HRSG User's Group: Improving steam-plant reliability, durability and profitability
Upgrade Duct-Burner Performance with Advanced Instrumentation
Roberto Roubicek, SEI
Many combined-cycle plants use duct burners to boost steam production (Fig 1). Unfortunately, the temperature instruments—or thermocouples—that control these burners typically are antiquated, causing such problems as flame instability, lost electricity sales, and HRSG damage by a phenomenon that the Techs wryly call ‘flame lick.’
Figure 1: Duct burners in HRSGs provide both operational and financial benefits, but their instrumentation and controls systems often need upgrading. Photo courtesy Scientific Environmental Instruments, Inc.
Fortunately, these duct-burner problems can be prevented, and steam production can be maximized, by replacing these antiquated thermocouples with advanced acoustic pyrometers. Acoustic pyrometers were discussed in great detail at the April 2019 HRSG User’s Group Conference & Expo, in a training session led by Roberto Roubicek, of Scientific Environmental Instruments, Inc (SEI). The session began with the announcement that both the Comisión Federal de Electricidad (CFE)—the enormous electricity company in Mexico—and Petróleos Mexicanos (Pemex)—the enormous oil & gas company in Mexico—have formed new business divisions to build more cogeneration plants in Mexico—good news to the HRSG and gas-turbine manufacturers in the crowd.
Acoustic pyrometers are like digital-age thermocouples, except that instead of providing merely a single-point measurement of temperature, the pyrometers provide lines of temperature-measurement across the entire combustion zone, which enables more precise control of fuel-trimming, thus better performance from the burners. For some years now, acoustic pyrometers have been optimizing burner-performance at oil refineries, blast furnaces, cement kilns, and coal-fired steam plants, but they’re relatively unknown within the combined-cycle industry. Their unfamiliarity to us is a bit surprising, given the fact that they were developed first by the combined-cycle conglomerate, Siemens, in the 1980s. Back then, Siemens tested the technology for measuring exhaust-gas temperatures on its new gas turbines. Siemens didn’t bring the technology to market, but a few years later, SEI did, and today SEI holds the patent on it. Here’s a primer on the technology:
Acoustic pyrometers operate on the principle that the speed of sound in a gas is proportional to the temperature of that gas. Acoustic transceivers are mounted on the outside walls of the heater, boiler, or HRSG and from there they launch a high-intensity sonic signal into the flame-stream. The temperature of the flame across the entire flame-stream is computed precisely from the signal’s time-of-travel. Acoustic pyrometers can display the real-time temperature-distribution profile (Fig 2a & 2b), the trends in temperature during user-selected periods, or the average gas temperatures within user-selected zones. These indications provide Plant Operators and Maintenance Techs more useful, visual information, compared to the legacy controls on most of today’s duct burners.
Click HERE for a short video of acoustic pyrometers in action.
Figure 2a & 2b: Acoustic pyrometers provide more useful, visual maps of flame temperatures, instead of the single-point measurement provided by thermocouples. Photos courtesy Scientific Environmental Instruments, Inc.
In the Q & A that followed this training session, a Sales Engineer from a duct-burner supplier commented, “I’m quite interested in these new pyrometers, because our PLCs [programmable logic controllers], overall, have been very reliable, but our customers definitely have had problems with the field sensing devices connected to the PLCs—what I call the brawn behind the brains—such as the temp sensors that need to be calibrated frequently, and the valve-positioners that need to be adjusted.” That comment sparked much interest from the crowd, prompting another attendee, from a 550-MW plant in the US Northeast, to comment that, several years ago, his plant had invested a lot of money upgrading the microprocessors and software in their DCS (distributed controls system, or ‘the brains,’ of the plant-wide controls system), but when the project was over, the plant got no benefit from it, because their field devices, or ‘the brawn,’ were still the same old, inconsistent technology. “We learned,” he opined, “that you can bring the digital revolution into the Control Room, but it won’t do you any good, if you don’t bring it into the field, too.”
For more details on bringing the digital revolution to your duct-burner temperature sensors, visit http://www.sciengr.com/. Or, you can contact Roberto Roubicek directly at email@example.com.