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Plastic compound materials developed for reciprocating compressors

A variety of plastic compounds are available for use as sealing and rider rings in reciprocating compressors; however, these materials have not been developed specifically for use in a reciprocating compressor.

05/30/2018


Figure 1: Persisto 850, shown in a heterogeneous combination of rings within a Redura Rod Sealing System. Courtesy: Burckhardt CompressionA variety of plastic compounds are available for use as sealing and rider rings in reciprocating compressors. In many cases, however, these materials have not been developed specifically for use in a reciprocating compressor. For reasons of cost effectiveness, they are optimized for as wide a range of applications as possible, e.g., in the automobile and the chemicals industries. The requirements for a dry-running material for reciprocating compressor sealing elements include:

  • Favorable tribological properties ensuring minimal friction and low wear
  • High degree of chemical resistance for use with as many different gases as possible
  • Good mechanical properties to withstand maximum load parameters
  • Good shape adaptability for optimal sealing performance
  • Minimum abrasiveness to avoid counter body wear.

To date, this general requirement profile is not fully satisfied by any dry-running material. In particular, no dry-running material is available that exhibits the same good wear properties in every gas and gas mixture environment. In an effort to improve service life, the dry-running materials often contain a significant proportion of abrasive fillers such as glass fibers or ceramic with the correspondingly negative consequences for soft counter body materials such as grey cast iron.

Many high-temperature polymers with good mechanical properties display a high elastic modulus. This leads to rigid sealing rings or sealing ring segments, which, in turn, display very poor adaptability. The latter has a particularly negative impact on oil-free compression of light gases at high pressures.

To this aim, Burckhardt Compression uses five reciprocating compressors to test the materials for each application. The Persisto materials currently cover a wide variety of gases in oil-lubricated as well as in dry-running environments. Figure 1 shows two compounds specifically developed for use in dry-running reciprocating compressors for the compression of hydrogen and tested on a hydrogen test compressor, whereby the alloy comprising polytetrafluorethylene (PTFE) and polyphenylene sulfide (PPS) is of particular significance. This alloy offers favorable properties for the requisite sealing functions and permits a higher load and a reduced creep compared to conventional filled PTFE materials.

Figure 2: Wear coefficients for Persisto 850 (red), Persisto 851 (grey) and typically used hydrogen sealing materials (blue). Courtesy: Buckhardt CompressionIf the wear compensation of sealing elements is based on shifts in ring components, rather than their elastic / plastic deformation, even high differences in pressure can be realized without failure by fracture. Discharge pressures of more than 300 barg (4350 psig) can be sealed in oil-free compression of hydrogen. In addition, the permissible pressure load for dry-running packings with a pressure difference of more than 200 barg (2900 psig) is remarkable. Figure 2 shows this using the example of a piston rod sealing in hydrogen with a discharge pressure of 80 barg (1160 psig).

Persisto 850 can be used in a wide range of gases and gas mixtures but is not recommended for compressing oxygen-containing gases. Apart from hydrogen, this new development is predestined for applications with various hydrocarbons (incl. boil-off gas), carbon monoxide, carbon dioxide or ammonia. Thanks to its special composition, the atmospheric dew point of the gases can be significantly below -70°C.

The Persisto materials allow Burckhardt Compression to offer solutions for complex sealing applications in reciprocating compressors without requiring the use of expensive counter body materials or coatings.

- Edited by CFE Media.



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