Carryover and Foaming After a Water Treatment Change
A plant replaces its water treatment contractor and switches from a phosphate-based program to a polymer-only program. Two weeks later the 8,000 kg/h boiler begins to show erratic water level, and the steam is wet enough to trip the superheat temperature alarm. The gauge glass shows a foamy layer and the waterline is hard to read. Steam samples show elevated sodium and conductivity. The boiler blowdown has been reduced to save energy, and the blowdown rate is now about 2% of feedwater. The softener has been regenerating on a fixed timer regardless of throughput, and the raw water hardness has risen because of a drought. The operator proposes adding an antifoam chemical and continuing at the same blowdown.
1. Diagnose the likely causes of the carryover and decide whether antifoam plus unchanged blowdown is an acceptable response.
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Antifoam plus unchanged blowdown is not acceptable as the primary response. Antifoam treats the symptom (foam) but does not remove the dissolved and suspended solids that cause carryover, and it can mask a condition that leads to priming, water hammer, and turbine or superheater deposits.
Likely causes: (1) the softener is regenerating on a fixed timer, so with rising raw-water hardness and varying throughput, hardness can slip through and form scale and sludge; (2) blowdown at 2% may be too low for the actual feedwater quality, allowing dissolved solids and silica to concentrate; (3) the switch to a polymer-only program without confirming the polymer is appropriate for this boiler and without monitoring may have changed the sludge condition and the carryover behavior; (4) foamy glass and erratic level point to high alkalinity, high dissolved solids, or organic contamination.
Acceptable response: sample and test feedwater, boiler water, and steam; check softener effluent hardness and regenerate based on throughput or hardness breakthrough, not just a timer; verify the polymer dose and product suitability with the treatment supplier; increase blowdown to control conductivity and silica within limits; investigate the source of any organic or oil contamination. Antifoam may be used only as a temporary adjunct while the root cause is corrected.
2. What operating limits and monitoring would you set to bring the boiler back under control, and how would you verify the steam quality has recovered?
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Set limits based on the boiler manufacturer's recommendations and the applicable water-quality guidance, and control to them with recorded tests.
Operating limits to establish: boiler water conductivity, total dissolved solids, silica, alkalinity, and pH within the treatment program's targets; feedwater hardness essentially zero; dissolved oxygen controlled by deaeration and oxygen scavenger; blowdown adjusted to hold conductivity at or below the target, not fixed at 2%. Because the raw water hardness is variable, set a softener effluent hardness limit and regenerate on breakthrough or throughput.
Monitoring: test boiler water and feedwater at least daily, and steam condensate conductivity and sodium periodically. Trend the data so a rising conductivity or sodium is caught before carryover. Check the gauge glass for foam and the waterline clarity each shift.
Verification of recovery: steam sodium and conductivity should fall toward baseline, superheat temperature should stabilize, and the water level should become steady with a clear glass. Confirm by sampling after the changes have had time to circulate, and document the results. If sodium remains high, look for mechanical carryover from a high water level or a damaged separator, not just chemistry.