REVIEW ON CONDENSER AIR LEAK TEST

Transcription

1 Volume 119 No , ISSN: (on-line version) url: REVIEW ON CONDENSER AIR LEAK TEST 1 Neethu Sathyan M, 2 Dr.Golda Dilip, 3 Dr.G.Victo Sudha George Research Scholar, Saveetha School of Engineering, Saveetha University, Chennai, India. Associate Professor, Saveetha School of Engineering, Saveetha University, Chennai, India. Professor, Dr.MGR Educational and Research Institute, University,Chennai,India 1 nshaiju@gmail.com, 2 goldadilip.sse@saveetha.com, 3 sudhajose72@gmail.com ABSTRACT In steam turbine based power generation systems, the performance of steam turbine drops due to low condenser vacuum because of air ingress. One of the major challenges in this area is early and accurate detection of air in leak point of condenser. The review focuses on different methods of condenser air in leak test and its importance. Different types of gaseous tracers test are popular in condenser leakage test. In Helium leak test, helium is sprayed around the surface of condenser, which is under vacuum condition. At the outlet of the vacuum pump, low concentration of the helium gas is detected by the mass spectrometer. SF6 is another major gaseous tracer used in industry. Ultrasonic Acoustics (UA) method can be used to detect air ingress into the condenser. As air is drawn into the condenser creates turbulence with strong ultrasonic components and which can be identified by specialized detection equipment. Infrared thermal image method is an emerging technology for condenser leakage test. High resolution thermal images of condenser surface can be captured by the advanced class of infrared cameras. The surface temperature pattern obtained from the thermal image of condenser is used to identify air ingress into the condenser. Tracer gas method is widely used in industry. Gas tracer test is time consuming and experts are required for detection of leakage in condenser. Tracer gases are costly and reversely affect the different components of power plant. High acoustic noise in plant area is the major challenge for Ultrasonic Acoustic method, which adversely affect accuracy of the condenser leakage test. Infrared thermal image method is none contacting and easy image capturing using infrared camera in which data is in digital form. Accuracy of the data depends upon the resolution of infrared camera. Application of advanced image processing tools can also improve the accuracy of condenser air leak test. Keywords: Air ingress, Condenser, Gas tracer, Infrared image,steam turbine, Ultrasonic Acoustic. 1. INTRODUCTION In steam turbine based power plant the basic principle of power generation is Rankine cycle. In Rankine cycle the major components are boiler feed pump, boiler, turbine and condenser. Boiler feed pump pressurize the water into very high level and transport to boiler. In boiler heat is added to the water and become high pressure super heated steam. Turbine extracts work from 3197

2 this high pressure super heated steam and convert it into low pressure wet steam. When the steam exits from the last stage of steam turbine, it becomes low pressure and low temperature wet steam. Condenser converts this low pressure wet steam in to water, which is then feed to the boiler to continue the cycle. In condenser the steam to water conversion leads to reduction of specific volume, hence condenser operates in vacuum. Deterioration of condenser vacuum is one of the major challenges in thermal power plants [1]. Condenser vacuum is maintained by vacuum pump, which removes dissolved gases from condenser. This vacuum increases the power out and efficiency of Rankine cycle, since pressure gradient across the turbine determine power output and efficiency. Hence condenser vacuum is one the important monitoring parameter in thermal power plant. The condenser is convert steam into water at a pressure less than atmospheric pressure. Air cooled condensers, water cooled condensers and evaporative condensers are different types of condenser. Evaporative condensers cooled by the evaporated water which taken away the heat from the condenser. An Air Cooled Condenser (ACC) is a direct dry cooling system where the steam is condensed inside air-cooled finned tubes. Water-cooled condensers are heat exchangers, which exchange heat to the cooled water and steam become condensed. Condenser performance can be influenced by a wide range of factors such as air in-leak, vacuum equipment capacity,air venting efficacy, other factors related to plant operating conditions. Air ingress to condenser has adverse affect on plant performance and equipment life. High air in leakage can increase condenser back pressure, which leads to the reduction of efficiency and power out of turbine. In addition to that air ingress increases the dissolved oxygen in the cycle, which in turn reduces life of equipment. Hence detection of air ingress into the condenser is an important work in power plant. The most common method to identify condenser air in-leakage test is gaseous tracer gas leak test, in which helium is used as a tracer gas [3]. In this method tracer gas is sprayed around the portion of condenser where leak in is suspecting and low concentrations of tracer gas is detected by mass spectrometer at the outlet vacuum pump. Based on the trace of helium, air leak in point can be identified. Helium is used as a tracer gas, because of the non-toxic, non-flammable nonreactive Condenser leakage test properties and quick diffusion though small leaks. SF6 is also used as tracer gas. In another condenser leakage test, instead of Helium use SF6 as tracer gas. It will follow the similar methods of helium tracer gas. Alternate method is ultrasonic leak detection, which is having limited success because noisy plant environment. 2. CONDENSER The condenser is convert steam into water at a pressure less than atmospheric pressure. In Condenser heat of low pressure steam is removed by circulating cooling water. During condensation process vapour is changed to water by removing latent heat energy. The condenser is a large shell-and-tube type heat exchanger. Steam flows through the shell side and cooling water flows through the tube side of the condenser. Water-cooled condensers are heat 3198

