Combustion Dynamics Monitoring – An Overview

Combustion dynamics monitoring is often perceived as a “black box” where magical things take place inside without a clear understanding of how they work or what they do. In this brief tutorial, we de-mystify the topic and explain why it is a vital part of protecting modern gas turbines.

Introduction

Although gas turbines have been used for power generation and mechanical drive applications since the late 1930s, and vibration monitoring has been standard on such machines since the 1960s, the necessity for combustion dynamics monitoring is a relatively new phenomenon, appearing in the mid-1990s.

At this point, it is natural for questions to arise such as “why did this type of monitoring wait to appear for nearly 30 years after vibration monitoring?” and “if industry lived without it for 30 years, is it really that important – or is it more of a luxury?”

The answer is quite simple: prior to the 1990s, combustion conditions and combustor designs made it unnecessary.

So what changed in the 1990s? Two words: firing temperatures.

Up until the 1990s, gas turbine firing temperatures did not exceed 2200° F (Table 1) and the exponentially greater production of NOx that occurs at elevated firing temperatures was not an issue. However, the need for greater efficiencies in gas turbines necessitated these increased firing temperatures, and various approaches were then required to mitigate the additional (and highly undesirable) NOx production as a pollutant.

The earliest mitigation technique was to simply cool the combustors with steam injection or water (known as Wet Low Emissions or WLE technology), but later so-called DLE (Dry Low Emissions) or alternatively DLN (Dry Low NOx) combustor designs were introduced. As with so many things in life, DLE was not without its problems, however. The nature of DLE / DLN combustion is to operate with as lean a fuel mixture as possible, and this results in so-called metastable flame conditions that must be monitored and controlled very closely to prevent flameout. Unfortunately, the instabilities that occur under lean burning conditions are essentially random in nature and affected by a set of factors that cannot be controlled, such as atmospheric conditions. As such, they cannot be easily modeled for control purposes – they must instead be monitored and used for feedback control.

While pressure pulsations inside the combustor(s) are always present, they do not become a problem until they start acoustically exciting the resonant frequencies of the combustor assembly, at which point they can (at worst) quickly destroy it if left unabated, or (at best, but still undesirable) contribute to the long-term fatigue of its elements and thus shorten its life. Additionally, if destruction occurs, the damage will not remain confined to the combustor. Instead, pieces will travel downstream into the turbine blades where they take an exceedingly expensive toll. To put this in perspective, Table 1 shows the approximate prices (circa 2015) for a first stage row of turbine blades as compiled in insurance tables. Thus, if anything, these are very conservative figures seven years later. As can be seen, combustion instability can result in millions of dollars of damage in replacement blade costs alone – not to mention the costs of lost production which may easily eclipse those of parts and labor, even with expedited shipping and overtime wages factored in.

Detection

To detect these combustion instabilities, gas turbine manufacturers began monitoring the characteristic frequencies associated with the instabilities using either dynamic pressure sensors directly observing conditions inside the combustors, or accelerometers mounted near the combustor section to indirectly pick up combustion issues mechanically transmitted to the turbine casing. As can be surmised, these frequencies are a complex function of the mechanical construction of the combustor as well as the way that air and fuel are injected, mixed, and burned inside the assembly to result in dynamic pressure pulsations that excite these frequencies. Because gas turbine manufacturers consider these details proprietary, the details of these frequencies are not readily available outside of an OEM’s personnel and partners.

Figure 1: Combustion dynamics are monitored via dynamic pressure sensors (left), accelerometers (right), or both depending on the OEM’s philosophy. Both sensor types must be able to endure extreme temperatures and harsh environments. Vibro-meter is a recognized leader in this field and our sensors are used by nearly every gas turbine manufacturer.

Detection of these frequencies is well within the capabilities of most vibration monitoring systems because they are designed to process and filter signals in roughly the same part of the spectrum (0-20kHz) as those in which these dynamic pressure pulsations occur. It was thus not a large step for vibration monitoring companies to adapt their instrumentation to accept and process the signals from dynamic pressure sensors rather than only from dynamic vibration sensors such as accelerometers and proximity probes. However, very narrow bandpass filtering is often required to isolate multiple specific frequencies from each sensor and thus the signal processing requirements incumbent upon combustion dynamics sensors will often equal or exceed those of vibration points.

A gas turbine manufacturer’s decision of whether to use dynamic pressure sensors, accelerometers, or both, is largely proprietary and based on deep knowledge of their machines’ designs and behaviors. As such, a variety of approaches are encountered in the field and vary not only by manufacturer, but by model of turbine. Vibro-meter has extensive experience with both types of sensors used to detect combustion instabilities and is indeed globally recognized as a leader in both high-temperature accelerometers and high-temperature dynamic pressure sensors able to operate for extended periods in the harsh environments of gas turbines.

Combustion Control

When the system detects combustion instability, a signal is sent to the turbine controller that the combustion is too lean and must be adjusted. In this manner, a closed feedback loop exists between the combustion control and the combustion monitoring systems. As the combustion is adjusted and exits the region of instability, the detection system will reflect this and no additional adjustment is necessary until something disrupts the combustion and incipient or full instability appears – at which time the combustion is adjusted again. The result is stable operation of the turbine with the leanest possible combustion and without damaging instabilities being allowed to persist for more than a few seconds.

