Labyrinthic gases

Impingement demister systems are designed to intercept liquid particles before the gas outlet. They are usually constructed from wire mesh or metal plates and liquid droplets impinge on the internal surfaces of the mist mats or plate labyrinth as the gas weaves through the system. The intercepted droplets coalesce and move downward under gravity into the liquid phase. The plate type devices or vane packs are used where the inlet stream is dirty as they are much less vulnerable to clogging than the mist mat. 

Fig. 10. The Groth ZG 5 centrifuge. R, rotor R, stationary shaft T, Teflon seal K, K, chambers for gas scoops S, S2, scoops V, gas supply M, manometer Z, Z2, tapping points for enriched and depleted gas P, P2, vacuum chambers E, electromagnet for eddy current heating Tb, Tb2, temperature measuring devices K, cooling coil and D, D2, labyrinth seals.

Rotor Seals To balance the thrust on the rotor, usually there are one or two labyrinth-type seals on the rotor. These seals often are damaged if there is dust in the incoming fluid or gas, and wear on the backside seal causes serious upsets in thrust-bearing loads. Provisions are available for coUecting and disposing of the dust which tends to accumulate in the seal so as to protect the seal from serious erosion. 

The shaft seal (see subsection Shaft Seals ) generally is a close-clearance labyrinth-type seal. It is desirable that there be available a suitable pressurized buffer gas for injection into the intermediate point in the seal, such gas to be available at an absolute pressure well above the highest shaft pressure to be sealed. Then the seal-gas system may consist of only a filter, a flow-indicating device, and a throttle... 

Changes in speed or process gas pressure can change the axial position of the turboexpander shaft. Labyrinth seal wear in the turboexpander may also cause thrust force unbalance. To decrease the load... 

Tail gas expander without or with adjustable inlet guide vanes (11) and guide vane setting motor (12), labyrinth shaft seals with air sealing... 

Nitrous gas compressor with air-sealed labyrinth shaft seals... 

Gas compressor steam cleaning facility with steam injection in the compressor suction main and in the discharge-side labyrinth shaft seal... 

Figure 4-18. Labyrinth seal for nitrous gas equipped with seal air connection.

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. 

To remove these undesirable but unavoidable salt deposits, a signi-fieant amount (about 1 % of mass flow) of boiler feedwater at 80-90°C is periodieally injeeted. Injeetion nozzles are typieally loeated in the inlet seetion, in the return ehannels, and in the labyrinth seals on the diseharge side of nitrous gas eompressors. This periodie flushing of the eompressor with water does have eertain disadvantages ... 

In the tapered labyrinth (Figure 5-5), a fixed labyrinth seals against a tapered shaft. As the shaft moves axially, the clearance of the seal changes. This allows an excess seal gas flow. The resulting venturi effect actually drags the lubricating oil into the process stream. Also, since the tapered shaft seal makes it difficult to provide an effective oil slinger, the problem is compounded. 

Damage to the labyrinth requires replacement of the expander heat barrier or compressor shaft seal. A replacement design (Figure 5-6) uses a replaceable stainless-steel labyrinth mounted on the shaft. Because the seal is not tapered, axial movement of the shaft has little or no measurable effect on seal gas flowrates. Each labyrinth has an integral oil slinger incorporated into the design. Also, each rotating... 

The labyrinth portion of the seal was designed to withstand the static and dynamic differential pressure (in the event of a major seal failure) while passing the minimum volume of purge gas. 

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... 

In this design, the inboard labyrinth of the DGS is fed with seal gas that is compatible with the process gas. The outboard labyrinth is injected with an inert gas. With this arrangement, the bearing housing is purged with inert gas, the seal gas leakage is minimized, and the mixture of seal gas and inert gas is vented to a flare or disposal system. Figure 6-21 shows a cross-section of a turboexpander with AMB and DGS. 

In large gas turbines labyrinth seals are used in statie as well as dynamie applieations. The essentially statie funetion oeeurs where the easing parts must remain unjoined to allow for differenees in thermal expansion. At this junetion loeation, the labyrinth minimizes leakage. Dynamie labyrinth applieations for both turbines and eompressors are interstage seals, shroud seals, balanee pistons, and end seals. 

A typical mechanical contact shaft seal has two major elements, as seen in Figure 13-30. These are the oil-to-pressure-gas seal and the oil-to-unconta-minated-seal-oil-drain seal or breakdown bushing. This type of seal will normally have buffering via a single ported labyrinth located inboard of the seal and a positive shutdown device, which will attempt to maintain gas... 

Gas compressors operating on highly toxic or flammable gases may require redundant systems to assure no leakages. In many applications, such as refrigeration gas, buffer seals are required with the liquid-buffered face seal. A popular technique is to use a buffered labyrinth seal with a liquid seal. 

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