3 exchangers, which exchange heat to the cooled water and steam become condensed. Figure 1 shows the images of a water cooled condenser. Figure 1. Water cooled Condenser Condenser performance can be influenced by a wide range of factors such as air in-leak, vacuum equipment capacity,air venting efficacy, other factors related to plant operating conditions. Air ingress to condenser has adverse affect on plant efficiency and equipment life. High air in leakage can increase condenser back pressure, which leads to the reduction of efficiency and power out of turbine. In addition to that air ingress increases the dissolved oxygen level in the cycle, which in turn reduces life of equipment. Hence detection of air ingress into the condenser is an important work in power plant. 3. EFFECT OF AIR INGRESS IN CONDENSER Deterioration of condenser vacuum is one of the major challenges in thermal power plants. The main causes of vacuum deterioration are low cooling water flow, fouling of condenser tubes, cooling tower problems and air ingress to the condenser. Air ingress to condenser reduces the heat transfer and increases the condenser back pressure. Air ingress to the condenser can be through different means such as flanges of valves, expansion bellow, joints, seals, passing of valves and other joints. Identification air ingress point of condenser is a difficult task for thermal power plant customers. By reducing the turbine exhaust pressure,the thermal efficiency of a power plant can be increased.it can be achieved by enhancing the heat transfer rate in the condenser tube. Air ingresses in condenser leads to increase the turbine exhaust pressure,which in turn decreases power output and plant efficiency. Figure 2 shows the impact of air ingress in output power generation, gross power output decreases with increase of condenser absolute pressure (decreases the condenser vacuum). 3199

4 Figure 2. The impact of air ingress in output power generation 4. CONDENSER AIR INGRESS TEST The most common method to identify condenser air in-leakage test is gaseous tracer gas leak test, in which helium or SF6 is used as a tracer gas. In this method tracer gas is sprayed around the portion of condenser where leak in is suspecting and mass spectrometer detects low concentrations of tracer gas at the outlet of vacuum pump. Alternate method is ultrasonic leak detection, which is having limited success because noisy plant environment. Infrared thermal image method is an emerging technology for condenser leakage test. High resolution thermal images of condenser surface can be captured by the advanced class of infrared cameras. 4.1 Gaseous tracer helium Helium leak test is considered as the most common method of condenser air in-leakage test, in which helium is used as a tracer gas and a mass spectrometer use to detect helium. In this method helium sprayed around the portion of condenser where leak in is suspecting and mass spectrometer detects low concentrations of helium at the outlet of vacuum pump. Based on the trace of helium, air leak in point can be identified [4]. The helium is used as tracer gas because of properties such as an inert gas, the non-toxic, non-flammable and quick diffusion though small leaks. Using the helium technique, the source of most air in-leaks can be located with the unit online. A helium monitor is installed in the discharge line of vacuum pump and trace of helium is monitored. The leak detection survey starts at the turbine deck level and proceeds from top to bottom of the unit, one deck at a time. Helium sprayed in one section at a time so that leak point can be identified without overlapping, otherwise, the ability to associate a response with a 3200

5 particular source may become impaired. Figure 3 shows Helium Mass Spectrometers Detect Trace Gases. Figure 3. Helium Mass Spectrometers Detect Trace Gases. In condenser helium leak test, heated water vapor from the vacuum pump discharge to be tested in the mass spectrometer to find the trace of helium. This hot water vapour is having potential to damage the mass spectrometer tubing. Another problem faced by this method is sensitivity variation of detection of helium gas trace based on the position of probe. Due to viscous laminar flow conditions helium detection sensitivity will be maximum, if the probe is place middle of cross section of pipe. The lowest detectible concentration of helium is one part per million above the background level, hence small leak in points cannot identify. The leak points are usually a long distance from the mass spectrometer, hence distance communication is required. 4.2 SF6 as Tracer gas Instead of helium tracer gas, Sulfur hexafluoride (SF6) is used as tracer gas. In this method Sulfur hexafluoride sprayed around the portion of condenser where leak in is suspecting and mass spectrometer detects low concentrations of Sulfur hexafluoride at the outlet of vacuum pump. Based on the trace of Sulfur hexafluoride, air leak in point can be identified [4]. It offers lower ambient background concentrations, lower cost per unit test than any other tracer. Detection accuracies can be 40 times better than helium tracer. Commercial SF6 analyzing equipment provides greater sensitivity and has lower maintenance requirements than any other tracer system. The low concentrations of SF6 necessary for detection, low solubility of SF6 in water and high efficiency of air removal systems on noncondensible gases can reduce the portability that measurable amount of tracer gas is carry over into feed water system. Hence reduce fluoride contamination on stainless steel component. 4.3 Ultrasonic Acoustics detection 3201

