For over 20 years, Mitsubishi Chemical Advanced Materials has supplied advanced thermoplastic materials to replace aluminum and other metals for use in turbo-compressor labyrinth seals. One of the advantages of polymer seals is that metal tooth deformation and mating shaft damage during shaft rubs is eliminated. This results in extreme improvement in overall compressor efficiency over the life of the seals, because initial clearances can be tightened and the loss of clearance over time is reduced. Another advantage of polymer seals is that mean time between repair (MTBR) is drastically reduced, because the seals maintain their high performance even after many years in service.
Mitsubishi Chemical Advanced Materials provided labyrinth seals to a leading OEM and tested two seal sets after they were pulled out of service after 11 and 15 years. The tests resulted in mechanical property data that shows the performance of the Duratron T4540 PAI seals (made with Torlon resin) after 11 years and 15 years in service compared to new “off-the-shelf” Duratron T4540 PAI. During their time in service, the first seal set was exposed to temperatures between 47 and 60 °C, the second to temperatures between 31 and 45 °C.
Visually, the seals were fine, with the exception of damage obtained during the removal process. They each exhibited some form of damage ranging from a few gouges to being completely broken apart.
To evaluate the integrity of the Duratron T4540 PAI seals, a few typical mechanical properties of the returned seals were compared to those of standard in-stock Duratron T4540 PAI. Only one seal, the initial set B-case, 2nd wheel, was large enough to allow full-sized tensile test bars to be machined, providing tensile properties including strength, modulus, and elongation values. Other mechanical properties were also evaluated, including compressive strength and modulus but due to the limited sample size of the other smaller cross-section seals, only compressive test samples could be machined and tested.
For the single seal in the 15-year second set, a full complement of testing was conducted except for the Tensile Modulus value that wasn’t obtained because the material couldn’t yield a full-size tensile bar.
The property values from the in-service Duratron T4540 PAI were then compared to a baseline set of data. The baseline values were obtained from recent compression molded tubular bars representing the same production process as the returned “older” samples. Two tubes were pulled from production and test plaques were machined, tested, and results documented (18” OD x 11” ID x 6” L and 12” OD x 3.5” ID x 6” L).
Table 2 below details the results of the mechanical property testing for both, the 11 year and 15 year in-service, polymer labyrinth seals.
Table 2: Mechanical property test results
(1) Mitsubishi Chemical Advanced Materials Production Sample data based on averages between 18" x 11" x 6" and 12" x 3.5" x 6" tubular bars.
(2) B-Case, 2nd wheel was the only component large enough to allow for a full-sized machined Tensile Bar, which allowed for tensile strength, elongation, and modulus data. As a result, only compressive data was generated on the other samples.
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. This can be attributed to slight embrittlement of the product in service. The flexural and compressive properties were higher than current production which can also be attributed to the same reason. The Tg or glass-transition temperature of the 11-year old material remained steady at 280°C which shows no major degradation of the polymeric structure. Also for these seals, the compressive strength and modulus values were tightly packed and within a 7% spread showing good data integrity amongst the returned seals.
For the 15-year old seal, both tensile and compressive data compared very well 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 both 11 and 15 years of service in a natural gas compressor, the integrity of the Duratron T4540 PAI seals appears to be very similar to current production.