by Henri Azibert (FSA Technical Director)
August 18, 2015

Recognized approximations in the packing formula are that it does not take account of lubricant levels, actual packing compression, type of liquid sealed, viscosity or temperature. But it can provide a figure for the amount of energy consumed by the packing. It tends to give power consumption levels that are approximately 10 times that of a balanced mechanical seal used under the same conditions. Test results show that the approximations in the formula are not sufficient to explain the deviations from the calculated values.

The differences in calculated results from the test measurements reported here vary by factors from 25 to 100 times.

While more work is planned, the conventional wisdom contained assumptions that are not verified through the experiments. Thus, the use of sealed pressure as the contact pressure for the packing along its entire axial length must be revised. A pressure drop coefficient of 0.2 gives much better correlation of calculated to testing results.

The coefficient of friction must also be re-evaluated when current advanced synthetic fiber materials are used.

For example, a coefficient of friction value of 0.03 for ePTFE/Graphite packing is more in agreement with testing results than the traditional value of 0.17. Other variables must also be considered, such as shaft speed and size as well as leakage levels because they have a direct impact on power consumption.

Further Work

Some further test work is planned on other packing types. The major thrust of this work is to develop a mathematical model that will provide an accurate tool for the calculation of packing power consumption. A revised formula will be finalized once testing is completed.

Initial friction tests for mechanical seal and graphite/ePTFE packingFigure 3a. Initial friction tests for mechanical seal and graphite/ePTFE packing

The unquestioned switch from compression packing to mechanical seals to save energy in sealing systems must be reconsidered. Users must take many factors into account when using one technology versus the other, including periodic maintenance, the availability of trained maintenance personnel and permissible leakage levels. But frictional energy saving is not as important as conventionally viewed. The choice of which technology to use must encompass all aspects of performance based on real results rather than perception.

Tests at 6 barFigure 3b. Tests at 6 bar

Acknowledgements

The author would like to express his appreciation to all of the ESA and FSA member companies involved in this project, in particular the members of the joint ESA/FSA Packings Technical Task Force and David Edwin-Scott of the European Sealing Association (UK), and Didier Fribourg of the Technical Center for Mechanical Industries – CETIM (France).

Next Month: How to Achieve Zero Emissions with Mechanical Seals

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