Measuring and testing – Simulating operating condition – Marine
Reexamination Certificate
1998-10-09
2001-04-24
McCall, Eric S. (Department: 2855)
Measuring and testing
Simulating operating condition
Marine
Reexamination Certificate
active
06220086
ABSTRACT:
TECHNICAL FIELD
The present invention relates to apparatus and methods for determining the surge pressure ratio of a compressor.
BACKGROUND OF THE INVENTION
The global market for efficient power generation equipment has been expanding in recent years and is anticipated to continue to expand in the future. The gas turbine combined cycle power plant is a preferred choice for this type of equipment due to relatively low plant investment costs and continuously improving operating efficiency of the gas turbine-based combined cycle which minimizes electricity production costs.
It is well known that elevated firing temperature in the gas turbine is a key element in providing higher output per unit mass flow, enabling increased combined cycle efficiency and that for a given firing temperature, there is an optimal cycle pressure ratio which maximizes combined cycle efficiency. The optimal cycle pressure ratio trends higher with increasing firing temperature. Compressors for these turbines are thus subjected to demands for higher levels of pressure ratio consistent with other goals, such as minimal parts count, operational simplicity and low overall cost. This optimal level of cycle pressure ratio requires improved compression efficiency in the compressor, recognizing that the compressor must perform in an aerodynamically and aeromechanically stable manner under a wide range of mass flow rates associated with varying power output characteristics of the combined cycle operation.
The maximum pressure ratio that the compressor can deliver in continuous duty is commonly defined in terms of a constant margin in pressure ratio from the surge pressure ratio line. Presently, the surge line cannot be analytically determined with certainty nor can the aeromechanical response of the blading at pressure ratios near the surge line be determined. Analytical estimates of the location of the surge line must therefore be experimentally verified through compressor testing or compressor mapping tests. As will be appreciated, compressor surge is that low frequency oscillation of flow where the flow separates from the blades and reverses flow direction through the machine, i.e., it serves as a physical limit to compressor operation at a given speed.
Full-scale mapping of a compressor including the surge line has been previously accomplished over the entire range of operating conditions of the compressor. That mapping facility included, among other things, a compressor inlet throttling system; a reduced turbine first-stage nozzle area, and compressor discharge valves for bypassing compressor discharge flow about the turbine, as well as ancillary features. In that facility, the inlet throttling system employed a valve to regulate and reduce the flow of inlet fluid to the compressor inlet. The reduction in the first-stage turbine nozzle throat area was effected from production hardware by reforming the trailing edge at different angles. These and other modifications permitted attainment of high-pressure ratios above the nominal operating line by a slow steady-state approach to the surge line without over-firing the turbine. Details of that facility are described in an article titled “CTV—A New Method for Mapping a Full Scale Prototype of an Axial Compressor,” authored by Vasco Mezzedimi, Pierluigi Nava and Dave Hamilla and appearing in an ASME article published in 1996, the disclosure of which is incorporated herein by reference. In efforts to map the surge line of a further compressor, it was found using that system, however, that the reduction in first-stage turbine area was inadequate to permit attainment of surge pressure ratios before limits on exhaust temperature were encountered. That is, before the surge pressure ratio could be obtained, the maximum operational exhaust temperature for the turbine was reached and this could not be exceeded without damage to the machine.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, apparatus and methods have been developed to map the surge line of compressors without violating any machine operational limits, including the exhaust temperature limit. To map the surge pressure ratio without exceeding machine operational limits, the inlet valve is throttled to a steady-state point along a speed line to a pressure ratio short of, but in proximity to the surge pressure ratio. This reduction of fluid inlet to the compressor may be, for example, to approximately one-half atmosphere. While further throttling to a surge pressure ratio for that speed line via steady-state increments of the throttle valve position could not be accomplished without encountering operational limits on exhaust temperature at the gas turbine, it was found that the surge pressure ratio along a given speed line could be obtained by impulsively further closing the throttle valve, i.e., holding it transiently in a further closed position and then returning the throttle valve to its prior position. As a result, a transient increase in pressure ratio occurs approaching the surge pressure ratio without correspondingly reaching the exhaust temperature limit. That is, while the exhaust gas temperature escalates during the transient, it does not escalate sufficiently to reach the exhaust temperature limit before the surge pressure ratio or a data point very close to the surge pressure ratio is obtained. If the surge pressure ratio is not reached by this single transient operation of the throttle valve, one or more additional transient conditions are created by further decreasing inlet flow over substantially the same transient time period, thereby increasing the pressure ratio during the transient until a surge condition is reached. Between these transients, the compressor is allowed to return to a steady-state condition corresponding to the start of the transient.
While not being bound by a theory of operation, by reducing the air flow into the inlet of the compressor, the fuel/air ratio in the combustor increases, with the combustion system therefore firing hotter. When firing hotter, back pressure is applied to the compressor which increases the pressure ratio at that speed, ultimately to a surge pressure ratio. The present invention, however, accomplishes this transiently such that the turbine does not have sufficient time to respond to the higher combustion temperatures and thus does not exceed operational limits such as the exhaust gas temperature limit. It will, of course, be appreciated that the transients are separated along various speed lines so that a complete mapping of the surge line can be obtained for all speeds.
In a preferred embodiment according to the present invention, there is provided a method for determining the surge pressure ratio of a compressor of a gas turbine, comprising the steps of transiently reducing flow of fluid into the inlet of the compressor of the gas turbine to increase compressor pressure ratios sufficiently to produce a surge pressure ratio and ascertaining the surge pressure ratio.
In a further preferred embodiment according to the present invention, there is provided a method of determining the surge pressure ratio of a compressor for a turbine, comprising the steps of (a) establishing a steady-state compressor operating condition with reduced flow of fluid to the compressor inlet, (b) substantially instantaneously further reducing the flow of fluid to the compressor inlet, (c) maintaining the further reduced flow of fluid to the compressor inlet for a period of time insufficient to exceed operational limits of the compressor and sufficient to increase the compressor pressure ratio, (d) immediately subsequent to step (c), increasing the flow of fluid to the compressor inlet to reestablish the steady-state operating condition of the compressor and (e) determining whether the increased pressure ratio constitutes a surge pressure ratio.
It is a primary object of the present invention to provide apparatus and methods of establishing surge pressure ratio in a compressor without exceeding operational limits and with minimal instrumentation and cos
Andrew Philip Lynn
Mazzoni Marco
Nava Pierluigi
Stampfli John David
General Electric Co.
McCall Eric S.
Nixon & Vanderhye
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