Why LNG HVAC is unlike anything else in the duct shop
Most HVAC duct shops in Australia run on a mental model that goes back to commercial buildings — hospitals, offices, schools, the occasional cool-room. Galvanised coil, Pittsburgh-locked seams, M8 threaded hangers, condensation on a chiller riser as the worst exposure case. That mental model is wrong by every meaningful metric on an Australian LNG mega-project, and the cost of bringing it onto site is measured in millions of dollars of rework once the EPC engineer rejects the first article.
An LNG export facility is a hazardous-classified, blast-rated, multi-billion-dollar process plant where every penetration through a wall has a hazardous-area zone on one side, a corrosion exposure category on both sides, an acoustic specification, an occupant risk envelope under API RP 752, and a structural anchor load engineered against tropical cyclone wind code AS/NZS 1170.2 region D. The duct does not just move air — it is one of the load-bearing risk barriers the operator pays you to deliver. The HVAC contractor who specifies 316L because "stainless is fine for outdoors" without reading the hazardous-area drawing has failed before the first sheet of coil leaves the slitter.
This guide is the field reference our engineers walk through with HVAC sub-contractors and EPC mechanical leads who are pricing or fabricating ductwork for the Australian LNG export sector. It covers North West Shelf, Pluto, Pluto Train 2, Wheatstone, Gorgon, Prelude FLNG, Ichthys, the Queensland trio of QPLNG, APLNG and GLNG, plus the future-development Browse, Sunrise and Crux projects — and the codes, materials and machine configuration that consistently get a duct package through EPC review at first submission.
The Australian LNG export landscape — operators, projects and EPC contractors
Australia is one of the world's three largest LNG exporters by tonnage, with installed liquefaction capacity in the order of 88 million tonnes per annum across the operating fleet and substantial capacity under development. The export base is concentrated on the north-west coast of Western Australia, the Northern Territory at Darwin, and Queensland's Gladstone port serving the coal-seam-gas fields of the Surat and Bowen basins. Every facility on the list below has commissioned HVAC ductwork, replaces ductwork on a 7 to 15 year envelope-corrosion cycle, and tenders new ductwork for de-bottleneck and module-addition projects on a more or less continuous rolling basis.
Operating LNG mega-projects — Western Australia
- North West Shelf LNG (Karratha, WA) — operated by Woodside, Australia's foundational LNG export project, five trains commissioned progressively from 1989 onwards. The facility sits on the Burrup Peninsula at the western edge of the Pilbara coast. Iron-rich red dust, chloride salt aerosol from the Indian Ocean, and tropical cyclone exposure define the corrosion and structural envelope. North West Shelf is the proving ground every Australian HVAC contractor has visited at some point in their career — and the project where the original galvanised duct on early trains had to be replaced wholesale within a decade.
- Pluto LNG (Karratha, WA) — Woodside-operated single-train liquefaction facility, commissioned 2012, sited adjacent to North West Shelf. Pluto draws gas from offshore Pluto and Xena fields and was originally built as a single 4.9 Mtpa train. The control room, marshalling building, laboratory and substation are all API 752-rated.
- Pluto Train 2 (Karratha, WA) — the second-train expansion supporting the Scarborough gas-field development. Train 2 is one of the largest oil-and-gas construction packages in Australia, with substantial HVAC ductwork demand for the process modules, the central control room expansion, the new marine off-loading facility and a significant set of equipment shelters.
- Wheatstone LNG (Onslow, WA) — Chevron-operated, two-train onshore liquefaction facility commissioned 2017, fed by the Wheatstone and Iago gas fields offshore. The plant footprint includes the LNG trains, a domestic gas plant for Western Australian gas market supply, and the marine terminal at Ashburton North. Wheatstone HVAC duct contracts are administered through Chevron and the principal EPC, with multi-discipline mechanical packages typically tendered separately from civil and structural.
- Gorgon LNG (Barrow Island, WA) — Chevron-operated, three-train liquefaction facility commissioned progressively from 2016, sited on Barrow Island Class-A nature reserve. Gorgon has the strictest biosecurity and quarantine regime of any operating LNG facility on the planet — every duct, every fixing, every gasket goes through a quarantine inspection before it lands on the island. Material certification is not optional. The CO2 reinjection facility associated with Gorgon adds an additional pressurised-gas service into the duct hazardous-area mix.
- Prelude FLNG (Browse Basin, WA offshore) — Shell-operated, the world's largest floating production vessel by displaced tonnage, commissioned 2018 over the Prelude and Concerto gas fields about 475 km north-north-east of Broome. Prelude is a fully marine HVAC application — IEC 61892 marine electrical and offshore-mechanical-cousin classification on every duct, with structural loading that includes both the ship's pitch-and-roll envelope and the cyclone survival design case. The Crux Gas Project, Shell-operated, is the back-fill supply for Prelude and will be tied in by subsea pipeline.
Operating LNG mega-projects — Northern Territory and Queensland
- Ichthys LNG (Darwin, NT) — INPEX-operated, two-train onshore liquefaction at Bladin Point near Darwin, commissioned 2018, fed by the Ichthys field about 220 km offshore via a 890 km subsea pipeline. The Darwin onshore plant includes the LNG trains, a condensate stabilisation unit, an LPG fractionation unit, and the largest standalone industrial control room complex in the Northern Territory.
