The Most Effective Seal Design When Misalignment Is Present

Written by Fluid Sealing Association

Page 2 of 2

Seal Designs

Most seal designs are manufactured to operate with a maximum angular misalignment of 0.003-in. (0.08-mm). This includes the out-of-square tolerance and the shaft-to-seal chamber bore concentricity. There are two primary designs used to accommodate misalignment. One design utilizes a rotating compression unit where the springs rotate with the shaft (see below).

The other is a stationary design (see below) that uses a stationary compression unit where the springs do not rotate with the shaft. The stationary design compensates for misalignment with one adjustment, while the rotating design must compensate at every revolution of the shaft. This movement predisposes the spring mechanism to fatigue, which can cause premature failure. The stationary design is the better choice when misalignment occurs.

The same concept also applies to welded metal bellows technology. Utilizing a stationary bellows allows for a one-time reposition of the stationary face, eliminating constant flexing/movement of the thin cross section bellows leaflets.

Most of the seal manufacturers use the flexible stationary design in their modular or cartridge seals because of space limitations. The excessive flexing and fatigue of the many thin cross-section springs is avoided, as is contact with the process fluid.

Stationary designs are gaining acceptance for several other reasons. First, they keep the small multiple springs out of direct contact with the product being pumped. Many fluids contain solids which can clog the small multiple springs, hang up the seal, and cause premature failure. Second, high speed applications-those with shaft speeds in excess of 4,500 feet per minute (23-m/s)-usually require special design attention. Above this speed, dynamic forces begin to exceed the limitations of a conventional rotating design. Utilizing the stationary design reduces the secondary seal and drive mechanism movement, which could cause excessive wear and fretting damage. The stationary design also provides better seal face tracking capability and improves seal life. Further, the drive mechanisms can be increased to handle the higher torques associated with these speeds.

Summary

Mechanical seal failures are usually the symptom and not the cause of maintenance problems, many of which are related to static misalignment. In operation, other forces such as radial loads, impeller balance and cavitation also affect seal alignment. While some seal designs are more tolerant of misalignment, they ultimately are not the solution for a misaligned system. Optimum performance, which includes maximum seal life, can only be achieved by correcting the root cause of the misalignment. Always try to obtain more detailed information about suitable seal designs from the seal manufacturers.

Next Month: What is the impact of flange finish on gasket performance?

We invite your questions on sealing issues and will provide best efforts answers based on FSA publications.

Fluid Sealing Association

Sealing Sense is produced by the Fluid Sealing Association as part of our commitment to industry consensus technical education for pump users, contractors, distributors, OEMs, and reps. This month's Sealing Sense was prepared by FSA Member Phil Peck. As a source of technical information on sealing systems and devices, and in cooperation with the European Sealing Association, the FSA also supports development of harmonized standards in all areas of fluid sealing technology. The education is provided in the public interest to enable a balanced assessment of the most effective solutions to pump technology issues on rational Total Life Cycle Cost (LCC) principles.

The Mechanical Seal Division of the FSA is one of five with a specific product technology focus. As part of their educational mission they develop publications such as the Mechanical Seal Handbook, a primer intended to complement the more detailed manufacturer's documents produced by the member companies. This handbook served as the basis for joint development of the more comprehensive Hydraulic Institute publication: Mechanical Seals for Pumps: Application Guidelines. Joint FSA/ESA publications such as the Seal Forum, a series of case studies in pump performance, are another example as is the Life Cycle Cost Estimator, a web-based software tool for determination of pump seal total Life Cycle Costs. The Sealing Systems Matter initiative also was launched to support the case for choosing mechanical seals that optimize life cycle cost, safety, and environmental compliance.

The following members of the Mechanical Seal Division sponsor this Sealing Sense series: