For some time, a considerable amount of confusion has existed over the appropriate way to inspect for the presence of a given failure mode. Should I perform some type of sensory inspection? Should I perform some type of quantitative inspection? Should I apply one or more condition-monitoring technologies? Should I apply some combination of these techniques to maximize the conditional probability of finding the defect?
Essentially, how do I identify the presence of a defect in such a way as to maximize the amount of time my planning department has to effectively and efficiently develop the job procedures, order the parts, and schedule and complete the work before the conditional probability of failure becomes too high? An explanation of the types of inspections and how they complement one another goes a long way toward clarifying which ones are most appropriate.
Sensory inspections have long been considered the backbone of any good inspection program. It was believed that sending someone around often enough to inspect for problems with machinery would result in identifying defects in plenty of time to effectively mitigate unplanned downtime. The inspector would use sight, sound and touch to determine if anything had changed since the last inspection. Any change would be recorded, reported and investigated by a craftsperson on the next scheduled outage.
While there is a tremendous amount of benefit to sending someone around to perform inspections, there are so many holes in this strategy that it should never be considered the backbone of the inspection program. Sensory inspections typically only identify the most obvious and drastic of problems. It is all but impossible for a sensory inspection to identify early, internal defects in machines.
Enhanced sensory inspections fill that gray zone. They are both a sensory inspection and a quantitative measurement with condition-monitoring characteristics. These inspections utilize instruments like spot radiometers, strobe lights, handheld vibration pens and simple ultrasonic meters to detect defects further up the P-F curve. While these tools multiply the power of the human senses, they have their limit. These simple tools do allow for different failure modes to be detected, but they shouldn't replace a comprehensive condition-monitoring program.
Quantitative inspections can provide useful information when it comes to generating data for trending and determining the characteristic life of a failure mode. Quantitative inspections require someone to measure something. Very common quantitative inspections include measuring the temperature of a seal on a pump or measuring the backplate clearance on a pump impeller. These measurements provide data to the planner and engineer and help determine the need for further maintenance action.
When designed properly, a quantitative inspection procedure details limits and typically expected measurements. Any inspection that requires someone to measure something should have the minimum, maximum and typical values, with conditional tasks defined for when the limits are exceeded. But a quantitative inspection performed at the proper inspection frequency rarely will have a measurement that exceeds the limits.
Condition monitoring, also known as predictive maintenance (PdM), is the application of condition-based monitoring technologies, statistical process control or equipment performance for the purpose of early detection and elimination of equipment defects that could lead to unplanned downtime or unnecessary expenditures. And generally speaking, you must conduct this while the equipment is in normal operation, with little to no process interruption. The purpose of these tools (vibration analysis, infrared thermography, motor circuit analysis, etc.) is to find defects not possibly found through previously available inspections methods, specifically while the machine is in normal operation.
Taking advantage of the available technology lets you accurately assess the condition of parts and the presence of defects heretofore impossible to detect. An example of the advantage these tools have in the area of quantitative inspections or sensory inspections is the use of vibration analysis to determine the presence of a defect on a rolling element bearing. Previously, mechanics and millwrights relied on "lift checks" to determine the amount of clearance in a bearing. Unfortunately, this technique is only valid for bearing defects that resulted in the removal of material from the raceways of the bearing; this bearing would be pretty bad off to have thousandths of inches of play in it. Sub-surface fatigue is easily seen with vibration analysis and at this point in the failure propagation has resulted in no removal of material from the raceways. This is the most common example of the advantages of condition-monitoring technologies.
In summary, there are different types of defect inspection techniques that can be brought to bear on a machine, and each has its advantages and disadvantages. However, these techniques aren't necessarily exact replacements for each other. Each determines the presence of the defect at different places along the P-F curve and, as a result, each gives the planning function different amounts of time to respond to the defect.
A failure modes, effects and criticality analysis (FMECA) can help you determine which inspection techniques should be applied, how often and with what degree of redundancy. Remember, the trick is to balance risk with rigor. How much risk you are willing to take with a given failure mode coupled with how much you are willing to pay for the inspection determines the appropriate strategy.