The varied availability of alternative energy sources, such as a solar and wind power, has contributed to combined-cycle power plants operating intermittently, sometimes many times a day. This cycling operation of gas turbines, for example, increases thermal stress to many components, specifically tube bundles within the associated heat recovery steam generators (HRSGs). Tube failures in HRSGs are among the most prevalent causes of unplanned outages at combined-cycle facilities.
A new, non-invasive technology to help eliminate these outages was discussed at the 2019 Emerson Global Users Exchange in Nashville, Tennessee. A large western utility worked with Emerson as well as Nooter/Eriksen in St. Louis, to deploy Rosemount 708 wireless acoustic transmitters to detect leaks in the HRSG tube bundles of the combined-cycle power plant.
"This is a classic example of pervasive sensing used in a new application," said Shane Hale, Emerson’s global director of business development for wireless, at Emerson. "This is a new application for wireless technology. It wouldn't make sense installing it in the traditional wired way. Because we don't have wires, we can install it in days, and, in many cases, less than a day."
With a legacy dating back more than 100 years, Nooter/Eriksen has long been a manufacturer and OEM for HRSGs. "They have supplied over 1,000 HRSGs each on the back end of a gas turbine as part of a combined-cycle facility,” said Chris Short, executive field sales for Emerson’s measurement and analytical business. “The HRSG creates steam from the hot exhaust of the gas turbine that is then fed to a generator. It also feeds high-pressure steam back to the turbine to generate additional power by helping with regeneration. These systems generate anywhere from 2 MW to 300 MW of power."
"These facilities were not designed for the thermal cycling they’re now seeing on a regular basis.” Emerson’s Chris Short, together with colleague Shane Hale explained the financial benefits of HRSG early-steam-leak detection at the 2019 Emerson Global Users Exchange.
Frequent cycling stresses tubes
The root cause of the tube-leakage problem is the variable availability of the alternative energy sources that were not around when these facilities were built. Today, intermittent production of wind and solar electricity requires these combined-cycle facilities frequently ramp up and down.
"The facilities were not designed for the thermal cycling they’re now seeing on a regular basis,” explained Short. “They were designed to run 90% of the time and not to be turning on when the wind stops or a large cloud passes overhead. The thermal shocks are stressing the system, and we have heard from our customers that tube leaks are the number-one failure leading to an unplanned outage in a combined-cycle facility. Typically, that means $100,000 to $200,000 of lost profit per day. And these unplanned outages can last anywhere from three to five days, depending on workforce and material availability."
The current way that most tube-bundle steam leaks are detected is by the presence of condensate in the exhaust gas found during manual inspection rounds. "A second way is by looking at the flow of steam makeup water, but small changes make small leaks very difficult to detect,” said Short. "A final way is to test for leaks when the turbine is down by pressurizing the tubes and listening for air leaks. But at that point, it is almost too late as it could take days to receive materials for repairs. All of these methods are very reactive."
A more proactive way is to use the acoustic sensor technology of the Rosemount 708 to detect steam leaks. "This wireless transmitter listens for ultrasonic acoustic signals, and it also has a temperature sensor," said Hale. "It was originally designed to detect failed steam traps, but, with the proven ability to listen, we looked for other applications. It turned out this is a really good application that otherwise took a manual check."
The sensor itself detects ultrasonic frequency sound in the 35 kHz to 45 kHz range. "It's beyond the audible range for humans, and beyond range for pump noise, for example, but right in the range of a steam leak," said Hale. "It talks wirelessly via a WirelessHART network, which makes installation quick and easy.”
Essentially this sensor listens for the ultrasonic whistle of a steam leak inside of an HRSG. "Two issues are how to detect a leak inside and where to mount the sensors," said Hale. "Fortunately, there are many places temperature is measured in an HRSG, and the thermocouple is a large metal tube, a thermowell, that extends into the HRSG. It turns out the thermowell is a good amplifier or microphone for an ultrasonic signal—an acoustic antenna. We simply strap the acoustic sensors to the thermocouples that already exist. It's a non-invasive installation that takes just minutes to install."
Dramatic project ROI
The acoustic transmitter does not provide a waveform or frequency of the sound detected because that would quickly drain the sensor's battery, explained Hale. "Instead, the sensor outputs acoustic counts between 0 and 255," he said. "The WirelessHART in the sensor is designed to give you as much information as you need, but it also needs to conserve battery. In this application, the battery will last five to 10 years depending on the configured update rate. If the actual waveform was transmitted, the battery would only last about a year."
Over time, when the monitored acoustic count goes up, there is a leak. "When the leak is detected, it doesn't mean the system needs to be shut down immediately; the increasing acoustic count indicates maintenance needs to plan to fix the leak during an upcoming shutdown. In the past, repairs of a leak found during a scheduled pressure test could take days."
“If you can detect the leak early, you can likely patch it to fix it quickly,” said Hale. "If not, the repair can be a major project, keeping the system down for several days instead of an eight-hour shift," he said. "It's a huge business advantage to know of the leak early."