A NACE International paper, HRSG Tube Failures: Prediction, Diagnosis and Corrective Actions highlights the challenge of these failures:
Many HRSGs were originally designed for base load operation. Recent economic factors have led to drastically increased cycling of many units. This has exacerbated the rate of failures.
These leaks can lead to forced outages that create associated costs including fines, lost time and replacement power purchases, which can exceed $100,000 USD per incident.
Heat recovery steam generator (HRSG) tubes are stressed by thermal expansion and contraction of the tubes from the combustion turbine exhaust gas. Optimizing steam temperature control can extend the life of the tubes. Monitoring the tubes with acoustic sensors can also help provide early warning to address leaks before they lead to unplanned downtime.
I connected with Emerson’s Juan Panama who explained that fatigue and stress cracking can occur in welds or the tubes themselves, or that poor welds could cause pinhole leaks. Leaks in the tubes themselves (outside of a weld area) are typically caused by corrosion processes, localized overheating, or incorrect metallurgy.
Juan noted that small super heater and re-heater tube leaks are difficult to detect, as no visible indications are present.
Without sensors to detect leaks, plant staff traditionally has checked by going into an outage and shutting down the HRSG (to allow steam to condense back into water to make leaks more visible), or if a leak is large enough by observing an increase in water usage at the control system.
For one combined-cycle utility plant in the U.S., when a leak was large enough, the plant would experience a forced outage, requiring a crew on site after the unit cooled down (regardless of overtime), the cost of replacement power, or the cost of contracted natural gas for the period of the outage.
With the emergence of Industrial Internet of Things (IIoT) technologies, wireless acoustic sensors made continuous monitoring for tube leaks possible. Traditional wired instrumentation is often difficult to justify the installation expenses.
Juan highlighted a utility that installed wireless acoustic leak detection system using 12 Rosemount 708 wireless acoustic transmitters on each HRSG, plus an Emerson Wireless Gateway. For this utility, these wireless transmitters and WirelessHART network made their installation economically viable.
Within the first six months, this Plantweb digital ecosystem solution helped detect four leaks between two HRSG units for the utility. Detecting these leaks allows the utility to quickly address the leaks before they become more significant and lead to lost generation and asset damage.
Visit the Steam Temperature Control section on Emerson.com for more on this and other solutions to reduce forced outages and increase reliability and efficiency. The case study on the U.S. combined-cycle power plant is another great source of an example of how this wireless acoustic technology can improve overall performance.