Pumps & Systems, July 2013
Defect elimination is a simple maintenance strategy with seemingly obvious value. However, how many facilities pursue it to the greatest degree of value? According to Webster’s online, a defect is “an imperfection that impairs worth or utility.”
The “utility” part of the definition is significant because every process industry professional focuses on uptime. If a machine has a defect, but it still works (still makes widgets) and that machine imparts defects into the widgets, the worth of that widget is reduced. Or if the machine still works, but production is slowed, the number of widgets produced over time is reduced. The worth of widgets sold is also reduced.
Defects cost money. If end users candidly investigate, they may see that they are investing money in keeping their defects. How? Money lost because the widgets are being sold at a reduced price or because the maximum number of widgets were not manufactured because of slowed production or downtime is money invested in keeping defects. Even worse, some operators are on long-term payment plans for their equipment, so they continue to invest in the same defects year after year.
The second part of the definition is elimination. According to Webster, eliminate means to put an end to or get rid of. The absence of defect also means the absence of the investment required to sustain the defect. An asset that is free of defects is an asset that can be optimally profitable. Defect elimination usually requires an investment. Rarely does simply tightening a bolt or performing an equally trivial task fix an expensive problem.
Defect exclusion with reliability engineering and precision maintenance offers the best chance for preventing defects in the first place. Often, process lines and equipment are purchased and assembled without much consideration for life-cycle costs or the impact of poor installation on life-cycle costs. The effect of this oversight is compounded each time sub-precision maintenance is performed.
Consider a direct-drive, 500-horsepower (four-pole motor) fan purchased to move a certain number of cubic feet per minute (cfm) in a production process. It was purchased from the lowest bidder at a cost of about $35 thousand and installed without precision installation practices.
Some possible defects of the equipment are divided into three categories and listed below:
- Inherent defects (such as unbalance, misalignment and resonance)
- Induced defects (such as improper installation—excessive or insufficient bearing clearance, foundation inadequacy, or improper lubrication)
- Age- and process-related defects (such as corrosion, fatigue, wear-out of bearings/seals/rotor and motor winding degradation)
The immediate reaction to misalignment’s inclusion in the inherent category may be negative. However, end users must understand that misalignment does not merely occur because of a terrible installation job. Misalignment will occur, and it will shorten equipment’s life unless the installer or maintenance professional does an excellent job.
What about the impact of defects on the $35-thousand machine? Improperly installed bearings can reduce the lifetime by as much as 99 percent. Even in the absence of this extreme, the operator may be unknowingly sacrificing half the bearing life. Consider the interaction between the defect categories and the part that precision maintenance plays or could play if implemented.
Misalignment, like unbalance and resonant frequencies, is always present to some degree. The key is to reduce it so that it has no measurable effect on the equipment’s lifetime. Consider how poor shaft to shaft alignment can accelerate many defects that should not occur within equipment for many years after a precision installation.
Misalignment restricts the rotation of shafts causing jerking or impacting that can often occur twice per revolution. If the $35-thousand machine runs at 1,790 rpm, it experiences 3,580 impacts on its bearings, seals and couplings during each minute. That means 214,900 hits per hour and 1,718,400 in an eight-hour day. This defect accelerates fatigue in many components. The restriction of the shafts probably causes pressure points on the seals, shortening their life, which in turn shortens the life of the bearings because of dirt ingress.
This defect may be shortening the life of the motor windings because of localized heating from the distortion caused by being bolted to an uneven base (a machine frame distortion, or soft foot). Misalignment is a reliability nightmare that can be avoided. Modern laser alignment equipment and techniques can effectively eliminate misalignment as a consideration in the lifetime of our assets.
Even when precision installation practices are followed, defects will occur. Under certain circumstances, the inherent defects can all be moved into the age- and process-related defect category. Rotors can become dirty or unevenly worn, increasing unbalance. Fasteners can loosen or break, causing a loss of machine stiffness, and resonance can become a factor in machine life when it previously had not been. Problems with fasteners, foundations, thermal growth and other factors can induce misalignment in a previously well-aligned machine.
Virtually every machine will, at some time in its life, develop defects. Many of them are reversible. If not detected and mitigated, they will shorten or end the effective lifetime of end users’ assets. For this reason, defect elimination is a desirable management tool—because end users either manage their defects, or they become out of control.