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Plant and Equipment Wellness, Part 1: Observing Variability
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Published January 8, 2008
A Surge Limit Line (SLL) is the line connecting the various surge points of a compressor at varying RPMs. The set point of the anti-surge controller is represented on the compressor map shown in Figure 4 by a line which runs parallel to the surge limit line. This line is called the Surge Controller Line (SCL). The controller is then able to calculate the deviation from the operating point to the SCL.
The compressor surge limit is not fixed with respect to any one measured variable such as compression ratio or pressure drop across the flow meter. Instead, it is a complex function that is dependent on the gas composition, RPM, suction temperature, and pressure. A closed loop PI controller would be unable to prevent surge during large or fast disturbances. Therefore, such a controller would be unable to stop surge. Rather, the controller would simply cycle the recycle valve open and closed in response to successive surge cycles. For a PI controller to act quickly, the "b" value would need to be high. This would result in a decreased operating region for the compressor when the recycle valve is closed.
Thus, an open loop control is used in conjunction with the closed loop in an anti-surge controller. The overall configuration is shown in Figure 5. A Recycle Trip Line (RTL) is used between the SLL and the SCL. Small or slow distrubances are managed by the closed loop controller which keeps the compressor operating point to the right of the RTL. For large or fast disturbances, the compressor operating point will reach the RTL. At this point, the open loop control will be initiated. This will add a step change which is a function of the compressor operating point at the moment it reaches the RTL. In this manner, the fast opening valve will be sufficient to stop surging.
Adaptive gain is also used in the anti-surge controller. When the operating point moves quickly toward the SCL, the adaptive gain move the SCL toward the operating point.
Anti-Surge Valve Requirements
1. The valve must be large enough to prevent surging under all possible operating conditions. However, a valve which is too oversized will result in poor control.
2. Stroke speed - A fast stroke speed is very important.
3. Ensure adequate air supply to properly operate the valve.
4. Tubing run should be minimized to reduce lag time.
5. One or more volume boosters are required to ensure fast response and equal opening and closing time.
6. Fail position should be open
By: Prabhat Yadav, Indian Oil Corporation Limited, Guest Author
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