Introduction: Doubling the Torch, Doubling the Discipline
On a long-distance pipeline spread, the external welding stations consume more crew-hours than any other activity after line-up. Every joint must be filled and capped across its full circumference, in every weather window the schedule allows, at a quality level that survives AUT inspection and hydrostatic testing. A full-automatic pipeline dual-torch external welder addresses this bottleneck directly: instead of one carriage making pass after pass around the pipe, two torches operate on the same joint in a coordinated fashion, roughly halving the number of orbital circuits per joint and, in practice, compressing fill-and-cap time by a large margin. This guide explains the architecture, synchronization logic, procedure windows, power supply integration, quality control, maintenance and economics of dual-torch external welding, using the DW series external welders and HW series engine-driven power sources supplied by Beijing Engine Welder Technology Co., Ltd. (brand DENVO / ENGINE WELDER) as the reference equipment family.
1. What Is a Full-Automatic Pipeline Dual-Torch External Welder?
A dual-torch external welder is an orbital welding carriage that carries two independently controlled GMAW/FCAW torches on a common chassis, riding on a rack-and-pinion track clamped around the pipe. “Full-automatic” means the carriage positions, torch parameters (current, voltage, wire feed speed, travel speed, oscillation), and pass sequencing are executed from stored procedure recipes with minimal operator intervention: the welder clamps the track, starts the cycle, and supervises. Each torch typically covers a quadrant or half of the pipe circumference, working simultaneously from the 12 o’clock and 6 o’clock positions (or two opposing start points) toward the meeting points, so that a full bead is deposited in roughly half the time of a single-torch circuit.
The machines in the DW-EW family accept pipe diameters from approximately 219 mm up to 1422 mm, use 0.9–1.2 mm solid or metal-cored wire, and store downloadable parameter sets on a touch-screen interface. When paired with a suitable engine-driven welding power source, one station can support two carriages; this is the configuration used on many modern mainline spreads.
2. Why Dual Torches: The Productivity Logic
The fill-and-cap phase of a large-diameter wall joint may require six to twelve passes depending on wall thickness (typically 10–30 mm on trunk lines). A single-torch carriage must travel the full circumference for every pass, plus repositioning time between passes. A dual-torch setup divides each pass into two half-circumference arcs welded simultaneously. The arithmetic is straightforward:
• Single torch: N passes × full circumference travel time + inter-pass handling.
• Dual torch: the same N passes × half-circumference travel time, with two torches running in parallel.
• Net effect: wall clock time for fill and cap is typically reduced by roughly 40–50%, subject to synchronization overhead and crossover handling.
On a 40-inch line with 300 joints in a production day, the difference between a 22-minute and a 13-minute external welding cycle per joint determines whether the spread needs six external stations or four — and each station on a spread carries its own power source, crew, and logistics footprint. Dual-torch welding therefore reduces not only welding time but the total capital and operating cost of the station train.
3. System Architecture: Track, Carriage, Twin Torch Mounts
A complete dual-torch external welding system consists of five subsystems:
3.1 Rack-and-pinion track. A split ring of gear-rack segments (often called a tooth-hole steel track) clamps around the pipe with quick-release links. The rack provides the positive engagement surface for the carriage pinion, ensuring that travel speed is exactly proportional to motor steps regardless of gravity loading — critical when a carriage must hold position at the 6 o’clock position or climb past 3 and 9 o’clock without slipping. Track segments are calibrated so that circumference alignment errors stay within fractions of a millimetre over the joint.
3.2 Carriage chassis. A lightweight, dust-sealed aluminum or steel chassis carrying the drive motor, two torch slide assemblies, the controller, and cable/ hose management. Weight matters: operators lift and clamp the carriage repeatedly on every joint, and on large diameters the carriage may be installed at working height on the pipe. The DW series emphasizes a lightweight, dust-resistant carriage with a single-gear rack engagement design that avoids lost motion (“skip”) during direction reversal.
3.3 Twin torch assemblies. Each torch mount provides multi-axis adjustment — torch extension (stick-out), work angle (transverse), travel angle (longitudinal), and lateral offset relative to the joint centreline — so that operators can dial in the exact wire position each procedure requires. Repeatable, indexed adjustment is essential because the two torches must be set to mirror-image geometry.
