02-05-2018

Persisto® - unique materials specifically developed for reciprocating compressors

A variety of high-quality plastic compounds is now available for use as sealing and rider rings in reciprocating compressors. But in many cases, 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. One well-known example is the so-called Bearing Grade PEEK which, despite its disadvantages, is also used for challenging loads in a reciprocating compressor.

The requirements that an optimal dry-running material for reciprocating compressor sealing elements should meet can be described as follows:
- 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, there is no dry-running material 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.

The Persisto® family stands for materials which have been specially developed for use as sealing elements in reciprocating compressors. To this aim, Burckhardt Compression uses five reciprocating compressors to test the materials for each particular application. The Persisto® materials currently available already cover a wide variety of gases in oil-lubricated as well as in dry-running environments. Figure 1 Persisto® 850 and Persisto® 851, the youngest members of the Persisto® family, were specifically developed for use in dry-running reciprocating compressors for the compression of hydrogen and tested on a hydrogen test compressor, whereby the highly-optimized alloy comprising polytetrafluorethylene and polyphenylene sulfide is of particular significance. This alloy offers particularly favorable properties for the requisite sealing functions and, in particular, permits a higher load and a reduced creep compared to conventional filled PTFE materials. At the same time, sealing and rider rings made of Persisto® 850 and Persisto® 851 display good flexibility and correspondingly good adaptability.

If the wear compensation of sealing elements is based on shifts in ring components, rather than their elastic / plastic deformation /1/, even high differences in pressure can be realized with Persisto® 850 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. Persisto® 851 is used in the pressure range of up to 50 barg (725 psig). The wear coefficients of both Persisto® materials are significantly lower than those of conventional polymer blends or carbon and fiber filled PTFE grades. 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 (bone-dry).

With its Persisto® 850 materials, Burckhardt Compression can offer new solutions for complex sealing applications in reciprocating compressors. These optimized, custom-made compounds are distinguished by superior sealing properties without requiring the use of expensive counter body materials or coatings. Only by the exclusively produced Persisto® materials, Redura® sealing systems achieve their maximum performance for an efficient, reliable and long-lasting operation in reciprocating compressors.