Shaft misalignment is one of the most common caus es of premature mechanical seal failure . Higher leak rates typify these failures. Rapid failures can occur in high pressure and speed applications. Misalignment ma y be stat ic or d ynamic in orig in. Static misalignment is measurable when the equipment is not in operation, while dynamic mi salignment i s detectable onl y during operat ion. Static misalignment is most prevalent and will be our focus. Causes of stat ic misalignment in clude parts that are out of tolerance , static shaft deflection and improper installat ion.
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.
Bent shaft
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:
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Excessive static shaft deflection
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Out-of-tolerance assemblies
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Deformation of the pump assembly due to high structural loads
Tolerances
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.
Mounting
Many pumps have four or more mounting pads for mounting the pump to the baseplate. A pump often does not have 100 percent contact with all of the baseplate mounting surfaces. This is often called a soft foot condition.
Bolting a pump to a base with a soft foot condition may result in misalignment of the pump shaft to the seal housing and place stress on the bearings and other components. To check for soft foot, mount a dial indicator on the baseplate and take a measurement from the pump adjacent to a hold-down fastener. A change of the indicator of more than 0.002-in. (0.05-mm) while tightening or loosening a hold-down fastener indicates a soft foot. Use stainless precision shims between the baseplate and pump to compensate. It is generally good practice to monitor shaft runout at the seal housing while installing a pump on its baseplate and connecting the pump to its piping.
Flanges
Mating flanges should be concentric and parallel. Bolting of misaligned piping to a pump will cause both the piping and the pump to distort. The amount of distortion is relative to the size and strength of the pump and piping, as well as the distance from the nearest fixed hold down. A good rule of thumb is that if mechanical leverage is needed to bring the flanges into position, then the nozzle loads should be checked.
If the piping connections are suspected, mount a dial indicator to check for shaft alignment at the seal housing to check shaft angularity and parallelism. If connecting or disconnecting the piping changes the shaft alignment, corrective action should be taken.
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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
