Question

With regards to boiler water level control. Explain the following:

(a) Shrink and swell phenomenon.

(b) Cascade control.

(c) Split control.

(d) Condensing chamber - Function and location.

Answer 1

Part (a)

The rapid change in drum pressure due to load variation leads to the expanding and shrinking of steam bubbles, which is termed as shrink and swell phenomenon

Swell:

The sudden rise in steam demand may cause a fall in steam pressure and the saturation temperature. Due to this, the water temperature at this moment may become higher than the saturation temperature.
The drop in saturation temperature will cause the formation of bubbles and will raise the boiler water level, which is termed as ‘swell effect’
The control system will shut the feed water control valve due to the swell effect when actually the amount of water has decreased. So water level may further decrease.

Shrink:

When the steam supply becomes normal, the saturation temperature rises, and the formation of steam bubbles drop
This will drop the water level in the drum, and the control system will open the feed water control valve.
Due to the introduction of cold water, steam bubbles will collapse, causing a further drop in water level, which is termed as the ‘shrink effect’
If the feed controller is unable to sense the phenomenon, there could be too high water level.

Part (b)

Cascade control is a two-level control system where the output of one controller becomes the target (setpoint) for a second controller, enhancing response accuracy. In boiler water level control, this technique helps counter the effects of shrink and swell by stabilizing feed water fluctuations. The primary controller monitors the main boiler water level, and a secondary controller tracks variations in feed water flow rate to adjust for changes in feed water supply pressure. This layered approach ensures precise feed water control, even when the system experiences large feed water pressure fluctuations, minimizing false indications and maintaining consistent boiler water levels.

Part (c)

Split control is applied when multiple control elements need to handle varying input ranges but produce a single output. For example, in boiler feed water systems, two feed water valves—a smaller start-up valve and a larger main valve—are controlled by a single controller.

In split control, two conditions are generally managed:

Start-up valve fully open at lower loads to handle minimal flow requirements.
Start-up valve closed at higher loads, with the main valve fully handling the feed water supply.

In split control, a single controller is used for more than one final control element making the control process more effective and at low cost.

Part (d)

Condensing chamber – Function and location

A condensing chamber (or condensing pot) is used in boiler drum level measurement systems with differential pressure transmitters to improve accuracy at high pressure and temperature. The chamber cools and condenses steam into water within the measuring line so that the differential pressure transmitter senses hydrostatic pressure of water only (excluding steam pressure variations). It is located at the end of the impulse lines connected to the boiler drum, usually near the transmitter. The condensing chamber stabilizes the measurement by preventing steam from entering the impulse line and causing measurement errors due to temperature and density changes.

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