4.4 SEALS 

The seals at the two shaft ends, at the points where shaft comes out of the casing, are applied to avoid or minimize leakage of compressed gas or air getting into the compressor casing. This seal can be of three types: labyrinth, oil and meohanical seal.

4.4.1 Labyrinth seals

If the use of labyrinth seals is allowed, these are applied to minimize leaks outside; in this instance the gas blowing through the shaft ends is cut off by a set of blades called labyrinths.

These seals are made of light-alloy or material resistant to corrosion, with hardness lower than the shaft, to avoid damaging it in case of accidental contacts. They can be easily removed. The blade number and clearance value depend on the operating conditions. If no slight gas leakage is allowed (poisonous, explosive gases etc.) the labyrinth seals are combined with extraction or injection systems.

The labyrinth seals are made of annealed alluminium alloy (70-80 Brinell hardness); if aluminium is not compatible with gas corrosivity, stainless steels are used with 18% Cr and 8% Ni content.

There is no limitation to use other materials such as bronze etc.

4.4.2 Oil seals

The oil seals consist of two floating rings (H.P. ring on high pressure side, L.P. ring on low pressure side) babbit-lined (see pict. 4.10 showing a typical drawing of a compressor and equipped with several L.P. seal rings).

The seal oil is introduced, at a pressure slightly over the gas, into the annular space between the two rings and percolates in the gap left between rings and shaft.

Oil coming from the low pressure side goes back to the reservoir muiisqecirculated; oil from h.p. side is drained by automatic traps.

Oil- is prevented from flowing into the gas by a large labyrinth seal placed between the oil seals and compressor inside, equipped with an intermediate pressure balancing chambe

The oil seals are made up of a carbon steel support ring, coated with a thin white metal layer (generally HOYT 11 R) or copperless white metal, if not compatible with the gas handled (OMNIA)

4.4.3 Mechanical seals

The mechanical seal consists chiefly of a carbon ring, generally stationary, kept in contact with a steel collar rotating with the compressor shaft This contact is granted by the combined action of elastic elements (springs or bellows) and the distribution of pressure acting on the ring.

Heat is generated by the contact between collar and ring and must be abstracted by cooling the oil seal. The differential pressure between oil and gas must be very high (3-5 ata), to lubricate the gap between collar and ring. To keep this Ap constant a pressure chamber has to be built up, making it necessary to have a low pressure seal between pressurized oil and atmosphere.

The mechanical seals are not manufactured direcly by Nuovo Pignone, but by qother qualified manufacturers .

Each manufacturer designs seals with particular structural features: e.g. pict. 4.11 shows a Koppers seal: as you can note the elastic elements are made up of springs and the L.P. seal by a carbon floating ring.

 The mechanical seals are applied where oil contaminating gas must be limited to the most; in fact, oil leaking from the H.P. ring is about 5-10 times less than from usual oil seals.

Besides, in case of compressor shutdown for lack of oil, the seal can go on cutting off the gas with the machine at a standstill (even if not perfectly as depending on scores or wears present on the faying surfaces between collar and ring). For more details click here

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