8.1 Baselines
Baseline data is that condition monitoring data representative of an equipment in a new and/or properly operating condition. The baseline data is the foundation of the predictive trending analysis required to forecast equipment condition. It is important that this baseline data is established as early as possible in the life of the equipment. The baseline readings and periodic monitoring data should be taken under the same conditions (or as close as can be achieved).
These conditions should be recorded. It is only under identical conditions that the relative comparison of data is valid. Significant changes in conditions often affect the data in
unquantifiable ways because of unknown or complex relationships. Baseline readings should be re-established each time equipment undergoes major maintenance.
8.2 Criteria by PT&I Technology
8.2.1 Vibration Monitoring
The development of specific vibration criteria for every machine at all possible operating speeds in all applications and in every mounting arrangement is not possible. Vibration amplitude varies with operating speed, load, and mounting arrangement, so developing criteria based solely on amplitude can be misleading. The frequency content of the spectrum is often as important as the amplitude. For example, the presence of a tone associated with an inner race defect on a new bearing is unacceptable regardless of the amplitude. Furthermore, one must consider the
difference between the vibration amplitude and frequency content of a reciprocating machine as compared to a centrifugal machine.
The vibration specifications provided in this guide are based on International Standards Organization (ISO), American Petroleum Institute (API), American Gear Manufacturers Association (AGMA), American National Standards Institute (ANSI), STD-167-1, MIL-STD 740-2 (Appendix C), General Motors Vibration Specifications, and field data acquired on a variety of machinery.
a. Developing Vibration Criteria—Specific vibration criteria are provided in this guide where possible. Where specific criteria are not provided, the procedure provided in paragraph 7.1.7.1.c.(4).(a) is recommended to the guide user for use in developing the vibration criteria.
b. Vibration Analysis of New Equipment—For all large or critical pieces of equipment assembled and run at the factory prior to shipment, a narrowband vibration spectrum should be acquired at the locations listed in Section 7.1.8 of this guide while the equipment is undergoing this factory performance testing. A baseline or reference spectrum should be retained for comparison with the post-installation vibration check.
Equipment failing the vibration criteria should be rejected by the Government prior to shipment.
Vibration tests are recommended under the situations listed in paragraph 7.1.7.1.c.(4)(b) if the equipment fails the initial test and/or if problems are encountered following installation.
c. Vibration Criteria for Electric Motors
1. General—All motor vibration spectra should be analyzed at the following forcing frequencies described in paragraph 7.1.7.1.c.(4)(c).
2. Balance - The vibration criteria listed in Table 7–1 are for the vibration amplitude at the fundamental rotational frequency or one times running speed (1X). This is a narrowband limit. An overall reading is not acceptable.
3. Additional Vibration Criteria—All testing should be conducted at normal operating speed under full load conditions. Suggested motor vibration criteria are provided in Table 7–2. In addition, Appendix E contains criteria for common machines and an example of how to calculate criteria.
d. Rewound Electric Motors—Due to the potential of both rotor and/or stator damage incurred during the motor rewinding process (usually resulting from the bake-out of the old insulation and subsequent distortion of the pole pieces) a rewound electrical motor should be checked both electrically and mechanically. The mechanical check consists of post-overhaul vibration measurements at the same location as for new motors. The vibration level at each measurement point should not exceed the reference spectrum for that motor by more than 10%. In addition, vibration amplitudes associated with electrical faults such as slip, rotor bar, and stator slot should be noted for any deviation from the reference spectrum.
Note: Rewinding a motor will not correct problems associated with thermal distortion of the iron.
e. General Equipment Vibration Standards
1. If rolling element bearings are utilized in either the driver or driven component of a unit of equipment (e.g., a pump/motor combination), no discrete bearing frequencies should be detectable. If a discrete bearing frequency is detected, the equipment should be deemed unacceptable.
2. For belt-driven equipment, belt rotational frequency and harmonics should be undetectable. If belt rotation and/or harmonics are detectable, the equipment should be deemed unacceptable.
3. If no specific criteria are available, the ISO 3945 acceptance Class A guidelines (Appendix C) should be combined with the motor criteria contained in Table 7–2 and used as the acceptance specification for procurement and overhaul.
f. Specific Equipment—Use the criteria shown in Table 7–3 on boiler feedwater, split case, and progressive cavity pumps.
g. Belt Driven Fans—Use the criteria in Table 7–4 for belt-driven fans:
h. Vibration Guidelines (ISO)—Table 7–5 is based on International Standards ISO 3945 (Appendix C) and should be used as a guideline (not as an absolute limit) for determining the acceptability of a machine for service. The vibration acceptance classes and ISO 3945 machine classes are shown in Tables 7–6 and 7–7, respectively. Note that the ISO amplitude values are overall measurements in inches/second RMS while the
recommended specifications for electric motors are narrowband measurements in inches/second Peak.
