Module 12 · Maintenance and Troubleshooting4 min read
Troubleshooting Low Efficiency and Pressure That Will Not Build
High fuel consumption and falling pressure are the two complaints every boiler house hears. Both have a short list of causes, and a few readings narrow it quickly. Work from measurements, not guesses.
You'll learn
- Diagnose low efficiency from stack readings
- Find water-side and fuel-side causes
- Diagnose pressure that will not build
- Check whether demand simply exceeds output
Before this: Efficiency Testing, Draft control and reading the stack, Boiler ratings: TPH and F&A 100 °C
BeginnerThe idea in plain words, and where you meet it. No maths.
Start with the stack
Two stack readings explain most efficiency problems: flue gas temperature and oxygen. Compare them with the commissioning or last good values.
| Reading | Likely cause | Check |
|---|---|---|
| Stack temperature up, O₂ normal | Fouled heat surfaces: soot on the gas side or scale on the water side | Clean tubes; water tests and inspection for scale |
| O₂ high | Too much excess air, or air leaking in | Combustion setting, damper, casing and duct leaks |
| O₂ low, smoke or CO | Too little air or poor mixing | Fan, damper, burner or grate condition |
| Both normal, fuel still high | Problem is outside the stack | Fuel quality, blowdown, steam losses, metering |
IntermediateThe formula, its units, and a worked example on real plant numbers.
Beyond the stack
- Fuel: wetter or lower-GCV fuel than assumed is the commonest hidden cause on biomass. Test moisture and GCV, not just the invoice.
- Unburnt fuel: high carbon in ash means fuel paid for and thrown out.
- Blowdown: continuous blowdown set far above what TDS requires.
- Steam losses after the boiler: leaks, failed-open traps, bare pipe and lost condensate all make the boiler burn more for the same production.
- Low feedwater temperature: condensate return falling off or a deaerator not working.
Worked example
How much one leak takes
Given
- A 6 mm hole in a 7 kg/cm²(g) line.
- Napier’s approximation: about 136 kg/h of steam, continuously.
Answer: Leaks rarely come alone; a survey usually finds many.
ExpertThe fine print: where the simple model breaks, and what it costs.
Pressure that will not build
If pressure falls under load, either the boiler is making less steam than it should, or the plant is drawing more than the boiler can make. Check the second first: it is common, and no amount of tuning fixes it.
Worked example
What a 5 TPH F&A boiler actually makes
Given
- A boiler rated 5,000 kg/h F&A 100 °C, running at 10.54 kg/cm²(g).
- With feedwater at 80 °C: 4,613 kg/h actual.
- With feedwater at 30 °C, after the condensate return failed: 4,250 kg/h actual.
Answer: The same boiler loses 363 kg/h of output — enough to make pressure sag at peak demand.
- Demand above capacity: new equipment added, or peaks coinciding. Meter steam flow against the boiler’s actual output.
- Firing limited: fuel feed, fan, damper or burner not reaching high fire; check the firing rate actually achieved.
- Heat transfer lost: scale or soot reducing output at full fire.
- Feed problems: feed pump unable to keep up, making the level control limit firing.
- Pressure gauge or switch fault: confirm with a calibrated gauge before chasing the rest.
Key takeaways
- Stack temperature and O₂ point to fouling, excess air or poor combustion.
- Check fuel quality, unburnt carbon, blowdown and steam-side losses next.
- Low pressure: first check whether demand exceeds actual output.
- Cold feedwater cuts a boiler’s actual output well below its F&A rating.