Abstract: A turbocompressor's Surge Limit Line, displayed in coordinates of reduced flow rate (q.sub.r) and reduced head (h.sub.r), can be difficult to characterize if the slope of the line is small; that is, nearly horizontal. And it can be especially difficult to characterize if the surge line exhibits a local maximum or minimum, or both. This is often the case with axial compressors having adjustable inlet guide vanes, and for centrifugal compressors with variable inlet guide vanes and diffuser vanes. With their prime objective being the prevention of surge-induced compressor damage and process upsets, antisurge control algorithms should compensate for variations in suction conditions by calculating both the operating point and the Surge Limit Line, utilizing specific (invariant) coordinates derived by using the notations of similitude or dimensional analysis. The result is that the surge limit is invariant (stationary) to suction conditions.

Abstract: A method is provided for controlling fuel flow to the combustor of a gas generator turbine during sudden changes in load or during a surge cycle of the process turbocompressor. Surge control is initiated by analog input signals emanating from various devices located throughout the compressor-process system. The fuel control system includes input signals from the gas turbine driver. These signals are acted upon by the fuel control system which transmits a signal to a fuel valve actuator that controls the valve that meters fuel to the combustor. In addition to this sequence of control communication, however, is a rate-of-change in the amount of the fuel provided. The rate of the increase of the amount of fuel is determined by current operating functions of the fuel control system. However, because of the characteristics of general load rejection and recovery, and the abrupt nature of surge, the rate of change in the flow rate of fuel is extremely high.

Abstract: An improved method is provided for controlling fuel flow to the combustor of a gas generator turbine during sudden changes in load or during a surge cycle of the process turbocompressor. Surge control is initiated by analog input signals emanating from various devices located throughout the compressor-process system. The fuel control system includes input signals from the gas turbine driver. These signals are acted upon by the fuel control system which transmits a signal to a fuel valve actuator that controls the valve that meters fuel to the combustor. In addition to this sequence of control communication, however, is a rate-of-change in the amount of the fuel provided. The rate of the increase of the amount of fuel is determined by current operating functions of the fuel control system. However, because of the characteristics of general load rejection and recovery, and the abrupt nature of surge, the rate of change in the flow rate of fuel is extremely high.

Abstract: Balancing the load between compressors is not trivial. An approach is disclosed to balance loads for compression systems which have the characteristic that the surge parameters, S, change in the same direction with rotational speed during the balancing process. Load balancing control involves equalizing the pressure ratio, rotational speed, or power (or functions of these) when the compressors are operating far from surge. Then, as surge is approached, all compressors are controlled, such that they arrive at their surge control lines simultaneously.

Abstract: A method and apparatus are disclosed for preventing overspeed of a hot gas expander and its resultant load when the load (or a portion of it) is shed suddenly. In this situation, power applied by the expander to the shaft must be reduced, but normal feedback to the speed controller may be too slow to be effective. The load rejection may be sensed by a feedforward system before a significant speed increase is detected by the speed control loop. For this approach, the expander can be instrumented with either a flow measurement device, a downstream pressure, or a downstream temperature. Inlet pressure and inlet temperature are required. The method uses the characteristic map describing the expander (shaft power versus mass flow rate) along with a feedforward control action to anticipate the speed increase. By using dimensional analysis, the characteristic curves will collapse into single curves describing the parameters of reduced flow, reduced power, and pressure ratio.

Abstract: A method and apparatus are disclosed for controlling the rotational speed of gas and steam turbines, and hot gas expanders, with each of these drivers driving a rotational load by way of a shaft. To accomplish this control technique, it is necessary to easily and accurately calculate the amount of power which must be shed by the driver(s) to maintain a constant speed, i.e., neither accelerating nor decelerating. Action must be taken to reduce the power applied to the shaft by this amount. The method incorporates an open-loop approach and uses a time derivative to moderate the open-loop action accordingly.

Abstract: Compressors for processes, as used for refrigeration systems applied to ethylene production, are multiple stage machines; furthermore, sidestreams enter/exit between the stages. Since a flow measurement device is not available between stages, and the gas temperature entering most stages is unknown, it is difficult to calculate an accurate value for reduced flow for antisurge control purposes. A new method is described, whereby reduced flow alone is replaced by the product of the reduced flow and the equivalent speed. This allows accurate calculation of the distance of the operating point to the surge line since the inlet temperatures into the separate compression units (except the first) are not necessary. The invention described herein can be applied to multistage compression systems for a variety of processes.

Abstract: A method and apparatus are disclosed for protecting turbocompressors from unstable flow conditions (surge and stall). To accomplish this, it is necessary to easily and accurately calculate a compressor's operating point and its distance from the interface between the surge region and the stable region--this interface is referred to as the Surge Limit Interface. The proximity of the operating point to the Surge Limit Interface is calculated using measurements of properties throughout the compressor-process system. It is crucial that the calculation be invariant to suction conditions, especially gas composition. Disclosed are three coordinates, T.sub.r (reduced torque), P.sub.r (reduced power), and N.sub.e (equivalent speed). Each of these can be combined with other invariant parameters to construct coordinate systems in which to define the Surge Limit Interface and measure the distance of the operating point to that interface.