Sequencing and allocating load across multiple boilers
Boiler sequencing is the control strategy that decides which boilers run, in what order, and at what load, so a multi-boiler plant meets demand at the highest combined efficiency. It avoids running too many boilers part-loaded or standing by hot, matching the number and loading of units to the actual steam demand.
What it is
A plant with several boilers can satisfy a given demand in many ways — one boiler near full load, or several lightly loaded, or units held hot on standby. Sequencing is the logic that chooses the best combination as demand changes through the day, bringing units on and off and sharing load to keep each running where it is most efficient. It treats the boiler house as a fleet, not a set of independent machines.
Why it is done
Boilers lose efficiency at low fire from higher relative standing and purge losses, and a hot standby boiler bleeds heat continuously while making no steam. Running four boilers at quarter load instead of one at full load can waste a meaningful fraction of fuel. Good sequencing keeps the fewest units running at their best load points, brings lead boilers on in the right order, and shuts or banks the rest, so the plant follows demand without a fleet of half-fired boilers.
How it is done
The efficiency-versus-load curve of each boiler is characterised so the controller knows where each unit performs best. A lead-lag order is set, often rotating to equalise running hours, and load is allocated to keep running units near their efficient band rather than spreading it thinly. Start and stop thresholds are tuned with hysteresis to avoid hunting, and standby strategy — hot, warm or cold — is decided against the cost of holding pressure versus restart time. Performance is then tracked against demand to confirm the strategy holds.
- Characterise efficiency curves
- Set lead-lag order
- Allocate load to best band
- Tune start/stop thresholds
- Decide standby strategy
- Track against demand
What to watch for
Tight start/stop thresholds make boilers cycle on and off, wasting purge and warm-up fuel and stressing the units — hysteresis is essential. Holding too many boilers on hot standby bleeds heat for no output, while too few leaves no margin for a demand spike. Sequencing on header pressure alone, without load-sharing logic, can leave units stranded at inefficient part load.
Related practices
Running a compressed-air leak survey programme
Retrofitting waste-heat recovery
Retrofitting variable-speed drives
Related topics
How to Improve Boiler Efficiency: A Practical Guide · Boiler Efficiency · Flue Gas Loss · Control Loop
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