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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Lip Wear
Figure 3. Inner diameter of seal exhibits uneven lip wear, as indicated by arrow.

TROUBLESHOOTING SEAL FAILURES
Despite all the advances in sealing system designs, materials and performance over the years, seals aren't immune to potential failure -- many times for reasons other than the seals themselves. Picking an inappropriate replacement, improper installation or switching or mixing lubricant can cause problems over time. When good seals go bad, the best troubleshooting practice is to ask the right questions and then follow a logical sequence of steps to analyze the failures and take remedial action.

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.

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