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General Machinery Monitoring Guidance

Where to Start?

If you are a newcomer to the field of machinery monitoring, you are probably overwhelmed by all the new concepts and terminology. To simplify matters, we will present a basic roadmap for machinery monitoring for centrifugal pumps, centrifugal compressors, steam and gas turbines, gear boxes, fans, reciprocating pumps, reciprocating compressors, and electric motors. (For equipment types that are not included in this section, contact the corresponding original equipment manufacturers for their recommended monitoring guidelines. Also, talk to other users of similar machine types employed in your industry about their monitoring experiences and best practices.)

Here are some machinery monitoring methods frequently used during field machine assessments:

•Vibration—This method uses dynamic data collected by measuring the motion of a vibrating surface. Analysis of this type of data requires complex signal processing and pattern recognition.

•Pressure—This approach can involve either static or dynamic data collected by inserting a pressure traducer into a fluid stream. This type of analysis also requires complex signal processing and pattern recognition.

•Temperature—This method is usually in the form of static data. Thermocouples or resistance temperature devices (RTDs) are often inserted either into a fluid stream or on or below metal surfaces to measure temperature. Portable infrared temperature guns can also be used to monitor machine surface temperatures. In some applications, thermography is employed to visualize temperature distributions across a machine in order to identify component issues such as failing bearings. Thermography has been successfully used to spot electrical problems in the field on motors and control panels.

•Oil analysis—Oil analysis requires that oil samples be sent off-site for lab testing. This step can lead to a delay in prompt information. Although most oil properties are usually determined by lab testing, some oil properties can be monitored in real time.

•Analysis of piping, duct work, and structural components—Equipment attached to the machine—such as piping, vents, duct work, and supporting structures—can be sources of information; they can path sources for vibrations, flow issues, and loadings.

•Electrical measurements—These measurements are usually in the form of dynamic data because electric power is sinusoidal. Voltage and current waveforms must be analyzed in real time so that they can be converted to real power, root mean square voltage, average current, power factor, etc.

•Performance analysis—The required data for this type of study are static in nature. They are usually taken with temporary or field mounted sensors.

Because there is such diversity in machine designs and applications, we will now provide more specific monitoring recommendations. Below is a listing of the most common process machinery along with recommendations as to what should be monitored and when. Keep in mind these are general recommendations; they should be compared with manufacturer’s guidelines and company best practices.

Monitoring Guidelines for Centrifugal Pumps

Figure 3.1 Typical Centrifugal Pump

Vibration Collection and Analysis: Most centrifugal pump 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. 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 made by using 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 (an example of 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 (an example of relative analysis criterion).

Inspection frequency: At the same time the vibration data is taken.

Oil Analysis: Oil analyses are typically reserved for critical machines with large oil reservoirs. Therefore, oil analyses are not usually warranted for most small spare pumps with small bearing sumps.

Inspection frequency: Monthly and quarterly inspection intervals are common.

Pressure Pulsation Analysis: This type of analysis is usually performed only when an internal pump issue is suspected or an unwanted pump/piping interaction is occurring. Clues as to when a pressure pulsation analysis is required are excessive piping vibration, broken bolts, high pulsations observed on a pressure gauge, high blade vane pass vibration in the vibration spectra, etc.

Inspection frequency: Only required when issues arise

Piping Vibration: This type of analysis is normally conducted only if piping failures have occurred or if the piping is deemed to be moving excessively. Piping analysis methods are similar to a vibration analysis performed on rotating machinery, except the assessment criteria are quite different.

Inspection frequency: Only required when issues arise

Performance Monitoring: To ensure that all your pumps are performing as advertised, it is important to assess their performance on a periodic basis. A performance analysis involves collecting pump pressures, flow, and horsepower data in order to determine overall efficiency and whether the pump delivers the performance required by the process.

Inspection frequency: Annual performance evaluations are common.

