1 anti-surge control system introduction

11 anti-surge control system instrument composition

Based on the SERIES3PLUS instrument, CCC has developed 24 anti-surge solutions for different operating conditions and control requirements of centrifugal compressors. The anti-surge control system of the company's compressor selects the FA32 control scheme, the compressor speed signal N output by the turbine speed regulator ST, the compressor's primary suction flow differential pressure signal p, and the compressor discharge. The pressure signal pd, the compressor suction pressure signal ps, is delivered to the anti-surge regulator UIC input channel, respectively. The output signal of the anti-surge regulator is sent to the valve positioner FY for controlling the opening of the regulating valve FV.

12 anti-surge regulator control function software structure

In order to take advantage of the control software of SERIES3PLUS instrument to ensure safe, smooth and efficient operation of the compressor, combined with the actual situation of the compressor, the company's anti-surge control function software uses the following combinations: 1) PI algorithm; 2) standard differential Response; 3) RECYCLETRIP response; 4) SAFETYON response; 5) discharge pressure limit PI control; 6) backup strategy control.

2 anti-surge regulator control function software

According to the characteristics of the compressor running curve, the SERIES3PLUS instrument adjusts the relative distance between the compressor operating point O and the anti-surge control line SCL by controlling the opening of the anti-surge regulating valve (Fig. 2) to prevent the compressor from running. Point to the purpose of entering the surge zone. However, the relative distance from the operating point of the compressor to the SCL line cannot be directly measured and must be calculated by multiple functions. According to BABCOCK's Compressor Design and Manufacturing Manual, the anti-surge control function line diagram SREIES3PLUS meter can calculate the Ss size according to the internal configuration of the multi-variable head characteristic function curve. Since the relative distance from the operating point of the compressor to the surge limit line SLL is (1-Ss), the compressor operating point is on the SLL line when Ss=1; when the compressor runs at (1-Ss)<0 On the left side of the SLL line, the operating point enters the surge zone; when (1-Ss) > 0, the compressor operating point is to the right of the SLL line. In anti-surge control, when the suction flow rate of the compressor is low, a sufficient circulation amount is required to ensure that the compressor does not surge, but on the other hand, the anti-surge valve opening degree of the compressor is required as much as possible. Small to minimize the energy consumption of the compressor and the highest efficiency. In order to properly handle the relationship between the two, an anti-surge control line SCL is set in the anti-surge control scheme (Fig. 2).

This line is formed by adding a safety margin value b to the SLL line. Similarly, if the ratio of the slope of the compressor running line OPL to the slope of the anti-surge control line is S, the relative distance of the compressor operating point to the anti-surge control line SCL is DEV=1-S, and the compressor operating point is The relative distance of the surge limit line SLL is equal to the relative distance from the compressor operating point to the anti-surge control line SCL plus the safety margin value, ie (1-Ss) = (1-S + b). When S=1, the compressor operating point is on the SCL line; when (1-S)<0, the compressor operating point is on the left side of the SCL line, the operating point may enter the surge zone, and the compressor suction flow rate must increase; When (1-S)>0, the compressor operating point is on the right side of the SCL line, and the compressor suction flow can be safely reduced.

21 anti-surge regulator PI control algorithm

Because the PI control algorithm is used to prevent small, slower interference is very effective, the SERIES3PLUS instrument uses the PI algorithm to overcome the small, slower interference common in the compressor operation, and always maintain the compressor as much as possible through the PI algorithm. The operating point operates on the anti-surge control line SCL.

22 standard differential response of anti-surge regulator

In normal operation, the operating point is basically near the SCL line. If the safety margin value b of the compressor can be kept as small as possible, the efficiency of the compressor operation is higher. When the compressor operating point moves rapidly to the surge zone under a large disturbance, if the compressor safety margin value increases rapidly, the PI algorithm can prevent the compressor from quickly entering the surge zone. The SERIES3PLUS instrument uses the standard differential response function to instantaneously increase the safety margin of the compressor. This ensures that the compressor has sufficient safety margin to reduce the impact of this interference on normal operation.

