Shrink and swell
Drum level moving opposite to the boiler as pressure flashes or collapses the steam bubbles. Why it forces three-element control on a drum level loop.
What it means
Shrink and swell is the name for the drum level moving in the direction opposite to what the boiler is really doing. It is a pressure effect, not a water inventory effect: change the pressure and the balance of steam bubbles in the drum changes, so the level moves before any real water has arrived or left.
Open the feedwater valve and the cooler water collapses the steam bubbles, so the level falls before it rises. Increase the firing and the extra heat swells the bubbles, so the level rises before it starts to fall. On a trend the initial move points the wrong way, and a level controller tuned to chase it responds to the swell instead of the boiler.
The practical consequence is that a boiler drum level loop is tuned for the swelling, not for the level — which is why drum level control leans on the flow signals of three-element control, and why level-only schemes are detuned and patient. A drum level loop tuned tight enough to respond to the swell is tuned wrong.
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Where this comes up
Written by PIDSnap control engineering. Published 2026-08-01, last reviewed 2026-10-22.