Monitoring Guidelines for Centrifugal Compressors

Figure 3.2 Centrifugal Compressor Rotor

Vibration Collection and Analysis: Due to their criticality, most centrifugal compressors have dedicated vibration monitoring systems consisting of either case-mounted accelerometers or non-contacting proximity probes at each bearing. If a significant change in overall vibration amplitude is reported by the monitoring system, a detailed frequency analysis is performed in an attempt to identify the nature of the malady. Critical monitoring systems usually have capabilities of generating trend plots, spectral comparisons, orbits, and other forms of data trending and presentation.

Inspection frequency: Additional monthly and quarterly inspections are often conducted by a technician to determine if any changes from the baseline have occurred.

Temperature Monitoring: Due to their criticality, centrifugal compressors typically have thermocouples or RTDs embedded in the bearing Babbitt near the location of where the maximum expected bearing temperature would occur. The proper placement of thermocouples and RTDs in the bearing allows accurate and repeatable bearing metal temperature measurements that can better track changing load and lubrication conditions.

Inspection frequency: At the same time the vibration data is taken.

Oil Analysis: Periodic oil analyses are typically performed on centrifugal compressors. These analyses can help detect oil contamination or bearing deterioration issues.

Inspection frequency: Monthly and quarterly inspection intervals are common.

Pressure Pulsation Analysis: These types of analyses are usually not conducted on a centrifugal compressor.

Inspection frequency: Only required when issues arise

Piping Vibration: These types of analyses are usually not conducted on a centrifugal compressor.

Inspection frequency: Only required when issues arise

Performance Monitoring: To insure that a compressor is performing as advertised, it is important to assess its performance on a periodic basis. A performance analysis involves collecting compressor pressures, flow, and horsepower data in order to determine overall efficiency and whether the compressor is delivering the performance required by the process.

Inspection frequency: Annual performance evaluations are common.

Monitoring Guidelines for Steam and Gas Turbines

Figure 3.3a Steam Turbine Blading

Vibration Collection and Analysis: Due to their criticality, most steam and gas turbines have dedicated vibration monitoring systems consisting of either case-mounted accelerometers or non-contacting proximity probes at each bearing including every thrust bearing. If a significant change in overall vibration amplitude or thrust position is reported by the monitoring system, a detailed frequency analysis is performed in an attempt to identify the nature of the malady. Critical monitoring systems usually have capabilities of generating trend plots, spectral comparisons, orbits, etc.

Inspection frequency: Additional monthly and quarterly inspections are often conducted by a technician to determine if any changes from the baseline have occurred.

Figure 3.3b Gas Turbine Power Recovery Buckets

Temperature Monitoring: Due to their criticality, steam turbines and gas turbines typically have thermocouples or RTDs embedded in the bearing Babbitt near the location of the maximum expected bearing temperature. The proper placement of thermocouples and RTDs in the bearing allows accurate and repeatable bearing metal temperature measurements that can better track changing load and lubrication conditions.

Inspection frequency: At the same time the vibration data is taken.

Oil Analysis: Periodic oil analyses are typically performed on steam turbines and gas turbines. These analyses can help detect oil contamination, oil breakdown, and bearing deterioration issues.

Inspection frequency: Monthly and quarterly inspection intervals are common.

Pressure Pulsation Analysis: This type of analysis is usually not conducted on steam turbines or gas turbines.

Inspection frequency: Only required when issues arise

Piping Vibration: This type of analysis is usually not conducted on steam turbines or gas turbines.

Inspection frequency: Only required when issues arise

Performance Monitoring: Performance monitoring can be conducted on steam turbines and gas turbines, but is both extremely complex and beyond the scope of this book. Usually this type of analysis is conducted by outside personnel with detailed knowledge of these machines.

Inspection frequency: Annual performance evaluations are common.

Additional Inspections: Due to the extreme conditions inside steam and gas turbines it is common to perform internal inspections at prescribed time intervals or “fired hours” intervals. This timing is because there are known failure modes that are not detectable with current online inspection methods. The original equipment manufacturer is a good starting point for recommended tear down inspection intervals for optimum reliability.

