Design of piston rings and packings
The sealing of the piston against the non moving cylinder wall is done usually by piston rings  touching the cylinder wall. These can be lubricated or non lubricated.
Lubricated piston rings are made of a special type of cast iron. Materials for non lubricated piston rings are : Fluor polymers (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 warrant 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.
The necessary number of piston rings influences to a large extent 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 . These consist of individual cups, which contain 1 anti extrusion ring and 2 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 .
Wear of non lubricated rings
For non lubricated compressors an estimate of the rate of wear on piston rings is given in the following equations.
The radial ring wear follows from the piston velocity and the total piston travel , which depends on the operating time, the mean radial pressure on the ring and the wear factor . 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 more or less similar conditions as found on the 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.