Improvements in floating-ring compressor seals

As is so often the case, compressor components undergo design changes as experience accumulates. Over the years, improved floating-ring shaft seals have become key elements in the uptime improvement inventory of modern plants.

Take, as an example, improvements in Elliott Isocarbon seals and associated seal systems. User feedback indicates that these seal configurations can, in fact, compete with dry gas seals in such areas as safety and reliability.

Safety attributes examined. While decades-old seal configurations (Fig. 1) may have resulted in mixed acceptance, the more recent versions of this product (Fig. 2) incorporate subtle (but important) changes in component geometry. Floating-ring seals are now favorably considered by many users for a number of reasons.

It should be noted that seal faces are made of stable materials. In Isocarbon seals, the rotating face is still made with cast iron, which is a very stable material that resists shocks and brittle fracture.

Dry gas seal rotating-face components are made of ceramics or tungsten carbide. Ceramics tend to be very brittle and sensitive to manufacturing variances. Unless expertly designed and manufactured, catastrophic failures and possible gas release can result with ceramics.

Reliability issues. A few additional advantages, disadvantages and concerns must be addressed when comparing floating-ring and dry gas seals:

•    Seal oil cools shaft ends; gas will not do so with the same effectiveness.

•    Vibration damping is enhanced with liquid (wet) seals. This may provide lower vibration through increased rotordynamic damping. However, as oil thickens with time, this damping varies. Such a change could influence equipment rotordynamics and create field problems. While dry gas seals have less damping, rotor characteristics do not change over time.

•    Process gas quality is essential to gas seal life. Unless absolutely clean, process gases can adversely affect gas seal operation. While this is generally not a major issue in the field if suitably designed filtration equipment is provided, filter system maintenance is a budget item that must be considered.

•    Owner-operators are often not allowed to service certain seal models. The gas seals are treated as a "black box" and must be returned to the manufacturer for refurbishment. With industry practices changing and owner-operator expertise often declining, some clients have opted to sign alliance agreements giving service and maintenance responsibilities to the vendor.

In essence, then, compressor sealing issues must be carefully weighed and modern technology updates are essential to making an informed choice.2 There can be no blanket endorsement of either sealing approach across the board. However, oil seals, including Isocarbon seals, can be very useful in certain
machines. They can be problem solvers in large compressors. One such application is in LNG where the advantages of damping and high cost of downtime justify a simpler oil seal arrangement. These seals also merit consideration where there is prolonged slow-roll, e.g., where steam turbine drivers are used and compressors are in hot stand-by service.

LITERATURE CITED

1    Bloch, H. P., A Practical Guide to Compressor Technology, 2nd Ed., John Wiley & Sons, Hoboken, New Jersey, 2006 (ISBN 0-471-727930-8).

2    Bloch, H. P., Compressors and Modern Process Applications, John Wiley & Sons, Hoboken, New Jersey, 2006 (ISBN 0-471-72792-X).