Due to the advancement of technology, the latest generation of ultrasound instrumentsis able to store more data than before, analyze data through signal analysis (fast Fourier transform (FFT) or timewaveform), take photographs, record temperature readings, record revolutions per minute (rpms), have advanced download/upload capabilities, include report writing, perform cost analysis of air and steam leaks, feature Wi-Fi and, in some cases, be able to use an accelerometer during the inspection for overall diagnostics of a motor or application. In many cases, it is a stand-alone technology.
Infrared, vibration and ultrasound are just a few of the instruments that makeup today’s maintenance team’s predictive maintenance instruments. While vibration and infrared technologies seem to be used as complementary technologies, ultrasound seems to be left out of the complementary technology equation.So why have so many techs left ultrasound technology in the closet?
Several times this year, the question, “Do you use ultrasound as a complementary technology?” was asked of attendees in training classes, at conferences and in e-mail surveys.Several times, the answer was no, mostly due to a lack of ultrasound familiarization. So, that begs the question, “Other than leak detection, what do you use ultrasound for?”
Two Technologies Complement Each Other in the Field
With regards to ultrasound for electrical inspection, ultrasound can be used prior to opening any electrical switchgear or cabinet to complement the widely-used infrared instruments. Another example is theuseof two complementary technologies, ultrasound and infrared imaging, to inspect air cooled condenser (ACC) tubes for leaks.
Many technicians are well versed in the latest technologies, yet are also “old school,”meaning,“you use the tools necessary to complete the task.” Educating yourself on the different technologies is not new. Using complementary technologies is not new either. But, there are far too few instances of actually seeing or hearing of technicians using complementary technologies.
Air Cooled Condenser Leak Detection
ACC technology is no longer just used in areas without an abundant water source.Rather, it is used more and more throughout the world as an efficient means for cost-effective heat transfer. The direct dry air cooled condenser technology condenses turbine exhaust steam inside finned tubes. These tubes are cooled by forced ambient air instead of water taken from streams, lakes, or oceans. This is also known as “once-through water.”
Figure 1: Air cooled condenser using forced air fans for cooling(Drawing courtesy of Jim Hall, Ultra-Sound Technologies)
Steam from the turbine enters a steam distribution manifold on top of the ACC structure (Figure 1.). Steam is distributed into the finned tube heat exchangers arranged in a “roof structure” with an A-shape configuration (similar to an A-coil used in home airconditioners).
It should be noted that there are multiple, individual cells in an ACC, depending on the size of the generating unit. For example, one 350 megawatt plant in Wyoming has 66 individual cells, which is a large physical size to survey.
Cells are not tube bundle sections, but individual modules with their own fans. Each cell is separated by a metal wall to direct airflow up and through the tubes. Typically, there is a catwalk going through the center of the cells, with a door on each end of the cell. The motor, gearbox and fan are supported by the catwalk’s structural steel in each cell. In colder climates, many companies switch to adjustable frequency drives or variable speed drives so the fans can run slower. This helpscompaniesreduce freezing problems with the tubes.Conversely,companies run 10 percent overspeed during hot weather months to increase cooling capabilities. Reducing back pressure on steam turbines allows for more generation capabilities.
It would take a very long time to check for leaks if only using ultrasound. The best time to check for leaks is when the ambient air is cool enough to allow plant operations personnel to shut the fans off in each individual cell so technicians cando theirsurvey. This also significantly reduces competing airborne ultrasound.
Figure 2: A structure with forced air fan(Drawing courtesy of Jim Hall, Ultra-Sound Technologies)
Steam flowing down inside the tubes condenses due to the cooling effect of the air across the external finned surfaces of the tubes by the forced air from the fans. Forced air fans are located at the of[MC1] the A-shape framework in the fan ring section. Condensate drains capture the condensate from the heat exchangers into condensate manifolds and then drain to a condensate tank. The condensate is then pumped to the feed heating plant or boiler.
An ACC is under vacuum, just as a surface condenser. Ambient air and other non-condensable gases enter the steam from several sources, including leaks through the system. Non-condensable gases evacuate in a separate section of the ACC called the secondary section, which is connected to vacuum pumps or air ejectors that exhaust the non-condensable gases to the atmosphere.1
Air Cooled Condenser Tube Leak Locate
Figure 3: The air cooled condenser tube bundle;the larger tube is the 3-inch steam supply tube and the other tubes are the air cooled condenser finned tubes (under vacuum), part of the tube bundle
Figure 3 was taken at a power plant in Montana that utilizes an ACC for coolingsteam after it passes through the turbine. Note the larger tube to the right of center in Figure 3 is much larger than the surrounding finned condenser tubes. This larger tube is the three-inch steam supply.
The plant arranged for a survey of all the cells (condenser tube bundles) in the condenser to check for vacuum leaks prior to a maintenance shutdown soany leaks found could be repaired during the shutdown. The plan was to use an infrared camera to do the initial survey to determine the general location of any leaks.
Because the tubes in an air cooled condenser are under vacuum, any leak would allow ambient or outsideair to flow into the tube,causing less steam to flow through the tube and making any leaking tube appear to be cooler (see Figure 4).Once cooler tubes were located,airborne ultrasound would be used to verify and pinpoint the leaks.
Figure 4: Any leak (blue tube) would allow air to flow into the tube,causing less steam to flow through and making any leaking tube appear to be cooler
As seen in Figure 4, a leaking tube can be seen as much cooler than the surrounding tubes. But because it is designed as a cooling element with cooling fins on the tube, the exact location of the leak is hard to determine with infrared alone. An Ultraprobe® model 10,000[MC1] , with the frequency set at 40 kHz, was usedas well to locate and pinpoint the leak for repair. If the background or competing ultrasound is too great, the frequency on the unit may need to be lowered or increasedby one to threekilohertz to find tune or focus in on the leak.
Since the finned condenser tubes are 30 or more feet long, other accessories, such as atelescoping flexible wand,may be used to reach and inspect the tubes. Inspecting the tubes using only ultrasound can take a considerable amount of time, so this is a good example where two technologies complement each other by working together to complete a task.
Whenever using an ultrasound instrument for vacuum leaks, the enduser needs to be close to the suspected leak area as possible. Ultrasound detects turbulence, white noise and friction in the air. Since a vacuum leak is internal, the turbulence will be harder to hear from a distance.
The use of a telescoping pole, flexiblewand and sensor, and other accessories, as well as manipulation of the frequency, may aid in detecting the leak. However, most ACCs have a movable maintenance ladder that will allow the end userto get closer to a suspected leak.
Infrared, depending upon the resolution, may detect a leaking tube from a distance, allowing the enduser to focus in on the leaking tube before using ultrasound to pinpoint the leak.
Today’s technician is better educated and better equipped to provide a world-class maintenance program for a company. Knowing which technology can complement other technologies in the field makes this technician invaluable.
The well versed ultrasound technician will experience many technologies when performing ultrasonic inspections. For instance, ultrasound can be mated to a vibration instrument to aid the enduser in diagnosing bearing faults, such as an outer race. Or, ultrasound can be used to detect arcing inside a 480v switchgear cabinet prior to opening the cabinet. Or, hear corona at a distance that is not visible to the infrared or corona camera due to the need for lineofsight.
So, what technologies do you use to complement other technologies in your facility?
- Wurtz, William and Peltier, Robert. “Air-Cooled Condensers Eliminate Plant Water Use.Power Magazine, September 15, 2008.