Pumps – Processes
Reexamination Certificate
1998-01-20
2001-04-17
Thorpe, Timothy S. (Department: 3746)
Pumps
Processes
C417S282000, C417S309000
Reexamination Certificate
active
06217288
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to a method and apparatus for controlling process-limiting variables of a turbocompressor network. More specifically, the invention relates to a method for limiting a critical variable of either a group of compressors (compressor station) or an individual compressor when these machines are subjected to large, fast disturbances.
BACKGROUND ART
The present-day method of limiting a critical variable—discharge pressure or suction pressure or pressure ratio—of either a group of compressors (compressor station) or an individual compressor, is carried out by two Proportional-Integral-Derivative (PID) loops: (1) a main-controller loop that acts on the performance of the compressors, such as rotational speed by way of speed controllers, or guide-vane position by way of performance controllers; and (2) a limiting-controller loop of the critical variable that acts on antisurge (recycle) valves by way of antisurge controllers. However, the effect of the main controller (as defined by gains of proportional, integral, and derivative components) is limited by stability conditions for controlling an inertial object like a gas pipeline, and by the rate of load changes (if high) of the gas pumping equipment (including drivers), which can reduce the pumping equipments' longevity.
In its current operating mode, a limiting controller often cannot prevent an emergency shutdown after encountering large, fast disturbances because its set point is very close to an emergency shutdown set point. Thus, the limiting controller initiates action to start opening the recycle valves when the critical variable is close to the emergency shutdown set point. But the stroke time of these valves is usually not less than 2 seconds, and there may be significant volume in the recycle loops; therefore, opening the valves might not prevent emergency shutdown.
Consequently, at the onset of sudden, large changes of mass flow at a compressor's or a compressor station's discharge, the effectiveness of the main and limiting controllers (acting independently) may be insufficient to prevent discharge pressure from reaching the shutdown set point which, in turn, disrupts the process supported by the compressor(s) and decreases the life of the driver(s). Similar problems might occur while limiting the suction pressure in the event of sudden, large changes of mass flow in a compressor's or a compressor station's suction.
DISCLOSURE OF THE INVENTION
The purpose of this invention is to improve upon the prior art by providing a group of compressors or an individual compressor with a method for limiting a critical variable when these machines are subjected to large, fast disturbances. An example of a group of compressors is a compressor station of a gas pipeline; examples of critical variables are discharge pressure, suction pressure, and pressure ratio.
Each compressor is equipped with an antisurge controller whose output modulates an associated antisurge (recycle) valve. And the performance of each compressor is varied by a guide-vane position or rotational speed. In applications with compressors having variable guide vanes, each compressor is equipped with a performance controller which affects the guide-vane positions. In variable speed applications, each driver is supplied with a rotational-speed controller that modulates a final control element. Every group of compressors, as well as every independent compressor, is equipped with two PID controllers: (1) a main controller for compressor critical variables that affects guide-vane position or rotational speed, through the corresponding controller; and (2) a critical-variable limiting controller that participates in modulating recycle valves. Increasing the effectiveness of limiting the critical variable is realized by the main controller's first and second output signals:
The first output signal is used as a set point for the compressor performance controllers to position compressor guide vanes, or as a set point for rotational-speed controllers.
The second output signal corrects the output signal of the limiting controller by comparing against the limiting controller's threshold values, and if higher, by (1) summing with the limiting controller's predefined set point, or (2) summing with the limiting controller's output signal, or (3) performing both operations simultaneously.
As described above, upon achieving the limiting controller's thresholds, the main controller's second output signal can correct the output signal of the limiting controller, thereby increasing the effectiveness of critical-variable limiting because the limiting controller is employed earlier. Accordingly, the collaboration of the main and limiting controllers ensures the reliability of compressor stations and of individual compressors when subjected to large, fast disturbances.
REFERENCES:
patent: 5032062 (1991-07-01), Peterson
patent: 5046928 (1991-09-01), Peterson
patent: 5052899 (1991-10-01), Peterson
patent: 5195875 (1993-03-01), Gaston
Mirsky Saul
Staroselsky Naum
Compressor Controls Corporation
Henderson & Sturm LLP
Thorpe Timothy S.
Tyler Cheryl J.
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