Gas seals

At the top of the furnace, the raw materials are charged into the furnace through a pressurized gas seal system, typically a double beU (see Fig. 6a). For very high pressure furnaces, three bells may be used. More recently, the Paul Wurth beU-less top has become popular, as its rotating chute design permits greater control over distribution of the burden (Fig. 6b). 

Fig. 6. Pressurized gas seal system at top of blast furnace (a) two-beU top and (b) beU-less top.

Three examples of ordinaiy gas seals are shown in Fig. 12-61. On direct rotary dryers, few gas seals are intended to be completely gastight, but by careful control of the internal pressure, generally Between 0.25 and 2.5 mm of water below atmosphere, dusting to the outside is prevented and in-leakage of outside air is minimized. 

FIG. 12-61 Rotary-dryer components, a) Alternative rotary gas seals, (h) Alternative trunnion roll bearings, (c) Thrust roll assembly. (ABB Raymond/Battlett-Snow TM.)... 

Because indirect-heat calciners frequently require close-fitting gas seals, it is customaiy to support aU parts on a selFcontained frame, for sizes up to approximately 2 m in diameter. The furnace can employ elec tric heating elements or oil and/or gas burners as the heat source for the process. The hardware would be zoned down the length of the furnace to match the heat requirements of the process. Process control is normaUy by shell temperature, measured by thermocouples or radiation pyrometers. When a special gas atmosphere must be maintained inside the cyhnder, positive rotaiy gas se s, with one or more pressurized and purged annular chambers, are employed. The diaphragm-type seal ABB Raymond (Bartlett-Snow TM) is suitable for pressures up to 5 cm of water, with no detectable leakage. 

To this day, balanced mechanical seals using o-rings are the standard in industry. Meehanical seals eontinue to evolve in sealing face technology, computer design, finite element analysis, cartridge designs, split seals, double or dual seals, and dry gas seals. 

The disbonding rate decreases with time [35], which can be attributed to the consumption of OH" ions by reaction with adhesive groups. This consumption is obviously partly compensated for by the formation of OH" ions through oxygen reduction these permeate inward from the outer surface of the coating. If this permeation is hindered by an aluminum foil gas seal, the disbonding rate falls off... 

Dry gas seals, when eeonomie eonsiderations allow, are another alternative for redueing gas leakage. Dry faee-seal systems have been used sueeessfully and field experienee has proven their effeetiveness. 

Kenneth, E. A. and Robert, X. P, Influenee of Gas Seals on Rotor Stability of a High Speed Hydrogen Reeyele Compressor, 17th Turbomaehinery Symposium, Texas A M University, 1982. 

Two or four lobe bearings are used in these expanders, determined by the funetion of the rotor-dynamie requirements. A double-aeting thrust bearing may be mounted, if required. Labyrinth seals with eonneetions for seal gas are standard equipment, however, dry gas seals are available. 

CASE STUDIES ON ACTIVE MAGNETIC BEARINGS (AMB) AND DRY GAS SEALS (DGS) ... 

The second option, dry face or gas seals, was developed in the early 1970s. In dry face seals, the hydrodynamic pressure that develops between the rotating and stationary seal faces produces positive sealing with minimal seal gas leakage. 

Active Magnetic Bearings (AMB) and Dry Gas Seals (DGS) in Hydrocarbon Processing... 

In this case, details of turboexpander design with dry gas seals and active magnetic bearings are presented in enough detail that the application to this specific process is clarified. 

Turboexpanders with Dry Gas Seals. As mentioned above, the application of dry gas seals in centrifugal compressors dates back to the early 1980s. They are useful in process compressors because seal gas leakage is considerably reduced. Also, the contact of process gas and other constituents that may dilute lubricating oil is minimized. 

The first documented use of dry gas seals in turboexpanders was in 1989. At that time, an ethylene plant in Scotland worked with a dry gas seal manufacturer to retrofit a turboexpander-integral gear-generator package. The partial success of that project was sufficiently encouraging for both user plant and turboexpander manufacturer to undertake a redesign of all three expander stages. Dry face or gas seals were installed at that time. 

This successful experience led to the design, manufacture, and use of turboexpanders with dry gas seals. Table 6-2 lists the installations and major design parameters of dry gas seals. 

Turboexpander manufacturers and dry gas seal vendors recognizing the above limitation, worked together to develop a dry gas seal design that is an excellent compromise given the space limitation and customer requirement. Figure 6-16 shows a cross-section of the dry gas seal developed for turboexpanders. 

The turboexpander dry gas seal consists of the conventional dry gas seal mating ring and primary ring, an outboard labyrinth, an inboard labyrinth, and tlie cavity to be vented, if desired. Tlie outboard labyrinth reduces warm seal gas leakage to the process side efficiency deterioration is thus minimized. The inboard labyrinth, on one hand, provides an additional seal between the process and lubricating fluids. On the other hand, it allows injection of an inert gas, if desired. In the latter case, inert gas leaks to the bearing side and to the cavity between the... 

Clair, M., Application of Magnetic Bearings and Dry Gas Seals to an Overhung Compressor, Revolve Proceedings, April 1992. 

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