- QPLNG (Curtis Island, Gladstone, QLD) — Queensland Gas Pipeline LNG, Origin Energy-operated coal-seam-gas-derived LNG, two trains commissioned 2014, on Curtis Island in the Gladstone Harbour port. The Queensland coal-seam-gas LNG projects share an operating envelope that is meaningfully different from the Western Australian coastal mega-projects — less salt aerosol, less iron-oxide dust, but substantial pipeline-coating chemical exposure where the gas-gathering network ties into the plant.
- Australia Pacific LNG (Curtis Island, Gladstone, QLD) — APLNG, a joint venture between Origin Energy, ConocoPhillips and a consortium partner, two trains commissioned 2015 to 2016. APLNG draws from the largest single coal-seam-gas resource on the Australian east coast and has a substantial gas-gathering and processing footprint inland from the LNG plant.
- GLNG Gladstone (Curtis Island, Gladstone, QLD) — Santos-operated, two-train coal-seam-gas-derived LNG project commissioned 2015 to 2016. The GLNG joint venture also includes equity holders from Asia and Malaysia. The plant configuration is similar to APLNG and QPLNG and the three Curtis Island plants share much of the marine off-loading and emergency response infrastructure.
Future-development and assessment-stage LNG projects
- Browse LNG (Browse Basin, WA) — Woodside-led future-development project across the Brecknock, Calliance and Torosa fields. Currently planned as a tie-back to the North West Shelf processing infrastructure rather than a standalone facility, but the engineering options for a future floating or onshore solution remain under assessment. HVAC duct demand on Browse, if developed, will be tied to the chosen execution route — but the project is large enough that it sits on every Australian fabricator's forward order book.
- Sunrise LNG (Timor Sea) — Woodside-led future development in the Sunrise and Troubadour fields of the Timor Sea, currently the subject of ongoing commercial and jurisdictional negotiations between the Timor-Leste government and the participants. Sunrise is one of the largest undeveloped gas resources in the region and the execution route — Darwin pipeline tie-back, Timor-Leste onshore plant, or floating LNG — will be defined by the resolution of the inter-governmental dispute.
- Crux Gas Project (Browse Basin, WA offshore) — Shell-operated, supports Prelude FLNG, currently in execution phase with subsea infrastructure and platform components. Crux is a back-fill resource rather than a standalone LNG project.
- Beach Energy Otway Basin (Victoria offshore) — Beach Energy domestic gas project, not an LNG export facility but a significant natural gas processing facility serving the Victorian and east coast Australian gas market. Otway has been an active project for HVAC fabricators servicing the Victorian oil-and-gas sector.
EPC contractors and HVAC sub-trade structure
The EPC contractor list on Australian LNG mega-projects is concentrated to a handful of internationally-credentialed engineering houses paired with Australian construction partners — KBR, Bechtel, Saipem, Worley, McConnell Dowell, John Holland, Civmec and Monadelphous. HVAC ductwork sits within the mechanical or building-services sub-contract package, typically administered through specialist HVAC subcontractors such as AlsipMcDermott, ESI, Nepean and MJM Marine for the offshore work. The duct-machine vendor — SBKJ in our case — sits one tier behind the HVAC sub-contractor and supplies the production line, tooling, welding station, controls and engineering certification. We do not negotiate directly with Woodside, Chevron, Shell or INPEX; we support the HVAC sub-contractor and the duct fabrication shop they nominate to satisfy the EPC mechanical specification at first submission.
Hazardous area classification — AS/NZS 60079 and what it means for ductwork
AS/NZS 60079 is the Australian adoption of the IEC 60079 international standard for electrical equipment in explosive atmospheres. The standard classifies the explosive atmosphere by zone, classifies the equipment by protection technique, and sets the inspection and maintenance regime for the operating life of the facility. For HVAC ductwork the relevant zones are 0, 1 and 2 for gas atmospheres.
Zone 0 is the rare case where a flammable atmosphere is continuously present — interior of a process vessel, vent stack interior, headspace of a sample point. Ductwork rarely enters Zone 0. Zone 1 is the common process-area envelope where flammable gas is likely in normal operation — within 3 to 5 metres of compressor seals, valve glands, sample points and vent terminations. Ductwork in Zone 1 must be conductive (grounded), bonded across joints, fitted with non-sparking access dampers, and supplied from a fan in a non-hazardous area through a non-return damper. Where it serves a Zone 1 enclosure with unclassified equipment inside, the enclosure must be Ex-pressurised — typically 50 to 75 Pa overpressure with loss-of-overpressure cut-out on the unclassified equipment.
Zone 2 is the broader process module envelope where flammable gas is present only in fault conditions. Zone 2 ductwork follows the same earthing and bonding philosophy as Zone 1 but with reduced ignition-source surveillance. Motor protection for fans in Zone 2 service can be Ex nA non-sparking rather than Ex d flameproof or Ex p pressurised types required in Zone 1. The classification drawing — produced by the process safety lead during FEED — is the controlling document. The HVAC contractor designs to the schedule once it is issued; "ATEX duct" as a generic category does not exist. Specify by zone, gas group, temperature class and protection technique.
