than the internal preset counter value, the count is cleared and begins again. However, a faulty machine will generate more excursions counts per cycle than a healthy machine. Therefore, the count is likely to exceed the preset in the specified time and the alarm will annunciate. Judicious setting of the system threshold peak level, allowable counts per measurement interval, and count time interval will permit reliable monitoring without excessive false alarms. This method has been proven to detect various internal faults, such as cracks, looseness issues, and excessive clearances.
Inspection frequency: Impact monitoring systems are normally permanently mounted in order to provide continuous protection.
Rod Drop Monitoring: Rod drop monitoring involves the use of permanently mounted eddy current probes to measure the relative position of compressor rods with respect to the compressor cylinder. For horizontal rods, the probe is mounted vertically above or below the rod so that a change in vertical position of the rod is reported as a change in probe driver output voltage. Ideally the gap should remain the same over the entire travel of the rod. In practice, the rod position will vary due to running clearances; it may even bow when subjected to the operating compression and tension forces. In spite of these complications, it is possible to identify piston and cylinder faults, such as rider band wear, by plotting the probe gap on a continuous basis and looking for changes in relative probe gap.
Inspection frequency: Rod monitoring systems are normally permanently mounted in order to provide continuous protection.
Monitoring Guidelines for Electric Motors
Figure 3.9 Typical Electric Motor
Vibration Collection and Analysis: Most electric motor vibration levels are monitored on a periodic basis using “walk around” data collection programs. Assessments are made by placing a vibration sensor, usually an accelerometer, with a magnetic base on the bearing housings at specified locations. Overall vibration amplitudes are trended to see if any changes are occurring. If a significant change in overall amplitude is observed, a frequency analysis is performed in an attempt to identify the nature of the malady.
Inspection frequency: Monthly and quarterly inspection intervals are common.
Temperature Monitoring: Bearing temperatures can be taken at the same time vibration data is collected. Assessments may be either absolute or relative criteria. For example, you may decide that if a bearing temperature exceeds 200°F (93.3°C), you will shut down (this is an absolute criterion). Or if you see a 20°F (6.7°C) increase in bearing temperature from one inspection to the next, you will investigate. (This is an example of a relative analysis criterion.)
Inspection frequency; At the same time the vibration data is taken.
Oil Analysis: Oil analysis is typically reserved for critical machines with large oil reservoirs. Therefore, periodic oil analysis is not warranted for electric motors with small bearing sumps.
Inspection frequency: Monthly and quarterly inspection intervals are common.
Motor Current Analysis: The Motor Current Signature Analysis (MCSA) is considered the most popular fault detection technology in use today on critical motors because it can readily identify common machine fault, such as cracked or broken rotor bars, turn-to-turn shorts circuits, and bearing deterioration.
Inspection frequency; Annually or semiannual intervals are common.
Performance Monitoring: This type of analysis does not normally apply to electric motors, however, load testing of electric motors is available to identify load related problems.
When Should a Machine Be Considered Critical Enough to Justify Continuous Monitoring?
The first questions to ask in determining a machine’s criticality are:
•What is the economic consequence if the machine fails without warning?
•Will an unexpected failure lead to a costly process interruption?
•Will an unexpected failure lead to a release of a regulated or dangerous fluid?
•Will an unexpected failure lead to a cost machine repair as a result of additional internal damage?
•Is there a significant threat to the safety of operating personnel or the community?
An affirmative answer to any of these questions means your machine is probably a critical machine and requires continuous monitoring. If you are still unsure, here are a few more questions to ask:
•Is the machine in a remote location that makes machine inspections infrequent or difficult?
•Are failures likely to occur between planned inspections?
•Is it difficult to detect early failure modes without instrumentation?
•Is the machine failure frequency uncertain and unpredictable?
•Has a decision been made to run until an early failure is detected?
A “yes” to one or more of these questions is further justification for continuous monitoring.
Machinery Monitoring Recommendations Based on Criticality
Table 3.1 allows you to quickly determine what machine monitoring methodology should be employed based on the potential risk. To use it, first rate the perceived risk—low, medium, or high—for each category. The category with the highest level of risk determines the overall machine risk rating. Finally, by determining if the machine is in a local production unit or remotely located, you can select an appropriate monitoring strategy.
Keep in mind this matrix is a starting point. Every organization will have to tweak the descriptors and limits shown here to match their accepted risk tolerance.
How to Use This Matrix
Example #1
Consider that there are two, 250-HP centrifugal pumps operating continuously in a processing unit. Only one pump is required to meet production needs, so they are considered spares for one another. The product is not flammable, but there is a real possibility of a significant release of a reportable product to the environment if a pump seal fails catastrophically. We must first rate pump criticality in all five categories (Economic Impact, Production Impact, etc). For this example, these pumps would receive the following criticality ratings:
Table 3.1 Machinery Monitoring Recommendations Based on Criticality
Note: Machine monitoring typically includes vibration, bearing temperatures, RPMs, driver load, and process pressure and flow measurements, which can either be taken and recorded periodically or be monitored continuously, depending on the risks involved.
Maintenance Impact: Low
