Three-element control

What it means

Three-element control is the boiler feedwater scheme that ties three signals together: drum level, steam flow and feedwater flow. Drum level trims the setpoint of a feedwater flow controller, and steam flow works as feedforward so the feedwater responds to load before the level has moved.

Each element answers a different question. Steam flow says what the boiler is being asked to do, feedwater flow says what the feedwater valve is actually doing, and drum level says whether the two are in balance. Only the last of those questions is the one a single-element scheme asks at all.

The scheme exists because drum level is unreliable during transients. When the steam demand jumps, the drum pressure drops and water flashes to steam, so the level rises at exactly the moment the feedwater should be increasing. A scheme waiting on level alone would move the wrong way, which is why the flow signals do the real work and the level only trims. The same scheme is searched as 3 element control loop, 3 element control, and three element control.

Questions that come up

What is three-element control on a boiler drum?

Drum level trims a feedwater flow controller, and steam flow feedforwards load. Level alone moves the wrong way on a swell, so the two flows do the real work.

Related

Where this comes up

• Tuning level control loops
• Averaging level control
• Boiler drum level control
• Feedforward control
• Shrink and swell