NFPA 59A — the LNG facility standard
NFPA 59A is the U.S.-origin Standard for the Production, Storage and Handling of Liquefied Natural Gas, adopted by reference in every Australian LNG operator's project specification alongside AS 2885 and AS/NZS 60079. The version cited in the project specification governs — there is no automatic update obligation when a new edition is issued.
The ductwork-relevant clauses cluster around five themes. First, vapour barriers and cryogenic-equipment isolation: where ductwork passes within 3 metres of LNG cold equipment, the duct insulation must be a closed-cell vapour-barrier-capable material (cellular glass) to prevent moisture migration and ice damage. Second, air change rates: compressor shelters and pump rooms require 12 ACH minimum in normal operation and 30 ACH minimum on emergency purge. Third, segregation of cold-zone and hot-zone ventilation — cross-contamination creates a path for cold methane to reach a hot-surface ignition source.
Fourth, methane detection at high points. Methane is lighter than air at ambient but heavier when cold — within roughly 60 degC of its boiling point. NFPA 59A requires detection at high points because the dispersion behaviour transitions as the gas warms; detection is wired through the ESD to trigger purge and isolation. Fifth, dedicated emergency purge ventilation, physically and electrically independent of normal HVAC, with redundant power and capacity to clear the space within 60 to 90 seconds. The standard does not specify duct material — it specifies performance. 316L stainless, properly sized and insulated, satisfies it. Galvanised does not, because envelope-corrosion perforation creates release paths the project safety case did not model.
API RP 752 — building siting and the control room duct envelope
API Recommended Practice 752 is the building siting and occupant risk standard used across petrochemical and LNG facilities for permanently occupied buildings. The operator runs a credible-release vapour-cloud-explosion study, identifies the overpressure expected at each building, and rates the building structure to withstand it. On a typical Australian LNG facility the central control room, laboratory, operations annexe, marshalling building and sometimes the substation are rated as blast-resistant under API 752, with overpressure capacities of 0.2 to 1.0 barg.
The HVAC implications are non-trivial. Every penetration through the blast-rated wall must use a blast damper — a fast-closing assembly that maintains the wall's pressure rating in a blast event. The HVAC route must preserve the blast rating, which typically means the duct is supported off independent steelwork and enters the wall at a position the blast analysis has accepted. The air intake siting follows from credible release modelling — placed on the side furthest from the assumed vapour cloud source, fitted with methane, hydrogen sulphide and oxygen-depletion sensors wired through the HVAC shut-down logic.
The building HVAC maintains a slight overpressure (25 to 50 Pa) when running normally so leak paths leak outward, not inward. On gas detection the building inverts to hold mode and overpressure is maintained passively by closed dampers and internal volume until the alarm clears. Duct, fans, dampers and gas detection are a single integrated safety system. The acoustic specification — NC-30 to NC-35 at operating consoles, NC-40 on the wider floor — drives silencer length and location, which has to be coordinated against the blast damper penetration constraints.
API 521 and pressure relief — the duct on the receiving end
API Standard 521 covers pressure-relieving and depressuring systems for gas-processing facilities. Ductwork does not handle the relief discharge itself — that is steel pipe to AS 4041 and ASME B31.3. But the duct around a flare knock-out drum, a relief header tie-in and the flare stack base sees radiant heat and thermal stratification from relief events. A 30-minute relief event can raise ambient by 80 to 120 degC in a poorly-ventilated structure close to the flare stack. Stainless duct handles the excursion; galvanised volatilises its zinc coating above 419 degC and releases zinc oxide fume — another reason galvanised is not credible on relief-adjacent duct.
API 521 also drives emergency purge ventilation sizing in spaces that could see a release event, feeding into the AS 1668.2 ventilation calculation that sets fan, duct and silencer dimensions.
AS 2885 — the Australian pipeline standard
AS 2885 is the Australian pipeline standard for high-pressure petroleum and gas pipelines. It is a pipeline design code, not an HVAC code, but it appears in the duct fabricator's reference list because the documentation chain — material certification, weld procedure qualification, NDT requirements, hydrostatic testing — sets the precedent the LNG plant adopts for adjacent piping and ducted services. The intersection for the HVAC fabricator is Mill Test Certificates compliant with EN 10204 type 3.1 traceability on duct in pressure-rated service, and qualified welders working to a documented WPS on any seam carrying classified pressure. The TIG seam welding station SBKJ supplies for stainless duct must be paired with a documented WPS, qualified welders, and a daily calibration log the EPC engineer can review without further explanation.
IEC 61892 and the FLNG vessel envelope
IEC 61892 is the international standard for mobile and fixed offshore units — electrical installations. HVAC ductwork on an FLNG vessel is subject to IEC 61892 for electrical fittings (fan motors, dampers, gas detection, fire detection) and to the marine-classed requirements of the vessel's classification society (DNV, Lloyd's, ABS or Bureau Veritas). Prelude FLNG is classified to a marine standard and any ductwork for Prelude topside or hull integration has to satisfy the surveyor's inspection.
