Components

Usually, piston rings touching the cylinder wall do the sealing of the piston against the non-moving cylinder wall [17]. These can be lubricated or non-lubricated (also called dry-running).

Lubricated piston rings are made of a special type of cast iron. Materials for non-lubricated piston rings are fluoropolymers (e.g. PTFE), polymer blends (blends of various polymers) and high-temperature polymers (e.g. PEEK).

In special cases, labyrinth seals are used, which run without touching the cylinder walls. For dry-running piston rings various types of cuts were developed, which enable the smallest possible increase in leakage with wear. Especially the double overlapping (gas tight) gap and the twin ring system are of advantage in this respect. However, with brittle ring material, the design has to be such that the gap is not liable to fracture, see Table 4.10.

The necessary number of piston rings influences largely the length of the piston. In splash lubricated plunger piston compressors one or two oil scraper rings are fitted underneath the piston rings.

For the sealing of the piston rod against the cylinder, pressure packings are used. They consist of individual cups, which contain one anti-extrusion ring and two split-sealing rings. With non-lubricated service, the packings have to be cooled. With highest-pressure compressors, the pressure packing principle also finds application as seal for the piston. The current state of design of pressure packings for piston rods is described in [18].

The table below shows the element names of Figure 4.27.

For non-lubricated compressors an estimation of the rate of wear on piston rings can be calculated according to the following equations, see Table 4.12.

The radial ring wear follows from the mean radial pressure p_r on the ring, from the wear factor K and from the total piston travel L, which depends on the mean piston velocity c_m and the operating time τ. An approximation can be used for the radial pressure of the most heavily loaded first ring (with double-acting pistons the first and the last ring).

The wear factor depends on the material and the operating conditions. It can be established with so-called tribometers. These are devices, which simulate the wear process under defined conditions close to them at the real compressor.

For low wear, one should aim to achieve low running surface temperatures through low compression ratios and low piston velocities. The running surface should have low surface roughness and the gas should not be too dry.