Condensate and the acid dew point
Where the acid forms, why standby is worse than running, and the details that let condensate leave instead of sitting.
Flue gas corrosion is a wet phenomenon. Dry gas at 150 °C is not aggressive to carbon steel; the same gas at 90 °C, below its acid dew point, will remove a millimetre of wall in a few years. The whole design effort is about keeping surfaces above the dew point or, where that is impossible, about letting the condensate leave.
The dew point is not fixed
It depends on the SO₃ concentration, which depends on the sulphur in the fuel, the excess air and the catalyst in the selective catalytic reduction system. A plant firing low-sulphur coal with a new SCR catalyst can have a dew point twenty degrees higher than the same plant two years later. Design for the worst case the plant can produce, not for the commissioning analysis.
Standby is the worst case
When the unit shuts down, the duct cools through the dew point band slowly and every surface gets wet. Gates that survive a year of running can corrode badly in a few weeks of standby if condensate sits in them. Drains at low points, sloped frame bottoms and a procedure for drying the duct before standby are as important as the material choice.
Sealing air and the dew point
Sealing air keeps the cavity dry only if it is above the dew point. Unheated air at 20 °C injected into a 50 °C saturated duct will cool the cavity below the dew point and make the corrosion worse, not better. That is why heating is specified, and why the heater interlock matters.
Inspection points
Mark four inspection points on each gate frame at the lowest elevation and record wall thickness at each outage. Corrosion is not uniform, and the lowest point of the lowest gate on the line is where it shows first. A four-point record over three outages predicts the fourth.
Talk to us about this duty
Send the duct section, design temperature, pressure difference and the duty point. We reply with the gate type, the material selection, the drive and a general arrangement drawing for approval.