FLNG ductwork is a marine project, not onshore. Differences include tighter material traceability (full mill-to-install paper trail), classification society survey at each fabrication milestone, full helium leak testing on hazardous-zone-supply ductwork, and structural supports engineered against the vessel's pitch-and-roll envelope plus the cyclone survival case. Australian FLNG work to date is concentrated on Prelude with associated Crux back-fill work. Future Browse (if floating) and Sunrise (if floating) would expand the Australian FLNG fabrication base. Active fabricators include MJM Marine and the marine divisions of Civmec and Monadelphous.
AS 1668.1 and AS 1668.2 — the Australian ventilation code
AS 1668.1 covers fire and smoke control; AS 1668.2 covers mechanical ventilation. Both are part of the Building Code of Australia by reference and apply to every occupied building on an LNG facility — control room, laboratory, operations annexe, marshalling, workshops, warehouse, gatehouse and accommodation. They do not apply directly to process modules, compressor halls or FLNG topside, but they apply wherever people work and wherever air handling has to coordinate with the wider emergency response.
The duct-relevant clauses cluster around minimum air change rates by occupancy, smoke control duct ratings (TIG-welded continuous seams are mandatory above 60/60/60 fire resistance) and the segregation of toxic and flammable risk between adjacent zones. SBKJ has published a separate reference on AS 1668.2 — see the cross-reference at the end of this guide. The LNG facility ductwork specification is the union of AS 1668, AS/NZS 60079, NFPA 59A, API RP 752 and the project-specific overlay. No single code controls — the integrated specification controls.
Control building (CCR) HVAC — blast-rated, 24/7 N+1, life-safety integrated
The central control room runs 24/7 with operating staff on console at all times. HVAC failure during a process upset is a credible contributor to a major hazard because operators cannot manage the plant from a smoke-filled or contaminated control room. Standard specification includes N+1 redundant air handling units, independent power from different switchboards, gas detection at intake wired through the shut-down logic, blast-rated penetrations, smoke-control dampers at every fire-rated barrier, NC-30 to NC-35 at operating consoles, and 22 plus or minus 2 degC with 50 plus or minus 10 percent RH year-round.
Ductwork inside the CCR is conditioned-space duct — 316L stainless or aluzinc, Pittsburgh seams acceptable on low-pressure supply/return, TIG-welded on exhaust and smoke-control routes. Outside the CCR perimeter the ductwork reverts to outdoor-exposure 316L stainless. The CCR must hold overpressure even with the intake closed on gas detection — typically for 30 to 60 minutes — so the duct envelope must hit a verified leakage class. Pass criterion is 0.5 to 1.0 m3/h/m2 at design pressure (AS 4254 Class C). The SBAL-V with the TIG seam welder and pressure-tested joint detailing hits Class C consistently and Class B (0.25 m3/h/m2) with the closed-cell gasket and welded-flange option.
Equipment shelter HVAC — Ex pressurised enclosures
The equipment shelter (local equipment room, instrumentation enclosure) is a weather-protected conditioned space housing process instrumentation, marshalling, LV switchgear and small UPS equipment. There are dozens across an LNG facility, each with its own HVAC spec. The shelter is often sited within a Zone 1 or Zone 2 envelope but the equipment inside is unclassified — so the shelter is Ex-pressurised at 50 to 75 Pa overpressure, continuous air supply from a non-hazardous fan, overpressure transmitter, with loss-of-overpressure shutdown isolating the unclassified equipment.
Ductwork at the hazardous-area crossing follows the more restrictive zone, plus a non-return damper, optional flame arrestor, and gas detection at intake. Material is 316L stainless minimum, fully welded seams, fully bonded across joints, structural support designed for cyclone region D (WA) or C (Darwin, Gladstone). EPC reviews consistently flag earthing-and-bonding detailing. Duct must be electrically continuous end-to-end (less than 10 ohms), bonded to building structure at both ends and across any flexible joint. SBAL-V fabrication includes bonding lugs at each flange and the verification test in the FAT.
Modular process building — factory-built sealed envelope
The trend across Australian LNG mega-projects has been towards modular construction: process modules are fabricated in a controlled yard (typically South-East Asia, increasingly Australia), shipped to site, and lifted onto pre-poured foundation. The module is a sealed envelope containing process equipment, pipework, instrumentation, cable trays and integrated HVAC ductwork. The duct is fabricated in the yard with the rest of the module — typically forced supply (non-hazardous fan), naturally extracted through louvred high-point openings, gas detection wired through the module ESD. Supply duct is 316L stainless, fully welded, fully earthed and bonded.
The tolerance challenge is severe. The module is fitted out, shipped as a single unit, and integrated into the wider plant — duct has to be dimensionally accurate within a few millimetres across tens of metres, has to survive a heavy-lift sea voyage of weeks duration, and has to align with site-installed adjacent ductwork on arrival. Accepted tolerances are tighter than commercial HVAC — plus or minus 2 mm on cross-section, plus or minus 5 mm running length, square within 2 mm on the diagonal. The SBAL-V hits these tolerances on galvanised out of the box; on stainless the higher work-hardening rate introduces variation unless the tooling is hardened and the line retuned. The standard stainless tooling package consistently hits the tolerance band on first-article test.
