Interpret Drawings and Specifications

Interpreting Drawings and Specifications

Module Introduction

Interpreting drawings and specifications is the first competency assessed in the Red Seal Interprovincial program for the boilermaker trade. This task represents approximately 12% of the exam. It requires rigorous reading of various technical documents: assembly drawings, fabrication drawings, descriptive specifications, welding standards, and material specifications. Your ability to extract relevant information, visualize parts in three dimensions, and apply dimensional tolerances will directly determine the quality of your work in the field. This chapter prepares you to master each type of document, decode symbols, and avoid interpretation errors that compromise the structural integrity of equipment.

Types of Drawings Used in Boilermaking

Assembly Drawings and Detail Drawings

The assembly drawing (or general arrangement drawing) shows the complete assembly of equipment — tank, boiler, heat exchanger, hopper, stack — with the relative positions of each component. It indicates overall dimensions, elevation levels, anchor points, and interfaces with adjacent structures. The detail drawing (or fabrication drawing) represents an individual part with all the dimensions necessary for its fabrication: thicknesses, bend radii, bevel angles, hole positions, and weld sizes.

On a job site, you will first consult the assembly drawing to understand the context, then the detail drawing to fabricate or install the part. The Red Seal exam verifies that you can make this transition without losing critical information.

Isometric Drawings and Orthographic Views

Orthographic views (right-angle projection) present the part from multiple planes: front view, top view, side view. The Canadian standard uses third-angle projection (American projection), where the top view is placed above the front view. Isometric drawings show the part in perspective with three axes at 120°, allowing you to visualize the general shape, particularly useful for piping and tubular structures.

For the exam, you must be able to:

Scales and Conversion Factors

Drawings are rarely at true scale (1:1). Common scales in boilermaking are:

Practical rule: never measure directly on the drawing with a graduated ruler. Use only the dimensions indicated. A drawing may have been reduced or enlarged during reproduction, making the scale inaccurate. If a dimension is missing, calculate it from adjacent dimensions or consult the specification.

Welding Symbols According to CSA W59

Structure of the Welding Symbol

The CSA W59 — Welded Steel Construction (Structural Steel) standard governs the symbolic representation of welds on engineering drawings in Canada. The complete symbol includes:

The basic symbol placed below the reference line indicates a weld on the arrow side. Placed above, it indicates a weld on the side opposite the arrow. A symbol on both sides (above and below) means a double weld.

Essential Basic Symbols

Weld Dimensions and Tolerances

For a fillet weld, the dimension indicated to the left of the symbol represents the throat size (dimension of the largest isosceles triangle inscribed in the cross-section). CSA W59 requires a minimum throat size based on the thickness of the thinner part:

The effective length of a fillet weld is the actual length minus twice the throat size (to account for start and crater areas at the ends). If the symbol indicates a length of 200 mm with a throat of 6 mm, the effective length is 200 − (2 × 6) = 188 mm.

Finish and Contour Symbols

The weld contour may be indicated by supplementary symbols:

The finishing method is noted in the tail: G (grinding), C (chipping), M (machining), or no indication if the finish is left as-welded.

Descriptive Specifications and Specifications

Structure of a Specification

The descriptive specification (or project specification) complements the drawings by specifying non-graphical requirements: material grades, fabrication processes, non-destructive testing, tolerances, painting, and marking. In Canada, specifications generally follow the MasterFormat system of the Bureau de normalisation du Québec (BNQ) or the format of the Construction Specifications Institute (CSI) . Relevant sections for the boilermaker include:

Each section contains numbered clauses (e.g., 3.1.2) that specify the requirements. The exam verifies your ability to quickly locate a requirement in a specification and apply it.

Material Specifications

Materials are designated by Canadian or North American standards:

The designation CSA G40.21 350W means: weldable structural steel, minimum yield strength of 350 MPa, grade W (weldable). The letter A indicates an improved grade through heat treatment, the letter R indicates atmospheric corrosion resistance.

