Thermography can be used to identify and analyze thermal anomalies for condition monitoring of machines. These thermal anomalies are usually caused by such mechanisms as operation, improper lubrication, misalignment, worn components or mechanical loading anomalies.
Infrared thermography (IRT) is based on measuring the distribution of radiant thermal energy (heat) emitted from a target surface and converting this to a surface temperature map or thermogram.
Thermal energy is present with the operation of all machines. It can be in the form of: friction losses within machines, energy losses within machines, as a characteristic of the process media, or any combination thereof.
As a result, temperature can be a key parameter for monitoring the performance of machines, the condition of machines and the diagnostics of machine problems. Temperature is also one of the key causes and symptoms of lubricant degradation and loss of lubrication function within a machine.
IRT is an ideal technology to investigate thermal anomalies on machines because it provides complete thermal images of a machine, or a machine component, with no physical attachments (nonintrusive), requires little setup and provides the results in a short time.
As such, thermography techniques can be used as part of a condition-monitoring process when such a process is implemented in accordance with ISO 17359.
Figure 1. Misaligned Pumps
The high coupling end pump and motor
bearing temperatures and the high
temperature difference between the
pump bearings are typical of
a misaligned pump.
Thermography Techniques
There are several recognized IRT techniques in use throughout industry. Comparative thermography is the most common technique and it is normally used to provide the best available data in lieu of ideal, or absolute, thermal measurements.
When encountering changing machinery operating conditions, the ability to perform rough emissivity estimates, and the ability to detect emissivity differences on machinery equipment, provide useful information for the condition monitoring and diagnostics of the machine under the less-than-ideal circumstances frequently encountered in the field.
Absolute thermography is used when it is essential to know as precisely as possible the true temperature of a target.
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Figure 2. Turbine Oil Coolers The two turbine oil coolers of identical design demonstrate that one of the turbines has an oil cooler problem. Insufficient thermal evidence existed at the time of the survey to determine whether the problem was related to oil flow, water flow, or both given that both machines were operating at identical loads. Under identical loads and oil condition, Turbine No. 2 operated with consistently higher vibration due to the effects of reduced oil viscosity resulting from the higher operating temperatures. |
Comparative Thermography
Comparative thermography can be either quantitative or qualitative. The quantitative technique requires the determination of a temperature value to distinguish the severity of a component’s condition.
This value is determined by comparing the target’s temperature to that of similar service equipment or baseline data. Although the temperature value is not exact, it is reasonably close to actual, and more importantly, the temperature differentials are accurate.
However, there are many applications where quantitative data is not required to monitor the condition of machinery, or to diagnose a problem and recommend the appropriate corrective action. In these cases, qualitative techniques may be more than adequate.
Comparative Quantitative Thermography
The comparative quantitative thermography method is effective for evaluating the condition of a machine or component by comparing approximate temperatures between identical items, reference values or baselines.
he precise determination of actual temperatures of a component, using IRT in the field, is considered difficult. This is due to a certain extent to the physics of IRT that take into consideration the multiple parameters that enable a true absolute temperature measurement. These IRT considerations are: emissivity, reflectivity and transmissivity.
As a result, estimates of these IRT considerations can be readily made to obtain a component’s approximate temperature, which in most cases is more than sufficient to determine the severity of an adverse condition.
Because it is not always practical to determine the exact temperature, or even emissivities, of each machine component, the alternative use of comparative thermography becomes more practical. Comparative measurement, unlike qualitative measurement, identifies a thermal deficiency by comparing the temperatures obtained using a consistent emissivity value e (default).
The temperature differential between two or more identical or similar components is measured numerically. Assuming that the environmental conditions for both components are similar, the differential temperature for the given piece of equipment is recorded as the amount above the normal operating temperature of the similar equipment.
Consider this example of comparative quantitative thermography: If two or more machines are operating in the same environment and under the same load conditions, and one is experiencing an elevated temperature, this usually indicates that a deteriorating condition may exist.
However, determining the temperature difference would assist in establishing the severity of the condition. In this example, a 5°C differential would be considered minor, whereas a 100°C differential may be considered critical. Knowing the approximate value of the elevated temperature would provide an indication that the temperature limit of a component may be approaching alarm values.
Therefore, while qualitative measurements can also detect deficiencies, it is the quantitative measurements that have the capability of determining severity.
Comparative Qualitative Thermography
Comparative qualitative measurement compares the infrared pattern, such as gear contact patterns, of one component to that of an identical or similar component under the same or similar operating conditions.
When searching for differing thermal patterns, an anomaly is identified by the intensity variations between any two or more similar objects, without assigning temperature values to the patterns. This technique is quick and easy and does not require any adjustments to the infrared instrument to compensate for atmospheric or environmental conditions or surface emissivities.
Although the result of this type of measurement can identify a deficiency, it does not provide a level of severity.
This IRT technique is used throughout most industries. It is effective in identifying hot bearings or other abnormally hot machine components, hot spots in electrical equipment, undesirable hot electrical connections, leaking or even clogged fluid heat exchange equipment and its components (tubes), and fluid leaks from pressure vessels, pipes and valves.