Monitoring Guidelines for Gear Boxes

Vibration Collection and Analysis: Due to their criticality, most large gear boxes have dedicated vibration monitoring systems consisting of either case-mounted accelerometers, non-contacting proximity probes at each bearing, or both types of sensors. Proximity probes are employed on units with fluid film bearings as a means of monitoring shaft vibration whereas case mounted accelerometers are used to monitor the condition of the gears.

Critical monitoring systems usually have capabilities of generating trend plots, spectral comparisons, orbits, etc. If a significant change in overall vibration amplitude is reported by the monitoring system, a detailed frequency analysis is performed in an attempt to identify the nature of the malady. Due to the sensitivity of gear dynamics to horsepower loading, it is particularly important to note power load levels when vibration data are collected.

Inspection frequency: Additional monthly and quarterly inspections are often conducted by a technician to determine of any changes from the baseline have occurred.

Figure 3.4 Double Herringbone Gears

Temperature Monitoring: Due to their criticality, large gear boxes typically have thermocouples or RTDs embedded in the bearing Babbitt near the location of the maximum expected bearing temperature. The proper placement of thermocouples and RTDs in the bearing allows accurate and repeatable bearing metal temperature measurements that can better track changing load and lubrication conditions.

Inspection frequency: At the same time the vibration data is taken.

Oil Analysis: Periodic oil analyses are typically performed on gear boxes. These analyses can help detect oil contamination, bearing deterioration issues, and gear degradation.

Inspection frequency: Monthly and quarterly inspection intervals are common.

Monitoring Guidelines for Fans

Figure 3.5 Typical Process Fan

Vibration Collection and Analysis: Most fan 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, onto 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: Many fans utilize greased bearings and are not conducive to periodic lubricant analysis. However, if the fan has a circulating lubrication oil system, periodic oil analysis is recommended.

Oil inspection frequency: Monthly and quarterly inspection intervals are common.

Pressure Pulsation Analysis: This type of analysis is rarely applied to fans.

Inspection frequency: Only required when issues arise

Piping or Ducting Vibration: This type of analysis rarely applies to fans.

Inspection frequency: Only required when issues arise

Performance Monitoring: Fan performance can be conducted with a manometer, flow indication, and a motor power meter. By comparing the measured performance with the shop test data, you can determine if performance has degraded.

Inspection frequency: Annual performance evaluations are common.

Monitoring Guidelines for Reciprocating Pumps

Figure 3.6 Typical Reciprocating Pump

Vibration Collection and Analysis: Most reciprocating pump 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 to 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. Smaller, less critical pumps like most reciprocating pumps are frequently protected with periodic oil changes.

Inspection frequency: Monthly and quarterly inspection intervals are common.

Pressure Pulsation Analysis: This type of field analysis is usually conducted whenever damaging pump/piping interactions are occurring. It is hoped that if a thorough pulsation study and design analysis was performed before machine installation, pulsation will be controlled to low, healthy levels. However, reciprocating pumps that rely on pulsations with gas-filled bladders are prone to failure that can result in excessive pressure pulsations. Clues as to when to conduct a pressure pulsation analysis will be excessive piping vibration, broken bolts, high pulsations observed on a pressure gauge, etc.

Inspection frequency: Only required when issues arise

Piping Vibration: This type of analysis is normally conducted only if piping failures have occurred or if the piping is deemed to be moving excessively. Piping analysis methods are similar to a vibration analysis performed on rotating machinery except that the assessment criteria are quite different.

Inspection frequency: Only required when issues arise

Performance Monitoring: Performance testing is normally not conducted on reciprocating pumps.

Monitoring Guidelines for Reciprocating Compressors

Figure 3.7 Reciprocating Gas Compressor

Vibration Collection and Analysis: Most reciprocating compressors moving flammable gases are equipped with permanently mounted vibration sensors, usually accelerometers or velocity transducers, on crankcases and crosshead guides. These sensors are then tied to local monitors that can be configured to alarm and/or trip the compressor whenever high vibration levels are encountered.

