Piping prefabrication is an important step in the construction of piping systems. Whenever feasible, sections of piping systems or piping spools are prefabricated by the contractor in the workshop. After prefabricating the spools, they are transported to the field and assembled to complete the piping system.
It is common practice in most organizations to field fabricate piping 1-1/2 inches NPS and smaller though there could be exceptions such as small bore stainless steel piping or small bore carbon steel piping with buttweld fittings rather than socket-weld fittings.
Assembling of piping sections in the field includes aligning, fit-up, welding and bolting up of flanged assemblies including valves, special items and instrumentation. Piping spools must be fabricated within certain level of tolerances to ensure that the final assembly conforms to piping design requirements. If the required fabrication tolerances are not maintained, there is a possibility of undesired strains being imposed on the piping systems during the field assembly. Imposing additional strains in the piping system during fabrication stage will invalidate the design assumptions and could result in unpredictable behaviour of the system during operation and in some cases failures with catastrophic consequences. Chapter V of ASME B31.3 covers requirements for the fabrication, assembly and erection of metallic piping in normal service. ASME B31.3 defines fabrication as the preparation of piping for assembly, including cutting, threading, grooving, forming, bending, and joining of components into subassemblies. Fabrication may be performed in the shop or in the field.
Fabrication tolerances can be broadly classified into:
- Linear Tolerances
- Angularity and Rotation Tolerances
Section below describes the various fabrication tolerances to be maintained on prefabricated piping assemblies in the fabrication shop.
Internal and External Misalginment at Circumferential Welds
Ends of pipe-to-pipe or pipe-to-flangess or fitting joints shall be aligned accurately. Internal misalignment criteria shall be governed by para. 328.4.3 of ASME B31.3 and the following:
- 24” NPS and smaller pipe-to-pipe or pipe-to-fitting: 1.6mm maximum deviation.
- 26” NPS and larger pipe-to-pipe or pipe-to-fitting: 3.2mm maximum deviation when permitted by the relevant WPS.
If the internal misalignment exceeds the above stated values, use one of the following rectification measures:
- As a first approach, rotate the pipe or fittings to reduce misalignment to the acceptable tolerances.
- Use spreaders or internal or external lineup clamps to correct moderate out-of-round condition.
- The end with the larger internal diameter may be built up internally by depositing weld metal to improve alignment.
- Taper the wall of the component internally after obtaining approval from COMPANY for the proposed method and ensure wall thickness is not reduced below the minimum required by the ASME B31.3 code.
- If allowable deviations are exceeded beyond repair methods, the fabrication shall be redone.
If the external surfaces of the components are not aligned, it is permissible to taper the welds between them as per ASME B31.3.
Tolerances on Linear Dimensions
The tolerance on center to center dimension, face-to-face dimension, center-to-face dimension and location of attachments shall be maintained within ±1/8" or ±3 mm maximum.
Tolerances on Bends
As per para. 332.2.2 of ASME B31.3. flattening of bends, measured as the difference between the largest and the smallest outside diameter at any cross section, shall not exceed 8% of the nominal diameter of the pipe for internal pressure. Flattening of bends at weld ends shall not exceed 3% of the nominal pipe diameter for external pressure. Removal of metal shall not be used to achieve these requirements. PFI Standard ES-24 provides guidelines on the ovality if the same are not defined in the governing code.
Tolerances on Mitred Joints
A mitred joint is formed by two or more straight sections of pipe matched and joined in a plane bisecting the angle of junction so as to produce a change in direction. As per para. 304.2.3, an angular offset of 3 deg or less (angle α in Fig. 304.2.3) does not require design consideration as a miter bend.
Tolerance on Flanges
Pipe flanges are oriented such that the bolt holes straddle the horizontal (plant north/south) when the flange face is horizontal and vertical centerlines when the flange face is vertical. Rotation of flanges, measured as the offset between elevation of bolt holes on opposite sides of a flange centerline, shall not exceed ±1/16" or ±1.6 mm. Additionally, the tilt of a flange measured at the periphery across any diameter shall not exceed 2.5mm/m or ±1/32" or ±0.8 mm from the square position.
Tolerances on pipe spools for field adjustment
Where a field fit-up is required, it is recommended that one spool piece shall have a plain end 100-150 mm (4-6 inches) longer than the dimension indicated on the piping drawing for adjustment at site. The installation contractor is required to verify the dimension the dimensions in the field before trimming and beveling the spool provided with cutting allowance. The fabrication contractor makes a provision in the material take off for these overlengths (cutting allowance). The adjacent spool piece is prepared with beveled end in the shop and need not have any overlengths for field adjustment.
Reference Standards
Pipe Fabrication Institute Standard PFI ES3 - Fabricating Tolerances
Pipe Fabrication Institute Standard PFI ES-24 - Pipe Bending Methods, Tolerances, Process and Material Requirements