Carryover, High TDS and Wet Steam on a Shell Boiler
A shell boiler rated 8 t/h at 10 bar gauge supplies process steam. Makeup is softened water; 60% condensate returns. The boiler is on manual bottom blowdown twice per shift. Recent logs show boiler water TDS 3,600 mg/L (limit 3,000), total alkalinity 850 mg/L as CaCO₃ (limit 700), silica 45 mg/L, and steam sodium 1.5 mg/kg (target below 0.5). The steam header has begun water hammering, the gauge glass shows an unstable level, and the steam looks wet at the sample point. Feedwater TDS is about 160 mg/L; condensate iron is 0.8 mg/L. Production wants full load for the next 48 hours and asks you to add antifoam and continue.
1. Diagnose the likely mechanism and rank the contributing causes.
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The combination of high boiler-water TDS, high alkalinity, unstable gauge glass and sodium in steam points to mechanical carryover rather than pure steam contamination. Silica at 45 mg/L is not volatile enough at 10 bar to explain 1.5 mg/kg sodium in steam; the sodium is coming with water droplets. The high TDS and alkalinity increase surface tension and foam stability, and the shell boiler has a small steam space relative to a water-tube boiler, so foam can build and prime.
Rank the causes: first, inadequate blowdown, since TDS is above limit and blowdown is manual twice per shift; second, high water level or level control instability, which reduces steam-space height; third, high instantaneous load, which increases steam velocity and droplet entrainment; fourth, possible condensate contamination suggested by 0.8 mg/L iron, which can indicate corrosion and adds dissolved solids. Antifoam may reduce foam temporarily but does not remove the dissolved solids or the mechanical entrainment risk.
2. Calculate the blowdown needed to hold TDS at 3,000 mg/L and state the immediate operating actions.
Read the worked answer
Use a solids balance. Let feedwater TDS be 160 mg/L and boiler water limit be 3,000 mg/L. The required blowdown fraction of feedwater is:
B = feed TDS / (boiler TDS − feed TDS) = 160 / (3,000 − 160) = 5.6%.
So blowdown should be about 5.6% of feedwater flow. At 8 t/h feedwater, that is about 0.45 t/h, or roughly 7.5 L/min, on a continuous basis. The present twice-per-shift manual blowdown is unlikely to hold this consistently, especially at high load.
Immediate actions: reduce firing/load until steam is dry and the gauge glass is steady; bring the water level to the normal midpoint, not the top of the glass; increase blowdown in short controlled periods while watching level; verify feedwater and condensate quality; and take an accurate steam sample after stabilization. Do not rely on antifoam as the primary control. If sodium remains above target after TDS is back in range, investigate mechanical causes such as a high water level, a damaged separator, or a sudden load swing.
3. Why is adding antifoam alone unacceptable, and what root-cause checks should follow?
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Antifoam treats the symptom of foam stability, not the dissolved-solids load or the steam-space velocity. If TDS and alkalinity remain high, carryover can still occur as mist and spray even if visible foam disappears. The dissolved solids will also concentrate further, risking caustic embrittlement, scale and priming. Adding antifoam while continuing at full load can therefore hide a deteriorating condition and produce wet steam that damages the header, traps and process equipment.
Root-cause checks should include: a proper blowdown audit against the calculated 5.6% rate; calibration of the level transmitter and a cross-check against the gauge glass; feedwater and condensate analysis for hardness, iron and organic contamination; steam purity testing after TDS is controlled; and inspection of the steam separator or baffles if the boiler has one. If condensate iron is rising, check for corrosion in the return system and whether contaminated condensate is being returned. The boiler should not be returned to full load until steam sodium is below target and the level is stable.