Condensers are used in power plants to recycle exhaust steam for thermoelectric heating, but they also are relied on to maximize turbine efficiency by maintaining a proper vacuum. Effective leak detection is needed to ensure that vacuum leaks do not occur.
Ninety percent of the power plants in the United States are fueled by coal, nuclear materials, natural gas and oil. These thermoelectric plants use the different types of fuel to boil water and create steam that turns the turbines to generate the electricity. Once the steam has passed through the turbine, it must be cooled back into water for reuse by the condenser. The exhaust steam from the turbine is condensed back into water by transferring the heat to the condenser coolant, typically cold water. A common condenser is the surface condenser, which also is called a water-cooled, shell-and-tube heat exchanger.
A secondary function of the condenser is to maximize turbine efficiency by maintaining a proper vacuum. Therefore, decreasing the operating pressure of the condenser (i.e., increasing the vacuum) increases the electric output of the turbine by increasing the enthalpy drop of the expanding steam. (Also called total heat, enthalpy is “a thermodynamic property of a system equal to the sum of its internal energy and the product of its pressure and volume.”)1 Operating the condenser at the highest vacuum increases plant efficiency, thereby allowing the plant to produce more electricity.
When a vacuum leak occurs in the condenser, noncondensable gases are introduced and must be vented. The gases increase the operating pressure, thereby reducing the turbine output and efficiency. The gases also decrease the heat transfer of the steam to the coolant and can cause corrosion in the generator.
Current Test Methods to Detect Leaks
Several methods for leak testing are used in the power plant. The most common method is helium leak testing.
Helium Leak Testing. For vacuum testing, a high vacuum pump and backing are used to evacuate the system of most gases. This creates the right condition for a mass spectrometer. Gases are ionized and accelerated through a magnetic field in the mass spectrometer, which isolates gas molecules by mass. This separation allows for the extremely small amounts of helium to be detected. Helium is introduced to the condenser system by spraying around the vacuum portions of the condenser. The mass spectrometer is placed at the gas outlet of the extractor or at other sites within the vacuum region of the condenser. For pressure testing, helium is introduced into the system and a sniffer probe is used on the outside to detect escaping helium gas.
Helium leak testing offers several advantages such as sensitivity (10-5 to 10-7 cc/sec), leak rate measurement, ability to seal leaks as soon as identified and ability to perform leak testing during normal plant operation. However, several disadvantages to helium leak testing exist, including:
- The results are operator dependent.
- Frequent calibration of equipment is necessary.
- The mass spectrometer is easily damaged in caustic environments.
- Use of one or more pumps along with the mass spectrometer or sniffer may require two people.
- Multiple leaks can be masked by one another if they are too close together.
- The helium may be blocked internally by a membrane or leak through an open valve before reaching the sniffer.
Ultrasound Detection. Ultrasound detection has become an alternative approach for vacuum leak detection. As tested and used by NASA on the International Space Station, ultrasound detection technology is now able to detect all turbulent flow gas and vacuum leaks2. Ultrasound is used by many power plants for condenser leak detection. In China’s power plants, for example, ultrasound was tested through a three-year pilot program for its performance to find condenser leaks, thereby improving plant efficiency and power generation output. Twenty-five percent of the Chinese power plants participated in the test with favorable results. The decision was made to institute a permanent program with the opportunity for all of the Chinese power plants to participate going forward.
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 ultrasound detector uses a transducer that is most sensitive to pressure changes around 40 kHz. The detected ultrasound is converted into the audible range of hearing (nominally, 20 Hz to 20 kHz) and output to a headset. The converted, amplified, filtered sound of the leak can be distinctly heard by a technician. Any sounds outside of 40 kHz (produced by the manufacturing environment or power generation plant) are inherently ignored by the ultrasound detector. Leaks can, therefore, be easily located in any noisy environment.
Advantages of ultrasound for leak detection include the following:
- Ease of Use. The user simply adjusts the sensitivity up or down on a small, handheld receiver to locate and pinpoint the leak. A user can quickly scan areas from distances up to 300’
- Directionality. Due to low amplitudes and short wavelengths, ultrasound travels in linear paths and does not tend to travel around corners or reflect. Leaks are not easily masked.
- Calibration Not Required. Ultrasound is used for indication and location, not measurement of leaks. The detectors are ruggedized for use in the most caustic of environments, including power plants.
- Sensitivity. Improvements to the technology have given the capability to find leaks faster and with greater confidence in order for condenser systems to run at normal and even improved vacuum levels to improve power turbine efficiency.
In addition, 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. In many power plants where ultrasound is deployed, a technician is used full time to scan the condenser, exchanger and multiple other systems for ultrasound. A 0.01 percent improvement in power generation pays for the integration and manpower of the technology in a matter of days.
Using Ultrasound Detection Technology
Power plants can take advantage of owning ultrasound technology by applying it to a variety of departments and systems when the detector is not in use for condenser vacuum leak detection. Ultrasound is produced by a variety of sources and can be used for steam traps and valves, condition-based monitoring and electrical inspection. The detector does not detect only leaks; it is an ultrasound detector.
Condition-based monitoring of critical bearings, motors and gearboxes — for indicating under lubrication, over lubrication and excessive wear — is a complement to infrared and vibration analysis. Ultrasound has an advantage over other predictive technologies due to the fact that faults appear first in the ultrasound range (about 40 kHz) before any indication of audible vibration, acceleration or heat. Additionally, ultrasound attenuates rapidly, allowing the user to pinpoint exactly which component is producing the bad ultrasound.
Ultrasound is also used to diagnose valves and steam traps. Contact the housing of the valve with the ultrasound receiver and a solid probe attachment. If the valve or steam trap is closed, there should be no ultrasound heard through the headset. If sound is heard, there is an internal bypass leak.
Electrical systems also can be tested for ultrasound produced by arcing, tracking or corona discharge. Infrared is used to indicate excessive resistance and load anomalies, but ultrasound is used to indicate leaking voltage, which can be a nuisance if it creates radio frequency interference (RFI). It may also cause catastrophic failure to transformers, relays and switchgear. Hissing or buzzing through the headset is an early indication of failure. A sound like frying eggs is an indication of imminent catastrophic failure.
Improving power generation efficiency and output is necessary to keep up with the growing demands of electricity usage. Leak detection in condensers can have a positive effect on the amount of electricity being generated. Helium leak detection is a sensitive method of finding the smallest vacuum or pressurized leaks in the condenser system. However, technological improvements to the sensitivity and signal-to-noise ratio of ultrasound detectors offer a faster, more portable alternative. Ultrasound technology should be considered for integration into the day-to-day maintenance and reliability tasks to improve power plant generation output.