Formed head inspection

The dimensional inspection for vessel are included before/after welding activities.

Before welding: formed head inspection, shell inspection and opening hole inspection etc.

After welding: overall dimension, nozzle to nozzle, TL to TL, etc.

In addition of dimensional inspection after hydro-static test.

I recommend to learn more specified rules and follow it. It is a best way to understand and prepare your documents.

In this topic, I would like to share you about an actual inspection of Ellipsoidal Head. How to minimize document and report to your AI, or during audit.

This is a basic sharing knowledge for welding inspector, below photo is ellipsoidal head.


Section VIII Div. 1 - Pressure Vessels: Stamp "U" for  pressure vessel.

Section VIII Div. 2 - Alternative Rules: Stamp for "U2" for pressure vessel.

Section VIII Div. 3 - Alternative Rules for Construction of High Pressure Vessels: stamp "U3" for High Pressure Vessels.

National Board Inspection Code - Stamp "R" for repair and alteration.

A qualified company have a ASME Manual as specified their qualification. I think it book is a better guide for you and follow up.

Section VIII Division 1

up to 3,000 psi (20 MPa)

Section VIII Division 2

up to 10,000 psi (70 MPa)

Section VIII Division 3

over 10,000 psi

A Formed Head Inspection

Inspection of the head sections to the specified shapes within the permissible tolerances UG-81.

Example: Internal diameter of shell vessel: D (ID) = 1960 mm. 

     >  Deviation outside of the specified shape ≤ 1 1/4% of D= 1960*1.25/100 =  24.5 mm.

     >  Deviation inside of specified shape ≤ 5/8% of D = 1960*5/(8*100) = 12.25 mm

So, Max. allowed deviation from template are: 24.5 +12.2 = 36.7 mm.

A Template Preparation

Next, we will prepare a template for checking.

Template for checking must be prepared with an offset at last 10 mm (from deviation inside of specified shape = 12.5 + 10 = 22.5 mm) for easy access during inspection due to knuckle of shape is not equally after forming. 

If your template are not offset from min. deviation inside of shape, it may not insert into the inside of head.

Thickness of template should be greater 5mm to enough strong for inspection and minimize deformation and distortion.

Verification of Template

Next: a template should cut by CNC machine to make sure the precisely arc length meet the design. The double verification the template must be manually verified this arc by drawn step by step length.

The Formed Head Inspection

Preparation of the plates for forming head must offset (extra) a minimum of 25 mm, the offset area will discard after forming head and groove beveling.

Step to be checked:

  • Groove beveling preparation must be done and meet the design drawing,
  • Surface conduction must be checked without crack, peeling, flake off, lamination etc. after forming,
  • Additional NDE for all surface or partial area (Knucle areas) by MT or PT,
  • Ultrasonic thickness measurement (UTM) must be done (check your ASME manual and with your AI), and available the UTM reports for reviewing.
  • Next > It is easy check by using template, put the template into the head, and align the template to the axial center and run it arround the circle of head.
  • Next > You only focus at a minimum gap and maximum gap.
    As above example: Internal diameter of shell vessel: D (ID) = 1960 mm.

- Min. gap shall be 10 mm. 

- Max. gap shall be 36.7 + 10 = 47.7 mm.

Done, You must record all values in an inspection report

Inspection reports

Yes, you want to get an approval from your AI and your Client before moving next step. That must be prepared a good report and provide that you are awareness and understood the standards.


       d*: are gap of between template and inside of formed head section.

       D*: are inside diameter of  formed head section.

Nguyen Duy Bien

Enjoy learning and researching, reading book. Knowledge welding inspection painting inspection.

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