Thermometry
Determining the absolute temperature of a target using IRT is difficult because of the many technical and environmental factors involved. As a result, absolute IRT measurements are done only if precise temperature values, or small temperature differentials, are critical to a process. These determinations are normally attempted only under extremely controlled laboratory-type conditions. This type of measurement is not normally used for condition monitoring.
Baseline Measurements
In all cases, it is strongly recommended that baseline measurements of critical plant equipment be taken. This is important when making later IRT surveys of machines or components and comparing them with previous thermograms of the same machines operating under the same load and environmental conditions. This condition-monitoring procedure is useful for identifying developing problems early, thus preventing major maintenance operations or catastrophic failures.
Assessment Criteria
When applying IRT to the condition monitoring and diagnostics of machines and their related components, it is strongly recommended that severity criteria be established.
The severity criteria can take two forms: They can be organized into general categories that identify temperature levels, or zones, vs. levels of criticality; and they can be applied to specific machines or components, or to like groups of machines or components. In either case, the levels are established through experience and data.
In practice, no singular acceptance criterion is universally applicable to the variety of items and applications existing in industry. Consequently, severity criteria must be developed for each category of equipment based upon its design, manufacturer, operation, installation, maintenance characteristics, failure modes and criticality.
Severity criteria can be established on individual machines or components. This method is based on many factors, including: temperature rise vs. historical data that establishes rate of deterioration and time to failure, criticality of the machine or component to the overall process, location with respect to other materials/equipment should a fire result, safety of personnel, environmental conditions, etc.
Applications could include temperature rises of critical machines, mechanical components, bearing temperature rises, electrical supply or connection rises, fluid leakage losses, or even the number of tubes clogged in fluid heat transfer-type equipment.
The infrared thermographer may use delta-T (temperature difference) criteria or classify the temperature severity of mechanical system anomalies. These delta-T criteria are usually reported as the temperature rise of the exception above the temperature of a defined reference.
By taking multiple measurements over time of similar components under similar operating and environmental conditions, statistical analysis can be used to set operational limits for trending and predicting the temperature performance of these components. A delta-T system may be used in conjunction with these absolute temperature criteria to rate the temperature severity of an exception above the maximum allowable temperature.
Relative Temperature Criteria
An example of a set of severity criteria based on categories and temperature rises above established references is as follows:
Advisory - Up to 10°C rise above a reference or baseline
Intermediate - 10°C to 20°C rise above a reference or baseline
Serious - 20°C to 40°C rise above a reference or baseline
Critical - In excess of 40°C rise above a reference or baseline
Reference temperatures should be based on temperatures specified by manufacturers of similar items or groups of equipment, or of components located on the same shaft.
Baseline temperatures should be based on historical or statistically derived temperatures established from the specific item or machine groups when operating under ideal conditions.
Absolute Temperature Criteria
The infrared thermographer may use absolute maximum allowable temperature criteria based on published data to identify mechanical system anomalies. It must be well-understood that there are two categories of criteria, material and design:
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Material criteria are used where the integrity of the material itself is of concern and is the focus of monitoring. Design criteria are used where the design integrity is the major concern and is the focus of the monitoring.
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Design criteria should be used in preference to material criteria because design criteria normally incorporate material requirements. Design criteria are usually cognizant of performance, operation, reliability and capacity criteria rather than just component material integrity.
When an exception is heating several adjacent system components and a material criteria is used, the component material having the lowest temperature specification should be referenced as the alarm criteria.
Caution: In most machines, the lubricant will have the lowest temperature specification. The maximum allowable temperature should be stated as the temperature above which an unacceptable loss of component life will be experienced due to a loss of lubricant performance. Such losses in performance may be immediate (viscosity) or long-term (oxidation, additive depletion, thermal degradation).
Such criteria tends to be design-based rather than material-based. This will require application-specific temperature criteria despite the possible use of common lubricants.
In many instances, the infrared thermographer cannot directly measure the surfaces of actual components. Care and good judgment must be used when applying any severity specifications to actual field temperature measurements, taking into account conduction paths, convection and radiation.
Profile Assessment Criteria
Profile assessment is a process of comparing temperature differences and patterns across a surface. As in any severity assessment process, the absolute and differential temperatures and profiles need to be determined for two key conditions: the new and the failed conditions. Severity assessment is the subsequent process of determining the condition of the equipment between these two conditions.
The key areas of profile assessment include temperature gradients, changes in profile, historical changes, localized differences, absolute temperatures, location of anomalies or profile characteristics relative to the item.
Diagnosis and Prognosis
Survey Intervals
Survey intervals should be cognizant of the rate of deterioration of the expected fault and the behavior over time of temperature as a representative symptom of the fault. The determination of survey interval is necessary primarily for prognosis accuracy rather than fault identification.
Image Interpretation
From a machinery viewpoint, thermal image interpretation is essentially a process of comparing absolute temperature and temperature profiles against design, manufacture, installation, operation and maintenance criteria.