3.4 Controller and HMI. A touch-screen controller stores pass-by-pass recipes: for each pass, current, voltage (or synergic setpoint), wire feed speed, travel speed, oscillation width and dwell, and torch position limits. Parameters can be uploaded and downloaded, which allows welding engineers to tune a procedure at the bench and push identical recipes to every carriage on the spread — a major factor in keeping production and repair-weld quality uniform.
3.5 Power and wire delivery. Each torch is fed by its own wire feeder and shielding gas line from the station’s power source. On mainline spreads the power source is typically an engine-driven welding generator such as the HW1000, whose output capacity is sized to run two carriages simultaneously.
4. The Synchronization Problem: Two Torches, One Bead
Dual-torch welding introduces a coordination requirement that single-torch welding does not have. The two half-beads must meet cleanly at the crossover points with matched height, width, and edge wetting. The synchronization logic covers four areas:
• Start-point staggering. On successive passes, the two start points (typically at 12 and 6 o’clock) are staggered by a set angular offset so that crossover seams from consecutive passes do not stack vertically — a known cause of lack-of-fusion and trap defects at the 12/6 quadrant boundaries.
• Speed matching. Both torches must deposit at matched mass rates. Because gravity assists the downward-running torch and resists the upward-running one, procedures compensate through per-quadrant parameter tables: slightly different voltage, wire feed, and travel speed for the downhill and uphill halves, plus oscillation dwell changes at the sides.
• Crossover control. As each torch approaches the meeting point, the controller ramps current and travel speed down (or applies a controlled stop-and-overlap sequence) so the terminating crater fills and the two beads merge with a smooth transition. Some procedures specify a fixed overlap of 10–15 mm of deposited bead.
• Interpass coordination. Both torches must finish and the joint must be within interpass temperature limits before the next pass starts. Because the halves finish nearly simultaneously, dual-torch welding actually improves interpass time discipline compared with a single torch that finishes halves sequentially.
5. Bead Sequence: Root, Hot, Fill, Cap
The external welder does not work alone: on most modern spreads, the root pass is deposited by an automatic internal welder (which clamps inside the pipe and welds the root from within against copper shoes), or by an external root process on smaller diameters. The dual-torch external machine then takes over:
• Hot pass: a fast, higher-current pass that straightens the root profile and burns out any edge contamination. Run immediately after root completion while the joint is hot.
• Fill passes: multiple passes that build the joint to near-flush with the pipe surface. Wall thickness and bevel geometry determine the number; the controller’s recipe table assigns parameters per pass and per quadrant.
• Cap pass: the final cosmetic-and-strength pass with wider oscillation, tuned to produce 1–2 mm reinforcement and smooth wetting into the base metal on both sides — the surface that AUT later scans and that field joint coating must bond to.
With two torches, hot/fill/cap passes are each executed as two simultaneous half-circuits. Some contractors run a “two-torches-two-passes” mode on thinner walls, where each torch completes an entire independent pass on opposite halves, further reducing cycle time.
6. Clock-Position Parameter Management
Orbital welding on fixed pipe is a study in changing gravity. The same procedure that produces a convex, well-crowned bead at 1 o’clock will sag and overlap at 5 o’clock if parameters do not change. Dual-torch external welders therefore divide the circumference into quadrants or finer sectors and apply per-sector parameter tables:
• 0–3 o’clock (descending): gravity pulls the pool forward and down; travel speed and voltage are raised, oscillation narrowed, to keep the pool behind the arc.
• 3 o’clock: the transition sector; typically the highest travel speed with short side dwells to control undercut on the vertical plane.
• 3–6 o’clock (upward on the uphill torch): gravity resists travel; speed is reduced, current slightly lowered, dwell increased to maintain convexity and avoid the pool running ahead.
• 6 o’clock: the crossover and pipework bottom sector, where both torch geometry and access are most constrained.
Stored recipes encode these tables per pass, so an operator starting a new wall-thickness class only needs to load the correct procedure file rather than re-tune every sector manually. The touch-screen’s download function then propagates the tuned recipe across the fleet of carriages.
7. Consumables and Shielding Gas
Dual-torch stations double consumable logistics, so specification discipline matters:
• Wire: 0.9 mm, 1.0 mm, or 1.2 mm wire, chosen per wall thickness and position. Metal-cored or low-alloy solid wire matching the pipe grade (e.g., X70/X80 class wire for corresponding line pipe) is standard on trunk lines. Two torches consume twice the wire per hour; feeder spool capacity and spare-spool staging on the welding vehicle directly affect arc-time efficiency.
