Boiler Level Control: Single, Two and Three Element
Feed control has to put back exactly the water the boiler sends out as steam. Watching level alone works on steady loads, but swell and shrink mislead it when load changes. Adding steam flow, and then feedwater flow, lets the control act on what is really happening.
You'll learn
- Describe single-, two- and three-element level control
- Explain why swell and shrink defeat single-element control
- Know when each is appropriate
- Understand how the three signals are combined
Before this: Load changes, Level measurement, On-off, modulating and PID
Putting back what leaves
In steady running, feedwater in equals steam out plus blowdown. The level control’s job is to keep it that way. The simplest control watches the level and opens the feed valve more when it falls, less when it rises: single-element control.
It works well on small shell boilers with large water volumes and steady loads. It struggles where loads swing, because the level itself is misleading during a swing.
The three schemes
| Scheme | Signals | Suits |
|---|---|---|
| Single element | Drum level | Small boilers, steady loads, large water volume |
| Two element | Drum level plus steam flow | Load swings; feed starts to follow steam demand at once |
| Three element | Drum level, steam flow and feedwater flow | Large water tube boilers, big swings, variable feed pressure |
In two-element control, a rise in steam flow immediately calls for more feed, so the control does not wait to see the level move — and is not fooled into cutting feed by swell. The level signal then trims the result.
Three-element control
Three-element control is a cascade: the level controller sets a feedwater flow demand from the steam flow plus a level correction; a separate flow controller then adjusts the feed valve to deliver exactly that flow, measured by a feedwater flow meter. Disturbances in feed pressure — another boiler starting, a pump changeover — are corrected by the flow loop before they ever reach the level.
- Steam and feed flow meters must be compensated and matched, or the control carries a permanent bias.
- Blowdown is a feed flow that does not appear as steam; continuous blowdown has to be allowed for in the balance.
- If a flow signal fails, the control should fall back to single-element rather than to manual.
Key takeaways
- Single element: level only; fine for small boilers on steady loads.
- Two element adds steam flow, so feed follows demand and is not fooled by swell.
- Three element adds feed flow in a cascade, rejecting feed pressure disturbances.
- Flow signals must be compensated and matched; fall back to single element on failure.