Balance of Plant & Stack
Everything that is not the turbine or the boiler - cooling, water treatment, fuel supply, compressed air, firefighting, electrical distribution and the stack. Individually ordinary, collectively most of the scope, and the part that decides whether the station can actually run.
Last updated 2026-08-25
Typical duration
Typically 12-20 months, beginning during the civils and often still finishing while commissioning is under way. Package count and interface management set the pace, not installed volume.
What is Balance of Plant & Stack?
Balance of plant is the name given to everything left over once the turbine-generator and the boiler have been accounted for, and the phrase does the scope no favours at all. It covers the cooling system, the raw water intake and storage, the treatment and demineralisation plant, the condensate and feedwater systems, chemical dosing and sampling, the fuel receipt and conditioning equipment, compressed air, firefighting, heating and ventilation, the auxiliary electrical distribution that powers the station's own machinery, the cabling that ties the whole plant to its control system, the pipe racks that carry all of it across the site, and the stack. On a drawing register it is the overwhelming majority of the documents. On a purchase schedule it is dozens or hundreds of separate packages. On a programme it runs longer than anything else on the project. And in a site photograph it is invisible, because none of it is the thing people came to look at.
What makes it difficult is that nobody owns it. The turbine arrives from one supplier with one set of drawings and one commissioning team behind it. The heat recovery unit arrives from another. Balance of plant arrives from everybody - a pump skid here, a treatment package there, a compressor, a set of transformers, a fire panel, a dosing unit, a crane, a set of analysers - each with its own documentation, its own delivery date, its own commissioning requirements, its own spares list and its own idea of where its responsibility stops. Turning that into working systems is the contractor's problem, and it is mostly an information problem rather than a construction one. The physical work is ordinary industrial pipework, cabling and mechanical installation done well. The difficulty is that there are hundreds of interfaces, every one of them is defined by somebody else, and a single small package arriving late or wrong will hold up the test of a system that is otherwise finished.
Two items inside the scope are large enough to shape the whole site. The first is cooling, because a thermal plant has to reject a great deal of heat and how it does that is decided very early. Cooling towers put substantial structures and a visible plume on the plot and need a continuous water supply. An air-cooled condenser needs almost no water and puts an enormous elevated steel structure beside the turbine hall instead, with fan decks, motors and very large ducting. Drawing directly from a river, an estuary or the sea needs an intake, screening plant, a pumphouse, long large-bore culverts and an outfall, and it brings environmental conditions with it that constrain when the work can be done. The second is the stack. It is the tallest thing on the site, it is governed by wind rather than weight, it carries the emissions monitoring equipment and the aviation lighting, and it is built by specialists in steel or concrete - normally while the heavy crane is already on site for the boiler, because bringing that crane back for the stack alone is not an argument anyone wins twice.
Compare the methods at a glance
When and why is Balance of Plant & Stack used?
Balance of plant starts earlier and finishes later than any other phase. Parts of it go in during the civils, because the buried pipework, culverts, duct banks and the firewater ring main have to be complete before the plot is closed off by structures. The bulk of it runs alongside the boiler and the turbine hall. And it is still being finished when commissioning is already under way, because the last small packages are always the last small packages. The sequence inside it is not driven by area but by system, and that distinction is the single most useful thing to understand about the phase. Commissioning cannot test half a system. It needs a complete one - every pump, valve, instrument, cable and control loop inside that system installed, tested and documented - so the installation is planned, tracked and handed over in system packages rather than by whichever corner of the site the gang happens to be standing in. Inside that logic, the station's own services come first. Auxiliary power, instrument air, service water, demineralised water and firefighting are what the commissioning of everything else runs on, so they are built, tested and made available ahead of the plant they will support. A project that leaves them until later finds its whole commissioning programme stalled for want of an air compressor. There is a second reason this phase deserves respect, and it lasts longer than the project. Balance of plant decides availability. A station is only worth what it can keep producing, and it will not produce anything without cooling, without treated water, without conditioned fuel and without reliable supplies to its own auxiliaries. The main machine rarely stops a power station. The small systems around it do.
Types of Balance of Plant & Stack
Explore each method in depth - benefits, limitations, plant and quality control on its own page.
Cooling system - towers, air-cooled condenser or direct cooling
The largest single decision in the package and one taken years earlier, because it shapes the site layout, the water consumption, the environmental consent and the plant's output. Wet cooling towers need a reliable water supply and produce a visible plume. An air-cooled condenser removes the water dependency and replaces it with a very large elevated steel structure, fan decks and extensive ducting. Direct cooling from a water body needs an intake, screening, a pumphouse, large-bore culverts and an outfall, and it arrives with conditions attached.
Water treatment and the water cycle
Raw water intake and storage, filtration, demineralisation, condensate polishing where the cycle needs it, chemical dosing, sampling and the effluent treatment that sends water back out again. Mostly delivered as vendor packages and skids, mostly modest in physical scale, and completely fundamental - a steam cycle runs on water of a quality the equipment suppliers specify, and everything that goes wrong with it goes wrong slowly, invisibly and expensively inside the plant.
