the premises or factory assembled. It does not include (1) self-contained or unit refrigeration systems subject to
the requirements of Underwriters Laboratories or any other nationally recognized testing laboratory, (2) water piping, or (3) piping designed for external or internal pressure not exceeding 103 kPa (15 psig), regardless of size. Other sections of the Code may provide requirements for refrigeration piping in their respective scopes.
831.8, Gas Transmission and Distribution Piping Systems. This section addresses gas compressor stations, gas metering and regulating stations, gas mains and service lines up to the outlet of the customers meter set assembly. Gas storage lines and gas storage equipment of the closed pipe type that are either fabricated or forged from pipe or fabricated from pipe and fittings are also included.
831.9, Building Services Piping. This section applies to piping systems for services in industrial, commercial, public, institutional and multi-unit residential buildings. It includes only those piping systems within the buildings or property limit.
When no section of the ASME Code for Pressure Piping specifically covers a piping system, at the discretion of the user, the user may select any section of the Code determined to be generally applicable. However, it is cautioned that supplementary requirements of the section chosen may be necessary to provide for a safe piping system for the intended application. Technical limitations of the various sections, legal requirements and possible applicability of other codes or standards are some of the factors to be considered by the user in determining the applicability of any section of this code.
All sections of the Code for Pressure Piping require qualification of the welding procedures and performance of welders and welding operators to be used in construction. Some sections require these qualifications to be performed in accordance with Section IX of the ASME Boiler and Pressure Vessel Code, while in others it is optional. The use of API Std 1104, Standard for Welding Pipelines and Related Facilities or AWS D10.9, Specification for Qualification of Welding Procedures and Welders for Piping and Tub-
ing is permitted in some sections as an alternative to Section IX. Each section of the Code should be consulted for the applicable qualification documents and detailed requirements for joint designs, welding procedures, heat treatment, quality control and operator
qualification.
All filler metal, including consumable insert material, should comply with the requirements of Section IX, ASME Boiler and Pressure Vessel Code. A filler metal not incorporated in Section IX may be used if a procedure qualification test is first successfully made in accordance with Section IX. Filler metals with less than 0.05% carbon content should not be used for high temperature applications (above 450°C
[850°F]) in low-alloy steels due to reduced creep rup- ture properties.
Backing Rings
The design and dimensions of backing rings vary according to the application; therefore if backing rings are to be used, the applicable subsection of the Code should be consulted to determine the required design and dimensions.
Ferrous metal backing rings which become a permanent part of the weld should be made from a material of weldable quality and should be compatible with the base material. The sulphur content should not exceed 0.05%.If two butting surfaces are to be welded to a third member used as a backing ring, and one or two of the three members are ferritic and the other one or two members are austenitic, the satisfactory use of
such materials should be determined by a welding procedure specification. Backing rings of nonferrous materials may be used for backing provided they are included in a welding procedure specification.
Consumable Inserts
Consumable inserts may be used provided they are made from material compatible with the chemical and physical properties of the base material. The welding
procedure for using consumable inserts must be procedure-specified.
Girth Butt Welds
Girth butt welds should be complete penetration welds and should be made with a single V, double V, or other suitable type of groove, with or without backing rings or consumable inserts. The depth of the weld measured between the inside surface of the weld preparation and the outside surface of the pipe should not be less than the minimum thickness required by the Code.
The rules for welding pressure piping systems are covered in detail in the ASME B3 1 Code for Pressure Piping. Any designer or fabricator planning to weld pressure piping should be familiar with the requirements of the Code, particularly with the Section which applies to the application involved.