Selecting and sizing the three gates
Inlet, outlet and bypass are three different duties. How to size each one and what figures the quotation actually depends on.
Treating the three FGD gate positions as one specification is the most expensive shortcut in a duct package. They sit in the same duct system but they see different gas, different pressures and different duty cycles.
Raw gas inlet
Size on the duct section and the design pressure difference, then add a margin for ash on the seat. A single louvre gate gives about 90 % closure and will not hold a maintenance isolation once ash has settled. We recommend a double louvre with sealing air, or a guillotine gate where the plate can be withdrawn completely. The inlet is a wear duty, so replaceable blade edge strips are worth specifying from the start.
Clean gas outlet
The outlet is a corrosion duty at 50 °C saturated gas. Size the same way, but the material selection changes: 316L for wetted surfaces, duplex for the shaft, solid alloy for the blade edges. The seat should be inspectable without dismantling the duct, because the outlet is the gate most likely to need attention.
Bypass
The bypass is sized by the opening time, not by the pressure. Start from the volume of gas that has to pass and the time available, then work back to a gate area that will pass it at the available differential. A bypass that is fast but undersized does not protect the absorber.
What the quotation depends on
Five figures decide the price and the design: duct section, design temperature, design pressure difference, sealing air pressure and temperature available, and the required bypass opening time. Without those five we can only quote a range, and a range is what causes a project to buy the wrong gate.
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.