Fuel receipt, conditioning and metering
The equipment that takes fuel from the connection point and delivers it to the machine in the condition its supplier requires - receipt, filtration and separation, conditioning, metering, and the safety and isolation arrangements around all of it. Compact but heavily instrumented, sited in a classified hazardous area with its own detection, ventilation and electrical equipment, and interfacing directly with another party at the site boundary.
The stack and emissions monitoring
A tall, slender, wind-governed structure in steel or concrete, carrying the flue from the boiler or heat recovery unit, the continuous emissions monitoring equipment and its sampling platform, aviation lighting, lightning protection and access. Built by specialists and normally erected while the heavy crane is still on site. The monitoring installation matters as much as the structure, because the site's environmental permit depends on it working and being demonstrably accurate.
Best suited for
- New build combined cycle and thermal stations, where balance of plant is the largest part of the scope
- Repowering schemes reusing some existing systems and replacing others, where the interfaces are the whole job
- Sites where the cooling arrangement shapes the layout, the consent and the plant output
- Projects committed to system-based completion and turnover rather than area-based progress reporting
- Schemes with an environmental permit whose conditions depend on the monitoring installation working properly
Balance of Plant & Stack: step by step
- 1
Step 1: Break the scope into systems and build the package register
Before anything is installed the scope is divided into the systems commissioning will eventually test, and every purchased package is allocated to one of them. The register that comes out of that exercise - package, supplier, system, delivery date, documentation status, interface owner - is the control document for the whole phase, and it is the only thing standing between a coherent programme and several hundred deliveries arriving in the order somebody happened to order them. System boundaries are drawn explicitly, including the awkward ones where a vendor package stops and site pipework starts, because that is exactly where scope disappears. Interface schedules are agreed with each supplier covering the physical connections, the electrical supplies, the control signals and the utilities each package expects to be handed. Most of the trouble in this phase is created here, months before anybody lifts a spanner, and most of it is created by an interface everyone assumed somebody else owned.
- 2
Step 2: Pipe racks, large-bore pipework and mechanical distribution
The racks go up and the distribution goes on them - steam, condensate, feedwater, cooling water, service water, compressed air, fuel and firewater running across the site between the plant items. Spools are fabricated to isometrics, delivered in a sequence that matches the erection rather than the fabrication, and joined by field welds carrying the same discipline as any other pressure joint: qualified procedures, qualified welders, examination to the regime the design states, and a record kept line by line. Supports, guides, anchors and expansion arrangements are installed to the stress engineer's drawings, because a line that is not free to move where it was designed to move will find somewhere else to move instead. Lines are cleaned and closed as they are completed, then tested by system rather than by area. Insulation, tracing and painting follow behind. It is unglamorous, high-volume work, and its quality is the difference between a plant that is tight and one that drips for thirty years.
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Step 3: Install the cooling system
Whichever arrangement was chosen, cooling is the biggest physical item in the package. Cooling towers involve a substantial structure, basins, distribution, fill, fans and drift eliminators, plus a large recirculating water system with its own chemistry and its own treatment. An air-cooled condenser is effectively a second structural steel project - a large elevated frame, fan decks, motors and gearboxes, and very large diameter ducting from the turbine exhaust that has to be supported, sealed and left free to move. Direct cooling means an intake structure, screening plant, a pumphouse, large-bore culverts and an outfall, most of it built in or beside water and much of it constrained by environmental conditions on when the work can happen at all. All three end at the same place: the system full, clean, chemically conditioned, with its pumps and fans run in and its control loops proved, well before the plant it cools needs it.
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Step 4: Water treatment, dosing and the water cycle
The water plant arrives as packages and skids, is set on its plinths, connected to the raw water supply, the distribution, the drainage and the electrical and control systems, and then handed to its suppliers to commission. The physical installation is straightforward. The commissioning is not, because water plant is slow to bring into a stable condition and it has to be stable before it can supply anything else. Sampling points, analysers and the laboratory arrangements go in alongside, because the operator will need to demonstrate water quality from the first day the cycle is filled. Chemical storage, bunding, containment, delivery points and the safety arrangements around them are designed rather than improvised, and the containment is built to the drawings the environmental permit assumed. This is one of the systems that has to be finished early, because the boiler cannot be filled and the cycle cannot be cleaned until the plant can produce water to the quality the equipment suppliers require.
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Step 5: Fuel receipt, conditioning and metering
Fuel arrives at a defined boundary, and on the far side of that boundary somebody else is responsible. The receipt station, the metering, the filtration and separation, the conditioning equipment and the isolation arrangements are installed to the equipment supplier's drawings, inside an area classified for the hazard it carries, with the detection, ventilation and electrical equipment selected for that classification. The pipework, valves and instrumentation are installed, tested and recorded to a higher standard than most of the site, and the whole installation is inspected against the hazardous area classification drawing before anything is introduced into it. Interface testing with the party on the other side of the boundary is arranged well in advance and is rarely quick, because their programme is not the project's programme. Commissioning of this system is a controlled activity in its own right, and it comes before first fire, since nothing downstream can happen until the machine can be given fuel in the condition its supplier requires.
