Product category:
Testing, analysing and monitoring equipment
News Release from: Flir Systems | Subject: Thermal imaging
Edited by the Engineeringtalk Editorial
Team on 03 April 2006
Severity criteria for IR maintenance
inspections
Thermal imaging has evolved into one of the most valuable diagnostic tools for predictive maintenance; but not every problem identified needs immediate rectification.
By detecting anomalies often invisible to the naked eye, thermography allows corrective action to be taken before costly system failures occur Identifying the problem, assessing the scale and consequences of a possible production line shutdown, image analysis, accurate reporting and recommending the most appropriate corrective action, are just some of the issues raised when using infra-red thermography
This article was originally published on Engineeringtalk on 29 Jun 2000 at 8.00am (UK)
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This document provides the answers to frequently asked questions in this regard.
"How should I report my findings to the maintenance team and how do I know what's really urgent and what's not?".
These are important questions for both the thermographer and his/her customer.
Finding hot spots is one thing, but assessing them properly is equally important.
Inspection results have to be classified and put into some kind of formal structure to allow decisions on how - and especially when - corrective action should be taken.
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The Infrared Training Centre, an independent infra-red training institute that also provides certification, suggests the following severity criteria for electromechanical components and installations.
Class A is a very serious anomaly that requires immediate attention.
Class B is a serious anomaly that requires attention as soon as possible.
And Class C is an anomaly that requires monitoring and a check-up at the earliest convenient time.
Severity criteria fulfil the need to prioritise findings in such a way that the most serious problems are dealt with first.
Direct, immediate repair of all detected anomalies in a plant is virtually impossible.
This would be too costly and inefficient.
The task of a thermographer and his customer or boss, is to keep a plant running as profitably as possible with minimal interruption, maximum safety and at the lowest cost.
This explains why a classification or prioritisation of faults is a logical consequence and vital to all companies.
Preventive maintenance is based on comparative evidence.
An infra-red camera operator gathers quantitative and qualitative information about scanned objects.
Quantitative information involves the scanned object's temperature, while qualitative information applies to discovering, describing and locating the faults.
Both are compared with "normal" temperature levels or function modes, within a certain range.
Measuring temperature levels is important in deciding how to deal with the problem once it is found.
The thermographer is of course generally familiar with the plant he or she is inspecting and knows that some components have varying functions and different temperature limits.
So a temperature limits table is clearly not sufficient.
And there are indeed no universally applicable severity criteria for a multitude of components.
Of course, if an object is "too" hot, it is usually a sign that something should be done about it - but not always immediately and not necessarily a repair.
It will certainly, however, require attention.
This can be defined as an alert to initiate the process of short- or medium-term solution planning.
The outcome may indeed be an immediate repair, but if it is not, another action could be determined.
This could be for example reducing load, applying additional cooling, ordering spare parts and labour, scheduling a convenient time for shutdown and repair or taking verification measurements.
"Immediate repair" and "needing attention" may be good indicators, but they are only a first step towards defining levels of urgency.
There are three main sources of information to determine severity criteria: rules set by standards organisations, operation manuals or construction plans and other literature and finally, previous field experience such as inspection reports and the thermographer's own experience in the sector.
Companies should certainly implement and maintain the following plan.
Keep up to date with publication from standards authorities such as relevant ISO (International Organisation for Standardisation), IEC (International Electrotechnical Commission), NETA (International Electrical Testing Association), IEEE (Institute of Electrical and Electronics Engineers), DIN (Deutsches Institut fur Normung), and similar national and international standards authorities.
Study available literature the materials and components you are inspecting.
Manufacturers' websites and manuals are an excellent source of information.
Document your field experience.
These data are of vital importance in determining, adjusting or even reassessing severity criteria.
Keep safe previous inspection reporting and store it on various media.
Infra-red camera manufacturers like Flir Systems offer various software packages for easy, clear and transparent predictive maintenance inspection reporting.
As a rule, criteria and guidelines should be kept alive by the accumulation of experience, gathered by thermographers.
Business sectors or organisations often develop their own set of criteria as part of their program guidelines and to cover their specific needs.
At the Eaton Group, an automotive supplier with 26 plants all throughout Europe, thermography inspection results are classified according to a four-level fault rating system.
A "minor problem" is usually rectified during the regular maintenance rounds by one of the group's 50 preventive maintenance employees.
An "intermediate problem" demands repair within two weeks after the inspection.
A "serious problem" requires a repair within one to two days following the survey.
And a "critical problem", at the top of the scale, requires immediate intervention as well as an additional thermographic inspection on site immediately after the repair work.
These uniform criteria for electrical installations have been established for the European Eaton Group subsidiaries by a consultancy.
"They are based on a long-term experience and they cover all our needs", says Peter Koelewijn, Field Service Supervisor and the group's chief thermographer.
"But that does not exclude taking my previous inspection reports with me when I'm surveying in a plant", he adds.
EON Bayern, a big European regional utility provider overseeing a 175,000km long network, has defined the following severity criteria to maintain its 43,000 low and middle voltage installations.
L1 stipulates a repair at the next annual, or other long-term regular inspection.
L2 requires a repair within 6 months.
L3 urges repair within one working week.
However, Uwe Thomas, Measurement Engineer responsible for the thermography inspections at EON Bayern, underlines that these criteria serve as orientation values rather than as clear-cut directives.
He adds that specific local conditions, such as climate or other atmospheric influences can alter the criteria.
"These criteria are not static, they are dynamic values with room for growth, re-assessment and if needed, change", says Thomas.
So can a thermographer, as the prime gatherer of information, determine the severity of a problem correctly?.
Yes, but he/she will have to know the installations more than thoroughly.
As a matter of fact the thermographer should be able to interpret the anomaly with regard to its impact on the entire operation or machinery unit.
Similar or identical hot spots on two identical components do not necessarily have identical consequences.
But only this, often unattainable, in-depth knowledge about the surveyed objects, empowers the thermographer to decide on the severity criterion.
This is a fact that is not often recognised, but good thermographers do understand this.
They know that identical or different types of components with different functions have different temperature limits.
They use their knowledge and experience to come up with reasonable solutions and reporting, within the framework of the customer's guidelines, when available.
In addition, it should be emphasised that severity criteria are not only important in providing a structure for classification of faults but also as a formalised communication tool between the thermographer and his customer, who is often expert and decision-maker about further measures and actions.
Severity criteria are guidelines.
They are useful indicators but they are insufficient.
They should be developed for each type of industry, company, operation, and even material and component.
The application of severity criteria should be a continuously evolving, dynamic process that takes new experience into account.
Much depends, as always, not only on the user skills of the thermographer, but also on his/her knowledge of and expertise in a relevant business sector. Request a free brochure from Flir Systems ...
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