8.2.2 Lubricant and Wear Particle Analysis
Lubricant analysis monitors the actual condition of the oil. Parameters measured include viscosity, moisture content, flash point, pH (acidity or alkalinity), and the presence of
contaminants such as fuel, solids, and water. In addition, the levels of additives in lubricants can be determined. Tracking the acid/alkaline nature of the lubricant permits the identification of an undesirable degree of oxidation, gauging the ability of the lubricant to neutralize contaminants, and aids in the verification of the use of the correct lubricant after a lubricant change. Viscosity (resistance to flow) provides a key to the lubricating qualities of the lubricant. These qualities may be adversely affected by contamination with water, fuel, or solvents and by thermal
breakdown or oxidation. The presence of water reduces the ability of the lubricant to effectively lubricate, promotes oxidation of additives, and encourages rust and corrosion of metal parts. In performing a spectrometric analysis, one burns a small amount of the fluid sample and analyzes the resulting light frequencies and intensities to determine the type and amount of compounds present based on their absorption of characteristic light frequencies. Sample results are compared to the characteristics of the same new lubricant to measure changes reflecting the lubricant’s reduced ability to protect the machine from the effects of friction. The continued presence of desirable lubricant additives is another key indicator of lubricant quality. Infrared spectrometry is also capable of detecting and measuring the presence of organic compounds such as fuel or soot in the lubricant sample.
In wear particle analysis, analysts examine the amount, makeup, shape, size, and other
characteristics of wear particles and solid contaminants in the lubricants as indicators of internal machine condition. With experience and historical information, one can project degradation rates and estimate the time until machine failure. Wear particle analysis includes ferrography, which is a technique used to analyze metal wear products and other particulates. Elemental spectrographic analysis is used to identify the composition of small wear particles and provide information regarding wear sources. Analyzing and trending the amount, size, and type of wear particles in a machine’s lubrication system can pinpoint how much and where degradation is
occurring. Figure 8–1 illustrates the relationship between wear particle size, concentration and equipment condition.
Figure 8 – 1. Wear Particle Size and Equipment Condition
8.2.3 Thermography
There are two basic criteria for evaluating temperature conditions. They are differential temperature (∆T) and absolute temperature. Each is described below.
a. Differential Temperature (∆∆T) - Temperature difference criteria are simple, easy to apply in the field, and provide an adequate qualitative screening system to identify thermal exceptions and problems. The ∆T criteria compares component temperature to the ambient temperature and may be used for electrical equipment. ∆T may also be used for mechanical components.
The typical ∆T criteria, which may be modified easily based on experience, are as shown in Table 8–1:
Wear Particle Size (µµm)
Temperature Rise Action
10-25F Repair at Convenience
25-40F Repair Next Scheduled Availability 40-80F Repair at First Availability
> 80F Repair Immediately
Table 8–1. Actions Required Based on Temperature Rise Under Load
b. Absolute Temperature—Absolute temperature criteria are generally specific to an equipment model, type of equipment, class of insulation, service use, or any of many other salient characteristics. As a result, absolute temperatures are more suited to
quantitative infrared thermography and critical temperature applications. The mechanical temperature specifications come primarily from manufacturer’s manuals. Electrical temperature specifications are set by three principal electrical standards organizations:
• National Electrical Manufacturers Association, (NEMA) (Appendix C).
• International Electrical and Electronic Engineers, (IEEE) (Appendix C).
• American National Standards Institute, (ANSI) (Appendix C).
A very useful summary of temperature criteria is found in the Guideline for Infrared Inspection of Electrical and Mechanical Systems, published by the Infraspection Institute of Shelburne, Vermont (Appendix C).
Table 8–2 provides absolute temperature limits for materials commonly found in the Government Plants:
8.2.4 Airborne Ultrasonics
The use of a passive ultrasonic instrument as a leak detector, i.e., listening for the ultrasonic noise characteristic of a pressure/vacuum leak, is qualitative. There are no numerical thresholds.
Many common passive ultrasonic devices operate on a relative, rather than a calibrated, absolute scale. However, by using relative changes in intensity from baseline readings, the degradation process may be trended and tracked.