Importance of a Good Underflow Seal

A cyclone installed to separate liquid from a carrier gas is normally equipped with a liquid drain pipe that is submerged at its bottom end in a pool of liquid. This type of seal is very similar to that shown in Fig. ll.l.le. The drain pipe must be of sufficient height above the gas-liquid interface level to overcome the suction created by the cyclone. In systems where foaming is possible, such an underflow seal must also take into consideration the decrease in liquid density brought about by foaming. 

As a rule, it is vitally important that a cyclone be operated at all times with a good underflow seal. All underflow seals in gas-cyclone systems allow some gas in-leakage, however, if the discharge pressure is greater than that which exists at the bottom of the cyclone. 

Counter-weighted flapper valves generally do not leak as much gas as fla per valves. This is because, unlike the flapper valve, the discharging solids tend to spread out around the entire perimeter of the circular opening when the valve opens (normally only slightly) and because the circular opening has a smaller perimeter compared to the elliptical flapper valve. 

If gas flow up the solids discharge opening is not adequately restricted, such upflow will reentrain some fraction of the solids attempting to make their way out the underflow. This will produce a degradation in overall separation performance as solids that otherwise would have been collected will be conveyed out the gas overflow pipe. In addition, upflow will set up a gas and solids recirculating flow pattern within the cyclone. These two effects are illustrated in Fig. 11.2.1. 

If they happen to be sticky solids, they will increase the rate of fouling of the downstream piping. 

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