The operation of an electric power generator produces large amounts of heat that must be removed to maintain efficiency. Depending on the rated capacity of the generator, it might be air-cooled, hydrogen-cooled, water-cooled, or, in the largest capacity generators, a combination of water for the stator windings and hydrogen for the rotor.
Hydrogen cooling offers excellent efficiency thanks to the gas’s low density and high specific heat and thermal conductivity. However, hydrogen is highly combustible when mixed with air and can be dangerous if the concentration level builds in an unwanted area. Turbine generators will leak some hydrogen during normal operation and rely on proper ventilation to keep the hydrogen levels from being a safety and explosion risk. Thus, hydrogen gas safety is critical for power plant operators.
Hydrogen molecules are very light and small, and therefore hard to contain. Between outages, the wear and tear on valves, seals, and equipment can allow large leaks to form and hydrogen levels to build in areas that could affect plant safety. The amount of hydrogen added each day is carefully monitored. An increase in makeup hydrogen would need to be investigated to find the source of the leak.
Traditional methods for leak detection and repair (LDAR) tend to be slow and may not find the leak quickly enough to avoid a shutdown. A shutdown period could last two to three weeks, with multiple days dedicated to leak detection alone. The cost associated with an unscheduled shutdown can run into the millions of dollars for a generating plant. The industry would prefer to perform LDAR online to avoid unscheduled outages, but until now the ability to do so was limited.
Traditional Detection Methods
Methods for detecting leaks range from using a soapy solution to create bubbles on each potential component to using microelectronic hydrogen sensors (sniffers) to detect hydrogen over a wide area. The soapy solution is sufficient for checking a single component, but checking for a leak in an unknown location could take weeks. Also, this method only works for tiny leaks because too much hydrogen flow will push the solution aside without forming bubbles.
The sniffer is a hand probe that produces an audio-signal when in proximity of a leak. Although this is a relatively affordable detection method, the sniffing test has some drawbacks. Generators are typically well ventilated. This can dilute the hydrogen, making it difficult to detect unless one is in close proximity to the source. Ventilation airflow can also move the hydrogen quite far from the source, leading to “hits” without adequately narrowing down which component needs repair. Sniffers do not allow operators to see a leak, so there is always some guesswork involved and time lost in the search for the source.
A New Approach
As a more recent evolution in gas detection technology, infrared cameras have become much more popular with maintenance teams. Infrared, or thermal imaging cameras as they are also called, have been used successfully to detect insufficient insulation in buildings or to find heat-based safety hazards in electrical installations.
Optical gas imaging with thermal cameras came into use a few years ago, using sulfur hexafluoride (SF6) as a tracer gas. However, some utilities have concerns with using SF6 as a tracer gas due to the cost, global-warming potential (GWP = 23,900), and, in some cases, restrictions on expanded use of SF6.
FLIR Systems partnered with the industry to develop a new generation of optical gas imagers (OGIs) using a tracer gas that eliminates those concerns. The new FLIR GF343 optical gas-imaging camera (Figure 1) uses CO2 as a tracer gas, which is readily available at generating stations. CO2 is inexpensive, has a much lower GWP, and has a lot fewer restrictions on use versus SF6. This will allow broader application of OGI for finding leaks.
|1. Seeing is believing. An optical gas-imaging camera allows detection of even small leaks from a safe distance. Courtesy: FLIR Systems|
Because only a small concentration of CO2 (generally 3% to 5%) needs to be added as a tracer gas to the hydrogen to make leaks visible to the OGI camera, the purity level of the hydrogen in the turbine is maintained, and normal generating operations are allowed to continue. The FLIR GF343 gives engineers a new tool for finding the source of leaks, without a shutdown.
Detecting CO2 Tracer Gas
By adding a small concentration of CO2 (less than 5%) as a tracer gas to the hydrogen supply, the generator will still operate at a safe and efficient level. This allows the operator and maintenance teams to monitor and check for hydrogen leaks during full operation.
During tests in the U.S. and Italy, it was proven that the FLIR GF343 can visualize a small amount (approximately 2.5%) of CO2 as a tracer gas in the system when there is a leak, therefore helping maintenance crews find and pinpoint leaks (Figures 2 and 3), tagging them for repair during shutdown or for immediate repair of any significant leaks.
|2. Needle in a haystack. A leak in a pressure gauge may not be easily found using traditional methods. Courtesy: FLIR Systems|
|3. Pinpointing trouble. Gas leaks are clearly visible on the thermal image. Courtesy: FLIR Systems|
The benefit that the GF343 offers over other detection technologies is that leak detection can now be performed under full operation, saving time and money by reducing shutdown time. Shutdown time could be reduced by two or even three days, and with each day of shutdown costing as much as $100,000 (depending on the type and size of the generator) the payback and return on investment afforded by using CO2 as a tracer gas and the FLIR GF343 CO2 camera is significant.
But small leaks are not only very frequent; they can also turn into large leaks. With the new camera, maintenance teams can keep the atmospheric hydrogen concentration below the explosion limit more easily.
How the FLIR GF343 Works
The FLIR GF343 camera uses a focal plane array indium antimonide (InSb) detector, which has a detector spectral response of 3–5 micrometers (μm) and is further spectrally adapted to approximately 4.3 μm by use of cold filtering and cooling of the detector by a Sterling engine to cryogenic temperatures (around –334F or –203C). The spectral tuning or cold filtering technique is critical to the optical gas imaging technique and, in the case of the FLIR GF343, this makes the camera specifically responsive and ultra-sensitive to CO2 gas infrared absorption.
Practically, the gas absorbs the background energy in view of the camera, such as from the sky, ground, or other sources. The camera shows this energy absorption by way of thermal contrast in the image. The camera not only shows the spectral absorption but also the motion of the gas; hence, you visualize the gas as a “smoke” plume.
The GF343 has an additional frame subtraction technique, which enhances the motion of the gas.
The high-sensitivity mode (HSM) has been the cornerstone of detecting the smallest of leaks. HSM is in part an image subtraction video processing technique that effectively enhances the thermal sensitivity of the camera. A percentage of individual pixel signals from frames in the video stream are subtracted from subsequent frames, thus enhancing the motion of the gas and improving the overall practical sensitivity of the camera and the ability to pinpoint the smallest of CO2 gas leaks, even without the use of a tripod. ■
—Steve Beynon is business development manager for GF cameras and optical gas imaging systems in the European, Middle Eastern, and African markets for FLIR Systems Ltd.