• Gas: typically an argon–CO2 mix (75/25 or 80/20 class) delivered from cylinders or an on-site manifold. Gas purity and hose integrity are non-negotiable: porosity is among the most common dual-station defects when a gas line leaks on one torch only, because the defect then appears as a half-joint porosity signature that is easy to misdiagnose.
• Contact tips and nozzles: per-shift inspection and replacement on a fixed count. A worn tip on one torch of a pair shows up as erratic arc length and spatter on only one half of the bead — again a half-joint signature.
8. Power Source Selection: Why the HW1000 Class Matters
Two carriages welding simultaneously draw roughly twice the arc power of one. The station power source must therefore deliver stable CV output to two independent feeders with fast dynamic response, plus auxiliary power for pumps, preheating units, and lighting. The HW1000 engine-driven welding machine is configured for exactly this duty:
• Multi-process output: CC and CV capability across a wide current range, with digital inverter control and low-spill waveform behavior suited to short-circuit and spray-arc GMAW/FCAW modes used in orbital fill and cap.
• Two-station capacity: the HW1000 is specified to support two sets of automatic external welding carriages at once, which is the defining requirement of a dual-torch station’s power side.
• Auxiliary power: 20 kVA class auxiliary output runs pipeline preheating equipment (including medium-frequency induction heating), tools, and camp lighting, so one machine inside a welding engineering vehicle covers the whole station.
• Deployment: mounted on a welding truck or skid, the machine reaches a working station, and the full station is deployable in roughly twenty minutes, including cable, gas, and track preparation.
Sizing rule of thumb: total simultaneous arc power plus auxiliary load should stay below the generator’s continuous rating with margin for transient starting loads. Where stations are mobile and spread logistics lean, pairing one HW1000 with two dual-torch carriages gives a four-torch production cell from a single diesel engine — a configuration that reduces fuel, noise, and maintenance points compared with four single-torch power packages.
9. Joint Preparation and Bevel Geometry
Automatic welding rewards tight, consistent bevels. Typical mainline preparations for automatic external welding are narrow-gap designs (compound or J-bevel style) with a small root face and 5–8° effective bevel angle, machined or ground on-site by pipe-facing machines. Consistency targets:
• Root gap controlled to tight tolerance; excessive gap shows up immediately as burn-through risk during the hot pass.
• Misalignment (high-low) limited per the applicable standard (commonly ≤ 1.5–3 mm depending on wall thickness and code); dual-torch procedures are less forgiving of high-low than manual welding because both torches follow the joint, not the operator’s eye.
• Bevel surface free of scale, moisture, and cutting oil; wipe-down and inspection before track clamping is standard practice.
Because the internal welder often deposits the root, bevel design must be jointly agreed between the internal-welding and external-welding procedure owners — a coordination step that experienced welding engineers treat as a gate item before production welding begins.
10. Productivity Mathematics: Planning a Dual-Torch Station
Spread planners model external welding capacity with simple but honest arithmetic. Take a 56-inch (Ø1422 mm) line, 20 mm wall, compound bevel, requiring one hot, five fill, and two cap passes (eight external passes):
• Mean circumference: π × 1.422 m ≈ 4.47 m. Single-torch travel per pass at ~0.35 m/min average incl. sector adjustments: ≈ 12.8 min, plus ~1.5 min inter-pass handling → ~14 min per pass → ~112 min per joint.
• Dual-torch: each torch covers 2.23 m per pass → ~6.4 min travel, plus synchronized handling ~1.5 min → ~8 min per pass → ~64 min per joint.
• Per-joint external welding time falls by ~43%; with crossover and start/stop overheads modeled, a conservative plan still yields 35–40% cycle reduction.
Multiply by joint count: on a 300-joint daily production requirement, the dual-torch station train saves several hundred crew-minutes per day, which converts directly into fewer stations, fewer power sources, and lower daily fuel burn. Real spreads add weather downtime, NDT-driven hold points, and crossover repairs, so production engineers typically derate theoretical gains by 10–15% when committing to schedules.
