Thermography Imaging and Survey

Thermographic inspection refers to the nondestructive testing of parts, materials or systems through the imaging of the thermal patterns at the object's surface. Strictly speaking, the term thermography alone, refers to all thermographic inspection techniques regardless of the physical phenomena used to monitor the thermal changes. For instance, the application of a temperature sensitive coating to a surface in order to measure its temperature is a thermographic inspection contact technique based on heat conduction where there is no infrared sensor involved. Infrared thermography on the other hand, is a nondestructive, nonintrusive, noncontact mapping of thermal patterns or "thermograms", on the surface of objects through the use of some kind of infrared detector.

In addition, there are two approaches in thermographic inspection: (1) passive, in which the features of interest are naturally at a higher or lower temperature than the background, for example: the surveillance of people on a scene; and (2) active, in which an energy source is required to produce a thermal contrast between the feature of interest and the background, for example: an aircraft part with internal flaws.

Thermal imaging cameras are commonly used for inspections of Electrical,Mechanical systems, Pipe work Faults , Refractory and petrochemical installations and components in all sizes and shapes Examples of failures in systems that can be detected with thermal imaging:

Infrared T-surveys are conducted to capture heat images of equipment, etc.to identify hotspots, so that appropriate and timely action is taken to avoid breakdowns. T-survey results provide indications of potential failures.

INFRARED THERMAL IMAGING-PRINCIPLE:

All objects that have a temperature above absolute zero (0 Degree Kelvin) emit infrared radiation, which can be measured on the infrared spectral band of the electromagnetic spectrum. The technique of measuring & viewing this energy is called infrared thermography or thermal imaging. Most thermal imaging systems respond to wavelengths of 3 - 5 micrometers or 8 –12 micrometers.

WHY THERMAL IMAGING?

Thermography is nothing but ‘capturing heat images on film’. Swedish power board began inspecting a large number of electrical components (1.5 lakhs plus) way back in 1965 & UK power generation board began effectively utilizing Thermography for predictive maintenance of transmission lines.

The application of both Thermal imaging & Ultra sonic detection techniques are based on a fundamental principle, ‘Electrical Failures are preceded by rise in temperature & sound’. If you understand this basic principle, carry out effective techniques to detect these failure ‘pre-cursors’ and take timely corrective action, electrical equipment failures can be avoided. Although T-imaging applications are numerous (quality checks, tank level monitoring, detection of steam leaks, integrity of furnace refractory lining, etc.), we will discuss electrical applications in this application note.

The electrical maintenance practices evolved over a period of time. Breakdown to preventive to predictive to risk /reliability based maintenance practices. Logically, high reliability plant & equipment requires to be maintained accordingly. In countries like USA, UK, France, Netherlands, etc., infrared Inspections are now recognized and recommended as a best practice for the inspection of electrical installations. In order to assess the equipment condition by predicting failures, without losing production, applications such as thermal imaging are gaining importance.

OBJECTIVE OF T-SURVEY

Generally T-Surveys are conducted with the following objectives:

·        To identify equipment / connections which requires Thermo-Survey

·        To carry out infrared thermal imaging of equipment (process, electrical, Mechanical) in operation to identify hotspots.

THERMAL IMAGES- RISK CATEGORIZATION PROCEDURE

WHY CATEGORIZE THERMAL IMAGES?

Although it is ideal that corrective action is taken on all thermal images that indicate a potential problem, categorizing helps the plant to take corrective action depending on the severity of the problem since all corrections may not require immediate action.

The risk levels are determined by the Thermographer based on the following factors and aspects:

·        Relative temperature difference (Delta T)

·        Industrial experience gained over the years to assess equipment conditions

·        Thermal imaging & analytical expertise

·        Equipment age

·        Environmental conditions (ambient temperature, dust, humidity, wind)

·        Load level

·        Criticality (from safety, reliability point of views)

·        Past equipment reliability data

·        Expertise gained as a Certified Reliability professional (CRP)

RISK LEVELS:

ALARMING LEVEL: (RED)

Delta temperature (differential temperature between the maximum &minimum / temperature near the object) more than 16 degree centigradeFailure could result in major fire / explosion / over-pressure situation /major abnormality .

CAUTION: (Blue)

Delta temperature (differential temperature between the maximum & minimum temperature) between 4 to 15 degree centigrade .

Impactmaybeconfinedtothe equipment

Failuremaynothavealargeimpacton safety/environment/productionbut repaircostmaynotbeinsignificant

CHECK & MONITOR: (Yellow)

Delta temperature(differential temperature between the maximum & minimum temperature) between 1 and 3 degree centigrade

Mostlyof aminor/ regulardisruptionthat will notaffectsafety/production/ environment .

NORMAL LEVEL: (Green)

Normal temperatures

THERMAL CAMERA SPECIFICATION:

-Extraordinary thermal sensitivity & imaging quality

-Real time imaging

-Flexible viewing options

-Allows special profiling and isotherm viewing

-Wide angle lens

-Powerful analyzing software

-Rapid Thermal Image report generating software

As per NFPA 70 (B) & NETA Standards, the Risk Category is defined as below:

Alarming       :DeltaT temperature is more than 15˚C.

Caution         :DeltaT temperature is between 4˚C to 15˚C.

Check & Monitor: DeltaT temperature is between 1˚C& 4˚C.

Normal          : Normaltemperature

DeltaT (differential temperature between the maximum & minimum/ temperature near the object) temperatures & corrective actions defined are based on National Fire Protection

Association(NFPA)70B, section 21.17.5.6 of NFPA 70B,2010 & International Electrical Testing association (NETA)standards.

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