Boiler & HRSG Erection
The tallest and most weld-intensive structure on the plot - a pressure system built from thousands of tubes, assembled into modules at ground level, lifted into a support frame and joined up at height, where the welding record is the product.
Last updated 2026-08-25
Typical duration
Typically 9-16 months from first steel to mechanical completion, and usually the longest single erection activity on the site. Welding volume and access set the pace, not the lifts.
What is Boiler & HRSG Erection?
On a combined cycle station the unit behind the gas turbine is a heat recovery steam generator: an unfired box that takes the turbine exhaust, runs it through banks of finned tube and turns feedwater into steam on its way to the stack. On a conventional station it is a fired boiler, larger again, with burners, water walls, air heaters and fans. Either way the thing being built is the same kind of thing - a tall structural frame with a pressure system inside it. Headers, drums, tube bundles, downcomers, risers, links and valves make up the pressure parts. Support steel, casing, ducting, insulation and access make up everything around them. It is the tallest item on the site after the stack, the heaviest concentration of steel on the plot, and normally the single longest erection duration in the programme.
Almost nobody builds a heat recovery unit tube by tube any more if they can avoid it. The supplier assembles harps, bundles and casing sections into modules in a fabrication shop, ships them as complete units, and the erection contractor sets them into the frame with a heavy crawler crane. Some modules go straight from the transport to the lift; others are landed in a pre-assembly area beside the unit, joined together at ground level and lifted as one. The logic is simple. A weld made on a level floor in a covered bay, by a welder standing up straight with a clear view of the joint, is a better weld than the same joint made at height in the wind, and it takes a fraction of the time. Every joint moved to the ground is a joint taken out of the risk. What it costs is planning. The modules are large, heavy and awkward, the lift plan drives the whole erection sequence, the crane standing and the laydown were fixed back in the civils phase, and the delivery order has to match the lift order because there is nowhere to park a module that arrived early.
What dominates the quality record is welding. Every pressure joint is a controlled item with a qualified procedure behind it, a qualified welder making it, an examination regime applied to it and a record that follows it for the life of the plant. The volume is large enough that the welding organisation - the booths, the shelters, the consumable control, the pre-heat and heat treatment arrangements, the inspectors and the tracking system - becomes a project inside the project. So does access. The scaffold in and around a heat recovery unit is one of the largest temporary works packages on the site, it is built, altered and struck in stages that have to match the erection and inspection sequence, and it frequently employs more people than the erection gang. Running quietly underneath all of it is one further discipline: the unit is designed to move as it heats and cools, so the supports, hangers, guides and expansion joints have to be installed to the supplier's arrangement and released at the stages the supplier states, or the unit will fight itself the first time it is fired.
Compare the methods at a glance
When and why is Boiler & HRSG Erection used?
The boiler or heat recovery unit starts as soon as its bases are handed over and finishes just in front of commissioning, and it is normally the longest single erection activity on the project. That puts it on the critical path from both ends. At the front it needs its foundations, its crane standing and its laydown before anything else can be sequenced, which is why so much of the civils package is built around it. At the back it has to be mechanically complete, welded out, examined, tested and closed up before the systems in front of it can be cleaned and the plant can be fired, so every week lost here lands directly on the commissioning window and on the connection date the network operator agreed years earlier. Heavy lifting adds its own tempo. A crawler crane large enough to set the modules is an expensive, long-lead item that has to be brought in, assembled, used and taken away again, and it is normally shared with the turbine hall and the stack, so the lifts are planned as one campaign rather than as a series of separate jobs. Wind decides when that campaign actually happens: a tall module presents a large sail area, the crane supplier states the conditions the lift can be made in, and a lift that has to wait three days for weather still has to have somewhere to wait. The argument for modularising as hard as the transport route allows is the same argument in a different form. Work done at ground level in a fabrication shop is quicker, safer and better inspected than the same work done at height, and the joints that stay at height are the ones the programme and the quality record will be judged on. Anything that can be finished before a module leaves the ground should be.
Types of Boiler & HRSG Erection
Explore each method in depth - benefits, limitations, plant and quality control on its own page.
Horizontal gas path heat recovery unit
The common arrangement on a combined cycle station. Turbine exhaust travels horizontally through a series of vertical tube bundles hung in line, so the unit is long and comparatively low and the modules land side by side in sequence. It suits a plot with room behind the turbine, it keeps individual lifts to a manageable height, and it gives good access down each side for erection and for the outages that follow.
Vertical gas path heat recovery unit
Exhaust travels upward through horizontal bundles, which produces a taller, narrower unit on a smaller footprint. Chosen where the plot is tight or the layout will not take a long horizontal run. The trade is height: more of the erection, welding and inspection happens well above the ground, the scaffold is larger, and the lifts reach further up.
Conventional fired boiler
A fired unit with burners, water walls, superheaters, air heaters and fans, normally hung from a heavy top-supported frame so that it can expand downwards as it heats. Much larger than a heat recovery unit in both structure and pressure parts, with a correspondingly larger field welding scope, and it brings the fuel handling, combustion air and flue gas equipment along with it.
