1. PURPOSE
1.1 This worldwide fabrication and erection specification defines requirements for measurement of pressure vessel shape imperfections and permitted tolerances for such imperfections.
2. scope
2.1 The supplier shall furnish all materials, plant, and labor associated with measurement of the pressure vessel shape imperfections. The supplier is also responsible for all costs associated with correcting nonpermitted imperfections.
2.1.1 All pressure vessels in severe cyclic service shall be subject, as a minimum, to the requirements of this specification.
2.2 The pressure vessel design and construction code will be specified in the project equipment specification (PES).
3. related documents
Applicable pressure vessel design and construction code
Project equipment specification (PES).
4. DEFINITIONS
4.1 Axial misalignment (da) is the centerline misalignment of abutting plates at a longitudinal weld seam of a shell.
4.2 Circumferential misalignment (dc) is the centerline misalignment of abutting shell courses at a circumferential weld seam between two courses.
4.3 Ovality, or out of roundness, is a measure of a cylindrical shell’s circularity. It is the difference between the maximum and minimum internal diameters measured at any one cross section, expressed as a percentage of the nominal shell outside diameter.
4.4 Local peaking (dp) is the deviation of a shell at its longitudinal weld seam (measured in a radial direction) from the true perfect cylinder. Peaking results from welding distortion and imperfect rolling/forming of the leading and trailing edges of the plate at a longitudinal seam.
- MEASUREMENT OF AXIAL MISALIGNMENT
5.1 Axial misalignment shall be measured before welding by placing a template (sized to the shell nominal outside diameter) against the shell, over the longitudinal seam, and perpendicular to it. Measurements shall be taken at the center and at 150 mm (6 in) from the end of each shell course. See Figure 1 for further details.
- MEASUREMENT OF CIRCUMFERENTIAL MISALIGNMENT
6.1 Circumferential misalignment shall be measured before welding by placing a straight edge against the shell with its edge running along the longitudinal axis of the shell. Measurements shall be taken in at least 45 degree intervals around each circumferential weld joint to establish the largest value of misalignment. See Figure 1 for further details.
- MEASUREMENT OF OVALITY
7.1 Ovality/out of roundness measurements shall be made to the inside surface of the shell and not to or near any welds or fittings. The shell diameter shall be measured in at least 45-degree intervals around the circumference to determine the maximum and minimum diameter at any location. Ovality measurements shall be made 150 mm (6 in) from the end of each shell course on the finished vessel [i.e., after postweld heat treatment (PWHT), if applicable]. See Figure 1 for further details.
- MEASUREMENT OF PEAKING AT WELD SEAMS
8.1 The supplier shall perform local peaking measurements using a contour gauge (see Section 9) for each shell strake/course of each vessel being supplied.
8.2 Measurements shall be taken on the outside of the vessel before hydrostatic testing.
8.3 Measurements shall be taken using the following procedure:
8.3.1 Sketch a weld map to identify all shell courses and the location for each measurement. This sketch shall be drawn in the upper left corner of each tracing sheet. Each tracing sheet shall show the manufacturer’s serial number, vessel item number, and seam identification number. Each tracing sheet shall be signed and dated by the approved Quality Control Inspector or the Air Products representative.
8.3.2 With the vessel lying in the horizontal position, it shall be rotated so that the longitudinal weld seam being measured is at 90 degrees to the vertical plane as shown in Figure 2.
8.3.3 Adjust the gauge pins to one side of their travel by pressing against a flat surface. Make one measurement of the vessel profile at 180 degrees from the longitudinal seam, 150 mm (6 in) from the end of the shell course, by pressing the gauge against the vessel. The gauge shall be in the vertical plane when conducting this measurement. Care shall be taken that all the pins of the gauge are in contact with the vessel wall.
8.3.4 After placing the contour gauge against the vessel, the “measured” outline shall be traced onto a sheet of paper. The position of the end and center pins shall be carefully marked on the tracing. Make the same measurement two more times to ensure that an accurate tracing of the shell profile has been obtained. If on each occasion the measurements are within 0.8 mm (1/32 inch) of the original traced curve, the curve is acceptable. Conduct similar measurements at the middle and at 150 mm (6 in) from the other end of the shell course and trace the profiles on the same sheet.
