Boiler Start-up: Pre-start Checks, Warm-up and Raising Pressure
Most boiler damage that is not caused by water is caused by starting too fast. A start-up is a sequence: check, fill, purge, light, warm, vent, raise pressure, warm the main, go on line. Each step protects something, and skipping one usually shows up later as a crack, a leak or a hammer.
You'll learn
- Run through pre-start checks
- Light up and warm a boiler from cold safely
- Raise pressure and put the boiler on line
- Understand why cold starts cost fuel and stress the boiler
Before this: Water level and pressure gauges, Burners and flame safety
Pre-start checks
- Water: level in the gauge glasses at the normal start-up mark; both glasses agree; feed pump and feedwater supply ready; softener in service.
- Valves: main steam stop valve closed; blowdown valves closed; vents and drains as the procedure requires (air vent open on a cold boiler).
- Fuel: supply available and fuel system ready; for oil, at the right temperature; for solid fuel, bunker full and grate clear.
- Air and draft: fans and dampers free; flue path clear and its dampers open.
- Safety: safety valves free; pressure gauge reading zero; low-water alarm and cut-out tested; flame safety system healthy.
- People: nobody inside the furnace, flues or drums; permits closed; access doors secured.
Lighting up and raising pressure
- Purge the furnace and flues with air before any light-up.
- Light at the lowest firing rate, and keep it low. On solid fuel, build a small fire and spread it gradually.
- Leave the air vent open while the water heats; close it when steam blows freely from it, so that air is not trapped in the steam space.
- Raise pressure slowly, at the rate the manufacturer specifies, watching the water level as the water expands.
- Blow down the gauge glasses once pressure is up, and check the safety valves are free.
- Warm and drain the steam main from the boiler outward before opening the main steam stop valve.
- Open the main stop valve slowly to put the boiler on line, then let the burner modulate with load.
The water level rises on its own as the boiler warms: water expands as it heats. Start with the level at the start-up mark rather than at normal working level, and blow down a little if it climbs too high.
Why slowly, and what a cold start costs
Thick parts heat more slowly than thin ones. During a fast start the inside of a tube plate or drum is much hotter than the outside, and furnace parts heat faster than the shell around them. The resulting thermal stress, repeated at every fast start, cracks tube plates, loosens tube joints and fatigues welds. Refractory spalls for the same reason. A slow, steady rise in pressure keeps the temperature differences small.
- An illustrative shell boiler with 10 t of water and 15 t of steel, from 30 °C to working pressure, 10 kg/cm²(g) (183 °C).
- Steel specific heat 0.49 kJ/(kg·K); coal at 4,000 kcal/kg burned at 75% efficiency (indicative figures).
- Water: 10,000 kg × 651.2 kJ/kg = 6.51 GJ.
- Steel: 15,000 × 0.49 × 153 = 1.13 GJ.
- Fuel: 7.64 GJ ÷ 4.1868 ÷ 4,000 ÷ 0.75 = about 608 kg of coal.
Key takeaways
- Check water, valves, fuel, air, safety devices and people before lighting up.
- Purge, light at low fire, vent air, raise pressure slowly, warm the main, then open the stop valve slowly.
- Slow starts keep thermal stress low in thick parts and refractory.
- A cold start of an illustrative 10-tonne-water boiler costs about 608 kg of coal before any steam is used.