How long is a piece of string? Answer: As long as you want to make it.
How long does a Mechanical Seal last? Same answer!
For some, this apparently flippant response will be downright irritating because the average life of their mechanical seals may be measured in only a few weeks or months. The very suggestion that it can be easily improved can be taken as insulting to their level of skill and expertise. In spite of this, there are a number of methods of extending mechanical seal life that have been proved for well over 15 years.
One of these, is to ensure that the Seal is mounted on a strong shaft. This is particularly necessary on a horizontal, end suction pump. As the impeller on an end suction pump is mounted at the end of an overhung section of the shaft, it is apparent that the shaft must be strong enough to resist any possible deflection.
However, there are two symptoms that become evident if the shaft is not strong enough.
1. When a variety of different seals fail within a similar interval of time. In other words, when you find yourself saying things like, “I’ve tried everything in that pump, and nothing seems to work.”.
2. When the packing leakage cannot be maintained at an acceptable level for any length of time, and eventually starts to look like Niagara Falls coming out of the stuffing box.
Under these situations, the problem is likely to be an undersized shaft that requires some attention.
The overhung section of the shaft can be strengthened by either increasing the diamter, or by reducing the length of the distance between the centerlines of the impeller and the closest bearing.
The latter is a little impractical as it requires some change in the design of the pump, the former can often be accomplished by simply removing the shaft sleeve and replacing the shaft with one that has a larger diameter (previously of the sleeve) thorughout the length of the overhung section.
This will increase the shaft strength, reduce the tendency for excessive deflection, cut down the amplitude of any possible vibration, and thus extend the seal life on an overhung pump design.
Author of “The Practical Pumping Handbook” and a specialistr in Pumping Reliability, Ross Mackay can be reached at www.practicalpumping.com or at 1-800-465-6260