Figure 2: Combustion dynamics monitoring is part of closed-loop control to ensure that damaging pulsations in low-NOx gas turbines do not prematurely age or destroy the combustor cans. The pulsations occur because today’s low NOx technologies rely on inherently metastable combustion conditions that burn fuel in the leanest possible manner but can produce an unstable flame if not meticulously controlled.

Monitoring

Both of vibro-meter’s flagship monitoring platforms support integrated vibration monitoring and combustion dynamics monitoring. For distributed applications, our VibroSmart platform is ideal with a DIN-rail mounted form factor. It is standard and preferred by a leading gas turbine manufacturer for several of its models where the controls are mounted on the gas turbine skid. For conventional centralized installations, our VM600^Mk2 platform is the right choice with a 19” rack-mounted form factor. It too is standard and preferred by several leading gas turbine manufacturers when the controls are mounted further from the machine using conventional field wiring runs.

Figure 3: The VM600Mk2 (left) and VibroSmart (right) platforms both offer integrated vibration and combustion dynamics monitoring. The VibroSmart platform is particularly suitable for packaged gas turbines that are skid-mounted and with controls that are likewise skid-mounted.

With the VibroSmart platform, combustion dynamics can be monitored by simply configuring the universal two-channel modules appropriately. With the VM600^Mk2 platform, combustion dynamics can be monitored either in our 4-channel MPC4^Mk2 modules occupying a single slot in the rack, or by means of our XMC16 modules – also occupying a single slot in the rack but able to address 16 channels instead of only 4 channels for larger installations. The combustion dynamics signals indicating normal or unstable conditions are in turn brought back to the turbine control system using a digital protocol such as Profibus or Modbus, allowing all channels over a single cable for minimal wiring.

Implementation

As noted earlier, the particulars of monitoring combustion instabilities are unique to each gas turbine OEM and are considered proprietary. Also, as was shown in Figure 2, the feedback from a combustion dynamics system serves as an input to the combustion control algorithm within the turbine control system. It is thus part of the OEM’s control scheme. For these reasons, although our platforms are able to implement the filtering, signal conditioning, and detection requirements of all leading gas turbine manufacturers, combustion instability is never a “do it yourself” exercise done in isolation from the turbine OEM. Simply know that regardless of whose gas turbines you may be using, whether Siemens, GE, MHI, Ansaldo, ABB, Alstom, Kawasaki, Hitachi, Rolls-Royce, Solar, Westinghouse, or any other², you can confidently choose vibro-meter sensor and monitors knowing that they will be able to address the OEM’s monitoring particulars.

Continued Innovation

While dynamic pressure sensors today rely on many of the same technologies as our piezoelectric acceleration sensors, this presents limitations and gas turbine OEMs can benefit from sensors that provide even more reliable detection of combustor problems. To this end, we are engaged in substantial R&D efforts surrounding the use of advanced, optical technologies for combustion dynamics sensors.

Figure 4: Fiber optic technology holds great promise for the next generation of dynamic pressure sensors being developed by vibro-meter for combustion dynamics monitoring. Shown here is our new Optima sensor, currently in development.

We anticipate being able to launch commercial versions of this technology to the marketplace shortly, and this will be the topic of future articles as we progress in our efforts. You can learn more about this technology in two recently presented technical papers^3,4.

Summary

This brief tutorial has explained what a combustion dynamics system does, how it exists in a closed loop with the turbine controller, and why it is necessary on modern gas turbines with higher firing temperatures and corresponding efficiencies while constrained by the need for lower emissions.

Vibro-meter was an early pioneer in the development of both suitably robust sensors and suitably capable monitoring systems for combustion dynamics when the need within industry first emerged in the 1990s with F-class turbine technology and its corresponding combustor designs. Our solutions can today be found on F-, G-, and H-class machines and we remain committed to this important segment of the market by offering leading technology and services trusted by gas turbine manufacturers around the globe.

You can learn more about our solutions for monitoring gas turbines on our gas turbine application page and in our 34-page whitepaper.

Footnotes

¹ Source: “Combined Cycle Power Plants” IMIA Working Group Paper 91 (15); pages 9 and 12, IMIA Annual Conference 2015, Merida (Yucatán), Mexico, 26-30 September 2015. Retrieved Sept 15, 2022.

² Many of the heritage brands listed here have been consolidated under Siemens, GE, and MHI following various acquisitions and mergers.

³ Nicchiotti, G, Page, SA, Solinski, K, Andracher, L, Paulitsch, N, & Giuliani, F. "Characterisation and Validation of an Optical Pressure Sensor for Combustion Monitoring at Low Frequency." Proceedings of the ASME Turbo Expo 2021: Turbomachinery Technical Conference and Exposition. Volume 4: Controls, Diagnostics, and Instrumentation; Cycle Innovations; Cycle Innovations: Energy Storage; Education; Electric Power. Virtual, Online. June 7–11, 2021. V004T05A005. ASME. https://doi.org/10.1115/GT2021-59103

⁴ Nicchiotti, G., Solinski, K. ., & Giuliani, F. (2021). “Lean Blowout Sensing and Processing via Optical Interferometry and Wavelet Analysis of Dynamic Pressure Data.” PHM Society European Conference, 6(1), 11. https://doi.org/10.36001/phme.2021.v6i1.2805

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