PIPING DESIGN BASIS

Selection of Piping Classes

The composition and properties of the process fluid shall be as specified by the process engineer. In consultation with the process engineer, the materials and corrosion engineer shall establish the required materials selection based on specific requirements. The design life and cost considerations shall also be taken in consideration.

After selection of the basic material for the specified service, a suitable piping class shall be selected considering the above specified requirements.

Design Codes

Metallic piping classes shall be designed in accordance with ASME B31.3.

The design limits specified in the piping classes shall be derived from the pressure/temperature (P/T) ratings for flanges given in ASME B16.5, unless otherwise stated in the piping class notes.

Where specified by ASME B31.3, bolting calculations shall be performed to verify the ability to seat the selected gasket and to maintain a sealed joint under the given P/T rating; ASME VIII, Division 1, shall be followed for this.

Allowable internal pressures for pipe and fittings shall be verified in accordance with ASME B31.3 to meet the design limits of piping classes. The lowest pressure at coinciding temperature selected from the above P/T ratings is governing for the piping class P/T rating.

Nominal wall thickness of pipes, outside diameters and nominal pipe sizes (DN) are in accordance with ASME B36.10 and ASME B36.19.

Allowable Stresses

Allowable stresses for the material specifications contained in the piping classes shall be established in line with ASME B31.3.

Piping Specialty Items

Piping specialty items shall be designed to ASME B 31.3. Piping spools not covered within the piping specification shall be treated as piping specialty items.

Branch Connections

Stub-in connections shall not be used, due to their stress intensification factors and the quality control of stub-in branches at the site. Reducing tees or equal tees reducers or weldolets are preferred, particularly under cyclic loading conditions, or when subject to vibrations.  Stub-in connections might be used in non-critical water services.  However, if the use of stub-ins cannot be avoided, check calculations shall be performed in accordance with ASME B31.3.

In each piping class a branch table is given in order to select the proper type of branch connection for the design limits of the class.

Internal Diameter Differences

For welding fittings and pipe within a piping class, identical wall thickness has been selected to prevent internal diameter differences at weld positions.

Heat Treatment

Where piping requires post-weld heat treatment, these shall be identified by the notation "PWHT" in isometric drawings and line lists.  Requirements for PWHT shall be as per ASME B31.3.

Bolt tensioning

In the piping classes, this additional bolt length shall be incorporated only for the sizes and ASME rating classes that are always recommended to be bolt-tensioned.

Compatibility of Materials

The possibility of galvanic corrosion shall be taken into account when different metals are coupled together. Where different materials are coupled together, a flange insulation kit made from non-conducting material shall be used.