Pressure Vessels with Thickness Greater than 1.5 inches

1. Scope

Pressure vessels with wall thickness greater than 1.5" (38 mm) should comply with the additional requirements specified herein, in addition to “Specification - Pressure Vessels”, which should apply to carbon steel, clad carbon steel, and stainless steel materials. The appropriate ASME Section VIII, Division 2 paragraphs should be substituted for ASME Section VIII, Division 1 referenced paragraphs, e.g., AM-204 through 218 for UCS-66 and 67.

2. Materials

All forgings should be forged as close as practicable to finished shape and size to develop metal flow in a direction most favorable for resisting the stresses encountered in service.

The tensile and Charpy V-notch impact tests required for certification of all plate, forging, and pipe grades of material according to Paragraph AF-101 should represent the vessel in the completed condition.

  • Compliance with this requirement should be verified by subjecting specimens for tensile and Charpy V-notch impact tests to the same complete thermal history that will be required for fabrication of the vessel.
  • The thermal history should include all hot forming and heat treatment steps, plus at least 2 additional postweld heat treatment cycles to allow for shop or field repairs.

For cold-formed components, stress relief heat treatment should be required if fiber elongation is between 5% and 10%.

  • Complete re-heat treatment in accordance with the Code (SA) material specification should be required if fiber elongation exceeds 10%.
  • Cold sizing should be permitted after stress relief heat treatment, if necessary to correct distortion, providing the fiber elongation does not exceed 5%.

For hot formed components, complete re-heat treatment in accordance with the Code (SA) material specification should be required if the temperature of hot forming exceeds the normalizing temperature for normalized material, or the tempering temperature for normalized and tempered or quenched and tempered material.

3. Design

Code Category C flange welds should not be allowed, unless specifically approved in writing by Buyer.

On vessels with a shell thickness of 2" (50 mm) and greater, all nozzles, manways, and other shell openings should be integrally reinforced type with full penetration welds.

Vessels in cyclic service should have nozzles that are fully radiograph-able per ASME Code.

4. Welding

When Charpy V-notch impact tests are required, welding current and travel speed should be considered essential variables for welding procedure qualification, to assure that production welds achieve substantially equivalent impact properties as the qualification welds.

Production test plates in accordance with Paragraph AT-203 should be required for each vessel when Charpy V-notch impact tests are required.

  • Production test plates should be subjected to the same total thermal history as the finished vessel.
  • Charpy V-notch impact tests should be made for the base metal, weld metal, and heat affected zone.

Postweld heat treatment should meet or exceed the minimum temperature and the minimum time required by AF-402. Buyer would not accept utilization of Code Table AF-402.2 for applying the postweld heat treatment at a lower temperature for an increased length of time.

Postweld heat treatment of the completed vessel as a whole should be preferred.

  • With written approval from Buyer, local heat treatment should be permitted only for Code Category B girth welds.
  • The locally heated band should not include any nozzles.

A sufficient number of thermocouples should be attached to the vessel during postweld heat treatment, and recorders should be used, to verify that the required temperature and holding time have been obtained.

The temperature difference between any two thermocouples should not exceed 2500F (1200C) at any time, especially during the heating and cooling cycles.

5. Inspection

All forging surfaces, regardless of thickness, should be magnetic particle inspected in accordance with the requirements of Appendix 9, Article 9-1, after final machining.

All Code Category A and B welds, including nozzles, should be fully (100%) radiographed after postweld heat treatment. When approved by Buyer, radiography may be performed before postweld heat treatment if ultrasonic inspection is performed after postweld heat treatment.

All Code Category D nozzle welds should be fully (100%) ultrasonically tested after postweld heat treatment.

Pressure retaining plate material should be ultrasonically examined by straight-beam method in accordance with ASTM SA578 Level B.

Plate edges, including weld bevel preparation, should be examined for laminar-type discontinuities using magnetic particle or liquid penetrant method.

All weld pad buildups for attachment of internal or external components and supports should be ultrasonically tested.

When possible by design, all skirt-to-vessel welds should be ultrasonically tested. Skirt-to-vessel welds that cannot be ultrasonically tested should be magnetic particle tested after the root pass is completed and every third layer thereafter. A dry-powder technique should be acceptable.

After the removal of temporary attachments and surface grinding of the attachment welds, all ground surfaces should be magnetic particle tested.



Tags: Pressure Vessel