Non-Destructive Testing (NDT) Requirements

The specification specifies the types of testing, their extents, and acceptance criteria. Common methods:

The specification may require a percentage of coverage (e.g., 10% of welds by RT) or full coverage (100%). Acceptance criteria are defined by the reference standard and quality level (e.g., level 2 per ISO 5817 for tank welds).

Dimensional and Geometric Tolerances

Fabrication Tolerances According to CSA W59

CSA W59 establishes fabrication tolerances for welded structures:

For pressure vessels built to the ASME Code Section VIII, Division 1, tolerances are more stringent: the deviation from circularity must not exceed 1% of the nominal diameter, and the alignment of longitudinal joints must not exceed 1.5 mm.

Geometric Tolerances (GD&T)

The geometric dimensioning and tolerancing (GD&T) system uses feature control frames to control the form, orientation, and position of features. Essential symbols for the boilermaker:

The feature control frame is read: geometric symbol, tolerance value, datum reference (e.g., ⟂ 0.5 A means perpendicularity of 0.5 mm relative to datum A).

Piping and Instrumentation Diagram (P&ID) Symbols

Reading Process Diagrams

Piping and instrumentation diagrams (P&IDs) represent the complete process: equipment, piping, valves, instruments, and control loops. The boilermaker must identify:

Line and Equipment Identification

The line identification system follows the format: nominal diameter (in) - fluid - line number - pressure class. The pressure class refers to the ASME B16.5 standard for flanges: class 150, 300, 600, 900, 1500, 2500. The higher the class, the greater the allowable working pressure.

Rigging and Erection Plans

Rigging Plans and Load Calculations

Rigging plans indicate anchor points, sling angles, and lifting equipment capacities. The boilermaker must verify:

Layout and Elevation Plans

Layout plans show the position of equipment on the foundation, with elevation dimensions (elevation relative to the reference level). Dimensions are expressed in meters with three decimals (e.g., 12.450 m). Benchmarks are indicated by the symbol BM followed by the elevation. The boilermaker must verify flange alignment, flow slopes (expressed as a percentage or in mm/m), and thermal expansion clearances.

Applicable Canadian Standards

CSA W59 — Welded Steel Construction

This standard covers the design of welded connections, welder qualifications, welding procedures, testing, and acceptance criteria. Key clauses:

CSA B51 — Boiler, Pressure Vessel, and Pressure Piping Code

CSA B51 governs the design, fabrication, and inspection of boilers and pressure vessels in Canada. It references the ASME Code Section VIII for design and CSA W47.1 for the certification of welding companies. The boilermaker must know the marking requirements (nameplate), fabrication records, and inspection requirements by the regulatory authority.

CSA W47.1 — Certification of Welding Companies

This standard establishes requirements for the certification of companies that weld steel structures. It defines company responsibilities, welding supervisor qualifications, and quality control requirements. The boilermaker must know that only companies certified to CSA W47.1 may perform welds on structures governed by CSA W59.

Canadian Electrical Code, Part I

Although the boilermaker is not an electrician, you must know electrical safety requirements when working near energized equipment. The Canadian Electrical Code, Part I (C22.1-21) defines minimum working distances near power lines. Rule 8-200 specifies minimum distances based on voltage:

Area, Volume, and Weight Calculations

Area and Volume of Common Shapes

The boilermaker frequently calculates areas and volumes to estimate material quantities and weights. Essential formulas:

Calculating Plate Weight

The weight of a carbon steel plate is calculated: weight (kg) = length (m) × width (m) × thickness (mm) × 7.85. The factor 7.85 represents the density of carbon steel (7850 kg/m³). For austenitic stainless steel (types 304, 316), the factor is 7.93. For aluminum, it is 2.70.

Example: A plate measuring 2.5 m × 1.2 m × 12 mm in carbon steel weighs: 2.5 × 1.2 × 12 × 7.85 = 282.6 kg.

Calculating Plate Development

Development is the length of plate required to form a curved part. For a 90° bend with inside radius R and thickness t, the developed length is: L = (π/2) × (R + k × t), where k is the neutral axis position factor (0.33 for R/t < 2, 0.40 for R/t between 2 and 4, 0.50 for R/t > 4).

Common Pitfalls to Avoid

Summary