Infra Red Thermography An Emerging Technique in Indian Nuclear Power Plants

Transcription

1 96 Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation NDE 2009, December 10-12, 2009 Infra Red Thermography An Emerging Technique in Indian Nuclear Power Plants M.A. Yadav, M.B. Jadhav, A.K. Deshmukh and C. Awasthi Quality Assuarance Directorate Nuclear Power Corporation Of India Ltd, Mumbai Abstract Nuclear Power Corporation Of India Ltd (NPCIL) has always been exploring challenging new techniques in the field of inspections. Faster inspections, reliable and repeatable results and lower equipment downtimes are the sought after objectives. Infra red Thermography (IRT)has gained momentum in a variety of applications in NPCIL stations. The paper describes IRT as a versatile tool applicable to situations where a problem or a condition can reveal itself by means of a thermal difference. Specific studies and applications in NPP s which are discussed are as follows: Electrical switchyard inspections, Study thermal insulation damage on high temperature pipelines, Experiment for estimation of heat load and visualisation / identification of areas contributing towards heat loss and applications in Control and Instrumentation systems panels and electronic units. At all occasions the technique has proved its worth by virtue of its inherent simplicity of usage and instantaneous results despite the complexity of test equipment. Commonly encountered sources of error are also mentioned along-with the discussion on the major requirements of IR Cameras for industrial applications and a brief discussion on the analysis features used. Some of the other useful applications amongst the vast variety are also touched upon. The paper attempts towards highlighting in general, the versatility, speed of obtaining useful results,flexibility of application and visualisation of the scanned surfaces, amongst other obvious advantages, of Infra Red Thermography as a fast emerging NDE technique. 1. Introduction Nuclear Power Corporation has been in the field of power production using Nuclear means for more than two decades and the organisation has been growing steadily over the years currently running 16 Nuclear power plants and having several Projects nearing completion. Future growth in this area of power production is bound to be phenomenal. Alongwith the growth come bigger responsibilities primarily towards safety followed by profitability. Greater are expectations towards generating power in an uninterrupted manner and without unplanned outages thus maximising the capacity and availability factors and minimising downtime. To reduce power plant equipment unavailability - emphasis is on faster maintenance, inspections, generation of reliable results and repeatability of the entire process. NPCIL has always been open towards induction of newer, challenging and more effective techniques for the inspection and testing of its entire range of equipment spread out in the field of nuclear systems, mechanical equipment, electrical equipment, control instrumentation and electronics and many others. Amongst the vast range of Non destructive test techniques, Infra Red Thermography (IRT) has made inroads into our NPCIL stations and has started gaining momentum as a versatile tool applicable in almost all areas where a problem condition can be revealed as a function of difference of temperatures The IRT technique was inducted way back in the mid 90 s for the ever popular application of electrical switchyard and switchgear applications. Since then NPCIL has moved into newer applications of IRT and the major areas being benefited and which are being discussed as a part of this paper include: Electrical switchyard and switchgear inspection Study of thermal insulation damage on high temperature pipelines and equipment. Experiment for estimation of heat load in the fuelling machine vault Location of hotspots in control and instrumentation systems. Last but not the least- a variety of other applications in this field 2. Infra Red thermography As is known IRT is the science of acquiring and analysing thermal information by using Non Contact Thermal imaging devices. Its unique advantages are: Non Contact,

