Polymer Seals Perform Reliably After Years of Use

Advanced thermoplastic materials offer several advantages over aluminum and other metals for turbo-compressor labyrinth seals. Polymer seals eliminate the risk of metal tooth deformation and mating shaft damage during shaft rubs. This enables end users to tighten initial clearances and reduce clearance loss over time. Overall compressor efficiency improves greatly over the life of the seals. This article provides mechanical property data from two different sets of seals. The integrity of the seals after 11 and 15 years in service was compared with new, off-the-shelf seals of the same material.

The first set of seals, including five different seals for evaluation, was installed in 1996 and remained in service for 11 years in a natural gas compressor. Visually, the seals were in good condition, with the exception of damage suffered during the removal process. They each exhibited damage ranging from a few gouges to being completely broken. Upon removal, the in-service seals showed little signs of wear and the teeth were well-defined and in good condition. The seals did incur some damage during the removal process: Some had just a few gouges, and others were completely broken. Another seal was evaluated from a different compressor, which had been in service for 15 years. This seal also showed some gouges from removal and handling during the compressor's rebuild.

Evaluating the Seals

To evaluate the integrity of the seals, the mechanical properties of the returned seals were compared with those of standard seals. Only one seal—from the initial set, B-case, 2nd wheel—was large enough to allow full-sized tensile test bars to be machined. Tensile properties are the fingerprint of a material's integrity. Full-sized tensile bars are needed to yield a full complement of tensile properties, including strength, modulus and elongation values. In addition, other mechanical properties were evaluated, including compressive strength and modulus. For the initial set of seals, only compressive test samples could be machined and tested because of the limited sample size of the other, smaller cross-section seals. The single, 15-year seal underwent a full complement of testing, except for tensile modulus testing. Without enough material to yield a full-sized tensile bar, the modulus value could not be determined. The property values from the in-service seals were then compared to a baseline set of data. To determine the baseline values, compression-molded tubular bars were used to replicate the same production process as the returned, older samples. Two tubes were pulled from production, test plaques were machined and tested, and the results were documented.

Results

The property values of both the 11-year-old and 15-year-old compressor seals revealed consistent performance compared with standard data for the product. For the 11-year-old seals, the tensile strength and elongation properties of the B-case sample were a little lower than baseline, but the tensile modulus was higher. The lower properties were due to slight embrittlement during service. The flexural and compressive properties were higher than current production, which can also be attributed to embrittlement. The glass-transition temperature (Tg) of the 11-year-old material remained steady at 280 degrees C, at which no major degradation of the polymeric structure occurred. The compressive strength and modulus values were also tightly packed and within a 7 percent spread, showing good data integrity among the returned seals. For the 15-year-old seal, both tensile and compressive data matched closely to baseline values, including the tensile elongation. Flexural strength was less than baseline, while flexural modulus was above baseline. However, the data was within acceptable variation considering the age of the polymer. After 11 and 15 years of service in a natural gas compressor, the integrity of seals appears to be similar to current production.