Measuring and testing – Vibration – Sensing apparatus
Reexamination Certificate
2001-06-12
2003-03-25
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
Sensing apparatus
C073S593000, C073S602000, C073S659000, C702S056000
Reexamination Certificate
active
06536284
ABSTRACT:
This invention relates to non-intrusive techniques for monitoring the rotating components of a machine. More particularly, the present invention relates to a method and apparatus for pro-actively monitoring the health and performance of a compressor by detecting precursors to rotating stall and surge using frequency demodulation of acoustic signatures present in the measured signal.
BACKGROUND OF THE INVENTION
The global market for efficient power generation equipment has been expanding at a rapid rate since the mid-1980's. This trend is projected to continue in the future. The Gas Turbine Combined-Cycle power plant, consisting of a Gas-Turbine based topping cycle and a Rankine-based bottoming cycle, continues to be the customer's preferred choice in power generation. This may be due to the relatively-low plant investment cost, and to the continuously-improving operating efficiency of the Gas Turbine based combined cycle, which combine to minimize the cost of electricity production.
In gas turbines used for power generation, a compressor must be allowed to operate at a higher pressure ratio to achieve a higher machine efficiency. During operation of a gas turbine, there may occur a phenomenon known as compressor stall, wherein the pressure ratio of the compressor initially exceeds some critical value at a given speed, resulting in a subsequent reduction of compressor pressure ratio and airflow delivered to the combustor. Compressor stall may result from a variety of reasons, such as when the engine is accelerated too rapidly, or when the inlet profile of air pressure or temperature becomes unduly distorted during normal operation of the engine. Compressor damage due to the ingestion of foreign objects or a malfunction of a portion of the engine control system may also result in a compressor stall and subsequent compressor degradation. If compressor stall remains undetected and permitted to continue, the combustor temperatures and the vibratory stresses induced in the compressor may become sufficiently high to cause damage to the gas turbine.
It is well known that elevated firing temperatures enable increases in combined cycle efficiency and specific power. It is further known that, for a given firing temperature, an optimal cycle pressure ratio is identified which maximizes combined-cycle efficiency. This optimal cycle pressure ratio is theoretically shown to increase with increasing firing temperature. Axial flow compressors, which are at the heart of industrial Gas Turbines, are thus subjected to demands for ever-increasing levels of pressure ratio, with the simultaneous goals of minimal parts count, operational simplicity, and low overall cost. Further, an axial flow compressor is expected to operate at a heightened level of cycle pressure ratio at a compression efficiency that augments the overall cycle efficiency. An axial flow compressor is also expected to perform in an aerodynamically and aero-mechanically stable manner over a wide range in mass flow rate associated with the varying power output characteristics of the combined cycle operation.
The general requirement that led to the present invention was the market need for industrial Gas Turbines of improved combined-cycle efficiency and based on proven technologies for high reliability and availability.
One approach monitors the health of a compressor by measuring the air flow and pressure rise through the compressor. A range of values for the pressure rise is selected a-priori, beyond which the compressor operation is deemed unhealthy and the machine is shut down. Such pressure variations may be attributed to a number of causes such as, for example, unstable combustion, or rotating stall and surge events on the compressor itself. To detect these events, the magnitude and rate of change of pressure rise through the compressor are monitored. When such an event occurs, the magnitude of the pressure rise may drop sharply, and an algorithm monitoring the magnitude and its rate of change may acknowledge the event. This approach, however, does not offer prediction capabilities of rotating stall or surge, and fails to offer information to a real-time control system with sufficient lead time to proactively deal with such events.
BRIEF SUMMARY OF THE INVENTION
The operating compressor pressure ratio of an industrial Gas Turbine engine is typically set at a pre-specified margin away from the surge/stall boundary, generally referred to as surge margin or stall margin, to avoid unstable compressor operation. Uprates on installed base and new products that leverage proven technologies by adhering to existing compressor footprints often require a reduction in the operating surge/stall margin to allow higher pressure ratios. At the heart of these uprates and new products is not only the ability to assess surge/stall margin requirements and corresponding risks of surge, but also the availability of tools to continuously predict and monitor the health of the compressors in field operations. The present invention affords a method of compressor health prediction, monitoring, and controls that may be leveraged to be acted upon for protecting the compressor from being damaged due to stall and/or surge.
Accordingly, the present invention solves the simultaneous need for high cycle pressure ratio commensurate with high efficiency and ample surge margin throughout the operating range of a compressor. More particularly, the present invention is directed to a system and method for pro-actively monitoring and controlling the health of a compressor by identifying stall precursors using frequency demodulation of acoustic signatures. In the exemplary embodiment, at least one sensor is disposed about a compressor casing for measuring at least one compressor parameter, such parameter may include, for example, pressure, velocity, force, vibration, etc. Sensors capable of measuring respective relevant parameters may be employed. For example, pressure sensors may be used to monitor pressure signals, flow sensors may be used to monitor velocity of gases. Upon collecting a pre-specified amount of data, the data are time series analyzed and processed to produce a signal whose amplitude corresponds to the instantaneous frequency of a “locally dominant” component of the input signal, where “locally dominant” is defined with respect to an established reference frequency lying within the spectral region (i.e., frequency range or bandwidth) passed by the band-pass filter (BPF). The frequency demodulated signal (y) is low-pass filtered to remove noise interference and subsequently processed to extract signal characteristics such as, for example, signal amplitude, rate of change, spectral content of the signal, the signal characteristics representing stall precursors.
The stall precursors are then compared with baseline compressor characteristics which are a priori computed as a function of the underlying compressor operating parameters, such as, for example, pressure ratio, air flow, etc., and the difference is used to estimate a degraded compressor operating map. A corresponding compressor operability measure is computed and measured with a design target. If the operability of the compressor is deemed insufficient, protective actions are issued by a real-time control system to mitigate risks to the compressor to maintain the required level of compressor operability.
In another embodiment, the frequency demodulation algorithm, band-pass and low-pass filtering operations may be implemented using analog circuitry to produce an output signal that is sampled and then processed to obtain stall precursors. The stall precursors are subsequently compared with baseline compressor values to determine the health of the compressor and initiate any protective actions deemed necessary.
Some of the corrective actions may include varying the operating line control parameters such as making adjustments to compressor variable vanes, inlet air heat, compressor air bleed, combustor fuel mix, etc., in order to operate the compressor at a near threshold level. Preferably
General Electric Company
Nixon & Vanderhye P.C.
Saint-Surin Jacques
Williams Hezron
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