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.
FGD Inlet & Outlet Isolation
A matched set of raw gas, clean gas and bypass gates that lets the absorber be opened while the unit stays on load.
The problem: the absorber has to be entered for inspection while the boiler keeps running, and any leakage past the inlet or outlet gate will put personnel at risk and corrode the duct.
Our approach is to treat the three gate positions as one system. The raw gas inlet gate is a double-louvre unit with a sealing air system at a pressure above the flue gas pressure. The clean gas outlet gate is the same construction but with wetted surfaces selected for the 50 °C saturated chloride environment. The bypass gate is a quick-opening unit that brings the flue back on line within two seconds if the absorber trips.
All three gates share one sealing air fan set, one actuator type and one control philosophy, with differential pressure monitoring on the sealing air so that the operator can see the seal is holding before a permit is issued.
The sealing air system is the solution, not an accessory
Most FGD isolation problems trace back to the sealing air, not to the gate. The fan is sized on the sum of the three gates plus a margin, then the ductwork is routed with long runs and several bends, and by the time the air reaches the last gate the pressure is not enough to hold the seal. We size the fan on the worst-case gate, measured at the seal, with the other gates assumed open.
Differential pressure, not flow
The instrument that matters is a differential transmitter across the seal cavity at each gate, wired to the DCS and trended. Flow is a poor indicator: a partly blocked nozzle still shows flow while the differential collapses. Operators should be able to see the differential at each gate before a permit is issued, and the alarm should be set on differential only.
Bypass response
The bypass gate has to protect the absorber when the plant trips. Required opening times between one and three seconds are typical. That is a pneumatic duty with a stored air receiver and a quick exhaust, and the receiver has to be sized for the volume of the actuator at the minimum plant air pressure, not the normal pressure.
Commissioning sequence
Prove the sealing air first, then the interlocks, then the gates. A gate proved before the air system is an invitation to run it dry. We supply a commissioning sheet listing the sequence, the acceptance figures and the record sheets for each gate, because the inspection body will ask for them.
Talk to us about this application
Send the duct section, design temperature and pressure difference, and the duty point. We will propose the gate type, the material selection and the actuation.
How we deliver it
The steps behind this solution
What we need from you, what we check, and what the finished installation has to prove before handover.
Installation and commissioning
Turning a delivered gate into a working seal: alignment, sealing air proving, interlock testing and the records to keep.