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Step 6: Auxiliary electrical distribution and cabling
The station has to power itself, and the auxiliary electrical system is what does it - the auxiliary transformers, the switchboards, the motor control centres, the uninterruptible supplies, the battery systems, the emergency generation, and the distribution out to every motor, valve, heater and panel on the plot. Alongside it goes the cabling, which is a large, tedious and completely unforgiving scope: routes, containment, tray and ladder, glanding, termination, segregation between power and instrumentation, fire barriers where routes cross compartments, and identification of every core at both ends. Cables are worked from a register and tested and recorded as they are terminated, because a cable found to be wrong after the boards are live is a very expensive mistake to unpick. The electrical system is also among the first things commissioned, since everything else depends on it, and that pulls it forward in the programme regardless of how the rest of the site is progressing.
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Step 7: The stack, ducting and emissions monitoring
The stack is erected by specialists, in steel or concrete, and normally while the heavy crane brought in for the boiler is still available. It is a wind-governed structure, so its foundation, its anchorage and its erection sequence are all designed around dynamic behaviour rather than weight, and dampers are fitted where the design calls for them. The flue or liner, the expansion arrangements at the base and the transition duct from the boiler go in with it, and the whole gas path has to be both tight and free to move. Access, platforms, ladders, lightning protection and aviation lighting follow. The emissions monitoring installation goes in at the level the design specifies, with its sampling arrangement, its shelter, its services and its data link back to the control system, and it is calibrated and proved by specialists. That installation is what the site's environmental permit is demonstrated against, so it is treated as a compliance item rather than as another instrument on a list.
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Step 8: Small packages, testing and system-by-system turnover
The end of the phase is a long tail of small things: compressed air, ventilation, the fire detection and suppression systems, cranes and hoists, workshop equipment, sump pumps, sampling panels, eyewash stations, lifting beams, lighting and small power. Individually trivial, collectively the reason power stations finish late, because every one of them carries a delivery, an installation, a commissioning visit and a document pack. Each system is walked down against its drawings and its punch list, tested as the design requires, and then formally turned over to the commissioning team as a package - marked-up drawings, test records, vendor documentation, spares and the operating and maintenance information all together. Systems are turned over in the order commissioning needs them rather than the order they happened to be finished in. The measure of this phase is not how much was installed. It is how many complete, documented, testable systems were handed across.
Plant & equipment
- Mobile cranes and telehandlers sized for vendor packages, skids and pipe spools rather than for single heavy lifts
- Pipe fabrication and handling equipment, welding plant and controlled consumable storage
- Test pumps, calibrated gauges, blanks and temporary strainers for system testing
- Cable drum stands, winches, rollers and pulling equipment for long and heavy cable runs
- Cable test and termination equipment, with results recorded core by core against the register
- Mobile elevating work platforms, scaffold and designed access for racks, elevated plant and the stack
- Specialist stack erection plant, whether climbing formwork or a purpose-built rig, depending on the structure
- Calibration equipment for instruments, analysers and the emissions monitoring installation
Quality control & testing
- A live package register tying every purchased item to a system, an interface owner and a current document revision
- Interface schedules agreed with each supplier covering connections, supplies, signals and expected utilities
- Pipework welded to qualified procedures and examined to the regime the design specifies, recorded line by line
- Supports, guides, anchors and expansion arrangements installed to the stress engineer's drawings and checked
- Systems tested by system rather than by area, with results filed against the system they belong to
- Cable installation, segregation, termination and testing recorded core by core against the cable register
- Hazardous area installations inspected against the classification drawing before fuel is introduced
- Formal turnover packages - marked-up drawings, test records, vendor documents and spares - handed to commissioning
Safety watchpoints
- A congested site with many small trades working simultaneously in the same areas and above each other
- Work at height on pipe racks, elevated plant, cooling structures and access platforms
- Stack erection, with high-level work, wind exposure and dropped-object risk across a wide area
- Hot work on pipework in and around installed plant, under permit and with fire watch
- Confined space entry into tanks, basins, culverts, sumps and ducting
- Chemical handling, storage and first delivery at the water treatment plant
- Hazardous area working around fuel systems, with ignition control and gas detection in place
- Progressive energisation, where parts of the site become permanently live while construction continues elsewhere
Common defects to hunt
- Interfaces nobody owned, discovered when two vendor packages are asked to connect to each other
- Installation planned by area rather than by system, leaving nothing complete enough to test
- Station services left late, stalling the commissioning of everything that depends on them
- Pipework supports, guides and anchors installed wrongly, so lines move where they were never designed to
- Cable segregation and identification done badly, producing interference and untraceable cores
- Firestopping missed where cable and pipe routes cross compartments, then buried by later work
- Vendor documentation missing or superseded, leaving systems installed but not ready to hand over
- Emissions monitoring installed as an instrument rather than as the permit compliance item it is
How long does Balance of Plant & Stack take?
Typical duration: Typically 12-20 months, beginning during the civils and often still finishing while commissioning is under way. Package count and interface management set the pace, not installed volume..