Pressure Vessel Dimension Inspection.docx

Pressure Vessel Dimension Inspection The Pressure Vessel Dimension Inspection article provides you with information about the dimensional requirements in pressure vessel construction and pressure vessel inspection. You may know some fabrication tolerances have not been addressed in ASME Code Section VIII. So you need to refer to other sources for inspection. This article provides you the most important dimensional inspection requirements. The dimensional check of a pressure vessel consists of the following items:

 Mill Undertolerance of Plates and Pipes  Tolerances for Formed Heads  Out of Roundness of shell  Nozzles and attachments Orientation  Nozzles and attachments Projection  Nozzles and attachments elevation  Nozzles and attachments levelness

 Weld mismatch  Weld reinforcement Mill Undertolerance of Plates and Pipes Pressure Vessel Dimension Inspection - Plate: Your pressure vessel plates with 0.01 inch or 6% undertolerance, whichever is smaller, may be used for full design pressure, instead of at the given design thickness specified. However, if the material specification allows greater undertolerance, then the ordered thickness for the material should be sufficiently greater. For example, if you have SA 516 Gr.70 plate with a 0.625 inch nominal thickness, and the actual thickness is 0.615, it is acceptable, and you may use this plate without any specific design consideration. But if the same plate has an actual thickness of 0.595 inches, you need to consider mill undertolarence in your design calculation. This plate thickness is acceptable based on the material specification (see table in SA 20 for thickness tolerances). We cannot reject this plate because it is in the permissible tolerance of plate specification, but based on ASME Code SEC VIII Div 1, you need to consider this 0.03 inches in your design calculation: e.g. Thickness = Min Thickness + Corrosion Allowance + Mill Undertolrance Pressure Vessel Dimension Inspection - Pipe: Contrary to the plate material, your pipe and tube mill undertolrances need to be taken into account for design consideration. Pipe and tube undertolrances are almost -12.5 % of the nominal wall thickness, but sometimes there are differences for different materials, so for any specific material, it shall be referred to the suggested pipe or tube material specification to obtain the exact amount of mill undertolrances.

For example, if there is a pressure vessel with nominal shell thickness of 0.5 inch, it needs a 6” SA 106 Gr.B nozzle to be attached to the shell plate. Assume that external loading is not a design controlling factor (is not governor), so as our minimum, nozzle thickness will be the same as the shell thickness. So with a consideration of -12.5%, the calculation will be: 0.5 / 0.875 = 0.571 inches. So when we refer to ASME B36.10 and select our pipe schedule it will be SA 106 Gr.B Schedule 160.

Pressure Vessel Dimension Inspection - Tolerances for Formed Head: You may know that there is a specific rule for dimensional checking of formed heads, based the fact that the UG-81 inner surface shall not deviate outside of the specified shape more than 11⁄4% of D and inside the shape more than 5⁄8%. You can use sweep boards, which are made by cutting a thin steel sheet or piece of wood for checking your crown and knuckle radius. For your skirt, the difference between the maximum and minimum inside diameter, should not be more than 1%, and you can use a tape measure or laser measure for dimensional control of the skirt. So to fully understand, let's check the following head together:

Head ID = 3364 mm Head Type: Torispherical Limit for Outside of specified Shape = 3364 x 1 ¼% = 42.05 mm Limit for Inside of specified Shape = 3364 x 5/8 % = 21.025 mm So we check the depth, and the drawing is 656.3 mm and the actual is 672 mm, so it is outside of the shape. We need to check if it is in the range of tolerance, so the maximum outside of the shape will be 42.05 + 656.3 = 698.35 so our actual value is 672 mm. 672