Boiler Water Carryover and Superheater Deposit After Softener Failure
A low-pressure fire-tube boiler supplies steam to a process and to a small turbine. The zeolite softener has been regenerating poorly for two weeks because the brine educator is partly plugged, and the boiler water total dissolved solids have risen from about 2,500 to 5,500 mg/L despite blowdown being increased. The operator reports priming and carryover: steam is wet, the steam header traps discharge continuously, and the turbine has lost power. The boiler water alkalinity is high and the chloride level has more than doubled. The feedwater is softened but not demineralized, and there is no continuous blowdown on this unit, only manual bottom blowdown.
1. Explain the chain from softener failure to turbine power loss, and distinguish the roles of TDS, alkalinity, and carryover.
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The plugged brine educator means the softener is not fully regenerated, so hardness leaks through and the softener's capacity falls. Hardness in the feedwater precipitates in the boiler as scale and sludge, and the rising TDS (2,500 to 5,500 mg/L) reflects both the reduced softener performance and the fact that manual bottom blowdown cannot control dissolved solids as effectively as continuous surface blowdown. High TDS and high alkalinity raise the foaming tendency of the boiler water, so steam bubbles are stabilized and carryover occurs: droplets of boiler water leave with the steam.
Carryover carries dissolved solids and alkalinity into the superheater and turbine. As the steam expands and cools in the turbine, dissolved salts precipitate on blades, roughening them and altering the flow path, which reduces power. Wet steam and continuous trap discharge confirm carryover rather than a pure steam-demand problem. Alkalinity contributes to foaming and to caustic attack risk; TDS is the primary driver of foaming here. The high chloride confirms that boiler water, not clean condensate, is reaching the steam side.
2. Give the corrective actions in priority order and the control changes to prevent recurrence.
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Priority: 1) Repair or replace the brine educator and verify softener regeneration with hardness tests on the softened water; do not rely on the softener until hardness is confirmed low. 2) Reduce boiler water TDS by blowing down and, if necessary, dumping and refilling with soft water; target TDS well below the point where foaming begins. 3) Inspect and clean the superheater and turbine; deposits must be removed before the turbine is returned to full load. 4) Check for scale/sludge in the boiler and clean as needed; verify the low-water cutoff and other safety devices still function.
Control changes: install continuous surface blowdown with a conductivity-based blowdown controller, because manual bottom blowdown alone cannot hold TDS on this unit. Add routine boiler water testing (TDS, alkalinity, chloride, hardness) with logged limits and alarms. Add a softener performance check to the daily routine and a regeneration verification step. Investigate whether an oxygen scavenger/alkalinity control program is needed, and document the event.