8.3.5 Now carry out a similar procedure of measurement on the vessel shell along the longitudinal weld seam at the same positions used previously [i.e., 150 mm (6 in) from the ends and at the middle of the shell course]; the gauge centerline being aligned with the weld centerline. At each position the measured vessel profile shall be traced over the curve previously traced by aligning the end points of the two profiles as shown in Figure 2. On each occasion, make the same measurement two more times to ensure that an accurate tracing of the shell profile has been obtained. If on each occasion the measurements are within 0.8 mm (1/32 inch) of the original traced curve, the curve is acceptable.
8.3.6 The measurements, as detailed above, shall be carried out on each shell course of the vessel, the end result being three tracings of the vessel profile for each shell course of the vessel.
8.3.7 The degree of peaking at each location is the difference between the profiles drawn 180 degrees apart.
8.4 The original copy of the peaking measurements shall be submitted for final review by Air Products.
- CONTOUR GAUGE
9.1 The contour gauge used by Air Products for measurement of longitudinal weld seam peaking is a McMaster Carr “Plastic Contour Gauge,” item #19065A33 in the McMaster Carr catalog. These items are readily available in the US. Alternative gauges may be used provided they have a gauge length of at least 460 mm (18 in), a maximum gauge pin width of 1.5 mm (1/16), and a measurable depth of at least 40 mm (1 9/16 in).
9.2 The center key of the gauge shall be marked with a permanent marking for consistency in the measurements.
- PERMITTED SHAPE IMPERFECTIONS
10.1 Unless specified otherwise in the PES or when the design and construction code has more stringent requirements, the permitted shape imperfections shall be as follows:
Axial misalignment (da) shall not exceed code allowable.
Circumferential misalignment (dc) shall not exceed code allowable.
Ovality (Dmax – Dmin) shall not exceed 0.5% of the nominal outside diameter (Dnom).
Local peaking (dp) at the longitudinal weld seam shall not exceed 3 mm (1/8 in).
- CORRECTIVE ACTIONS
11.1 Shape imperfections reduce the useful life of pressure vessels in fatigue service because they produce secondary bending stresses, which are additive to the general pressure-induced membrane stresses. If the tolerances specified are not met, the vessel design life will be reduced. Depending on particular project circumstances, Air Products might reject the entire vessel or vessel sections or, exceptionally, agree to a reduction in the vessel purchase price to reflect its shortened service life.
11.2 If any corrective actions are required, the measurement of shape imperfections shall be performed again after the action has been taken.
11.3 Axial misalignment (da) cannot be corrected once the longitudinal seam has been welded. Out-of-tolerance misalignment shall be reported to the Air Products representative immediately and might lead to rejection of that course.
11.4 Circumferential misalignment (dc) cannot be corrected once the circumferential seam has been welded. Out-of-tolerance misalignment shall be reported to the Air Products representative immediately and might lead to rejection of the weld seam(s)/courses concerned.
11.5 Out-of-tolerance ovality in individual shell courses shall be corrected by rerolling and/or the use of rounding rings and spiders. All shell courses shall be within tolerance before completing any circumferential weld seams. Once the vessel has been completed, it might not be possible to correct out-of-tolerance ovality, which might cause the vessel to be rejected.
Note: Particular attention shall be given to the stiffening and/or support of vessels during PWHT so as to maintain ovality within permitted levels.
11.6 Longitudinal seams exceeding the local peaking (dp) limit shall be corrected by rerolling the shell course/strake, or else the shell course shall be rejected.
- DOCUMENTATION
12.1 A copy of all shape imperfection measurements, including copies of the peaking measurement tracings, shall be included in the pressure vessel dossier.
Figure 1
Pressure Vessel Shape Imperfections
Figure 2
Longitudinal Weld Seam Peaking Measurement Example