2 NDE 2009, December 10-12, Real Time and Two dimensional images.a further classification would be : Active thermography : which uses simulated outside heat sources to generate heat in the specimen either continuously of in discreet steps. This is mostly used in the investigation of debonding. Voids, broken fibres etc in laminates, FRP sheets components etc. Passive thermography : where the inherent heat emitted by the object of interest is itself used in the testing process. We are into applications using this approach and hence further discussion would deal with this approach. 3. IR Camera The equipment being used by NPCIL, currently is a state of the art hand held IR camera Model : P65 from M/s FLIR, Sweden. The earlier generation IR Camera system possessed by NPCIL was the AGEMA Thermovision 870 The major specifications for this current equipment FLIR P65 are as follows: IR Detector Focal Plane Array (Uncooled) 320 X 240 Pixels Spectral range um The IR spectrum is as follows Temperature Ranges Accuracy Corrections -40 Deg C to 2000 Deg C in 5 ranges +/- 2Deg C or +/- 2% of reading Emmisivity, Reflected ambient temp, Auto Optics / External optics transmission correction Image Frequency 50/60 Hz Built in Digital video 640 X 480 Pixels full color Laser Locator Yes IR Lenses Operating conditions 24 deg with optional 7Deg and 45 Deg -15 to 50 Deg C 98% humidity We are currently operating in the LW range. This is the thermal imaging region, in which sensors can obtain a completely passive picture of the outside world based on thermal emissions only and requiring no external thermal source. This is also that part of the IR spectrum which is least affected by atmosphere as against the SW region which shows many peaks and dips in the transmission throughout. The other regions (commonly referred) in the IR Spectrum are : Near-infrared: um in wavelength, commonly used in fibre optic telecommunication because of low attenuation losses in the SiO 2 glass medium. Examples include night vision devices such as night vision goggles and IR control devices. Short-wavelength infrared: µm, used mostly in high temp applications. Mid-wavelength infrared also called intermediate infrared: 3-8 µm. In guided missile technology this is the heat seeking region in which the homing heads of passive IR homing missiles are designed to work, homing on to the IR signature of the target aircraft, typically the jet engine exhaust plume. The analysis features of the system include many useful features viz. Spot meter : One or many spots placed on the display which indicate the temperature of the marked location Area measurement: An area (Square or circular ) wherein the Min/ Max/ Avg temperature is indicated. Isotherms: Areas of same apparent temp highlighted by a contrasting color used to indicate Temperatures Above/ Below / between preset levels. Pallettes : Different Pallettes (Iron, Rainbow, Single color etc) to aid the user in analysing the temp distributions on

3 98 Yadav et al. : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation various subjects eg. Thermal insulation Iron palette to indicate distribution of heat coming out of the weal areas, Electrical switchyard Red color to highlight the spots of high temperature, Complex geometry Rainbow palette to clearly highlight areas of nearly same temperatures with same colors. Line profile: Useful in plotting the temp profile along a line, on long objects, to aid in visualising the temp distribution along the length. Histograms: An invaluable tool indicating the frequency of occurrence of the different temperatures in the acquired image. The important parameters to be preset before embarking on IR thermography are: Emmissivity : To enable measurement of temperature close to its true value Reflected apparent temperature : To minimise errors in measurement due to reflection from hot / cold objects in the vicinity of the subject Range, Focus, Composition : all to be optimised for obtaining accurate results. Another very important parameter is the Field of View which is a affected by the IR detector size and the IR lens being used. Its significance is similar to the FOV in conventional photography but becomes more important in obtaining accurate temperature readings for smaller areas at larger distances from the camera. Considering all of the above we carried out IR thermography for four different applications and the activity resulted in excellent results in each of the activity. 4. IR Applicatoins in NPCIL: 4.1 Electrical Switchyard and electrical switchgear scanning for locating hot spots potential sites for failures: The electrical switchyard is the interface between the power generating station and the consumer. It houses a myriad of high voltage high current equipment cable connections, transformers, insulators etc running in hundreds. All of these are potential candidates for failure due to excessive heating. This is caused due to improper / loose connections terminations. Heat generation due to resistive heating of the loose connections / faulty components is a precursor to failure of the component leading to economic loss both to the power station and to the consumer. These hundreds of connections cannot be subject to temperature measurement by conventional means, moreover the resistance heating is caused only when the line is live rendering the equipment unavailable to contact type measurement. At NPCIL coastal site, IRT was planned as a preventive measure during the pre-monsoon period just before station outage. IRT activity was planned in the late evening hours after Sun set thus avoiding possible errors due to reflection of sun and also improving the visibility of the LCD screen as compared to bright sunlight. Windy period was avoided to reduce errors in marginal cases when wind would cause reduction in temperature. Analysis was improved by using the Red palette as against the Iron palette to quickly pinpoint the hottest spots of the image. Further improvisation in the resolution using 7 deg telephoto lens (as against standard 24 deg lens) was a big advantage for imagimg the small size drop jumper connection in overhead lines. Result : Several overheated drop jumpers with loose connections, Brush connections, overheated insulators, bus connections, traps etc could be easily located within a short inspection period of 3-4 hours. These were rectified subsequently and the healthiness verified again using IRT. 4.2 Study of thermal insulation on high temperature pipelines and equipment. The locations inside containment houses a variety of the high temperature pipelines and insulated systems and equipment viz. discharge valves, secondary steam generators etc. This area was reported to have higher than expected ambient temperature and causes for the same were to be investigated. IRT was planned immediately after station outage to be able to catch the emanating heat from weak insulation before the circuit cools down. Within 5-6 hours of plant shutdown, a quick IRT scan using conventional 24 deg lens was carried out for all the piping connections, equipment surfaces, valve bodies etc. It was a surprise to find quite a few weak areas in the IR images showing weakened / deteriorated insulation, exposed discharge valve bodies, gaps in insulation at equipment and pipeline connections, broken patches in insulation and even patches in pipelines where insulation coverage had been missed! All the areas were emmiting a large amount of heat to the surroundings on a continuous basis during plant operation thus causing a substantial increase in the ambient temperature. Repairs and rectification was taken up and the immediate effect was a reduction in the ambient temperature thus serving the intended purpose.