11. Defect Control in Dual-Torch Work
AUT inspection of dual-torch joints reveals characteristic defect patterns, each with a known control lever:
• Lack of side-wall fusion: usually at fill/cap edges where dwell was insufficient or the bevel angle was too narrow for the pass width. Control: increase side dwell time 0.1–0.2 s, reduce pass width per layer, verify bevel angle against procedure.
• Crossover lack-of-fusion: at the 12/6 meeting points where two beads join. Control: stagger successive start points, enforce 10–15 mm overlap, reduce current ramp at crossover.
• Porosity: half-joint distribution indicates a gas-line, tip, or nozzle problem on one torch. Control: per-shift gas-leak check on both trains, tip replacement counters, nozzle cleaning at every pass change.
• Undercut at 3/9 o’clock: gravity-assisted side flow on vertical planes. Control: raise travel speed, reduce voltage slightly in vertical sectors, narrow oscillation.
• Excessive cap reinforcement or concavity: cap-pass oscillation width and wire feed mismatch. Control: recipe-level cap parameters with per-quadrant correction, verified on test coupons before production.
The discipline that makes dual-torch quality manageable is data: modern carriages log actual parameters per pass, so when AUT flags an indication, the engineer can retrieve the logged amperage, voltage, and travel speed for that pass and that clock sector, and correct the recipe rather than guessing.
12. Procedure Qualification and WPS Discipline
A dual-torch automatic procedure is qualified under the governing pipeline code (e.g., API 1104 in many international projects, with project-specific supplements). The WPS must define, for each pass and each sector: process (GMAW/FCAW), wire class and diameter, gas composition and flow, current/voltage or synergic window, travel speed range, oscillation parameters, torch angles and extensions, interpass temperature limits, and the start/stop and overlap rules specific to dual-torch operation. Because two torches work the joint, qualification coupons must reproduce the dual-torch sequence — including crossover regions — so that the qualified envelope covers the actual production geometry.
Change control is equally important: a wire brand change, gas mix change, or bevel redesign usually triggers requalification or a supplement. The parameter download capability of the DW-EW carriages helps here in a subtle way: because the HMI can lock recipe files and distribute them, field supervisors can verify that every carriage is running the qualified file version and nothing else.
13. Working With the Internal Welder: Station Choreography
On large-diameter spreads using internal root welding, the station runs a fixed choreography per joint:
1. Line-up and internal clamp closing; internal welder positioned at the front of the station performs the root (typical cycle within about two minutes on flexible-mountain versions such as the DW48/56-8H-D for suitable diameters).
2. While root and hot pass complete, the external crew clamps tracks and carriages for the next joints in the rolling station.
3. Dual-torch carriages run hot → fill → cap passes with per-pass recipes.
4. NDT (AUT) scans the completed joint behind the station; indicated defects go to a repair cell with defined repair procedures.
5. Field joint coating follows after inspection clearance.
The dual-torch external station’s compressed cycle allows it to keep pace with the internal welder’s rapid root cycle without becoming the station bottleneck — which is precisely its role: the internal welder sets a fast cadence, and the external cell must match it.
14. Site Deployment: A Seven-Step Commissioning Routine
Crews that deploy dual-torch stations in a repeatable sequence lose far less time to setup chaos. A field-proven routine:
1. Position the welding engineering vehicle at the station; ground it, extend leveling, start the HW1000 and verify auxiliary power.
2. Inspect and clean the pipe surface where the track will seat; remove coating overhang within the specified band.
3. Clamp the split track; verify rack alignment and run a dry circuit of the carriage to confirm smooth engagement with no skip.
4. Mount both carriages; set mirror-image torch geometry per the WPS (extension, work angle, travel angle, offset), then verify with a gauge.
5. Connect gas and wire to both trains; purge lines; run wire through to the tips and check feed tension.
6. Load the qualified recipe file; dry-run one pass without arc to confirm travel limits, crossover logic, and stop positions.
7. Execute a short test arc on scrap or the first joint per supervisor instruction; verify parameters on the HMI against the WPS sheet before committing the joint.
On organized spreads this routine settles into a 15–25 minute station setup, after which the station runs joint after joint with only consumable top-ups and track moves.
