Designing the sealing air system
Fan sizing, header layout, heating and the instrumentation that lets an operator trust the seal.
Sealing air is what turns a gate from a restriction into an isolation device. It is also the part of the package most often designed last, by which time the fan has been bought and the header route is fixed.
Fan sizing
Size on the worst-case gate measured at the seal, with the other gates assumed open, and add the duct losses at the actual route rather than at a nominal length. The common failure is a fan sized on total flow with no allowance for the pressure drop in a long, bendy header. If the run is more than twenty metres or has more than three bends, calculate it properly or move the fan closer.
Header layout
Feed each gate from a dedicated branch off a common header with a balance damper and a gauge at the branch. Without a gauge at the branch nobody can prove an individual gate at a later date, and proving one gate is exactly what a permit requires.
Heating
On the clean gas side the air is heated to at least 80 °C so the cavity stays above the acid dew point. The heater is interlocked so that the gate cannot be counted as an isolation device unless the heater is running. Without the interlock the heater will eventually be switched off to save power and the gate will corrode quietly.
Instrumentation
One differential transmitter per gate, trended in the DCS, with the alarm on differential rather than on flow. A flow switch is cheap and useless: it shows flow through a blocked nozzle.
Loss of sealing air
Decide in advance what happens when the fan stops. The options are to alarm only, to close the gate, or to trip the unit. Whichever is chosen, it should be written into the control philosophy and tested, because the moment the air is lost is not the moment to discover that nobody decided.
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.