Post Weld Heat Treating: Can It Be Done On-Site?

Yes. Post weld heat treatment can absolutely be done on-site — and for large, heavy pressure vessels, it’s almost always the right call. Gulf Coast Combustion has been performing on-site PWHT using direct gas fire combustion since 2014, mobilizing directly to fabrication shops and plant sites across the Gulf Coast and the U.S. Here’s how it works and when it makes sense for your project.

When On-Site PWHT Is the Right Call

On-site is the right call in most large vessel situations. Here are the scenarios where it consistently wins:

The vessel is too large to transport. Oversized load permitting, rigging, crane time, and specialized transport equipment all add cost — and risk. For vessels over 25,000 lbs, on-site heat treating is almost always more cost-effective. Gulf Coast Combustion has performed PWHT on vessels over 120 feet long and up to 600,000 pounds. There is no furnace that handles that.

The vessel is already set in position. Repair welds, maintenance PWHT, and in-plant work don’t have a transport option. If the vessel is already positioned at a facility, on-site is the only practical method.

Your customer wants to witness the process. When your end client requires QC witnessing of the heat treat cycle, on-site PWHT puts your team and theirs in the same room. The strip chart recorder runs in front of everyone, and documentation is in hand before the crew leaves.

Your schedule can’t absorb furnace lead time. Furnace facilities have queues. Coordinating transport, furnace availability, and return transport adds days or weeks to a project. On-site PWHT is scheduled around your timeline.

The vessel carries transportation risk. A finished, precision-fabricated pressure vessel is not a simple load. Transit damage happens. On-site eliminates that exposure entirely.

The job involves multiple vessels at one location. Batching on-site PWHT across multiple vessels at a single shop reduces mobilization cost and often makes the per-vessel economics significantly better than furnace PWHT.

On-Site vs. Shop Furnace — Side by Side

How On-Site Direct Gas Fire PWHT Works

Gulf Coast Combustion uses high-velocity direct gas fire combustion for on-site pressure vessel PWHT. High-velocity gas burners fire directly through the vessel’s available nozzles and manways. The vessel, wrapped in 1-inch 8-lb Kaowool ceramic fiber insulation, becomes its own furnace. Type K thermocouples attached directly to the vessel surface via TAU capacitor discharge welding monitor temperature at multiple points simultaneously.

The heat cycle records continuously on a Chino strip chart recorder from first heat through cool-down. Every job follows a custom execution plan submitted to the client for approval before work begins, defining soak temperature, hold time, heat-up and cool-down rates, and thermocouple placement.

For carbon steel pressure vessels under ASME Section VIII Division 1, GCC’s standard parameters:

• Soak temperature: 1,100°F–1,200°F (GCC standard: 1,150°F ±50°F)
• Heat-up rate: 400°F/hr ÷ governing wall thickness; never exceeds 400°F/hr
• Hold time: 1 hour/inch for first 2 inches, then 15 minutes/inch after 2 inches
• Cool-down rate: 500°F/hr ÷ governing wall thickness; never exceeds 500°F/hr
• Max temperature differential during soak: 250°F

GCC’s gas train consoles deliver up to 10 million BTU — enough to execute the heat cycle correctly on the largest vessels in fabrication. Watch the time lapse below of a complete on-site PWHT job on a 600,000 lb vessel, from insulation wrap to strip out.

What You Receive at Job Completion

Every Gulf Coast Combustion job ships with a full documentation package before the crew leaves the job site:

• Heat Treatment Record (HTR) — signed by GCC and the client representative
• Strip chart recorder trace — the complete time-temperature history for every thermocouple channel
• NIST traceable calibration certificate — confirming recorder accuracy
• Execution plan — the pre-approved technical plan specific to that vessel

That documentation becomes part of the vessel’s permanent record. No waiting for paperwork from an outside facility.

Frequently Asked Questions

Is on-site PWHT as code-compliant as furnace PWHT?

Yes. ASME Section VIII Division 1 does not specify a method — it specifies parameters. Whether PWHT is performed in a furnace or on-site via direct gas fire combustion, the code requirements are identical: controlled heat-up rate, verified soak temperature and hold time, controlled cool-down rate, and full documentation. Gulf Coast Combustion executes every job to ASME UCS-56 standards with a complete documentation package on every job.

What size vessels can be heat treated on-site?

On-site direct gas fire combustion has no furnace-size limitation. Gulf Coast Combustion has performed PWHT on vessels over 120 feet long and up to 600,000 pounds. Burner count and placement are calculated based on vessel weight and geometry to ensure uniform heating. If a vessel can be built and set at your facility, it can be heat treated there.

How long does on-site PWHT take from mobilization to completion?

Most on-site jobs run 36 to 72 hours from setup to strip out, including insulation wrap, thermocouple attachment, the controlled heat-up cycle, soak period, controlled cool-down, and equipment removal. Wall thickness is the biggest variable — thicker walls require slower rates and longer soak times. GCC provides a custom execution plan before every job with the expected timeline.

Can PWHT be performed on a vessel already set in position at a plant?

Yes. This is one of the main advantages of on-site heat treating. Gulf Coast Combustion has performed PWHT on vessels already set in position at plant sites and fabrication shops across the United States — including repair welds, nozzle modifications, and in-plant maintenance jobs where the vessel cannot be moved.

Where does Gulf Coast Combustion perform on-site PWHT?

GCC is based in Spring, TX and mobilizes anywhere in the United States. Primary markets include Houston, Beaumont/Port Arthur, Corpus Christi, Midland/Odessa, Dallas, Baton Rouge, Oklahoma, and Wyoming. For a full list, see our service areas page.

For more on the full on-site PWHT process, code requirements, and how to evaluate a heat treating contractor, read The Fabricator’s Complete Guide to Pressure Vessel PWHT. For common questions answered in plain language, see our PWHT FAQ. For a full overview of GCC’s heat treating services — including localized PWHT, refractory dry outs, and in-house furnace options — visit our services page.