LNG cold zone — minus 161 degC and boil-off gas management
LNG boils at minus 161 degC at one atmosphere. Wherever the plant transfers LNG, the surrounding envelope is a cold zone — equipment cold, pipework cold, supports cold, and any release creates a cold gas cloud heavier than ambient air until it warms above roughly minus 100 degC. The HVAC challenge is twofold. Duct material has to handle radiant cold without losing mechanical properties — 316L stainless retains yield and ductility to cryogenic temperatures; galvanised becomes brittle. Second, ventilation has to manage boil-off gas (BOG) — methane vapour from the surface of any LNG storage or transfer.
Standard ventilation rate is 30 ACH normal, 60 ACH emergency purge. Supply from a non-hazardous source, extract at the high points opposite the supply to sweep the space, gas detection wired through the ESD. Insulation on supply duct within 3 m of LNG cold equipment must be vapour-barrier-capable closed-cell — typically cellular glass at 50 to 100 mm depending on exposure. Shop-applied cellular glass jacketed in aluminium or stainless cladding outperforms site-applied; our SBAL-V line works upstream of a cellular-glass insulation station fed by line outputs.
Compressor building HVAC — high noise, high heat, dedicated ventilation
The compressor building houses gas-turbine-driven refrigerant compressors — typical aero-derivative drives of 50 to 100 MW mechanical output, generating substantial heat and noise. Ventilation rate is set by the heat load: 60 to 90 ACH normal, much higher on emergency purge. Supply is drawn from a non-hazardous source through Ex-pressurised ducting; extract through louvred or fan-extracted openings. Methane and oxygen-depletion detection is wired through the building shut-down. Duct material is 316L stainless on supply and extract, fully welded, fully earthed and bonded.
The acoustic challenge is severe — exterior NR-85 plant-side reduced to NR-55 at the site boundary, with consoles behind acoustic enclosures at NC-65 or better. Silencers are 1.5 to 3 m long with octave-band insertion loss specified in detail, in stainless to meet corrosion and fire requirements. The supply and extract fans are significant equipment — 50,000 to 150,000 m3/h, with motors in Ex d flameproof or Ex p pressurised protection. The fan room is a hazardous-area enclosure with the motor isolated from the airstream and bearings ventilated through a separate purge. Ductwork between the fan room and the compressor building is high-volume, high-pressure run with substantial structural support requirements.
Process Quench Tower (Q-Tower) — ammonia capture in coal-seam-gas LNG
The Queensland coal-seam-gas LNG projects — QPLNG, APLNG, GLNG — process gas with substantially higher dissolved CO2 and trace ammonia than conventional Western Australian gas. The process includes a Process Quench Tower (Q-Tower or amine absorber) capturing ammonia and acid-gas components before liquefaction. The HVAC duct around a Q-Tower is a more aggressive chemical environment than the rest of the plant — ammonia is a chloride-cousin for stress-corrosion-cracking risk on austenitic stainless. 316L is the bare minimum; duplex 2205 is frequently specified for duct in direct contact with Q-Tower exhaust or amine vapour drift. Insulation must be ammonia-compatible — cellular glass or faced mineral wool, not polyurethane or polystyrene. SBAL-V configured for duplex 2205 supply is the configuration we recommend for any coal-seam-gas LNG plant in the Surat or Bowen basin gas-gathering footprint.
FLNG vessel HVAC — integrated with ship HVAC
The FLNG vessel — Prelude today, potentially Browse and Sunrise in future — integrates gas processing, liquefaction, storage and off-loading on a single floating unit. HVAC is integrated with ship HVAC: accommodation, engine room, topside process and marine off-loading are designed as a coordinated system. Ductwork implications: full marine traceability with classification society survey at each step, tighter dimensional tolerances, full helium leak testing on hazardous-zone supply, structural supports engineered against pitch-roll plus cyclone survival, and A60 or H60 fire-rated duct construction through fire-rated bulkheads.
Default material is super duplex 2507 for splash-zone and main-deck exterior, duplex 2205 for topside interior, 316L for accommodation. Seams are TIG-welded throughout. Fabrication is in the marine yard with material control, WPS qualification, classification society liaison and hold-point inspection tied to the vessel's construction schedule. SBKJ has supplied stainless duct lines to several marine fabricators in this market — SBAL-V with stainless tooling, TIG seam welder, and tighter tolerance band (plus or minus 1.5 mm cross-section instead of the standard 2 mm).
Why galvanised fails on Australian LNG sites
The Australian LNG export coast — Karratha, Onslow, Barrow Island, Darwin — sits in ISO 12944 C5-M (very high, marine) to CX (extreme). Queensland Curtis Island is C5 to C5-M. Hot-dipped galvanised has a C5-M service life of 5 to 10 years before zinc consumption; on wetted faces it is 18 to 36 months to visible zinc loss and 4 to 7 years to perforation. Painted galvanised extends slightly but the recoat cycle is not credible where duct is inaccessible without scaffolding or hot-work permits.
