Wednesday, August 31, 2011

Tolerances, Procedures and Travelers

After the holiday is finished, I will try to make another post about fabrication and today Piping & Fabrication will talk about Tolerances, Procedures and Travelers. I hope that would be enough for today.

Tolerances. In order to assure installation of a system within a reasonable degree of accuracy, all the components involved must be fabricated to some set of tolerances on those dimensions which affect the system length. Tolerances on valve dimensions are given in B16.34, those of welding fittings in B16.9, and those for flanges and flanged fittings in B16.5, B16.1, etc. The assembly of these components will result in ‘‘tolerance stack-up,’’ which could have a significant impact on the overall dimensions, particularly in a closely coupled system.

Piping subassembly tolerances normally conform to PFI-ES-3 ‘‘Fabricating Tolerances.’’ Usually the terminal dimensions are held to + 1/8 in, but can be held more closely upon agreement with the fabricator. In order to assure that tolerance stack-up is held to a minimum, the manner in which shop details are dimensioned should be carefully studied. As an example, assemblies with multiple nozzles can result in large deviations if these are dimensioned center to center. A better way is to select a base point and dimension all nozzles from this location. This assures that all nozzles are + 1/8 in (3.0 mm) from the base point.

For angle bends, terminal dimensions and often a chord dimension are required, since a small variation in angle with long ends can result in serious misalignment.  Sometimes assemblies which have been fabricated within tolerance may not fit in the field because of tolerance stack-ups on equipment to which they are attached. This will be addressed in the section, ‘‘Installation.’’

Procedures and Travelers. The need to assure better control of fabrication processes has led the use of written procedures for most operations. Fabricators will have a library of written procedures controlling cutting, welding, bending, heat treatment, nondestructive examination, and testing. Welding procedures in most codes are qualified under ASME Section IX, which requires written Welding Procedure Specifications (WPSs) backed up by Procedure Qualification Records (PQRs).

Similarly ASME Section V requires NDE to be performed to written procedures. Frequently, piping fabricators use a system of travelers to control flow through the shop. This practice is well-suited to fabrication of piping subassemblies under QA or QC programs, where record keeping is required. It also affords the purchaser and the third-party inspector opportunities for establishing ‘‘hold points’’ where they may wish to witness certain operations or review certain records.

1 comment:

  1. good thought and nice blog.