Compressor Controls Corporation

Brett W Batson

Saul Mirsky

Krishnan Narayanan

Naum Staroselsky

Byron K Broussard

Serge Staroselsky

Vadim Shapiro

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. This disclosure describes a new method of antisurge control for turbocompressors, which uses combinations of invariant coordinates that differ from those revealed in the prior art. Subsequently, the key to this invention is that any combination (linear or nonlinear) of invariant coordinates is also invariant.

Method and apparatus for antisurge control of turbocompressors having

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. This rapid increase of fuel to the combustor may lead to dangerous excursions in temperature, resulting in high temperatures in gas turbine elements. Consequently, a new function is added to the fuel control system's operation whereby improved regulation of the rate of increase of fuel is achieved. This new function receives a signal that either surge or a change in load has been detected. Next, a signal is transmitted to the fuel valve actuator, regulating fuel to the combustor, thus power to the power turbine, thereby preventing overspeeding. After that, the rate of change of the fuel valve is limited for a period of time. Once recovery has been accomplished, the change rate of the fuel valve is gradually returned to a normal operating setting.

Prevention of parameter excursions during process compressor surge in

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. This rapid increase of fuel to the combustor may lead to dangerous excursions in temperature, resulting in high temperatures in gas turbine elements. Consequently, a new function is added to the fuel control system's operation whereby improved regulation of the rate of increase of fuel is achieved. This new function receives a signal that either surge or a change in load has been detected. Next, a signal is transmitted to the fuel valve actuator, regulating fuel to the combustor, thus power to the power turbine, thereby preventing overspeeding. After that, the rate of change of the fuel valve is limited for a period of time. Once recovery has been accomplished, the change rate of the fuel valve is gradually returned to a normal operating setting.

Prevention of parameter excursions in gas turbines

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.

Method and apparatus for load balancing among multiple compressors

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. Speed increase can now be largely avoided by employing the resultant values from any of the pairs of reduced power relationships (reduced power versus reduced flow rate squared, reduced power versus pressure ratio, and reduced power versus temperature ratio). The reduced flow rate needed to keep the expander and its loads from increasing speed can now be correlated to a valve position resulting in the reduced flow required to maintain speed. This valve position setpoint is applied as an open loop step change.

Hot gas expander power recovery and control

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.

Method and apparatus for overspeed prevention using open-loop response

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.

Method and apparatus for antisurge control of multistage compressors

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.

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Dallas Center, IA, United States of America

Brett W Batson