Trace H2S in sour-gas service forms zinc sulphide and accelerates coating loss. Pipeline coating chemicals (mercaptans, glycols, methanol) accelerate it further. The failure mode is multi-mechanism — sulphide pitting, chloride pitting and underdeposit corrosion all contributing. The accepted minimum for outdoor and exposed duct is 316L stainless (the "L" suffix suppresses welding sensitisation, the gateway to intergranular SCC in chloride exposure). 316L is the workhorse — roughly 70 percent of stainless tonnage on a typical LNG plant. Duplex 2205 covers splash zones, FLNG topside exterior and worst H2S excursions — 20 to 25 percent. Super duplex 2507 is reserved for FLNG main deck exterior and chloride-extreme service — 5 to 10 percent.
316L stainless mandatory — and what that means in the fabrication shop
Specifying 316L is the easy part. Fabricating to 316L on a galvanised-tuned line is hard. The behavioural differences mean a galvanised-tuned auto duct line will not fabricate acceptable stainless without retuning, retooling and often re-training. First, work-hardening rate. 316L work-hardens roughly 2.5 times faster than carbon steel and twice as fast as galvanised. The Pittsburgh-hook roll stations on galvanised will not form a clean hook on 316L without higher forming forces and additional passes. Line speed comes down 25 to 40 percent versus galvanised. The SBAL-V stainless configuration ships with hardened tooling rated for 100,000+ metres of stainless throughput.
Second, galling. 316L galls against carbon steel tooling — particles transfer and accumulate as drag marks the EPC inspector will flag. Solution is hardened tool-steel forming rolls with the right surface finish, plus a chlorine-free synthetic lubricant (chlorine-bearing lubricants introduce SCC risk). Third, heat-affected zone on welding. TIG welding of 316L produces a 3 to 8 mm HAZ; chrome carbide precipitation in 425 to 815 degC triggers sensitisation. On 316L the precipitation kinetics are slow enough that a properly-set TIG weld does not produce significant sensitisation. The SBKJ TIG station is set up for the 316L parameter band. Fourth, the cost of mistakes — stainless coil is 3 to 4 times the cost of galvanised, a botched run goes to scrap, and the FAT package SBKJ delivers as part of the stainless configuration is correspondingly more rigorous.
Duplex 2205 and Super Duplex 2507 — the chloride-extreme specifications
Duplex 2205 and super duplex 2507 are mixed austenite-ferrite microstructure stainless steels. 2205 has roughly twice the yield strength of 316L and substantially better chloride SCC resistance — the standard choice for splash zones, FLNG topside exterior and high-chloride duct. 2507 takes strength and chloride resistance another step up. Fabrication implications: higher forming forces (higher yield, higher forming load), more critical welding parameter control (the microstructure is sensitive to cooling rate and heat input), and tighter post-weld inspection (ferrite content measurement on weld and HAZ). The SBAL-V configured for duplex 2205 is a higher-capability stainless configuration — upgraded forming stations and a more rigorous welding parameter window, 16 to 20 weeks lead time vs 14 to 18 for standard 316L.
Super duplex 2507 volumes are usually small enough that hand-laid welded duct is more economical than auto-line fabrication. Where the project requires significant 2507 tonnage, set up a small parallel fabrication cell for 2507 and run the main duct line on 316L and 2205.
TIG welded seam — the welding standard for hazardous-zone duct
Seam construction on duct serving hazardous zones is universally TIG-welded continuous seam. Pittsburgh, button-punch, S-cleat and drive-cleat are acceptable on internal conditioned-space duct in non-hazardous areas only — not on duct that crosses the hazardous-area boundary, supplies an Ex-pressurised enclosure, or extracts from a hazardous-zone space.
TIG welding (GTAW) uses a non-consumable tungsten electrode with argon shield for stainless. The station is a programmable longitudinal seam welder with backing-gas purge, tungsten tracked along the seam at constant speed and arc-length feedback maintaining stable arc. Output is a continuous full-penetration weld. Typical WPS: 60-120 A, 10-14 V, 200-400 mm/min travel, 10-15 L/min argon shield, 5-10 L/min argon backing, 2.4 mm 2% thoriated tungsten with 30-degree cone. Welder qualification under AS/NZS 1554, re-qualify every 6 months or after WPS change.
The SBKJ TIG seam welder is an inline longitudinal welder with backing-gas purge integrated with the SBAL-V output. Welded duct comes off the line as a single continuous fabrication — no handling between forming and welding. This matters for dimensional tolerance because handling-induced distortion is the biggest single contributor to variation on stainless duct.
ATEX and IECEx certification — what to ask for and verify
ATEX is European Directive 2014/34/EU; IECEx is the international scheme for the same scope. The two are largely harmonised. IECEx is generally accepted on Australian LNG facilities; ATEX-only equipment is sometimes accepted with project-specific equivalency assessment. ATEX/IECEx certification is typically required on the fan motor, damper actuators, gas detection sensors, access door interlocks and bonding-related fittings. The duct itself (a passive metal envelope) does not require certification — it requires a manufacturer's declaration of conformity stating that the duct, when properly installed and bonded, is compatible with the certified electrical equipment.