Stick-built erection
Where the transport route, the crane availability or the plot will not take large modules, the unit is built up from small pieces on site. It removes the heavy lift and the delivery constraint and replaces them with a far larger volume of field welding, examination and scaffold at height. Slower, more weather-dependent and harder to inspect - chosen when the alternative is not available rather than because it is better.
Best suited for
- New build combined cycle stations where the heat recovery unit is the longest erection activity on the programme
- Conventional fired stations with a large field welding scope and a heavy top-supported frame
- Repowering schemes fitting a new heat recovery unit to an existing steam cycle or an existing stack
- Sites with a transport route, a crane standing and a laydown area that will take large pre-assembled modules
- Projects where the pressure-parts record has to be complete and traceable before commissioning can begin
Boiler & HRSG Erection: step by step
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Step 1: Plan the lifts, the access and the scaffold as one job
The erection sequence, the crane, the delivery schedule and the scaffold design are settled together, months before the first module arrives, because each one constrains the others. The lift plan comes first: what has to be lifted, in what order, from where, and what the crane has to be to do it. That fixes the crane, and the crane fixes the standing, the assembly area, the counterweight space and the swing paths, all of which were reserved during the civils phase and are now confirmed against the crane supplier's actual figures. Deliveries are planned backwards from the lift order, with the route from the works or the port checked for headroom, bridges, turning and overhead lines, and with a holding position for anything that arrives ahead of its slot. The scaffold is designed alongside as a permanent feature of the erection rather than an afterthought, in lifts that can be built, used, altered and struck in step with the welding and inspection sequence. Anything left out of this plan turns up later as a crane standing idle.
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Step 2: Erect the support steel and the frame
The frame goes up first, set out from the as-built base survey and checked against the steelwork model in the same way the turbine hall was. On a top-supported unit the frame is doing something unusual - it will carry the whole pressure system hung beneath it, and that system will grow downwards and sideways as it heats, so the frame, the hangers and the guides are all part of one designed arrangement rather than a stiff cage. Columns are set, plumbed and packed, bracing goes in to the sequence the temporary works design allows, and the frame is released stage by stage only as permanent bracing is completed. Because modules will be threaded into this frame afterwards, the erection sequence has to leave the openings the lift plan needs and keep the bracing that stability needs at the same time, which is a real design problem and not something to be settled on the day. Levels and lines are surveyed as the frame rises, because a module built to fine tolerances in a shop will not absorb a frame that has drifted.
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Step 3: Receive, inspect and pre-assemble the modules
Modules arrive on low-loaders or by barge and are checked in before anything else happens. Receipt inspection covers transport damage, distortion, the condition of protective covers and end caps, the preservation of the internals, and the documentation that travels with each unit - material certification, shop weld records and the supplier's inspection release. Anything wrong is raised immediately, because the answer may have to come from the works rather than from the site. Modules then move to the pre-assembly area beside the unit, are set on stools, joined into the larger assemblies the lift plan calls for, and finished as far as they usefully can be: links welded, supports fitted, insulation cleats attached, sometimes a share of the platework and access hung on before the lift. Every joint completed here is a joint that does not have to be made at height. The assemblies are then rigged, with lifting points, spreader arrangements and any temporary stiffening designed rather than improvised, and checked before the crane takes the weight.
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Step 4: Set the modules - the heavy lift campaign
The lifts are the visible part of the phase and the shortest. Each one runs to a written lift plan covering the weight, the centre of gravity, the rigging, the crane configuration, the exclusion zone, the tag lines and the conditions the crane supplier will work in, with a nominated lift supervisor in charge and everybody else out of the way. Modules are threaded into the frame, landed on their supports or hung from their hangers, and secured before the crane is released, and the temporary restraint that holds a module before its permanent supports are made up is designed as part of the plan rather than found on the day. The campaign is run as a campaign - the crane arrives, does everything it was brought for including whatever the turbine hall and the stack need, and leaves - because the cost of having it stand waiting on a delivery is enormous. Weather stops it more often than anything else, and the discipline that matters is calling the stop early rather than starting a lift the wind is about to interrupt.
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Step 5: Field welding of the pressure parts
Once the modules are in, the pressure system is joined up: module to module links, header connections, downcomers, risers, drum connections, valve stations and the interface spools out to the steam and feedwater systems. This is where the phase spends most of its hours. Each joint is made to a welding procedure qualified for that material and that configuration, by a welder qualified for the procedure, with consumables controlled and issued rather than left in a van, and with the pre-heat and any heat treatment carried out to the arrangement the design calls for and recorded as it happens. The organisation behind that - the booths, the habitats that keep wind and rain off a joint at height, the power supplies, the storage ovens, the inspectors and the tracking system that knows the status of every joint on the unit - is a substantial operation in its own right, and it is set up before the first joint is made rather than after the first joint is rejected.
