B Moment Balance on Flapper Valve Plate

In those conflgurations shown above requiring a vent line, it is almost always necessary to install one or more block valves so as to help set the vent gas flow and for isolation purposes. If the solids are erosive, one must limit the velocity within the vent line to acceptable levels (generally under 30 m/s) so as not to erode the vent line piping. Vent line elbows, valves and any flow restriction orifice that may be present are especially vulnerable to erosive attack. Special erosion resistant, double-block valves may be necessary under highly erosive conditions. 

In this case, one would want to make certain that the bottom elevation of the second-stage cyclones (top of diplegs) is located at least 5 m above the bed surface. In practice, this design would call for a 6 m, or greater, elevation difference. 

If we perform a moment balance about the hinge rings of a typical flapper valve plate, we can quantitatively predict the effect that the flapper plate 

The first term accounts for the height the catalyst must back up above the bed in order to overcome the weight (more precisely, torque ) imposed by the flapper plate. 

The second term is the manometer effect and accounts for the difference in solids densities in the bed outside the flapper valve and that inside the valve. If there is no difference in these densities, or if the dipleg is not submerged, this term vanishes. In lightly-loaded (that is, second or third-stage) cyclones for which Eq. (11.B.8) mainly applies, the dipleg solids density is normally less than that of the bed. This results in a positive contribution to the solids backup, h. We may note that the backup increases as the submergence increases and with a decrease in dipleg solids density. 

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