Verification on receipt: check the certificate matches the equipment supplied (not a generic certificate for a different model or protection class), the certifying body is an accredited notified body (NANDO for ATEX, IECEx Online Certificate System for IECEx), and the certificate is current. Counterfeit certificates are a real risk and the verification step is fast and free.
HEPA-equivalent particulate filtration on lube and seal-oil contamination
Compressor seals and gas-turbine lube-oil systems are sealed pressurised systems, but in normal operation a small amount of oil mist or seal gas escapes into the building ventilation envelope. Standard approach is HEPA-equivalent filtration (H13 or H14 EN 1822) on the extract path, bag-in-bag-out housing for safe replacement, full ducting in 316L stainless. The HEPA filter and housing are supplied by a specialist filtration vendor; our role is duct upstream and downstream sized and tolerance-matched to the housing flanges. Upstream duct must deliver a uniform face velocity within plus or minus 20 percent — non-uniform velocity reduces filtration capacity and accelerates differential pressure rise.
Acoustic specification — NC-30 control room, NC-50 wider plant
Acoustic spec is one of the most consistent drivers across LNG projects: CCR consoles NC-30 to NC-35, control floor NC-40, lab and offices NC-40, marshalling and ops annexe NC-45. Compressor building exterior NR-85 plant-side reduced to NR-55 at site boundary, with consoles inside behind acoustic enclosures NC-65 or better. Silencers are packed dissipative with stainless or galvanised perforated facing, mineral wool packing, structural casing — sized to the target octave-band insertion loss. CCR supply silencer 1.5-2.5 m long, 100-150 mm packing; compressor extract silencer 2.5-4 m, 200-300 mm packing.
Silencers are fabricated to the same corrosion and welding spec as the duct they serve — 316L stainless casing outdoor, TIG-welded seams in hazardous zones, full earthing continuity end-to-end. We supply silencer cases as part of duct fabrication scope; packing material is supplied separately by the acoustic specialist.
SBKJ machine configuration for LNG ductwork fabrication
The standard SBKJ configuration for fabricators entering the Australian LNG market is the SBAL-V auto duct line in its stainless variant — full auto production line covering de-coiling, levelling, cutting, slitting, notching, roll-forming, seam-forming and flange-attaching, configured for the higher work-hardening rate and precision demands of stainless. Capacity is 50 to 100 m/h of finished duct depending on cross-section and gauge.
The stainless tooling package upgrades the standard tooling to hardened tool steel with surface finish suitable for stainless contact, rated for 100,000+ metres of throughput. The TIG seam welder is an inline longitudinal welder integrated with the SBAL-V output, set up for 316L parameter band with optional duplex 2205 capability through programmable parameter sets — backing gas purge, arc-length feedback, welding-parameter logging supporting AS/NZS 1554 WPS verification. The ATEX-certified construction option upgrades switchgear, lighting, motors and instrumentation to ATEX/IECEx equivalents suitable for installation in a Zone 2 fabrication area. Certification is per-component, so the buyer specifies which sub-systems require ATEX.
The line is supplied with a Siemens or Mitsubishi PLC (buyer choice) with open programme architecture, Windows-based HMI in English plus buyer's second language, full English FAT report, stainless TIG WPS and operator manual. Lead time on the fully-specified stainless SBAL-V is 14 to 18 weeks ex-works Australia, 16 to 20 weeks with international shipping and on-site commissioning. Sized to the buyer's coil — typical Australian LNG spec is 1,250 mm slit, 0.8 to 1.5 mm gauge, 316L 2B or 2D finish.
Lead time, FAT and on-site commissioning
Procurement runs 4 to 6 months from contract signature to commissioning: 2 to 3 weeks spec finalisation and contract execution, 14 to 18 weeks fabrication and FAT at SBKJ, 2 to 4 weeks shipping and customs, 1 to 2 weeks rigging and installation, 2 to 4 weeks commissioning and operator training. Total purchase order to first-article duct is 22 to 30 weeks.
FAT is conducted with the buyer's nominated engineer (typically senior fabrication engineer from the HVAC contractor or EPC mechanical lead) witnessing: full line cycle on the buyer's coil spec, sample duct to project specification with dimensional measurement, weld procedure qualification with macro-section and bend test, control system functional testing including safety interlock chain, signed FAT report. Typically 2 to 4 days. On-site commissioning is 1 to 2 SBKJ engineers for 7 to 14 days covering installation supervision, mechanical and electrical commissioning, 16 to 24 hours of structured operator training, first-article duct with buyer's QA witness, and signed commissioning report.
Common EPC review findings and how to avoid them
Across the LNG packages we have supported, the EPC engineer's review of a duct fabricator's first-article submission consistently flags the same handful of items. Understanding the pattern shortens the iteration cycle from three rounds of comments to one — and the difference in the project schedule is usually four to six weeks. The first finding is incomplete material traceability — the Mill Test Certificate exists but cannot be tied back to the specific duct section presented for inspection. The fix is heat-number marking on every duct section at the fabrication stage, with a fabrication log that ties each duct ID to the heat number and the MTC. The SBAL-V can be configured with an inline marking station that imprints the heat number on each duct as it leaves the line.
