Mechanical contact seals

Fhe mechanical contact seal is a very positive seal. The seal is normally oil-buffered. The mechanical seal, which is the most complex and expensive, is used where gas leakage to the atmosphere cannot be tolerated. This may be due to the cost of the gas, as in closed-loop refrigeration, or where the process gas is toxic or flammable. The mechanical contact seal requires more power than the other seals, which is a deterrent to its use on lower power compressors. 

The dry gas seal is a variation of the mechanical contact seal. It differs in that it uses a microscopically thin layer of gas to separate and lubricate the faces. The seal is configured in a tandem or double-opposed seal arrangement. More complete details are covered in Chapter 5 under Dry Gas Seals. 

Figure S-S, Schematic of a combined lube and seai system for a compressor with mechanical contact seals. (Modified courtesy of Elliott Compand...

Mechanical Face Seals This type of seal forms a running seal between flat precision-finished surfaces. It is an excellent seal against leakages. The seahng surfaces are planes perpendicular to the rotating shaft, and the forces that hold the contact faces are parallel to the shaft axis. For a seal to function properly, there are four sealing points ... 

Figure 1-8. Special mechanical seals (contact seals) used in hot gas turboexpanders. (Source GHH-Borsig.)...

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... 

In dry compressors, shaft end seals are generally one of five type.s. These are labyrinth, restrictive ring, mechanical contact, liquid film, and dry gas seal. The labyrinth type is the most simple but has the highest leakage. The labyrinth seal is generally ported at an axial point between the seals in order to use an eductor or ejector to control leakage and direct it to the suction or a suitable disposal area. Alternatively, a buffer gas is used to prevent the loss of process gas. Appendix D presents a calculation method for use with labyrinth seals. 

Because most axial compressors are in air service, most are equipped with labyrinth type end seals. There are no interstage seals in the machines with unshrouded stator blades. The balance piston seal, a labyrinth type, is the only internal seal. There is no reason that axials cannot use some of the other seals as described in Chapter 5, such as the controlled leakage or the mechanical contact type, if the gas being handled by the compressor needs a more positive seal. If there is any prob-... 

Mechanical wet-contact seal 0 High pressure, high speeds 12-54... 

Noncontact Seals Noncontact seals are used extensively in gas service in high speed rotating equipment. These seals have good mechanical reliability and minimum impact on the rotor dynamics of the system. They are not positive sealing. There are two types of non-contact seals (1) labyrinth seals and (2) ring seals. 

FIGURE 2.33 Creation of a hydrophilic PDMS channel with hydrophobic sealing, (a) The native PDMS surface with methyl groups, (b) After treatment with a 02-plasma, a hydrophilic surface with exposed OH-groups is obtained, (c) The hydrophilic PDMS surface is in mechanical contact with a native PDMS surface, (d) Rearrangement of the PDMS surface leads to its hydrophobic recovery, (e) A hydrophilic channel with hydro-phobic sealing is obtained [267], Reprinted with permission from Elsevier Science. 

Seals for pumps are generally mechanical. If the fluid contains salt crystals, which are abrasive, it is recommended that water be used to flush the seal. Both seal faces on the pump side should be of a hard material using carbon for one of the seal faces has not proven to be successful. The reader is again urged to contact seal manufacturers for their recommendation. 

A squeeze bottle is regarded as an open system. There is no valve or similar mechanism that seals acceptably and prevents contamination of the contents. The nasal tip, which is usually in contact with the nostril during actuation, naturally becomes contaminated. After releasing the pressure on the bottle, the back flow of the liquid enables bacteria to enter the system. After appropriate incubation time, contamination will occur in the container. Of course, preservatives do oppose the bioburden, but the efficacy of a preservative is limited. 

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