15. Data Logging and Traceability
Modern pipeline owners increasingly require weld-parameter traceability: for each joint, which carriage, which pass, what actual current/voltage/speed. Dual-torch carriages with touch-screen controllers and parameter upload/download provide the capture point. Practical traceability practice:
• Assign each carriage a fleet ID and record the carriage-pass mapping with joint numbers at shift start.
• Export parameter logs at shift end to the welding engineer’s laptop; reconcile against AUT indications.
• Version-control recipe files: any change to a qualified recipe gets a new version number and distribution date; carriages run only the current version.
• Keep consumable lot records alongside parameter logs, so a porosity cluster can be traced to a wire lot or gas cylinder batch if needed.
This traceability turns AUT from a pass/fail filter into a feedback loop: sector-level defect trends point at specific parameters, and the recipe fixes propagate through download to the whole fleet in minutes.
16. Operator Training: From Welder to Welding-Machine Supervisor
Automatic welding changes the operator’s role from depositing metal to supervising a machine that deposits metal. Training programs for dual-torch crews typically cover:
• Track and carriage installation, alignment, and dry-run verification.
• Torch geometry setting to mirror-image tolerances, and why a 2–3° work-angle error on one torch shows up as asymmetric bead profile.
• Recipe loading, sector-table reading, and parameter verification against the WPS.
• Recognizing arc-sound and bead-appearance anomalies per sector; interrupting cycles safely when a torch misbehaves.
• Crossover quality judgment: when to accept a seam, when to grind and re-run locally per the repair procedure.
• Consumable handling: tip change cadence, gas-line checks, spool changes without losing pass continuity.
• Data export and daily reporting.
Crews converting from single-torch to dual-torch need an additional calibration block: the synchronization behaviors — staggered starts, speed matching, crossover ramps — are new failure modes, and a two-week supervised production block on non-critical or test joints is a common investment that pays back quickly in avoided repairs.
17. Maintenance and Spare-Part Strategy
A dual-torch station has more moving wear items than a single-torch one, and station downtime is expensive on a moving spread. A practical maintenance regime:
• Per shift: clean nozzles, inspect tips, check gas fittings, wipe rack teeth, verify carriage wheel and pinion condition visually.
• Weekly: check drive motor brushes (where applicable) or encoder function, slide backlash on both torch mounts, cable insulation, gas-hose integrity, track clamp threads.
• Per project phase: replace high-wear kits — tips, nozzles, drive wheels, pinions, slide bearings — based on joint-count counters rather than failure.
• Spares on the vehicle: one full spare carriage or a complete set of chassis-level modules, torch spares for both trains, track segments for the largest diameter in use, and controller with pre-loaded recipe library.
Because both torch trains run simultaneously, their wear clocks are synchronized — which is actually an advantage: replace both trains’ consumables on the same cadence and the two arcs stay matched.
18. Safety Engineering Around Automatic Carriages
Automatic external welding introduces its own safety profile. Core controls:
• Arc radiation: both arcs produce intense UV; station screens and full PPE for all personnel in the cell, including support staff who may underestimate exposure from the second arc.
• Fume extraction: two arcs double fume generation; mobile extraction positioned per the ventilation plan, with attention to the pipe interior when internal welding has just finished.
• Pinch and rotation hazards: a carriage moving under power on a rack can pinch fingers during adjustment; procedures require power-off torch adjustments.
• Gas asphyxiation in confined spaces: argon-rich gas is heavier than air; when working near the pipe ends or in trenches, monitor and ventilate.
• Electrical safety: high open-circuit voltages on some processes; ground-return clamps placed per procedure and inspected each shift.
• Manual handling: track segments and carriages lifted by two persons or with aids; on large diameters, working-at-height rules apply to carriage mounting on top-dead-centre positions.
19. Weather, Season, and Environment
Dual-torch stations inherit pipeline welding’s environmental sensitivities, amplified by automation’s need for stable arc behavior:
• Wind: shielding-gas loss is the primary wind risk; windscreens around the joint, higher gas flow within procedure limits, and wind-speed thresholds for stopping are standard.
• Moisture and dew: preheat and interpass temperature control manage hydrogen risk; in humid climates, stored recipes often include longer preheat holds.
• Cold: below specified temperatures, preheat requirements rise and wire becomes brittle to handle; crewed enclosures or heated shelters are used on winter spreads.
• Dust and sand: the carriages’ dust-sealed design and per-shift rack cleaning address the abrasion problem that kills slide and pinion life in desert work.