4 NDE 2009, December 10-12, Possibility of improvements in electronics design for Control & instrumentation systems As a curiosity, the IR camera was taken to the C&I lab where electronics systems and panels were being developed and tested. A quick scan was a revelation of areas emmiting heat and probable room for improvement. Distinct temperature differences at different identical components / integrated circuits was observed. Overheating on Power supply bus, Power transistors and heat dissipating heatsinks were revealed Overheating of IC s like memory chips microprocessor chips etc could be easily established. Malfunctioning components due to over heating can cause the entire Data acquisition system to go haywire posing as a nightmare to the operating engineers. Realising the ease with which the emitted heat being captured by IR camera can reveal weak spots in electronic design a more detailed study of the panels and systems is being planned. 4.4 Experiment for estimation of heat load and visualisation of areas contributing to heat emission In our power stations there are several equipment carrying high temperature. Heat loss from these areas though fully covered with insulation would cause considerable load on the area coolers. It was also felt that the code to estimate this heat load should be validated and appropriate corrections be implemented for future stations. IR thermography was planned to help in providing an insight into the above application. An experiment was planned which involved the IRT of the various surfaces A detailed 3D domain was visualised and in addition to IRT the measurement of temperature using conventional means using RTD s was planned. A good match of the temperatures measured by these RTD s and by IRT was observed. An attempt was also made to estimate the heat content in the free space by suspending Al foils in the 3D domain and capturing IR images of these foils. The experiment was planned for a period of 8 hrs wherein two sets of readings were collected. The IR images revealed several areas of heat escaping out of the joints in insulation as well as areas indicating very good insulation properties thus maintaining the temp well within specified limits. The capabilities of IRT was established well beyond the expectations and the ease of usage and convenience coupled with the usefulness of the results was appreciated by all concerned personnel. 5. Many other Applications of IR: Connection problem with - Internal breaker Lug connection getting Hot Detection of Hot drive belts, conveyer rollers getting hot. Electrical Motor - Internal winding problem

5 100 Yadav et al. : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation Liquid Storage tank -Determination of liquid level. Cold cooling fins due to low oil level in a transformer Steam Traps detection of Leakage from a steam trap. In steel-making operations to minimize slag carry-over and maximize iron yields Detection of Asphalt that is cooler during rolling process thus prone to premature failure useful for inspection of Highway during asphalt rolling. Possibility of detecting the extents of air-voids between the bond-line of advanced composite materials (FRPs) and concrete substrate. Flaw detection and evaluation of airport pavements In Automobile industry- evaluating simple items such as transmission belts to complex ones such as turbochargers or catalytic converters, New engines put to trial for heat build-up and distribution and also for testing interior furnishing materials - leather, wood and synthetics on wear and tear and material resistance, including extreme climatic conditions In the mining industry : for inspection of motors and the respective bearings as an excellent complement to vibration analysis and for conveyors which can be many kilometers in length and have literally tens of thousands of rollers. In paper manufacturing processes an excellent means of evaluating one of the most difficult parts of the paper manufacturing process, the drying stage. Refineries - typically for process furnace maintenance - heater tubes for carbon scale buildup known as coking, refractory loss management, tank level verification, condenser fin diagnosis, and electrical distribution system maintenance. Coal based Power plant inspections - monitoring the coal stock on self-combustion and complex appli-cations such as turbine assessment. Electronics industry- Accurately understanding heat loads by visualization and quantification of heat patterns in electronic devices and managing heat dissipation without sacrificing performance or cost. Steel industry - routinely inspect iron ladles and torpedo cars and detect refractory break down before it occurs. And the Buildings inspections as part of heat loss studies and energy audits is a rapidly growing industry abroad Conclusion The endless applications using IR Thermography speak for themselves and confirm that this NDE technique is going to grow and contribute significantly towards reliable inspections, large cost savings, faster inspections, lower down times, higher productivity and convincing results thro visualisation and imaging. Acknowledgements We gratefully acknowledge the following websites wherein useful information was available related to the topic of the paper and could make it more informative. Proceedings of Inframation 2002 and 2003 Thanks are also due to all our seniors and colleagues at our sites and HQ without whose help the above work could never materialise.