Overall vibration amplitudes can also be 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 and stage discharge temperatures are both commonly monitored continuously. High bearing temperatures may be an indication of a failing bearing or lack of lubrication. A high cylinder discharge temperature can either signal a cylinder valve failure, cylinder wear issue, or excessive stage compression ratio.

Inspection frequency: At the same time the vibration data is taken

Oil Analysis: Periodic oil analyses are typically performed on reciprocating compressor. These analyses can help detect oil degradation or contamination or machine wear issues.

Inspection frequency: Monthly and quarterly inspection intervals are common.

Pressure Pulsation Analysis: This type of field analysis is usually conducted whenever damaging compressor/piping interactions are occurring. It is hoped that if a thorough pulsation study and design analysis was performed before machine installation, pulsation will be controlled to low, healthy levels. Clues as to when to conduct a pressure pulsation analysis will be excessive piping vibration, broken bolts, high pulsations observed on a pressure gauge, etc.

Inspection frequency: Only required when issues arise

Piping Vibration: This type of analysis is normally conducted only if piping failures have occurred or if the piping is deemed to be moving excessively. Piping analysis methods are similar to a vibration analysis performed on rotating machinery except that the assessment criteria are quite different.

Inspection frequency: Only required when issues arise

Condition monitoring methods unique to reciprocating compressors

Performance Monitoring: Performance monitoring of reciprocating compressors requires specialized equipment and training. This topic is beyond the scope of this book; however the basic methodology will be presented here. To analyze reciprocating compressor performance, dynamic pressure transducers must be installed in the head-end and crank-end cylinder volumes and a means of determining the crank’s angular position is required. By plotting the in-cylinder pressure versus crank position, a pressure-volume (P-V) diagram can be generated similar to the one shown in Figure 3.8. P-V diagrams help enlighten the analyst as to the condition of critical internal components, such as valves and rings. Additionally, vibration versus crank position is collected to determine the condition of critical bushings.

Inspection frequency: Quarterly performance evaluations are common.

Figure 3.8 Typical Pressure Volume Plot

Impact Monitoring: Simply measuring vibration velocity can be unreliable; the increase in velocity from incipient failures is usually small and will be buried in the larger signal due to machine movement. By the time the fault has been detected, major secondary damage may have already occurred. Impact monitoring overcomes this problem. First it takes raw (i.e., unprocessed) acceleration signals from an accelerometer permanently mounted in the compressor frame. It then counts the number of excursions that exceed an alarm threshold in a set time. If there are fewer events detected 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

Production Impact: Low

Environmental: High Fire Risk: Low Safety Impact: Low

The controlling category is Environmental. Therefore, these pumps receive an overall criticality rating of High. It is recommended that they have permanent monitoring instrumentation and shutdowns installed (see Table 3.2 and follow the black path).

Example #2

There is a single multistage centrifugal pump that boosts kerosene flow from the crude oil processing unit to storage. If the pump is unavailable for service, the crude oil unit will have to reduce production by about 50%.

The highest risk is the potential of a greater than 24-hour loss of production if the pump shuts down unexpectedly. For this example, the pump receives the following criticality ratings:

Maintenance Impact: Low

Production Impact: Medium Environmental: Low Fire Risk: Low Safety Impact: Low

Because the pump is in a remote location, the recommendation is to install permanent monitoring instrumentation and shutdowns, and initiate weekly or daily operator inspections (see Table 3.2 and follow the grey line path).

Simplified Method of Economic Justification

In the business world, a detailed economic analysis is often required to determine if a machine is critical enough to warrant permanent monitoring. This type of analysis requires 1) an income stream, which contains the economic consequence of an undetected failure times the probability of the event occurring for each year of the analysis, and 2) the estimated cost of the monitoring system. Many electronic spreadsheets are available to assist you in performing this type of economic analysis. Every organization has its own hurdle for justification. However, in our experience, any monitoring project that has a payout of 2 years or less will probably be approved.

