Carryover After a Sudden Load Shed
Your 60,000 lb/hr packaged watertube boiler has run steadily at 45,000 lb/hr with steam purity acceptable at the superheater outlet. At 02:00 the plant loses a large process load; firing falls to low-fire and drum pressure climbs from 150 to 158 psig before the burner modulates back. Within ten minutes the operator reports the sight glass on the steam header moisture separator filling, rising superheat temperature, and a conductivity alarm on the saturated steam sample that has gone from 0.4 to 3.2 micromhos/cm. Drum level at the time of the alarm reads 3 inches above the centerline on the remote indicator, but the local gage glass shows a level that is foaming and hard to read. Feedwater conductivity is normal, and the chemical log shows the last phosphate treatment was four hours ago with a normal residual.
1. What is the most probable cause of the steam contamination, and what evidence in the scenario supports it over a chemistry upset?
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The pattern points to mechanical carryover driven by a sudden load reduction, not a chemistry excursion.
- Feedwater conductivity is normal and phosphate residual is in range, so dissolved solids in the drum did not spike.
- The event is time-locked to the load shed and pressure bump. A rapid firing reduction with a hot, high-pressure drum causes drum level swell and foaming, and the remote indicator (differential-pressure type) can read high while the actual water/steam interface is frothy and unreliable.
- Steam conductivity rose sharply while boiler water chemistry stayed stable - classic signature of water droplets being entrained rather than vaporized solids.
A chemistry upset would normally show elevated boiler water conductivity, a falling phosphate residual, or a feedwater change first. None of those are present. The local gage glass foaming confirms the mechanism.
2. List, in order, the corrective actions you would direct, and state what you would NOT do and why.
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- Reduce firing further / hold at low fire and stabilize drum pressure rather than chasing level.
- Verify level on the local gage glass, not the remote indicator, before any feedwater adjustment.
- Blow down the drum (short, controlled bottom blowdown) to lower the water/steam interface and reduce foaming, watching level closely.
- Take a fresh boiler water and steam sample after 15–20 minutes to confirm the conductivity is falling.
- Do not add feedwater to chase a high remote reading - the frothy level can be falsely high, and adding water risks a true high-level trip or wet steam to the superheater.
- Do not trip the boiler on conductivity alone if pressure and level can be held; a controlled load reduction is safer than an unnecessary trip.
Document the event and notify the chemist to check for any trace contaminant (oil, organics) that could stabilize foam.