6 Ultrasound detection has become an alternative approach for vacuum leak detection. Ultrasound is used by many power plants for condenser leak detection [5]. When used for process condition monitoring, an ultrasound detector works by detecting ultrasound produced by the turbulent flow of a pressure or vacuum leak. As a gas or liquid escapes from one higher-pressure system to the lower-pressure side, the molecules become agitated. The turbulence produces sound pressure variations at frequencies all along the spectrum from about 20 Hz up to 100 khz. The amplitude or intensity of the sound at the source of the leak is dependent upon a number of factors, including pressure differential, directional radiation pattern, humidity, temperature and the physical characteristics of the crack. The cost of a high-end ultrasound detector required for condenser leak detection is less than a helium leak detection system and requires less training. 4.4 Infrared image Recent evolutions in infrared cameras have become popular with power plant industry for hot spot detection. Infrared cameras have been used successfully to capture the thermal image of any object. Thermal image of condenser surface can be easily capture by the infrared camera and transfer to computer for analysis [5] [6]. The comparative analysis of condenser thermal image can detect the air in leak points. Figure 4 Thermal Image of a condenser. Comparative analysis required between non air ingress image and latest thermal image of a condenser. Images of condenser for different operating condition were captured using infrared camera and those images were stored in a database. During testing the latest captured image can compared with the images in the database. Condenser air in leak can be identified by spatial comparison of non air leak images with the testing image. The Figure.4 shows the infrared thermal image of a small section condenser. The equipments required for this method is advanced class thermal image camera such as Fluke TiX 1000.Fluke TiX 1000 is having high resolution of 2048X1536 and laser distance meter to measure distance between camera and the object. Infrared camera Fluke TiX 1000 is shown in figure

7 Figure 5.Fluke TiX COMPARATIVE STUDY Tracer gases are sprayed around the portion of condenser where leak in is suspecting and mass spectrometer detects low concentrations of gas tracer at the outlet of vacuum pump.gaseous tracers test are popular in condenser leakage test, it is time consuming and experts are required for detection of leakage in condenser. Tracer gases are costly and reversely affect the different components of power plant. In Ultrasonic Acoustic method, sound generated due to air flow from higher-pressure system to the lower-pressure side is used for detection of air ingress. The turbulence produces sound pressure variations at frequencies all along the spectrum. High acoustic noise in plant area is the major challenge for Ultrasonic Acoustic method, which adversely affect accuracy of the condenser leakage test. This method is fast but experts are required for detection of leakage in condenser. Thermal image of condenser surface can be easily capture by the infrared camera and the comparative analysis of condenser thermal images can detect the air in leak points. Infrared thermal image method is none contacting and easy image capturing using infrared camera in which data is in digital form. Accuracy of the image depends upon the resolution of infrared camera and application of advanced image processing tools can improve the accuracy. 6. CONCLUSION The review paper focused on different methods of condenser air in leak test and its importance. Different types of gaseous tracers test are popular in condenser leakage test. Helium leak test and 3203

8 SF6 leak test are the major gaseous tracers based methods. Ultrasonic Acoustics (UA) method can be used to detect air ingress into the condenser and it is considered as one of the advanced method. As air is drawn into the condenser creates turbulence with strong ultrasonic components and which can be identified easily by specialized detection equipment. Infrared thermal image method is an emerging technology for condenser leakage test. High resolution thermal images of condenser surface can be captured by the advanced class of infrared cameras. The surface temperature pattern obtained from the thermal image of condenser is used to identify air ingress into the condenser. The comparative study indicate that, tracer gas methods are widely used in industry but it is costly, time consuming and experts are required for detection of leakage in condenser. The accuracy of the Ultrasonic Acoustic method is reversely affected by high acoustic noise in plant area. Infrared thermal image method is fast, none contacting and the image capturing is in digital form. Accuracy of the image depends upon the resolution of infrared camera. Early identification of air ingress using thermal image comparison is a difficult task. Application of advanced image processing tools can improve the accuracy of Infrared thermal image condenser air leak test. REFERENCE [1] Matt Crockett Leak Testing Steam Turbine Condensers BRO Forum AEP Plant Engineering Programs Columbus (2009) [2] How to Diagnose/Troubleshoot Air Ingress 7th annual Air-Cooled Condenser Users Group, September 2015 [3] R Hollis, D Tondi Condenser Leak Detection Guidelines Using Sulfur Hexafluoride as a Tracer gas, Final Report, September 1988 [4] Anthony La Porte,Richard E. Putman The Practical Application of SF6 and Helium for Condenser Tube and Air Inleakage Detection EPRI, Condenser Technology Conference, August [5] Matt Crockett Predictive maintenance of BRO activities BRO Forum AEP Plant Engineering Programs Columbus (2009) [6] S. Bagavathiappan Infrared thermography for condition monitoring Infrared Physics & Technology Infrared Physics & Technology 60 (2013)

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