Here is a simplified method that can be used to justify condition monitoring hardware projects. First you need the following information:

•Annualized economic risk of the “do nothing” case

•Annualized economic risk of the improvement case

•Payback period required by management

•Probability of detecting the machine fault in its early stages

Table 3.2 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.

Economic risk is a product of two quantities: consequence and frequency (or probability). For justification, consequence is usually presented in monetary units. If a pump fails catastrophically, a potential consequence is $50,000. If this event occurs at a frequency of once every ten years, this represents an annual economic risk of $5,000/yr, but if it occurs every two years, the annual risk is $25,000/yr.

Assume the annualized economic risk for a given machine can be lowered from $30,000 to $5,000—with a certainty of 80 percent—by installing condition monitoring hardware. This scenario represents a project income of ($30,000 – $5,000)/yr ×.80 = $20,000/yr. If management requires a two-year simple payback, then we can justify $20,000 × 2 ($40,000) for a condition monitoring project. If we go on to assume the installed cost is twice the cost of the basic hardware, we know we can only justify the purchase of $20,000 in hardware.

Let’s explore a simplified justification formula. To use this formula, we need to know a) the estimated probable maximum loss (PML) per failure event without monitoring hardware and b) the estimated probable maximum loss (PML_CM) per failure with the proposed monitoring hardware. (Probable maximum loss events are the worst losses that can be expected, not the worst losses that can be imagined.) Starting with these assumptions, we propose the maximum amount of condition monitoring equipment justified (CM) is determined by the following formula:

where:

1.CM is the maximum amount of condition monitoring equipment justified.

2.PML is the probable maximum loss predicted without condition monitoring. (Include probable losses due to fire and environmental release fines, etc.) The equipment OEM is one good source for this information.

3.PML_CM is the probable maximum loss predicted with condition monitoring. Again, the equipment OEM is one good source for this information.

4.D is the probability of detecting the machine fault in the primary state.

5.t_PB is the payback period required by management

6.T_PML is the expected time between PML events. Use five years for unproven, isolated, or severe service machines; ten years for typical industrial duty machines; and 20 years for proven, light-duty service machines.

7.Sum these terms for all components in the machine train to determine the total amount CM equipment justified.

Consider a spare pump example involving two 500-hp spare pumps. No production loss is associated with pump failures. If we consider only secondary damage due to an undetected primary failure, we decide (PML – PML_CM) is $75,000. Now assume this installation is an isolated one. Therefore, select a TPML of five years, a D of 75 percent, and a management-required payback of two years. This analysis will lead us to the conclusion that an investment in CM hardware of up to [($75,000 × 0.75 × 2) / 10] = $11,250 is justified.

Additional Analysis Notes

•In cases where multiple machines can be monitored with a common monitoring system, first calculate the CM value for each machine in the area, then sum the values to determine the total amount of condition monitoring equipment justified.

•If a permanent system cannot be justified, consider a walk-around monitoring approach.

•More condition monitoring hardware may be justified if a more detailed risk assessment is performed.

Once the maximum amount of condition monitoring equipment justified is determined, work with the condition monitoring hardware provider to determine how to best spend this money. The types of condition monitoring hardware will depend on the answers to the following questions:

1.What types of machines are going to be monitored? Compressors, pumps, gearbox, etc.?

2.What are most predominant machine failure modes? Bearings, seals, etc.?

3.What bearing types do you have? Rolling element bearings (REB) or hydrodynamic?

4.What is the transmissibility ratio at the bearings, i.e., the ratio of casing vibration to shaft vibration?

5.What are the machine/shaft operating speeds?

6.How will the failure of the critical components manifest themselves? Vibration, temperature, acoustic energy?

7.How do these components usually fail? Gradually or suddenly?

Finally, determine if the proposed condition monitoring installation will actually yield the reduction in annualized risk assumed in the original analysis. If not, the user must either consider other design options or rerun the analysis considering the new assumptions.