5.2 Orthographic, Isometric, Arrangement, and Foundation Drawings
Key Takeaways
- An orthographic drawing shows an object in separate flat views, while an isometric drawing shows all three dimensions in a single pictorial view.
- Piping isometrics are drawn not to scale and carry the dimensions, weld locations, and spool numbers needed for fabrication.
- Single-line piping drawings represent pipe with one line and symbols; double-line drawings show the pipe outline and are used for large sizes and congested areas.
- A foundation drawing gives the anchor bolt pattern, projection, embedment, and the elevation of the grout surface.
- Equipment arrangement drawings locate equipment relative to plant coordinates and column lines, which is how a mechanic finds the right machine.
Orthographic versus isometric
An orthographic drawing projects the object onto flat planes and shows each face in a separate view: front, top, right side. Dimensions are true length in each view, which makes orthographic the right choice for machining and fabrication detail.
An isometric drawing shows three faces at once in a single pictorial view. The three axes are drawn at 120 degrees to one another, and lines along those axes are dimensioned to true length while diagonals are not. Isometrics are far easier to visualize, which is why piping fabrication runs on them.
| Drawing type | Shows | Best for |
|---|---|---|
| Orthographic | Separate true-shape views | Machined parts, weldments, structural detail |
| Isometric | Three dimensions pictorially | Piping fabrication, field routing, one-line clarity |
| Oblique | One true face, depth at an angle | Simple illustration |
| Section | Internal construction | Valves, casings, gearboxes |
| Exploded | Assembly order of components | Repair procedures and parts identification |
Piping drawings: single-line and double-line
- A single-line (simplified) piping drawing shows each pipe as one heavy line, with standardized symbols for elbows, tees, valves, and reducers. It is compact and is the norm for most process piping.
- A double-line drawing shows the pipe as two lines representing its outside diameter. It is used for large-diameter lines, congested spaces, and presentation drawings where actual clearance must be visible.
A mechanic must be able to recognize how the symbols change with the viewing direction. In a single-line orthographic plan view, a 90-degree elbow turning toward the viewer is drawn as a circle with a dot at the center; turning away, it is drawn as a circle without the dot. That convention distinguishes an up-turn from a down-turn on the same plan.
Piping isometrics and spool drawings
A piping isometric covers one line number from one point to another. Its defining characteristics:
- It is not to scale. A 40-foot straight run and a 2-foot nipple may be drawn the same length. All dimensions are written.
- It carries the line number, service, pipe specification, insulation requirement, and the coordinates or elevations at each end.
- It shows field welds and shop welds distinctly, usually with a field weld marked by a filled flag or the letters FW.
- It is divided into spools — the sub-assemblies that are welded in the shop and then transported and joined in the field. Each spool has its own number.
- A north arrow on the isometric orients the drawing to plant coordinates.
The mechanic's practical use of an isometric is to identify which flange to break, which spool to remove, and which welds are field welds that can be cut without scrapping a shop assembly.
Equipment arrangement drawings
An equipment arrangement (or general arrangement) drawing is the plan and elevation map of a unit. It locates equipment by:
- Plant coordinates, given as north-south and east-west dimensions from a plant origin.
- Column line references, such as the intersection of line C and line 7.
- Elevations, referenced to plant grade or to a benchmark.
This drawing is how the maintenance crew finds P-101B in a unit with four identical pumps, and how they confirm that a crane can reach it.
Foundation and anchor bolt drawings
Before a baseplate can be set — Chapter 8 — a mechanic reads the foundation drawing for:
| Item | Why it matters |
|---|---|
| Anchor bolt pattern and spacing | Determines the template and whether the baseplate will drop over the bolts |
| Bolt diameter, projection, and embedment | Projection sets how much thread is available above the grout; embedment sets holding strength |
| Sleeve detail | Sleeved bolts allow lateral adjustment; solid-set bolts do not |
| Top-of-concrete elevation | Establishes the grout thickness under the baseplate |
| Finished grout surface elevation | The datum the machine centerline is set from |
| Reinforcement and block dimensions | Drilling a new hole through rebar is a stop-work condition |
Projection is the length of anchor bolt sticking above the concrete. Insufficient projection means the nut cannot be fully engaged after the grout, shims, and baseplate stack up. Checking projection against the drawing before the machine arrives avoids a very expensive discovery.
The six principal views and drawing only what is needed
Orthographic projection makes six principal views available — front, top, bottom, rear, left side, and right side — but a competent drawing shows only the views required to describe the part without ambiguity. A shaft is fully described by one view plus a diameter note. A gearbox housing may need four. Extra views cost drafting time and invite contradiction between sheets, so their absence is deliberate rather than an omission.
The rule that makes multiple views usable is that adjacent views share a dimension:
| Pair of views | Dimension they share |
|---|---|
| Front and top | Width (left-to-right) |
| Front and side | Height (top-to-bottom) |
| Top and side | Depth (front-to-back) |
That sharing is why the views are aligned rather than scattered on the sheet, and it is how a mechanic transfers information between them.
Worked example. A bearing housing drawing gives the bolt-circle diameter and the four bolt positions in the top view, and the depth of the bore and the housing height in the front view. To find how far a bolt hole sits above the base, project the hole straight down from the top view into the front view, then read the height dimension in that view. Neither view alone answers the question; the alignment between them does.
Third-angle and first-angle projection
Which side of the front view a given view lands on depends on the projection convention, and getting it wrong mirrors the part.
| Third angle (ANSI, North American practice) | First angle (ISO, common in Europe and Asia) | |
|---|---|---|
| Right-side view is placed | To the right of the front view | To the left of the front view |
| Top view is placed | Above the front view | Below the front view |
| Mental model | Each view shows the nearest face | Each view is projected past the object onto the far plane |
Drawings identify their convention with a projection symbol in or next to the title block: two views of a truncated cone lying on a horizontal axis, one a trapezoid and one a pair of concentric circles. The narrow end of the trapezoid points left in the third-angle symbol and right in the first-angle symbol.
This is not academic. Imported pumps, gearboxes, and compressors arrive with first-angle drawings, and reading one as third angle reverses the handedness of every offset feature — the oil drain, the keyway, the discharge orientation. Check the projection symbol on any drawing from a foreign original equipment manufacturer before laying out a single hole.
Why is a piping isometric drawn not to scale?
On a single-line orthographic plan view of piping, an elbow is drawn as a circle with a dot at its center. What does this indicate?
Which dimension on a foundation drawing determines whether the anchor bolt nut can be fully engaged after the baseplate, shims, and grout are in place?
A mechanic is issued a gearbox drawing from a European manufacturer and lays out the mounting holes assuming the usual North American convention. What is the likely consequence?