Shaft misalignment is one of the most common causes of premature mechanical seal failure. Higher leak rates typify these failures. Rapid failures can occur in high pressure and speed applications. Misalignment may be static or dynamic in origin. Static misalignment is measurable when the equipment is not in operation, while dynamic misalignment is detectable only during operation. Static misalignment is most prevalent and will be our focus. Causes of static misalignment include parts that are out of tolerance, static shaft deflection and improper installation.
Evidence that misalignment was the cause of a seal failure includes one or more of the following: broken springs, worn and/or extruded dynamic gaskets, fretting corrosion of metal surfaces adjacent to the dynamic gasket or worn drive mechanisms. No single mechanical seal design can fully compensate for poor alignment. Some designs are more forgiving of misalignment than others, but there can be performance compromises when selecting the seal design that best compensates for poor machine alignment. The first priority should always be to correct the misalignment so optimum seal performance can be achieved.
Causes of Misalignment
Seal housing misalignment may be caused by a permanent bend in the shaft, excessive shaft deflection or by misalignment between the rotor assembly and the seal housing. Determining shaft condition should be the first step when checking for misalignment at the seal housing.
A bent shaft is easy to detect but often difficult to repair. To determine if a shaft is bent at the seal housing, mount a dial indicator on a stationary surface and measure the runout on the shaft sleeve while turning it 360-deg. Total Indicated Runout (TIR) of more than 0.002-in (0.05-mm) warrants a repair or replacement of the pump shaft.
It is possible for the shaft to be bent at a location that will not be evident when checked at the shaft sleeve. A shaft also should be checked for straightness if vibration readings indicate an out-of-balance condition. The detection procedure is the same. Mount the indicator on a stationary surface and take a reading from the shaft while turning it.
It is usually less expensive and more reliable to replace a bent shaft than to straighten it.
Shaft Concentricity and Parallelism
Misalignment from a lack of concentricity or parallelism can be measured by a dial indicator on the shaft. While rotating the shaft and dial indicator, measure runout on the seal housing bore or register for concentricity at the seal housing face for parallelism (angularity).
If the seal housing concentric runout is greater than 0.005-in (0.125-mm) or the face runout is greater than 0.0005-in/in (15µm/3cm) of seal housing bore diameter, further investigation and corrective action is warranted. Potential causes of an out-of-tolerance seal housing runout reading include:
Excessive static shaft deflection
Deformation of the pump assembly due to high structural loads
Misalignment from the tolerances of the assembled machined components is harder to detect. There are usually several register fits between the bearing supports and the seal housing. These fits need to be machined concentric and square. Measurements should be made on a machine. Although an individual register may appear to be out of tolerance by only a small amount, it is important to remember that the total misalignment of the assembled parts will be the sum, or stack up, of the individual component misalignments. Register fits are subject to wear and corrosion over time and should be checked as part of any major maintenance.