Feedwater Quality Excursion and Drum Carryover
A 300 t/h industrial boiler with a drum pressure of 6.2 MPa operates on a makeup of 40% demineralized water and 60% condensate return. At 06:00 the condensate return begins carrying a process contaminant: cation conductivity rises from 0.15 to 1.8 µS/cm, sodium rises from 3 to 45 ppb, and the drum blowdown conductivity climbs. Drum level is steady, steam flow is steady, but superheated steam temperature has dropped 12 °C and the attemperator spray flow has increased. The plant chemist is off shift. The shift supervisor must decide whether to continue at load, increase blowdown, or trip the unit, and what immediate chemistry actions are needed.
1. Interpret the chemistry trend and connect it to the steam temperature drop and increased attemperator spray.
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The cation conductivity jump from 0.15 to 1.8 µS/cm and sodium from 3 to 45 ppb indicate a real contaminant ingress from the condensate return - likely a process leak into the condensate. The drum is concentrating these impurities, and the blowdown conductivity rise confirms it.
The superheat temperature drop and increased attemperator spray are the boiler telling you the steam is contaminated. Dissolved solids in the drum water raise the risk of carryover - mechanical and vaporous - so the steam now carries water droplets and dissolved salts into the superheater. The extra moisture lowers superheated steam temperature and forces more attemperator spray to hold the setpoint. That is the link: chemistry excursion → carryover → wetter steam → lower temperature, more spray.
The judgement is to treat the chemistry as the cause and the temperature as the symptom. Continuing to spray more to hold temperature masks the problem while salts deposit in the superheater and turbine.
2. Recommend immediate actions and the decision criteria for reducing load versus tripping.
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Immediate actions: isolate the contaminated condensate return and switch to demineralized makeup to protect the drum, accepting the makeup capacity limit. Increase continuous blowdown to lower drum conductivity, but watch drum level and feedwater demand. Sample drum water and saturated steam for sodium and conductivity to quantify carryover. Notify the chemist and call them in. Check the attemperator spray water quality - contaminated spray water adds to the problem.
Decision criteria: if steam sodium and cation conductivity can be brought within the operating limits by isolating the return and increasing blowdown, continue at reduced load. If steam cation conductivity keeps rising, or sodium in steam exceeds the limit after isolation, reduce load to lower carryover and protect the superheater and turbine. Trip if drum level cannot be maintained with the blowdown required, or if steam quality limits are exceeded and cannot be corrected - depositing salts in a superheater or turbine is far more costly than a controlled shutdown. Document the event and the samples.