Therefore, in the standard differential response equation, when the derivative of Ss is positive (ie, the compressor operating point moves to the left), the output value of the standard differential response will automatically select the maximum value that has been calculated. If its derivative decreases, the standard differential response output value remains the same. However, when the derivative value is negative (ie, the compressor operating point moves to the right), the standard differential response value will be gradually reduced to 0 by the constant AG (AG = 0.5) set inside the meter. When the compressor is running When the point moves to the left of the RTL line, the standard differential response is aborted.

23 anti-surge regulator RECYCLETRIP response

The RECYCLETRIP response is used to prevent sudden pulse disturbances that are too fast for the PI algorithm to be too large and cause the compressor operating point to cross the RTL line to further protect the compressor from surge. Because the RTL line is an RT value attached to the SCL line (RT=15)

To determine the relative distance of the compressor operating point to the RTL line: dRT = 1 - S + RT = 1 + RT - b1 - nb2 - Ss. When dRT < 0, the compressor operating point crosses the RTL line and enters its left side. Then, the RT response is triggered, and the RT response output value will quickly open the anti-surge valve in a series of step changes, increasing the circulation of the compressor and preventing the compressor from surging.

The time interval during which each step output value changes is determined by the internally set constant C2 (C2 = 0.48). When the compressor running point returns to the right side of the RTL line, the RT response is aborted, and the output value of the RT response is exponentially attenuated by the attenuation rate TL (TL=120) set by the meter, so that the output is gradually reduced, and the compressor operating point is again Gradually return to the vicinity of the SCL line under the influence of the PI algorithm.

24 anti-surge regulator SAFETYON response

Although the PI algorithm, standard differential and RT response have been used to eliminate external interference with compressor operation, unexpected production process changes, compressor wear, severe interference, and occasional compressor surge, in order to limit compression The number of surges and the possibility of surge, the SERIES3PLUS instrument control software sets the last line of SAFETYON response lines (Figure 2). Since the SOL line is determined by adding the SO value (SO=10) on the SLL line, the relative distance from the compressor operating point to the SOL line: d=1-Ss+ SO. When d<0, the compressor operating point is located. On the left side of the SOL line, the SAFETYON response is triggered. Since the SAFETYON response formula is: nb2, which is the second term in equation (9), and the compressor operation has entered the surge region (n#1), the SAFETYON response is rapidly increased by the change in the number of surges n. The safety margin value b causes the SCL line to move rapidly to the right, thereby keeping the compressor operating point away from the surge zone, preventing the compressor from surging further.

The number of compressor surges n is determined by the SERIES3PLUS meter in each scan cycle (40ms) based on the compressor's suction flow rate or discharge pressure rate of change. When the derivative of the suction flow rate or the discharge pressure is less than the internal constant A1, A3 (A1 = -15, A3 = -0.09), the meter determines that the compressor has a surge and automatically accumulates the value of n. After the surge occurs, the cause of the surge must be judged. After the corresponding measures are taken, press the RESETSAFETYON button on the SERIES3PLUS panel to restore the surge n to the initial value even if the SCL line returns to the original position.

25 anti-surge controller discharge pressure limit control

To prevent the compressor from surging due to excessive discharge pressure, the regulator is equipped with a compressor discharge pressure limit control using a conventional PI algorithm. When the discharge pressure exceeds the set value, the regulator output increases, and the anti-surge valve is opened to reduce the discharge pressure to avoid compressor surge.

26 anti-surge controller backup strategy control

In order to minimize the possibility of the anti-surge system itself affecting the normal operation of the anti-surge system, the regulator control software sets the following two backup strategies:

1) When the suction flow exceeds the defined upper and lower limits, the regulator automatically switches to manual mode, using CONST1 (CONST1=95)

As the output signal of the regulator.

2) When the suction pressure does not exist, the regulator automatically switches to the minimum flow control.

3 Conclusion

The company's anti-surge system control scheme is more abundant and economical than the common fixed safety margin anti-surge scheme in China. Therefore, this system makes the compressor run well. Protection, the compressor works more efficiently, but the energy consumption is lower. From the perspective of more than three years of operation, the system works very stable and has strong anti-interference ability. The compressor operating point is almost working on the SCL line. From the unexpected situation that the compressor suction flow rate drops rapidly, the system has quickly suppressed the surge of the compressor. The continuous surge data of the SERIES3PLUS meter shows that the compressor has no more than 2 consecutive surges. .

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