"Ensuring Quality: Acceptable Dimensional Tolerances for Pressure Vessel Fabrication"

 Tolerance On Dimensions For Pressure Vessels

In the world of industrial engineering, the fabrication of pressure vessels is a critical process that demands precision and adherence to stringent standards. Ensuring that these vessels meet acceptable dimensional tolerances is vital for safety, performance, and compliance with industry regulations. This article explores the essential dimensional tolerances in pressure vessel fabrication, highlighting the importance of these standards in manufacturing processes.

1. MATERIAL:

 a. Plate thickness:                  As per material specifications.

 b. Pipe dimensions:               As per material specifications.

Choosing the right materials is the first step in pressure vessel fabrication. Material specifications, including plate thickness and pipe dimensions, must conform to the relevant standards to ensure the structural integrity of the vessel. For instance, plate thickness should align with material specifications, while pipe dimensions should be verified against standards

 

2. SHELL DIMENSIONS -                  COMPLETED VESSEL:

 

a. Ovality: ID max.                  1% of nominal ID subject to a max. of 19 mm or as                                 ID min                              mentioned in the drawings.

 

b. Local deviation:

 
The profile shall be checked by means of a gauge (template) of the designed form of the shell and having a length equal to one-quarter of the internal diameter of the vessel.

 

c. Flat spots or peaks:                        Not acceptable at weld seams.

 d. Height:                                +/- 1 mm/1000 but not to exceed 8 mm.

 

The completed vessel's shell dimensions are crucial for maintaining its shape and functionality. Key tolerances include ovality, local deviation, flat spots or peaks at weld seams, and height. For example, the maximum allowable deviation for ovality is 1% of the nominal ID, subject to a maximum of 19 mm.

 

3. DISHED ENDS:

 a. Circumference:

The outside circumference of the finished end shall not depart from the circumference calculated from the outside diameter by more than the following amounts:

Outside diameter (mm)                     Circumferential tolerances (mm)

300 up to and                                     4.8 mm

 including 600 and above 600           1/4%

             

b. Circularity:

The difference between max. and min. diameter of the straight flange shall not exceed 1% of the nominal diameter and shall in no case be greater than 19 mm.

 

c. Thickness:

The thickness is the thickness of the dished end after manufacture and is applicable over the whole area of the dished end up to the point where the crown radius joins the knuckle radius. From this point, a gradual thinning is permissible up to a max. of 10% of the thickness at the point where the knuckle radius joins the straight face. In any case, the thickness at any point shall not be less than the calculated minimum thickness for the dished end.

 

d. Profile:

Depth:                         Not less than the theoretical depth. Shall not be greater than 1-1/4% of the diameter of the dished end.

Knuckle radius:          As per drawing, as measured by the template and shall not be less than that specified in the drawing.

Crown radius:            Shall not be more than that specified in the drawing.

 

The dished ends of pressure vessels must adhere to tolerances for circumference, circularity, thickness, and profile. For instance, the outside circumference should not deviate more than 4.8 mm for diameters up to 600 mm. Thickness tolerances ensure the integrity of the dished end throughout its area.

 

4. ALIGNMENT TOLERANCES WELD SET UPS:

          

a. L/Seam                                                                   Misalignment - max:

Up to and including 12.5 mm                                    1/4t

Over 12.5 mm up to and including 19 mm              3 mm

Over 19 mm up to and including 62.5 mm              3 mm

 

b. C/Seam:

Up to and including 19 mm                                       1/4t

Over 19 mm up to and including 38 mm                  4.5 mm

Over 38 mm up to and including 62.5 mm              1/8t

Over 62.5 mm                                                            Lower value of 1/8t and 20 mm

 

 

5. STRAIGHTNESS - COMPLETED VESSELS:

The max. deviation of shell from a straight line shall not exceed 0.3% of the cylindrical length on any individual 5M length of the vessel.

 

 

6. ATTACHMENT WELDS:

 

Dimensional deviation - max. allowable

i. Stand out:                                                                +/- 5 mm

ii. Orientation measured on shell:                            1°

iii. Inclination of flange from specified surface:     1/2°

 

 

a. Stiffener rings:

The welding of stiffener rings on the inside or outside of the vessel shall be continuous or intermittent, in which case the total weld length shall not be less than half the circumference in case of external rings and not less than one-third the circumference in case of internal rings.

 

  

b. Reinforcement pads:

The reinforcement pad shall conform to the curvature of the shell and each shall be provided with a tell-tale hole.

 

c. Leg Support:

a. Height from specified line to bottom support -   maximum deviation of : +/- 10 mm

b. Bolt hole location including diagonal internal:   +/- 3 mm

 

If closer tolerances than those mentioned above are specified in the fabrication drawings, those mentioned in the drawings shall stand.

 

Maintaining the straightness of the completed vessel is vital. The maximum deviation from a straight line should not exceed 0.3% of the cylindrical length over any 5-meter section of the vessel.

 

7. RADIOGRAPHY ACCEPTANCE STANDARDS (ASME SECTION VIII DIV 1 REQUIREMENTS):

 

a. Spot radiography:

The welds whose radiographs reveal the following defects shall be repaired and re-radiographed:

Any cracks, incomplete fusion, or penetration.

Slag inclusions or cavities if such imperfections are greater than 2/3t, where t is the thickness of the thinner plate welded. If several imperfections within the above limitations exist in a line, and the sum of the largest dimensions of all such imperfections is not more than t in a length of 6t separated by a length of 3L, and if the largest imperfections considered are of acceptable weld material, where L is the length of the longest imperfection.

 

b. 100% Radiography:

Welds that are shown by radiography to have the following defects shall be repaired and re-radiographed:

Any type of cracks, incomplete fusion, or penetration.

Any elongated slag inclusion with a length greater than:

1/4t for t up to 3/4"

1/3t for t from 3/4" to 2-1/4"

3/4" for t over 2-1/4", where t is the thickness of the weld.

Any group of slag inclusions in line that have an aggregate length greater than t in a length of 6t, except when the distance between successive imperfections exceeds 6L, where L is the length of the largest imperfection in the group.

Any rounded indications in excess of those specified in Appendix-4 of ASME Section VIII, Div. 1.

 

Adhering to these dimensional tolerances ensures that pressure vessels comply with ASME standards and other relevant regulations. This compliance is crucial for safety, preventing failures and ensuring the vessel performs as intended under pressure.

Consider a case where a manufacturing defect due to improper alignment tolerances led to a vessel's failure under pressure. This underscores the importance of strict adherence to the specified tolerances in preventing such incidents.

Maintaining acceptable dimensional tolerances in pressure vessel fabrication is essential for safety, performance, and compliance. By following these standards, manufacturers can ensure their vessels meet industry requirements and operate reliably.

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