Section 2 Manufacture and workmanship of fusion welded pressure vessels

2.1.1 Prior to commencing construction, the design of the vessel is to be approved, where required, by Pt 5, Ch 8, 1.2 Plans and Pt 5, Ch 9, 1.6 Plans.

2.1.2 Pressure vessels will be accepted only if manufactured by firms that have been assessed and approved as indicated in Pt 5, Ch 14, 2.1 General requirements 2.1.3 .

2.1.3 Class 1 and Class 2/1 fusion welded pressure vessels constructed to Class 1 and Class 2/1 requirements will be accepted only if manufactured by firms equipped and competent to undertake the quality welding required for the Class of vessels proposed. In order that firms may be approved for this purpose, it will be necessary for the Surveyor to visit the works for the purpose of inspecting the welding plant, equipment and procedures, and to arrange for the carrying out of preliminary tests as referred to in Pt 5, Ch 14, 2.1 General requirements 2.1.8. For Class 1 independent cargo tanks on Type G tankers or for pressure vessels containing LNG for propulsion purposes, the Manufacturer’s works are to be assessed and approved in accordance with the Materials and Qualification Procedures for Ships, Book A Procedure MQPS 0-4, “Approval of works for the Manufacture of Fusion Welded Pressure Vessels ”.

2.1.4 Class 2/2 pressure vessels made in accordance with Class 2/2 requirements, will be accepted only if constructed by firms whose works are properly equipped to undertake the welding of pressure vessels of this Class.

2.1.5 It will be necessary for the Surveyor to visit the works for the purpose of inspecting the welding plant, equipment and procedures, and to arrange for the carrying out of preliminary tests as referred to in Pt 5, Ch 14, 2.1 General requirements 2.1.8.

2.1.6 Class 3 pressure vessels will be accepted if constructed by firms whose works are equipped to undertake the welding of pressure vessels of this Class.

2.1.7 It will be necessary for the Surveyor to visit the works for the purpose of inspecting the welding plant, equipment and procedures, and to arrange for the carrying out of preliminary tests to demonstrate the quality of the welding. These tests are to be carried out by the firm under the supervision of the Surveyor. The test requirements will be based on the welding process used, but are to be similar to those described in Ch 13, 4.8 Mechanical requirements 4.8.4 and Ch 13, 4.8 Mechanical requirements 4.8.5 of the Rules for Materials.

2.1.8 For preliminary tests, see the Materials and Qualification Procedures for Ships, Book A, MQPS 0-4, Section 4.1.

2.2.1 Where the construction requires post-weld heat treatment, consideration should be given to certifying the material after subjecting the test pieces to a simulated heat treatment.

2.3.1 Measurements are to be made to the surface of the parent plate and not to a weld, fitting or other raised part.

2.3.2 In assessing the out-of-roundness of pressure vessels, the difference between the maximum and minimum internal diameters measured at one cross-section is not to exceed the amount given in Table 14.2.1 Tolerances for cylindrical shells.

2.3.3 The profile measured on the inside or outside of the shell, by means of a gauge of the designed form of the shell, and having a chord length equal to one-quarter of the internal diameter of the vessel, is not to depart from the designed form by more than the amount given in Table 14.2.1 Tolerances for cylindrical shells. This amount corresponds to x inFigure 14.2.1 Tolerances for cylindrical shells.

Figure 14.2.1 Tolerances for cylindrical shells

2.3.4 Shell sections are to be measured for out-ofroundness, either when laid flat on their sides or when set up on end. When the shell sections are checked while lying on their sides, each measurement for diameter is to be repeated after turning the shell through 90° about its longitudinal axis. The two measurements for each diameter are to be averaged, and the amount of out-of-roundness calculated from the average values so determined.

2.3.5 Where there is any local departure from circularity due to the presence of flats or peaks at welded seams, the departure from designed form shall not exceed that of Table 14.2.1 Tolerances for cylindrical shells.

2.3.6 The external circumference of the completed shell is not to depart from the calculated circumference (based upon nominal inside diameter and the actual plate thickness) by more than the amounts given in Table 14.2.2 Circumferential tolerances.