14.1 Process Piping: Materials, Sizing, Fittings & Joining

Task D-19 (Services process piping, tanks and containers) carries 4 exam questions across eight sub-tasks — install, diagnose, maintain and repair, for both piping and vessels. Millwrights routinely install pump suction and discharge piping, lubrication and cooling lines, compressed air mains and process headers.

Pipe Sizing: NPS and Schedule

Nominal Pipe Size (NPS) is a dimensionless designator, not a measurement. For a given NPS, the outside diameter is fixed, and the schedule number sets the wall thickness. Increasing the schedule thickens the wall inward, shrinking the bore while the OD stays the same — this is what makes standard fittings work across schedules.

At and above NPS 14 the designator equals the actual OD in inches. Note that at NPS 2 and below, the nominal size is neither the OD nor the ID — a common source of ordering errors.

Tubing is entirely different: tube is specified by actual outside diameter and wall thickness (or by BWG/gauge). A 1/2 in tube measures 0.500 in OD; a 1/2 in pipe measures 0.840 in OD.

Materials

Never use PVC for compressed gas. PVC fails by brittle shatter, propelling fragments; compressed air stores enough energy to make that failure lethal. This is a routinely examined safety rule.

Codes and Jurisdiction

• ASME B31.3 (Process Piping) governs design, materials, fabrication, examination and testing of process piping.
• ASME B31.1 (Power Piping) governs boiler external piping and power plant service.
• CSA B51 is the Canadian code for boilers, pressure vessels and pressure piping. Pressure-retaining designs are registered with a Canadian Registration Number (CRN), and welding is performed by shops qualified under CSA W47.1 to CSA W59 procedures.
• Enforcement is provincial: TSSA in Ontario, ABSA in Alberta, Technical Safety BC, and equivalent authorities elsewhere. A millwright must know that pressure-retaining welds require a qualified welder and procedure, and are not general shop work.

Fittings

Joining Methods

Valves: Isolation versus Throttling

Stem style matters for clearance and diagnostics: a rising stem shows valve position visually and needs headroom; a non-rising stem fits confined spaces but gives no visual indication.

Thermal Expansion and Pipe Supports

Piping grows when it heats. Ignoring that growth breaks nozzles off pumps and vessels — a mechanical failure the millwright is called to repair.

$\Delta L = \alpha \times L \times \Delta T$

Worked example. A 30 m carbon steel steam condensate line (alpha = 11.7 x 10⁻⁶ mm/mm/C) is installed at 20 C and operates at 120 C.

$\Delta L = (11.7 \times 10^{-6}) \times 30{,}000\text{ mm} \times 100 = 35.1\text{ mm}$

That 35 mm has to go somewhere. Accommodation methods:

• Expansion loop — a U-shaped offset that flexes; the simplest and most reliable, needs space.
• Bellows expansion joint — a metal bellows that compresses axially; must be installed with the shipping bars removed only after the anchors and guides are complete, or it will squirm and fail.
• Slip joint — a packed telescoping joint; requires periodic packing service.
• Offsets and changes of direction — natural flexibility in a routed system.

Support hardware:

Pipe strain on rotating equipment is a classic millwright failure mode. Piping must be supported so that it comes to the pump or compressor flange free and in alignment, with no need to pull the flanges together. The standard check is to set a dial indicator on the pump shaft, loosen the suction and discharge flange bolts, and watch for shaft movement. Movement greater than about 0.05 mm (0.002 in) means the piping is forcing the casing, which will distort the housing, destroy the alignment, and wreck the bearings and seals. Correct the piping — never shim the pump to chase a pipe-strain misalignment.

Line Breaking and Isolation

Opening a process line is a controlled activity governed by A-1.04 zero-energy principles:

1. Identify the contents and hazards from the P&ID and the SDS.
2. Isolate with double block and bleed where the service is hazardous, or install a blind/spade for positive isolation.
3. Drain, vent, flush and purge; verify with a gauge that pressure is zero.
4. Lock out and tag every isolation point, including the drivers of any pump that can repressurize the line.
5. Wear PPE for the contents, and crack the flange on the far side from your body, breaking the joint at the bottom last so residual liquid drains away from you.