Absolute tightness In practice absolute tightness can never be achieved, and there will always be some degree of leakage. However, something approaching zero leakage can be obtained through welding. The type of assembly shown in Figure 8.11 is difficult to disassemble and requires the welds at the periphery of the thin plates to be broken. Shaft seals
It is possible to achieve a virtually leak proof fan by employing a direct driven fan having a flanged end shield motor. Even if gas escapes through the seal at the shaft extension, it is still contained within a totally enclosed motor housing. This should be naturally cooled and there is then no shaft seal at the non drive end.
Figure 8.11 Welded flange with added plates
The bolting together of two surfaces such as flange faces, only provides a limited tightness even when the flanges have a high degree of surface finish and the bolts are “torqued-up” to a significant value.
Sealing without joints
In certain cases, it is possible to achieve a reasonable degree of gas tightness by using a knife edge plane contact as shown in Figure 8.12. This design requires that the geometry of the contact surfaces is very good and that the surface roughness is minimal.
Other methods may also be used for fans in the gas industry, see Figure 8.13, which shows a fan arranged with shaft seals and drive through a coupling.
Figure 8.13 Direct driven leak proof fan for the gas industry
The shaft closing washer described in Section 8.3.3 as part of the spark minimising features may also be used as a simple seal. Provided it is made from a soft brass or similar, the hole can be of exactly the same diameter as the shaft. It will easily “run in” without causing any damage. Provided the ratio of critical speed to running speed is high, the shaft deflection is low and the balance grade better than G 6.3 (preferably G 2.5), elongation of the hole will be minimal.
Figure 8.12 Knife edge plane contact
The example shown has a knife edge in contact with a plane surface. One of the two pieces should preferably be much more ductile than the other. This type of assembly should be restricted to parts less that about 100 mm for the maximum dimension.
A box is filled with a soft packing, such as greased rope. This packing is compressed against the shaft by a gland. The gland is usually split as illustrated in Figure 8.14 and held in place by swivel bolts.
The gland tightness is critical — too tight and heat will be generated. There will also be a frictional power loss. If insufficiently
With a sealing gasket, a high level of gas tightness can be achieved with less than perfect surface quality even on larger areas. The gasket material must have good elasticity, plasticity
And low permeability. It must also have good resistance to the Figure 8.14 Components of split stuffing box and gland
Tightened there will be considerable leakage. Maintenance is therefore greater than for other types.
Figure 8.18 Section through a mechanical seal
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These are most commonly used and many variants exist. All however require a polished shaft, see Figure 8.15. The labyrinth ring is in two parts, typically stainless steel or PTFE.
Carbon ring in 2 or 3 parts