Handling Load Changes on a Boiler
Process plants rarely draw steam steadily. Batches start, machines stop, shifts change. A boiler has to follow those swings while holding pressure and water level, and how well it does depends on its design, its controls and how the operators load it.
You'll learn
- Describe what happens inside a boiler when demand changes
- Understand swell and shrink
- Explain why part-load running costs efficiency
- Handle load swings without trips or carryover
Before this: Water level and pressure gauges, Start-up: checks, warm-up, raising pressure
What happens when demand jumps
When a process valve opens and more steam is drawn than the boiler is making, pressure falls. The control system sees the fall and increases firing, but fuel and air take time to change and the heat takes time to reach the water. In the meantime the boiler supplies the extra steam partly from its own stored heat: as pressure falls, some of the hot water flashes to steam.
When demand drops suddenly, the opposite happens: pressure rises, firing is cut back, and if it cannot be cut fast enough the pressure can climb toward the safety valve setting.
Swell and shrink
The water in a working boiler is full of steam bubbles. When pressure falls, more water flashes, the bubbles grow, and the water level rises — swell — even though water is leaving the boiler faster than ever. When pressure rises, the bubbles shrink and the level falls — shrink — even though the boiler holds more water than before.
A simple level control that only watches the gauge does the wrong thing on both occasions: it cuts feed during swell, just when the boiler needs more, and adds feed during shrink. On boilers with large load swings, two-element control adds steam flow so the feed can follow demand, and three-element control also measures feedwater flow. The instrumentation module covers it in detail.
Part load and how to load a boiler
A boiler loses a fairly constant amount of heat from its hot casing whatever it is producing. At low output that fixed loss is a bigger share of the fuel. UNEP puts the shell loss at about 1.5% of fuel input at full rating, rising to around 6% at a quarter load, and gives an optimum loading of about 65–85% of rating.
| Load, % of rating | Shell loss, % of input |
|---|---|
| 100% | 1.5% |
| 75% | 2.0% |
| 50% | 3.0% |
| 25% | 6.0% |
A fixed loss divided by the load fraction, from UNEP’s stated 1.5% at full rating and about 6% at quarter load.
- Stagger process start-ups instead of opening every user at once.
- Open large steam valves gradually; give the burner time to follow.
- With several boilers, run fewer of them at a good load rather than all of them lightly loaded.
- Avoid burner cycling at low load: a burner with good turndown, or a smaller boiler for the night load, saves fuel and wear.
- If load swings are large and unavoidable, a steam accumulator can store steam at peaks of supply and release it at peaks of demand.
Key takeaways
- A demand jump drops pressure; stored water flashes to supply steam while firing catches up.
- Swell raises the level as pressure falls; shrink lowers it as pressure rises.
- Part load raises the fixed shell loss as a share of fuel; aim for about 65–85% load.
- Stagger start-ups, open valves gradually, and match the number of boilers to the load.