Tube failure continues to be the leading cause of unplanned downtime in thermal power plants. Leaks occur in the tubes due to tube failures such as bending, cracking, bulging or wear. Reasons for the failures include ash, overheating, corrosion and erosion.
The reheater is critical to the performance and efficiency of the power-generation process in a thermal power plant. A typical thermal power plant uses the Rankine cycle model, which describes how steam-operated heat engines generate power by converting heat into mechanical work. In short, fuel such as coal is put into a boiler furnace to heat water (i.e., steam) to the right temperature and pressure. The steam is heated further with an economizer and put into a furnace, where the radiant energy is absorbed into the firewalls. By this point, the temperature of the steam has been raised from 700 to 800°F (371 to 426°C).
The steam goes into a superheater at just about the critical point in order to heat up to 1000°F (537°C). If the steam pressure and temperature are higher, there is greater efficiency by the engine for converting the steam’s heat into mechanical work. The steam then goes through the turbine, where it loses energy and, therefore, temperature and pressure. (Enthalpy is a measurement of energy, typically represented in Joules as the sum of internal energy and product of pressure and volume.)
From the turbine, most of the steam is exhausted to a reheater to be used again for a second, lower pressure series of turbines. The reheater pressure is just one-fourth of the original boiler pressure, but the temperatures are the same. This prevents the vapor from condensing during expansion and improves efficiency. The combination of turbines — high pressure/high temperature and low pressure/high temperature — is more efficient than reheating through burning fuel.
The steam then is exhausted from the low pressure turbines into the condenser, where cooling water is used to condense the steam into water. The phase change from steam to water creates a natural vacuum in the condenser, which also is aided by vacuum pumps. The condensed steam — referred to at this point as condensate water — is cleaned and sent back through the heating process. There, it will be converted from water to a supercritical fluid, which will continue the once-through cycle over again. The cooling water —referred to as circulating water — is sent from the condenser to the cooling tower. There, the water is air cooled using evaporative cooling to lower its temperature before the water is pumped back through the condenser.
Leaks Threaten Plant Efficiency
Vacuum can be lost in the turbine due to condenser air-in-leakage. If left unchecked, there will be a strain on the turbine blades. This strain can lead to erosion, eventual blade cracking or even blade liberation due to the strain caused by condenser backpressure (low vacuum caused by air leaks and inadequate cooling).
Reheater leaks are another problem. When the reheater is online, the boiler tube pressure is positive. When the reheater is offline, the valves are closed to keep water out of the system. The steam cools and condenses, creating a natural vacuum. A vacuum is further created by a vacuum pump. The typical vacuum level should be equal to the optimum condenser vacuum level (~ 25” Hg).
Power plants remain diligent about indication and location of leaks in boiler tubes because they are the major cause for outage and generation loss. An estimated 60 percent of boiler outages are due to tube leaks. The costs of repair, replacement and maintenance are extremely high. Reheater materials typically are replaced every 15 to 20 years at an estimated cost of $20 million.
Whenever the reheater is offline, personnel should perform a leak check using ultrasound in order to prevent possible minute leaks from getting worse. A gross leak will cause the gas temperatures and water level to drop. The gas temperatures should be in the range of 2000°F (1093°C). When the water level drops, more water must be added. In the case of a leak, about 100 to 500 gallons of water can be added per hour to make up for the loss.
Ultrasonic Leak Testing Method
An ultrasound leak detector with appropriate sensitivity can be used to indicate and locate leaks whenever there is a scheduled shutdown of the reheater. An ultrasound leak detector also can be used to detect and locate leaks whenever there is a drop in gas temperatures at water level (a common indication of a leak). The steps are as follows:
- The boiler is shut down.
- The valves are closed automatically in order to stop steam from flowing.
- The boiler goes into a cooldown process. This requires six to 10 hours, depending upon the ambient temperature. In order to preclude other problems from occurring, a 200°F per hour cooling rate is maintained.
- Once the cooldown process is complete, lock out/tag out of the reheater is performed. This allows the technician to walk or crawl into the confined spaces.
- At the same time, a vacuum is pulled from the condenser through the turbines to the reheater, caused by the condensing of steam and vacuum pumps. Anything connected to the turbines is under vacuum when offline. The vacuum level is verified to be a minimum 20” Hg, but the desired vacuum is 25” Hg.
- The ultrasound leak detector is used by the technician to sweep the reheater. A reheater can be more than 70’ long and 20’ deep. If there are no leaks, the entire leak detection sweep will take just a few minutes.
- If a leak is indicated — the technician hears a hissing sound within the headset — the location of the leak must be determined. This is performed using the principles of ultrasound (i.e.,directionality, sensitivity adjustments, shielding, different acoustic attachments, etc.) to pinpoint the exact location within the tube bundles. This process can take up to an hour, depending upon the location of the leak. In some cases, a solid-probe attachment is used to indicate the tube that is leaking. The location of the leak can be traced by moving along the tube bundle toward the loudest sound.
- Repairs should be completed as soon as possible. A code repair is performed, and documentation of the repair must be submitted to the insurance provider. Confirmation of the repair is indicated by a vacuum test or, in rare cases, a hydrostatic test.
- The ultrasound leak detector is used to confirm there is no sound coming from the leak location, and no leaks were created during the welding or installation of a new tube.
Taking Advantage of Ultrasound Detection Technology
Personnel at power plants can maximize the facility’s investment in ultrasound technology by applying it to other departments and systems when the detector is not in use for reheater leak detection. Ultrasound is produced by a variety of sources, and the leak detection technology can be used for steam traps and valves, condition-based monitoring and electrical inspection.
Condition-based monitoring of critical bearings, motors and gearboxes for indicating inadequate or excessive lubrication and excessive wear is a compliment to infrared and vibration analysis. Ultrasound has an advantage over other predictive technologies because faults appear first in the ultrasound range (about 40 kHz) before any audible vibration, acceleration or heat is experienced. Additionally, ultrasound attenuates rapidly, allowing the user to pinpoint the exact source.
Ultrasound also is used to diagnose valves and steam traps by contacting 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. (Leaking voltage can be a nuisance by creating radio frequency interference or even causing catastrophic failure to transformers, relays and switchgear.)
In conclusion, reheater and boiler tube failures continue to be the leading cause of unplanned downtime in power plants. A routine leak-detection program using ultrasound can be implemented to prevent catastrophic failure and improve the overall efficiency of the power-generation process. A robust, intrinsically safe ultrasound detector can offer quick payback with the detection of just one leak.PC
For more on condenser leaks check out the article http://www.process-cooling.com/articles/88004-comparing-leak-detection-methods-for-power-generation-condensers