TL;DR
Water level fluctuation (swell and shrink) is often caused by a faulty modulating feedwater valve, a high total dissolved solids level increasing foaming, or a worn feedwater pump. Check TDS and the level control valve response first.
What you might see
- water level gauge glass showing oscillating level
- low-water cutoff tripping on swell or shrink
- feedwater pump cycling rapidly
- water level alarm activating intermittently
Likely causes
Faulty modulating feedwater control valve hunting or sticking, over- and under-feeding water
High TDS causing priming or foaming in the boiler drum and giving a false level indication
Fluctuating steam demand causing rapid swell-and-shrink cycles that overwhelm the control
Failed or contaminated water level sensor probe causing the level controller to respond incorrectly
Required tools
- Conductivity meter for TDS measurement
- Boiler water sample container
- Multimeter for probe continuity
- Calibrated pressure gauge
Safety first
- Never adjust water level controls with the boiler under pressure unless that procedure is explicitly supported by the Cleaver-Brooks manual for the specific control type.
- A low-water condition on a fired boiler can cause a furnace explosion or tube rupture. Never bypass or disable the low-water cutoff.
Procedure
- 1
Read the boiler TDS (total dissolved solids) with a conductivity meter on a boiler water sample. Target TDS is equipment-specific; above 3000 ppm is typically too high and promotes foaming.[1]
- 2
If TDS is high, increase bottom blowdown frequency and surface skimming to bring TDS within range.
- 3
Observe the modulating feedwater valve response. It should move smoothly in response to level changes. A valve that hunts, sticks, or slams fully open or closed is faulty.[1]
- 4
Inspect the water level sensor probes (conductance probes or float-type). Remove and clean the probes. Scale or contamination on a conductance probe changes its response curve.
- 5
Test the low-water cutoff function by slowly lowering the water level to the cutoff point and confirming the boiler shuts down cleanly.
Warning: The low-water cutoff test must be performed exactly per the Cleaver-Brooks procedure. Do not perform it by bypassing safeties. - 6
If the feedwater valve is hunting, the PID controller tuning may need adjustment. Slow the integral gain to reduce oscillation without degrading response.
Sources
Cleaver-Brooks CB Firetube Boiler Operation, Service, and Parts Manual, Cleaver-Brooks
Cleaver-Brooks CB Boiler Operation, Maintenance and Parts Manual, water level control and blowdown procedures (general)
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