When using thermography for machinery condition-monitoring, the operating conditions at the time of each survey need to be known in detail because many changes in thermal profile are operating condition-dependent.
Understanding a machine’s design is essential to understanding component loading which in turn is the primary contributor to thermal profile.
When using thermography to assess machine condition, the machine must be viewed as a whole and each image must be analyzed as part of a series rather than an individual representation of a localized condition.
Thermal imaging is a powerful correlation technique for other condition-monitoring methods. It is exceedingly useful for solving lubrication issues that are temperature-related as it can pinpoint location, source and extent of the influencing thermal anomalies as well as determining effectiveness of circulation, control and cooling systems.
The key point to successful machinery investigation and solution design - using thermal imaging - is knowledge of design, manufacturing, installation, operation and maintenance-induced failure modes and their thermal symptoms.
This knowledge is far more important than specific knowledge of the technique, its applications and its limitations.
Comparative Quantitative Case Study - Mill Drive Train Misalignment
This case study involves a grinding mill drive train in a gold mine consisting of a motor, a three-shaft reduction gearbox, two pinion bearings and a pinion gear. All bearings were rolling element types and the mill drive pinion bearings were double-row spherical roller bearings.
The gearbox had been experiencing high gear and bearing vibration, unacceptable bearing failure rates, gear wear and rapid oil degradation.
Figure 3. Temperatures Recorded
Figure 3 shows the temperatures that were recorded. These temperatures and profiles indicate a misalignment between the motor and gearbox, a misalignment between the gearbox and the mill drive pinion, and a misalignment between the mill drive pinion gear and driven girth gear.
The combined motor to gearbox to mill drive pinion misalignments resulted in a loss of end float within the gearbox trapping the intermediate shaft gear. The 10.2°C temperature difference across the mill drive pinion gear was also excessive and indicated severe pinion to girth gear misalignment.
The misalignment conditions, gear loading and bearing distress were all confirmed using vibration analysis. Subsequent drive train overhaul and realignment reduced the temperatures, vibration and oil degradation rates to acceptable levels.
Sidebar 1
Machinery Lubrication spoke with several users of infrared cameras. They include John Bankhead, president of Reliability Maintenance Services in Fouke, Arkansas; Joel Kinnaird, thermographer for Leadership Electric in Dallas, Texas; and Dennis Rejcek, a reliability technician for M&M Mars in Waco, Texas.
When asked what benefits they’ve experienced from using an infrared camera, here is what they had to say:
John: Because my company is a service company, it’s my customers who reap the benefits. Using IR cameras allows one to examine overheated bearings, and look at couplings and alignment issues, among others.
IR is used in performing electrical surveys and can be expanded to include mechanical surveys. Consider the process issues specific to industry; for example, a tissue machine at a pulp and paper mill. Moisture retention in sheets of tissue is determined by thermography. IR is not as trendable compared to other technologies, such as vibration analysis.
The main advantage of IR is that it is a one-shot technology. It determines if a condition is good or bad, without much in between. Regarding the cost of ordinance, the cost of using IR cameras is difficult to quantify before it becomes a problem. The saying “a picture is worth a thousand words” is especially true of IR cameras.
A qualified thermographer using an IR camera is the low-hanging fruit in a PdM program. With IR cameras, there is immediate payback. IRT is one of the best ways to determine machine efficiency. Consider the steam traps in all plants and industries. That presents a huge opportunity and a lot of money to be saved.
Joel: My company sells services, such as conducting inspections and performing repairs, and thermography is a large part of what we do. With an IR camera, you can take sample shoots of images in your own plant.
In electronic IR inspections, we open panels, switch gears, and look for hot spots in electrical connections. This enables us to let the customer know an approximate time before a problem occurs, and helps in recommending a solution. Scheduled repairs are cheaper than emergency repairs, which are caused by failures, because emergency repair crews are costly.
This is especially important in a hospital or in the computer operations of large companies. IR has numerous applications wherever temperature differences can be analyzed. It is used in the manufacturing of laminated materials for aircraft such as wings and fuselage; wherever microscopic voids conduct a temperature difference.
IR cameras are used for quality-control purposes, for example, in circuit control fields. They detect when motor bearings are drying up, when they need lubrication and when they should be replaced. IRT is actually more proactive than preventive in savings. We’ve experienced multiple savings, as our company sells preventive practices.
One example of savings is on the insurance required of property owners and property managers. Preventive maintenance, such as IRT and specifically the use of IR cameras, reduces insurance premiums. The money our customers save in reduced premiums often pays for the inspection.
Dennis: IR cameras allow you to shoot images of the equipment to see how warm bearings are. Two different views enable the operator to take a picture of what he’s shooting. Using IR reduces downtime because the cameras let you see the problem before it actually appears.
Sidebar 2
Survey: How Does Your Infrared Inspection Program Stack Up?
The charts above are the results of an online Infrared User survey conducted from December 2002 to January 2003. The survey was hosted at www.maintenancebenchmarking.com, an online survey site and was written and conducted at Reliabilityweb.com. The survey received more than 100 responses, and was designed to provide a snapshot of current infrared best practices.
