Use of Codes and Standards in Piping System Design | Calgary, AB


Use of Codes and Standards in Piping System Design

In practice, the assurance that the design and construction of a piping system will meet prescribed pressure-integrity requirements is achieved through the use of published codes and standards. Numerous codes and standards have been formulated and published by major interest groups of the piping and pressure vessel industry. The most widely used codes and standards for piping system design are published by the American Society of Mechanical Engineers. The American National Standards Institute (ANSI) accredits m any of these codes and standards.

Differentiation between Codes and Standards

Codes and standards both provide criteria through which pressure integrity can be ensured and simplified design rules to ensure adherence to the criteria. Many designers and engineers think the terms code and standard are synonymous, or at least somewhat interchangeable, but this understanding is incorrect.

Codes. Piping codes provide specific design criteria such as permissible materials of construction, allowable working stresses, and load sets that must be considered in design. In addition, rules are provided to determine the minimum wall thickness and structural behavior due to the effects of internal pressure, dead weight, seismic loads, live loads, thermal expansion, and other imposed internal or external loads. Piping codes provide design rules for nonstandard components and for the reinforcement of openings in the pipe wall. They do not provide design rules for standard in-line components such as valves, flanges, and standard fittings; rather, they define the design requirements for these classes of components by reference to industry standards.

The use of specific codes for the design and construction of piping systems is frequently mandated by statute or regulations imposed by regulatory and enforcement agencies.

Typically codes are structured around technology or industry user lines. For example, ASME B31.1, Power Piping, covers piping systems in power plants, district heating plants, district distribution piping systems, and general industrial piping systems while ASME B31.3, Process Piping, is structured around the chemical, petroleum, and petrochemical industries. Any one of the above-named industrial facilities might have a pipeline with similar service requirements such as a high-pressure steam main, a boiler feed water line, or a cooling water line. How-ever, the requirements of the specific code, as influenced by the needs and experience of the user industry, will dictate the pipeline’s design and construction requirements.

Many piping design and construction codes are listed in the section ‘‘Reference Codes and Standards.’’ The systems and subsystems covered by these codes are defined in their scope sections. The scope sections of all potentially applicable codes should be reviewed early in the design phase of a piping project to determine which code, or codes, should be applied to the piping design and construction.

In some cases, multiple codes may be required for the design and construction of the same piping system, depending upon its location. For example, a steam main serving a petrochemical plant from a major utility’s district heating system would be designed and constructed to ASME B31.1, up to the petrochemical plant property line. The balance of the piping on the petrochemical plant’s property would be designed to ASME B31.3. In the case of a natural gas main serving a utility power-house, the outdoor piping is designed and constructed to ASME B31.8 up to and including the meter set, and the in-plant piping is designed and constructed to ASME B31.1.

Sometimes, different piping systems within the same building or facility will be designed and constructed to different codes. For example, most of the piping systems in a utility power plant are designed and constructed to ASME B31.l. However, the building heating and air conditioning piping systems are designed and constructed to ASME B31.9, Building Services Piping.

Standards. Standards provide specific design criteria and rules for individual components or classes of components such as valves, flanges, and fittings. There are two general types of standards: dimensional and pressure integrity. Dimensional standards provide configuration control parameters for components. The main purpose of dimensional standards is to ensure that similar components manufactured by different suppliers will be physically interchangeable. Conformity to a particular dimensional standard during the manufacture of a product does not imply that all such similarly configured products will provide equal performance. For example, two different styles of NPS 10 (DN 250) Class 150 flanged-end gate valves could be manufactured, in part, to ASME B16.10, Face-to-Face and End-to-End Dimensions of Valves. The valves would be physically interchangeable between mating flanges in a particular piping system. However, because of completely different seat and disk design, one valve might be capable of meeting far more stringent seat leakage criteria than the other. Pressure-integrity standards provide uniform minimum-performance criteria. Components designed and manufactured to the same standards will function in an equivalent manner. For example, all NPS 10 (DN 250) Class 150 ASTM A105 flanges, which are constructed in accordance with ASME B16.5, Pipe Flanges and Flanged Fittings, have a pressure-temperature rating of 230 psig (1590 kPa gage) at 300 deg F (149 deg C). Statute or regulation does not normally mandate standards; rather they are usually invoked by a construction code or purchaser’s specification.

The ASME Pressure Classification System

The ASME pressure classification system meets the needs of industry by providing quantitative performance standards for a wide range of piping components, based upon a manageable number of operational variables. This system defines predetermined pressure-temperature ratings that components are designed to meet.

A number of different ASME standards for piping components provide pressure-temperature ratings. The standards in current use in the piping industry are listedin the section ‘‘Reference Codes and Standards.’’ In this section the pressure classi-fication system in ASME B16.5, Pipe Flanges and Flanged Fittings,1 will be usedfor illustration. However, the concepts covered are generally applicable to all theASME pressure-integrity standards.

Flanges manufactured in accordance with ASME B16.5 are made from materials categorized into 34 material or material alloy groups. There are 8 carbon and low-alloy steel material groups, 10 high-alloy steel material groups, and 16 nonferrous metal groups. Within each of the 34 material groups is a subgroup listing of ASTM materials specifications for forgings, castings, and plates. In addition, acceptable bolting materials and bolting dimensional recommendations are specified. Partial listings of the various material groups, subgroups, and bolting materials are shown in Tables B2.1, B2.2a, and B2.2b. For the complete list, see ASME B16.5.

For any single material group, all flanges made from any material in the group, which carry the same ASME flange pressure class, have the same pressure-temperature rating.

ASME B16.5 provides seven pressure classes for flanges. They are Classes 150, 300, 400, 600, 900, 1500, and 2500. The pressure-temperature ratings for flanges representing all material groups are organized within 34 tables, one for each material group. Table B2.3 is adapted from ASME Standard B16.5 and is typical of the 34 flange rating tables. It provides the pressure-temperature ratings for flanges in material group 1.1. The table is organized with the pressure classes listed across the top and the maximum working temperatures along the left-hand border. The body of the table provides the pressure ratings for flanges from each pressure class, at the given temperature.

In practice, the use of ASME B16.5 to determine a flange rating is quite simple.

The procedure is outlined below:

  1. Determine the maximum operating pressure and temperature for the required flange.

  2. Select a flange material and therefore a material group from one of the 34 listed material groups. Be aware that some of the qualifying notes concerning maximum operating temperatures for various materials may influence the final material selection.

  3. Enter the appropriate material group table at the increment of temperature listed which is higher than the desired maximum operating temperature. Start with the Class 150 column and proceed to the right until a pressure rating for the desired temperature is found which equals or exceeds the required operating pressure. The column in which this condition is satisfied dictates the required pressure class and specifies the actual pressure-temperature rating of the flange.

Example B2.1. Assume that an ASTM A105 carbon-steel flange is required to satisfy a pressure rating of 1060 psig (7310 kPa gage) at