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Step 6: Examination, testing and the weld record
Examination runs alongside the welding rather than trailing behind it. The design states which joints are examined, by what method and against what acceptance, and the work is done by qualified people whose reports build a record joint by joint. Repairs are tracked the same way, because a repair rate that starts to climb is telling the site something about a procedure, a consumable batch, a welder or the conditions people are working in, and it is far cheaper to learn that at joint fifty than at joint five hundred. When the system is complete it is tested to the regime the design specifies, under the control of the engineer responsible for it, with the area cleared and the test witnessed and recorded. What is handed on at the end is not simply a welded unit. It is a complete traceable record tying every pressure joint to its procedure, its welder, its examination and its test, and that record stays with the plant for its life. It is also the one thing that cannot be reconstructed afterwards if it was not built properly at the time.
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Step 7: Ducting, casing, insulation and the transition to the stack
With the pressure parts complete the gas path is closed up. Inlet ducting from the turbine, casing panels, internal baffles and liners, the expansion joints between sections that move relative to each other, and the transition duct out to the stack all go in, and the whole path has to be gas tight and free to move at the same time. Expansion joints are the item most often damaged during erection and the item that causes the most trouble later, so they are fitted late, protected while everything around them is finished, and their shipping restraints removed at the stage the supplier states rather than whenever somebody notices them. Insulation and cladding follow, and the quality of that work decides both the efficiency of the unit and the surface temperatures people will be working beside for the next thirty years. Before anything is closed and lagged the internals are inspected and cleaned out, because everything left inside a duct or a casing during erection stays there, and the first time the unit runs it will find it.
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Step 8: Supports, hangers and handover to commissioning
The last mechanical act is to set the unit up to move properly. Spring hangers, rigid supports, guides, stops and sliding arrangements are checked against the supplier's drawings, adjusted to the settings the supplier issues, and the transport and erection restraints are released in the order and at the stage they state - not before, and not forgotten. Getting this wrong is invisible on the day. It shows up the first time the unit is heated, as a support that has gone solid, a hanger sitting at the end of its travel, or a duct pulling against something it was never meant to touch. Then the unit is walked down as a system: punch listing, insulation completeness, access and handrails, drains and vents, instrument connections, and the temporary items that have to come out before it is closed. The scaffold comes down in the order the inspection sequence allows, not in the order that suits the scaffolder. What is handed to the commissioning team is a complete, tested, documented pressure system with its record in order and its supports set, ready to be cleaned and eventually fired.
Plant & equipment
- Heavy crawler cranes configured for the module weights and reaches in the lift plan, to the crane supplier's stated configuration
- Self-propelled transporters, low-loaders, stools and jacking equipment for moving, pre-assembling and setting modules
- Rigging, spreader beams, lifting points and temporary stiffening designed for each assembly rather than picked on the day
- A designed access scaffold, built, altered and struck in step with the erection and inspection sequence
- Welding plant, controlled consumable storage and issue, and pre-heat and heat treatment equipment
- Welding habitats and weather shelters that keep wind and rain off joints made at height
- Non-destructive examination equipment, with the controlled areas and exclusions some methods require
- Test pumps, calibrated gauges, blanks and instrumentation under the responsible engineer's control
Quality control & testing
- As-built base and frame surveys reconciled against the supplier's module dimensions before the first lift
- Receipt inspection of every module for transport damage, distortion, preservation and accompanying documentation
- Welding procedures qualified for the material and configuration, and welders qualified for the procedure they are using
- Consumables stored, issued and returned under control, with batch traceability maintained throughout
- Pre-heat and heat treatment carried out to the design arrangement and recorded as it happens, not afterwards
- Examination applied to the regime the design specifies, with repair rates tracked and investigated rather than absorbed
- Pressure-parts testing carried out under the responsible engineer's control, with the area cleared and results recorded
- Hanger, support and expansion settings checked against the supplier's arrangement, with restraint releases signed off
Safety watchpoints
- Working at height across the full elevation of the unit, on scaffold that is being altered around the work
- Heavy lifting of large modules with significant wind exposure, controlled by the lift plan and stopped by the weather
- Dropped objects onto trades working below, in a structure where people are stacked vertically for months
- Hot work and fire risk in a structure full of insulation, timber decking and temporary sheeting
- Welding fume, arc radiation and noise inside casing, ducting and other enclosed spaces
- Confined space entry into drums, headers, ducts and casing voids, under permit and with rescue arranged
- Radiographic examination requiring the surrounding area to be cleared and controlled, usually out of hours
- Stored energy during pressure testing, with access controlled by the engineer responsible for the test
Common defects to hunt
- Weld defects and rising repair rates absorbed as normal instead of investigated at the procedure or the conditions
- Incomplete weld records, leaving joints that cannot be traced to a procedure, a welder or an examination
- Modules distorted in transport or lifting and then forced into place rather than referred back to the supplier
- Expansion joints damaged during erection, or shipping restraints left in and only discovered at first fire
- Hangers and supports set wrongly, so the unit fights itself the first time it is heated
- Debris, tools and offcuts left inside ducts, casings and pressure parts before they are closed up
- Gas path leakage at casing joints and penetrations, costing efficiency and creating hot surfaces
- Insulation and cladding finished badly, leaving hot spots, water ingress and corrosion under the lagging
How long does Boiler & HRSG Erection take?
Typical duration: Typically 9-16 months from first steel to mechanical completion, and usually the longest single erection activity on the site. Welding volume and access set the pace, not the lifts..