Seal Off Centrifugal Pump Problems

Paying proper attention to seals can improve pump performance and life.

By Rick D. Farris, SKF USA Inc.

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Questions that will help to pinpoint failure causes include:

How well has the seal performed in the past and is it the correct seal for the application? If there's a history of failures with a particular seal, the culprit may not be the seal itself -- unless the seal isn't the right design or the material is inappropriate for the application. At the first signs of failure, such as intrusion of foreign matter (Figure 2), check the seal's part number and review recommended applications to exclude the seal itself as suspect. Then, via a process of elimination, focus on the many influences that can impact seal performance and service life.

Always check whether operating conditions conform to the optimum range specified for the seal. Subjecting a seal to operating conditions outside that range surely will result in its failure. For example, when operating temperature or pressure exceeds the lip material's maximum, the seal may exhibit heat cracking, which is indicated by a hardened seal lip or fine cracks visible in the seal lip surface. Excessive surface speeds or insufficient lubrication at the seal lip can eventually lead to heat cracking and damage.

Shaft-to-bore misalignment or dynamic run-out can cause early lip leakage, excessive and uneven lip wear on one side of the seal (Figure 3) or excessive but consistent lip wear all around. (Shaft-to-bore misalignment results from inaccurate machining, shaft bending, lack of shaft balance or worn bearings; dynamic run-out is a similar condition where the shaft doesn't rotate around its true center.) The seal's lip area with the greatest wear will indicate the direction of the misalignment.

A breakdown in lubrication or improper lubricant also can lead to problems. Sometimes heat may be high enough to break down the lubricant but not enough to harden the seal's lip. In such cases, sludge or varnish-like deposits will accumulate on the seal lip and damage will occur. Using the proper lubricant and regularly changing it are among the best practices to help avoid lube-related seal failures.

Improperly installed seals likely will fail quickly. Symptoms of damage caused by a hammer blow during installation include visible dents on the seal back, a distorted sealing element or a garter spring that pops out. All are causes for concern and necessitate seal replacement.

Other factors ranging from possible media intrusion to undue pressure within a seal cavity can compromise seal performance. Also, it pays to review maintenance and operating practices to see if they could adversely impact sealing systems.

The central message here is to confirm the seal has been installed properly, runs within specified operating condition ranges and benefits from the proper lubricant.

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