Pipefitting & Welding: The Craft

Pipefitting and welding are skilled trades central to the construction and maintenance of piping systems that carry fluids (liquids, gases, steam, chemicals, etc.) in industrial, commercial, and power-generation facilities.

• Pipefitting involves:

  • Laying out, cutting, threading, and assembling piping systems.

  • Reading and interpreting piping isometric drawings and P&IDs (Piping and Instrumentation Diagrams).

  • Installing supports, hangers, and valves.

  • Preparing joints (beveling, alignment, fit-up) before welding or coupling.

• Welding in piping involves:

  • Joining pipes, fittings, and components by fusing metals with heat (arc welding is most common).

  • Using techniques like SMAW (stick), GTAW (TIG), GMAW (MIG), and FCAW (flux-core).

  • Ensuring welds meet quality standards (X-ray/UT tested in critical applications).

  • Working with tight tolerances to prevent leaks or structural failures.

Together, pipefitters and welders create piping systems that must withstand pressure, temperature, and corrosion conditions safely and reliably.

Materials Commonly Used in Piping & Welding

A good craftsperson must know how different piping materials behave when being cut, joined, or exposed to service conditions. JDI has teams of mechanical contractors led by superintendents and project managers that know the specs necessary. Key materials include:

1. Carbon Steel (CS)

• • Most common in industrial piping.

  • Affordable, strong, and weldable.

  • Susceptible to corrosion (often needs coatings or cathodic protection).

2. Stainless Steel (SS)

• • • Corrosion-resistant (used in food, pharma, chemical service).

    • Types: 304, 316, duplex, super-duplex.

    • More difficult to weld (susceptible to warping and carbide precipitation).

3. Alloy Steels

• • Used in high-temperature or high-pressure systems (e.g., power plants).

  • Chrome-moly steels (like P11, P22) resist creep and scaling.

4. Non-ferrous Metals

• • Copper, brass, bronze → plumbing, HVAC.

  • Aluminum → lightweight, corrosion-resistant.

  • Nickel alloys (Inconel, Hastelloy, Monel) → high-temperature, highly corrosive environments.

5. Plastics & Composites (non-welded, but joined with solvent cement or heat fusion)

• • PVC, CPVC, HDPE, PP → chemical handling, water treatment.

Each material demands specific knowledge of cutting, handling, preheating, interpass temperature control, post-weld heat treatment (PWHT), and welding filler selection.

Understanding ASME B31.1 & B31.3 Piping Codes

The ASME B31 Code for Pressure Piping is the standard governing safe design, fabrication, and installation of piping. The two most relevant parts for pipefitters/welders are:

1. ASME B31.1 – Power Piping

• Covers piping in electric power plants, industrial plants, geothermal heating systems, and high-pressure/high-temperature applications.

• Emphasis on safety and reliability under high-pressure steam and boiler-related systems.

• Materials: usually carbon steel and alloy steels.

• Requires strict welding qualifications and inspections.

• Example: piping in a power plant or textile plant steam loop.

2. ASME B31.3 – Process Piping

• Covers process piping in chemical plants, refineries, pharmaceutical, textile, paper, and cryogenic plants.

• Broader scope with a focus on hazardous chemicals, corrosive fluids, and specialty materials.

• Has different “fluid service categories” (Normal, Category M (toxic), High Pressure, Severe Cyclic).

• Example: piping in a petrochemical refinery or chemical processing plant.

👉 Key Differences:

• B31.1 → Power piping (steam, boiler systems, high pressure/temperature).

• B31.3 → Process piping (chemical, industrial, hazardous fluid service).

• B31.1 is generally more conservative in stress allowances, while B31.3 allows more flexibility but requires careful material and inspection controls.

✅ In practice: A skilled pipefitter/welder needs to:

• Understand piping drawings, specs, and the systems.

• Select correct materials, joints, and welding methods.

• Follow the applicable ASME code requirements.

• Ensure quality through inspections (visual, NDE).

• Prioritize safety, since piping failures can cause catastrophic accidents.

Here’s a quick reference comparison chart between ASME B31.1 (Power Piping) and ASME B31.3 (Process Piping):

ASME B31.1 vs B31.3 – Comparison Chart

👉 Rule of Thumb:

• If you’re working in power generation (steam, turbines, boilers) → B31.1 applies.

• If you’re working in process industries (refineries, chemicals, pharma, food, etc.) → B31.3 applies.

Contact JDI Industrial Services, today, to address your pipefitting needs or wants, and we will help you however we can. When you’re asked, “what is pipefitting” by someone, tell them about JDI.