Compression packings have suffered from a reputation of being an old-fashioned technology unsuited to modern industrial processes. In the case of rotating equipment, they are largely superseded by mechanical seals. In particular, many believe packings are inefficient because of high frictional losses. Much of this perception is based on outdated products and not on modern types that use sophisticated synthetic yarns combined with complex lubricants.
This article describes the development of a straightforward test procedure for compression packings used in rotary applications. The procedure was used to study the frictional characteristics of several packing types in comparison with various mechanical seals using a test rig specifically designed for the purpose. The results from the friction testing on a number of packing types and mechanical seals will also be discussed. These results call into question the theoretical methods currently used to calculate packing friction.
Test Procedure Development
As early as 2004, the European Sealing Association (ESA) along with its U.S. counterpart, the Fluid Sealing Association (FSA), formed a joint task force to develop a realistic, performance-based test method for compression packings used in rotary applications. The driving force for this project was to enable manufacturers to publish true comparative data on packing performance and allow end users to better differentiate between products when making selections for their applications.
The specification was developed through a number of iterations. At each stage, the validity, accuracy and repeatability were tested using "round-robin" tests. Each member company tested the same product from a single source, and the results were compared. Any deviations from consistency were discussed and the specification refined for the next validation round. To maintain impartiality, all of the test results were submitted to an independent body for analysis—French research organization Centre Technique des Industries M\'e9caniques (CETIM), who also carried out their own tests in each round. Figure 1 shows a typical test setup.
The first drafts of the specification allowed test conditions that reflected those commonly encountered in field applications but with water as the test medium. The following parameters were to be measured and recorded at specified intervals during each test run after the break-in period and at the end of the test:
- Total leakage (milliliters)
- Leak rate (milliliters per hour)
- Gland temperature (degrees Celsius)
- Number of gland adjustments
- Amount of each adjustment (millimeters)
- Normalized power consumption (watts per millimeter squared)
Leakage from the static outer side (gland) and the dynamic inner side (shaft) was recorded separately.
For the first series of tests, the packing selected was one of known good performance and of material and construction typically used by all of the participating manufacturers. A graphite/expanded polytetrafluoroethylene (ePTFE) cross-plaited packing was selected, and test packings were manufactured by one manufacturer from the same batch of yarn to suit each of the participants test rigs.
The general trends from these early tests provided composite results for 12 tests at six test facilities under the same conditions of 6 bar pressure for 100 hours at different speeds. While consistency within each individual laboratory was satisfactory, the variation between them was substantial. The specification was, therefore, refined to better control the test conditions and procedures, and the importance of the initial fitting of the packing and the break-in period was emphasized.
Three leakage classes were introduced to allow for differing target leakage levels depending on the criticality of the intended application area of the packing.
- L1 = less than or equal to 5 milliliters per minute (ml/min)
- L2 = less than or equal to 15 ml/min
- L3 = less than or equal to 30 ml/min
Gradually, other packings were tested and eventually a final specification was reached. Figure 2 shows results from testing a graphite/ePTFE packing under the final specification conditions, with good repeatability of results.
The final specification was issued and is freely available to download from the FSA website. The specification was also put forward to CEN Technical Committee TC 197 – 'Pumps' to be adopted as a full European Standard. This was approved, and TC 197/WG 3 has prepared a Final Draft EN 16752 Centrifugal pumps- Test procedure for seal packings, which is currently going through the standardization approval process and should see final publication in 2015.
While the final test procedure produced good correlation of results in terms of packing leakage, temperature and post-test packing condition, the one performance aspect that continued to cause debate was frictional level and power consumption. Throughout the round-robin test program the results reported for frictional torque or absorbed power showed significant variability, partly because of the different methods used to measure it.
This uncertainty about packing friction is concerning, because the generally accepted wisdom is that packings are inefficient in terms of power consumption. But little research has been conducted on the more sophisticated products currently available that use exfoliated graphite, ePTFE, aramid and other synthetic yarns and modern lubricant systems.