Clean gas outlet and bypass gates
Saturated gas at 50 °C with chlorides on one side, full-temperature raw gas on the other. Material and seal selection for both.
The outlet gate lives in the least forgiving environment on the plant: saturated gas at about 50 °C, carrying chloride, sulphate and gypsum carry-over, and sitting at a pressure only slightly below atmospheric. Condensate forms on anything that is not kept warm, and once it forms the corrosion rate is set by the chloride concentration.
Material
We default to 316L for blades, frame and seat on the clean gas side, with the shaft in 2205 duplex because the shaft is the one part that cannot be inspected without opening the duct. Solid alloy is used on blade edges rather than a coating, because a coated edge wears through at the strip and that is where the corrosion starts. Where chloride levels are monitored above 20,000 mg/l, we quote duplex for the whole wetted surface.
Keeping it dry
Sealing air on the outlet side is heated to at least 80 °C so that the cavity stays above the dew point. The air is taken downstream of the sealing air fan and passed through a small electric heater, interlocked so the gate cannot be relied on for isolation unless the heater is running. This is the detail most often left out of a duct package and the one that decides whether the gate is still usable after five years.
Bypass gate
The bypass handles raw gas at full temperature and has to open within seconds of an absorber trip. Blade and frame are carbon steel or low alloy, the seat is metal-to-metal because a soft seat will not survive the temperature, and the actuator is pneumatic with a stored air receiver. Leakage through a closed bypass is tolerated on most plants, but it must be measured: we supply a baseline figure at commissioning so that deterioration can be tracked.
Inspection
Both gates should be inspected at every absorber outage. The items worth checking are blade edge thickness at three points, seat flatness with a straight edge, and the condition of the shaft gland. A photographic record from the first inspection makes every later one faster.
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.