The second consistent finding is inconsistent weld parameters across the production run. The TIG seam welder operates within a narrow parameter window and the daily calibration log has to demonstrate that the current, voltage, travel speed and shield gas flow were within tolerance for every duct in the run. The fix is automatic parameter logging on the welder with a printable daily log that the EPC engineer can review. Our standard welder package includes the data logger and the report template. The third is earthing-and-bonding discontinuity at flexible joints and access doors. The duct must be electrically continuous, and the path through flexible joints, expansion joints and access doors is the weak point. The fix is bonding straps across every flexible joint, lugs welded to every access door frame, and a resistance measurement test at every joint as part of the FAT. Standard SBKJ practice is to include the bonding straps and the resistance test in the fabrication package.
The fourth is dimensional drift across long fabrication runs. Stainless coil is supplied in approximately 5 to 7 tonne master coils, and the dimensional behaviour of the coil can shift slightly across the run as the work-hardening accumulates. The fix is mid-run dimensional verification — every 50 to 100 metres of fabrication, pull a duct section, measure cross-section and squareness, and confirm against the project tolerance band. The SBAL-V includes an inline dimensional check that flags out-of-tolerance sections before they accumulate. The fifth is documentation completeness — the EPC engineer expects the complete documentation package (FAT report, material certificates, weld procedure qualification, welder qualification records, calibration certificates, fabrication logs) at first submission. Missing one document triggers a complete re-submission cycle. Standard SBKJ practice is to package the documentation alongside the duct fabrication itself, with each duct ID cross-referenced to its supporting documents.
Cross-references and related guides
This guide sits within a broader SBKJ engineering reference set covering hazardous-area, marine, cryogenic and code-driven HVAC ductwork applications. The directly-related guides are listed below.
- Hydrogen Production HVAC Duct Guide — companion reference covering the emerging hydrogen production sector, where the hazardous-area and material-selection logic overlap substantially with LNG but the explosive gas characteristics (lower flammability limit, ignition energy, dispersion behaviour) differ.
- Marine and Offshore HVAC Duct Guide — the marine and offshore equivalent of this guide, with full coverage of IEC 61892, classification society requirements, and the marine fabrication standards that apply to FLNG and floating production work.
- Cold Storage and Cold Chain HVAC Duct Guide — the cryogenic and refrigerated facility reference, with material and insulation logic that overlaps the LNG cold-zone work covered here.
- SBKJ Marine Industry Page — the SBKJ industry landing page for the marine and offshore sector, with product configuration and case study references.
- AS 1668.2 Australian Ventilation Code Reference — the detailed reference on the Australian ventilation code that controls the occupied-building HVAC on every LNG facility.
FAQ
Why does galvanised duct fail on Australian LNG sites?
Australian LNG export plants sit in ISO 12944 C5-M (very high, marine) or CX (extreme) corrosivity categories. Chloride salt aerosol, elevated humidity and trace H2S in sour gas service consume the zinc coating on galvanised duct within 18 to 36 months on wetted faces and perforate the steel within 5 years. The accepted minimum for outdoor and exposed ductwork on Australian LNG mega-projects is 316L stainless, with duplex 2205 for splash zones and super duplex 2507 for chloride-extreme service.
What hazardous-area classification applies to LNG ductwork?
AS/NZS 60079 governs hazardous-area classification. Zone 1 covers areas where flammable gas is likely in normal operation; Zone 2 covers areas where gas is present only in fault conditions. Ductwork in Zone 1 must be bonded, earthed, fitted with non-sparking access dampers, supplied from a non-hazardous fan and Ex-pressurised where it serves an unclassified-equipment enclosure. The classification drawing controls — the HVAC contractor designs to the drawing, not the other way around.
What is API RP 752 and why does it drive control-room duct design?
API RP 752 is the building siting and occupant risk standard for permanently occupied buildings in petrochemical and LNG facilities. Blast-rated buildings (typical CCR, lab, ops annexe) require blast dampers on every penetration, controlled air intake siting based on credible release modelling, gas-detection-triggered HVAC shut-down, and pressure-holding capability on intake closure. Typical overpressure rating is 0.2 to 1.0 barg.
What does NFPA 59A require for LNG facility ventilation?
NFPA 59A requires vapour-barrier insulation within 3 m of LNG cold equipment, 12 to 30 air changes per hour in compressor shelters with higher rates on emergency purge, segregation of cold-zone and hot-zone ventilation, methane detection at high points (methane is lighter at ambient but heavier when cold), and dedicated emergency purge ventilation independent of normal HVAC. The standard sets performance — 316L stainless duct properly sized and insulated satisfies it.
What machine configuration does SBKJ supply for LNG ductwork fabrication?
SBAL-V auto duct line in stainless configuration with hardened 316L tooling, integrated inline TIG seam welder with backing gas purge, ATEX-certified switchgear option for hazardous-area shops, Siemens or Mitsubishi PLC with open programme. Standard 316L coil with optional duplex 2205 capability. Lead time 14 to 18 weeks ex-works Australia for the fully-specified stainless configuration.
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