20. Total Cost of Ownership: Dual-Torch Economics
Contractors evaluate dual-torch adoption on total cost, not sticker price. The TCO equation includes:
• Capital: carriages, tracks per diameter, power source (one HW1000 class per station), welding vehicle.
• Productivity credit: 35–45% external-welding cycle reduction converts into fewer stations for the same daily joint target.
• Consumables: roughly double per-station wire and gas consumption, but spread-level consumption falls because fewer stations produce the same output.
• Fuel: one engine per station instead of one per carriage-train pair; station-level fuel burn drops measurably versus single-torch fleets.
• Crew: same operator count supervises higher output; training investment is front-loaded.
• Repair rate: disciplined dual-torch spreads hold rework rates low; the data-logging loop is the key driver — parameters traceability shortens defect-diagnosis time and keeps repair cell throughput high.
In practice, the productivity credit dominates on long trunk lines with high daily joint counts, while on short tie-in projects the simpler single-torch or manual option may still win. Matching the machine to the project length and diameter is the honest engineering answer.
21. Selection Checklist for Procurement Teams
Before committing to a dual-torch external welding system, verify:
1. Diameter coverage against your pipe range (e.g., Φ219–1422 mm class coverage) and track availability per diameter.
2. Wire diameter range (0.9–1.2 mm) and feeder compatibility with your qualified consumables.
3. Recipe management: touch-screen storage, upload/download, recipe version control, and parameter logging export.
4. Rack engagement design: positive pinion-rack engagement without lost motion on reversal; dust sealing appropriate to your environment.
5. Torch adjustment repeatability: indexed settings for extension, work angle, travel angle, offset.
6. Synchronization features: staggered start logic, crossover ramps, per-sector parameter tables.
7. Power source pairing: confirmed two-carriage simultaneous capacity, CV dynamic response, auxiliary power for preheat (e.g., HW1000 class with 20 kVA and two-carriage automatic external support).
8. Training and documentation: crew training program, WPS-support materials, and responsive spares supply.
9. Compliance with your governing code’s qualification requirements for dual-torch sequences.
22. Frequently Asked Questions
Q: Can dual-torch machines also run single-torch procedures?
A: Yes. Carriages in the DW-EW family can run one torch train with the other idle or removed, which is useful for repair welding, tie-ins, and small diameters where a second torch adds no time benefit.
Q: What pipe sizes benefit most from dual torches?
A: Typically from about 30 inches upward, where circumference travel time is long enough that halving it dominates the synchronization overhead. Below that, single-torch setups often remain simpler and equally fast in practice.
Q: Does the internal welder still do the root?
A: On large-diameter mainline spreads, yes — internal root welding plus external dual-torch fill/cap is the standard high-productivity train. External root variants exist for diameters where internal access is impractical.
Q: How is weld quality compared with single-torch welding?
A: With qualified procedures and the synchronization disciplines described above, dual-torch joints meet the same AUT acceptance criteria. The defect patterns differ (crossover-related indications instead of start/stop-related ones), and procedures are qualified accordingly.
Q: What maintenance skills does the crew need?
A: Mechanical (track, slides, pinions), electrical basics (cables, encoders, controllers), and parameter-management skills. Suppliers typically provide commissioning training; a designated station technician per shift is a common staffing pattern.
Conclusion: A Cadence Machine for Modern Spreads
The full-automatic pipeline dual-torch external welder is not a novelty; it is the productivity response to a simple fact of pipeline economics — external fill and cap are where the crew-hours go, and halving them restructures the whole station train. Success depends less on the machine itself than on the disciplines around it: qualified dual-torch procedures, mirror-image torch setup, sector-parameter tables, crossover control, power sources sized for two simultaneous arcs, and a data loop that turns AUT findings into recipe corrections. Contractors who build those disciplines consistently report external welding cycles compressed by roughly 40% and station counts reduced accordingly — the kind of margin that decides competitive bids on long-distance pipeline work.
Beijing Engine Welder Technology Co., Ltd. (DENVO / ENGINE WELDER) supplies DW series external welding machines — including single-torch and dual-torch configurations with touch-screen parameter management — together with HW series engine-driven power sources such as the HW1000, which supports two sets of automatic external welding with 20 kVA auxiliary output for preheating and station power.
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