Underflow sealing

In this chapter we shall examine a number of issues pertaining to underflow design, including some commonly used geometries, the problem of gas leakage into the bottom of the cyclone, evidence of an improper underflow seal, means of eliminating underflow related problems and how to prevent them in the first place. 

Figure 11.1.1 illustrates six common industrial underflow or seal configurations. Most industrial cyclone systems utilize one of these seal designs or some variant thereof. The primary purpose of these underflow sealing devices is to isolate, more or less, the cyclone from the conditions that exist downstream of the sealing device. Without such a seal, either gas would blow out the bottom of the cyclone or it would flow up the cyclone. Neither of these conditions is normally desirable although a controlled amount of gas blowdown may be specified in some instances for solids conveying purposes. 

Fig. 11.1.1. Conventional cyclone featming six common types of underflow seal configurations ...

Fig. 11.1.4. Flapper-type underflow sealing valves featuring free-swinging hinged seal plates. Ducon type CA Trickle Valve (left) and type FA Trickle Valve (right). Courtesy Ducon Technologies Inc.

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. 

As mentioned above, a vapor-liquid cyclone of the conventional reverse-flow variety must be designed to handle liquid films attempting to make their way out the vortex tube (i.e., layer losses ). Additionally, the cyclone must be designed so that the vortex tail (the end of the vortex) is isolated or decoupled from any liquid that is allowed to collect in the lower section of the separator or from the liquid already flowing down the walls. See, for example. Fig. 13.1.1. Furthermore, proper underflow sealing is just as important with vapor-liquid cyclones as it is for gas-sohds cyclones. 

In order to facilitate replacement and/or cleaning of the individual units, the cyclones are sealed into the tubesheets with rubber gaskets and secured with bolts so that they can be individually removed if necessary. A further variation of this design is possible in using four tubesheets and two sets of hydrocyclones, with either the underflows or the overflows opening into a common space inside the vessel. 

If the pressure drop across the distributor is increased so that the product of (AP/Lg)njf (the maximum AP/Lg possible in the standpipe) and the standpipe length, /fgp, is less than the sum of AP and AP, then the underflow standpipe will not seal. 

The rotary lock, star or feeder valve and the screw conveying discharge device, shown in Figs. 11.1.1a and 11.1.1b and 11.1.2, are two devices often used to seal against high differential pressures existing between the cyclone and the underflow. These devices are very suitable for some cyclone applications such as the flour mill application shown in Fig. 11.1.3. However, rotary valves do require motors that render them unsuitable for use inside of vessels... 

Flapper valves are used to seal both high and low solids-loaded cyclone underflows although the valve type shown on the left hand side of Fig. 11.1.4 is more commonly used with lightly-loaded, second-stage cyclones operating in a fluid bed environment. 

The cyclone underflows are not isolated (sealed) from one another. 

Obviously, the same sort of analysis and observations apply to any number of cyclone pairs whose underflows discharge into a common liquid seal pool. One just needs to work through the detailed computations on a case-by-case basis along the lines presented above. 

Before closing this discussion we wish to call attention to the fact that the liquid seal at the bottom of each cyclone causes their performance to be rather unaffected by flow imbalances of the type we just observed. This would not be the case had the cyclone underflows not been isolated from one another. In this case, any pressme imbalance that would exist between the cyclone underflows will cause gas to flow down some of the underflow openings and up other underflow openings. This, in turn can lead to a serious degradation of separation performance and is the reason why most multiclone systems do not perform as well as one of its individual cyclones tested in isolation. 

Fig. 14.4.1. A cyclonic type separator with liquid sealed underflow...

---