Fabricate and Fit Boiler and Pressure Vessel Components

Fabricating and Fitting Boiler and Pressure Vessel Components

Introduction to the Fabrication Module

This chapter covers all fabrication and fitting operations for boiler and pressure vessel components, as assessed on the Red Seal exam. You must master layout, cutting, forming, assembly, and dimensional verification techniques while applying the requirements of the ASME Code, Section VIII, Division 1 (for pressure vessels) and the ASME Code, Section I (for boilers). The Canadian Electrical Code, Chapter V, applies to related electrical aspects but will not be detailed here.

Fabricating a pressure vessel requires dimensional precision in the range of ±1.5 mm on diameters and joint alignment of ±2 mm. Any deviation can compromise structural integrity and pressure-tightness.

2. Reading Drawings and Layout

2.1 Interpreting Fabrication Drawings

The fabrication drawing (or shop drawing) contains all necessary information: dimensions, tolerances, materials, welding processes, heat treatment, and non-destructive examination (NDE) requirements. You must identify:

2.2 Laying Out Developments

Layout is the operation of transferring the actual dimensions of parts to be cut onto the plate. For a cylinder, the development is a rectangle where:

Calculation example: For a cylinder with an outside diameter of 1,200 mm:

For a flanged and dished head (ASME F&D type), the development is more complex. The simplified formula is used:

Development = D_ext + (2 × straight flange height) + (0.5 × knuckle radius)

Table 1 – Typical layout allowances

2.3 Layout Instruments

Exam trap: Layout must always be done from the neutral surface (plate centreline) for formed parts, not from the outside surface, because thickness affects the development.

3. Cutting and Edge Preparation

3.1 Cutting Processes

The choice of process depends on thickness, material, and required precision.

ASME Rule: After thermal cutting (oxy-fuel, plasma), a heat-affected zone (HAZ) forms. For carbon steels over 25 mm thick, grinding or machining of a minimum of 1.5 mm is required to remove this zone before welding.

3.2 Bevel Preparation

Edge preparation (bevel) is essential for achieving complete penetration. Common types:

Standard angles: The total angle of a V-bevel is 60° ± 5° (30° ± 2.5° per side). The root face must be 1.5 to 2.5 mm.

Formula for calculating deposited metal volume (V) for a V-bevel:

V = (b × h) + (h² × tan(α/2))

Where:

Example: Thickness 20 mm, root face 2 mm, angle 60°:

3.3 Edge Cleaning

Before welding, edges must be free of:

ASME Requirement: Edges must be dry and clean over a minimum width of 25 mm on each side of the joint.

4. Plate Forming

4.1 Cold Forming vs. Hot Forming

ASME Section VIII Rule: Cold forming of carbon steel where deformation exceeds 5% requires stress-relief heat treatment after forming, unless the material is used at a temperature below 50 °C.

Calculating deformation (ε) when forming a cylinder:

ε = (e / (2 × R_mean)) × 100%

Where:

Example: 25 mm plate formed to a mean radius of 500 mm:

Since this value is less than 5%, no heat treatment is required.

4.2 Forming Processes

Ovality control: Maximum allowable ovality is 1% of the inside diameter (ASME Section VIII, UG-80). For a cylinder of 1,500 mm diameter, ovality must not exceed 15 mm.

4.3 Forming Flanged and Dished Heads

Flanged and dished heads (ASME F&D, elliptical, hemispherical) are formed by dishing/spinning. The height-to-diameter ratio determines the type:

Minimum thickness requirement after forming: The thickness at the crown of a flanged and dished head must not be less than 90% of the nominal thickness (ASME Section VIII, UG-81).

5. Assembly and Fitting

5.1 Joint Alignment

Alignment of plates before welding is critical. Alignment tolerances (mismatch) according to ASME Section VIII:

Measurement method: Use a 300 mm straight edge and a dial indicator. Measure on both the inside and outside of the joint.

5.2 Root Gap

The root gap is the space between the two pieces at the base of the bevel. It must be:

Exam trap: A root gap that is too large (> 3 mm) causes blowholes and lack of fusion at the root. A gap that is too small (< 1 mm) prevents complete penetration.

5.3 Tack Welding

Tack welds hold parts in position during welding. Requirements:

ASME Rule: Tack welds are an integral part of the final weld. They must be made by qualified welders and be free of defects.

5.4 Circularity Verification

After assembly, circularity is verified using a template or a measuring arm. The tolerance is 1% of the inside diameter for pressure vessels (ASME UG-80).

Ovality formula (O):

O = (D_max − D_min) / D_nominal × 100%

Where D_max and D_min are the maximum and minimum measured diameters.

Example: D_nominal = 2,000 mm, D_max = 2,015 mm, D_min = 1,990 mm:

This value exceeds 1%, so the vessel must be reformed or rejected.

6. Dimensional Checks and Testing

6.1 Final Dimension Checks

The following checks are performed after welding:

6.2 Concentricity Check

For jacketed vessels, concentricity between the two cylinders must be verified. The tolerance is ± 3 mm on the annular gap.

6.3 Pressure Testing

Before the pressure test, the vessel must be fully assembled and all welded joints completed. The hydrostatic test is performed at a pressure of:

P_test = 1.3 × P_design × (S_test / S_design)

Where:

Example: P_design = 1.5 MPa, S_test = 138 MPa, S_design = 120 MPa:

Test duration: Maintain the pressure for at least 30 minutes for vessels, then visually inspect all joints.

7. Post-Weld Heat Treatment

7.1 Stress Relief

Stress relief is required when:

Stress-relieving temperature: 600 to 650 °C for carbon steels. Hold for 1 hour per 25 mm of thickness (minimum 1 hour).

7.2 Preheating

Preheating reduces the cooling rate and prevents cold cracking. Typical temperatures:

ASME Rule: The preheat temperature must be checked at 75 mm from the joint, on the face opposite the heat source.

8. Pitfalls to Avoid

9. Summary

10. Self-Assessment Questions

Answers: