Application of DCS system in anti-surge control system of air separation compressor

Keywords: DCS system, air separation compressor, anti-surge control system

Preface

   The 6000m3/h air separation plant of Xilin Iron and Steel Company was completed in May 2001 by Hangzhou Oxygen Concentrator Group Company. The factory design was designed by Beijing Iron and Steel Design Institute. It was successfully put into operation on September 2, 2001, and it took more than half a year to run. It shows that the unit is operating stably, and all indicators have reached or exceeded the design value. The entire control system uses Honeywell’s TDC3000 and two GUS stations.

1. The composition of the control system

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1. The composition of the automatic control system.

The automatic control system is composed of the controlled object, detection components, controller and regulating valve. As shown in Figure 1.

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Application of DCS system in anti-surge control system of air separation compressor

  Figure 1 Block diagram of automatic control system

   a. Controlled object: equipment, machine or production process that needs to be controlled.

b. Controlled variable: The physical quantity in the object that needs to maintain the set value.

c. Manipulated variable: The amount of material or energy that is manipulated by the controller to keep the controlled variable at the set value.

d. Disturbance (disturbance), in addition to manipulated variables, factors that act on the object and can cause changes in the controlled variable, such as load changes are a typical disturbance.

e. Set value: the target value of the controlled variable.

f. Deviation: The deviation should theoretically be the difference between the set value and the actual value of the controlled variable.

2. Cascade control system

The cascade control system is the earliest, most effective, and most widely used complex control system. Its characteristic is that two controllers are connected in series, and the output of the main controller is used as the setting of the sub-controller, which is suitable for the time constant. And the controlled object with larger pure lag.

2. Logic point function

   Logic point provides logic capability, and it cooperates with digital combination point to provide combinational logic function. Logic points are composed of logic blocks, FLAG, numbers, input connections and output connections. Logic points have up to 12 inputs, 16 logic blocks, and 12 output connections.

3. Anti-surge control of hollow-separated core compressor

   The centrifugal compressor is selected for air separation. The centrifugal compressor has high working efficiency, runs smoothly under normal working conditions, and has no pulsation in the compressed air flow. It is relatively sensitive to changes in pressure, flow, and temperature of the conveyed medium. Surging easily occurs. When surge occurs, the flow fluctuates greatly, and the unit vibrates severely. If measures are not taken to control it in time, the compressor rotor and stator will be subjected to alternating stress and fracture; the pressure between the poles will be abnormal and cause strong vibration, leading to seals and thrust bearings Damage; collision of moving components and stationary components, causing serious accidents. Therefore, every effort should be made to prevent the compressor from entering surge conditions. The surge phenomenon can be effectively controlled. As shown in Figure 2, according to the performance curve of the centrifugal compressor under different working conditions, as long as we control the minimum flow of the compressor in the working area (in the control line), compression The machine can work normally. The sign of surge is a minimum flow point, below which surge will occur. Therefore, it is necessary to have a control system to prevent the compressor from surging, and limit the flow rate of the compressor not to be reduced to the lowest allowable value under this working condition. That is, the compressor will not enter the surge operating region.

Application of DCS system in anti-surge control system of air separation compressor

  Figure 2 Centrifugal compressor performance curve and anti-surge control principle diagram    The anti-surge condition of the compressor is: △P≥a(p2±bp1)

Where △p——The pressure difference between the front and back of the orifice plate for measuring flow in the inlet pipeline

p1——pressure at the inlet

p2——pressure at the outlet

a, b-parameters related to pressure ratio, temperature, orifice plate coefficient of orifice flowmeter, can be obtained through thermal computer and experiment.

It can be seen from Figure 2 that due to the unstable operating conditions (p, T, Q) of the centrifugal compressor during the gas transmission process, if the control is not good, the performance of the compressor will be significantly deteriorated, and the airflow parameters (pressure, flow) will be generated. Large-scale pulsation, increased noise and vibration, and severe enough to damage the compressor. The traditional anti-surge method generally uses the compressor rated minimum flow control method. The disadvantage of this control method is that it cannot sufficiently make the gas compressor work in its working area. Frequent activation of the anti-surge valve (vent valve) wastes energy. The economy of gas transmission is reduced. Moreover, with the long-term wear and tear of the compressor, its performance will change, that is, the characteristics of the compressor's rated minimum flow rate (surge line) will deviate. If no real-time correction is made, it will inevitably cause control failure and make the compressor work in a surge. Vibration area, its consequences can be imagined. Therefore, for the possible surge caused by changes in operating conditions and changes in compressor performance during the gas transmission process of the hollow-separated core compressor, we adopted the following control strategies:

(1) Determine the best working area (control margin line) of the compressor according to the rated minimum flow characteristic curve of the centrifugal compressor and the specific parameters of the gas transmission system;

(2) Use mathematical methods to fit the surge line and anti-surge control line under different working conditions (p, T);

(3) Use the massive data processing method to process the compressor's historical data online to calculate the deviation between the actual minimum flow and the minimum rated flow of the centrifugal compressor;

(4) Real-time compensation for surge line and anti-surge control line according to the deviation of the inlet and outlet temperature of the centrifugal compressor and the actual minimum flow;

(5) Compare the minimum flow rate of the surge point obtained by the operating point with the minimum flow rate of the actual operating point;

(6) The result of the above comparison (change in flow rate and rate of change) is used as the input of the controller, and the centrifugal compression is achieved by controlling the compressor inlet guide vane (intake valve) and anti-surge valve (vent valve) Anti-surge control of the machine.

Application of DCS system in anti-surge control system of air separation compressor

  Figure 3 The control logic diagram of the main compressor inlet guide vane   

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As shown in Figure 3: When the main compressor starts, IIC2301 is in the automatic position, IIC2301.OP=100; PIC1101 is in the program position, set its output in CL language, which is equal to the output of MIGVRAMP2, when PI1101.PV=PI1101.SP, PIC1101 is automatically set to automatic, M-IGV (inlet guide vane) is automatically controlled by PIC1101, HC1031.OP=100. When the motor allowable load signal is "ON", MIGVRAMP1 and MIGVRAMP2 start, and their output starts to climb and increase. Within one second, MIGVRAMP1 increases from 0 to 20, and then increases from 20 to 100 within 15 minutes and 59 seconds. After MIGVRAMP2 delays for another second, it also increases from 20 to 100 within 15 minutes and 59 seconds. When the air separation fails, MIGVRAMP2 is "SHUTDOWN" and its output is 20. When the empty fault is removed, press HS1105 (air separation fault confirmation), MIGVRAMP2 is activated, and its output increases from 20 to 100 within 15 minutes and 59 seconds after a 1 second delay. The SUB 1000 module makes the inlet guide vane opening equal to the opening of PIC1101 minus "100 minus the opening of IIC2301". LO SELECT is a low selection logic block, which selects the low opening and outputs to MIG. When the motor allows the loading signal "OFF", the compressor stops and MIGVRAMP1 is "SHUTDOWN". When its output is 0, the inlet guide vane is fully closed. When the HC1031 is a stand-alone test run and normal, the operator uses it to relieve pressure and unload. Manually, the MIGV can only be turned off but not turned on.

When the main compressor is started, HC1046.op=0, PIC1044 is in automatic state, when PIC1044.op=0, FICI1044 is in program state, set FIC1044.op=M-BOVRAMP output in CL language. When FIC1044.pv≤FIC1044.sp, FIC1044 is automatically set to cascade control. When the motor is allowed to load signal "ON" or HS1105 (air separation fault confirmation) "ON", M-BOVRAMP starts and begins to crawl, and its output controls GI1046, where PI1044.pv=PIC1044.sp, FI1044.pv≤FIC1044. In sp, FIC1044 replaces RAMP to control GI1046. When PIC1044.pv≥PIC1044.sp, the output pressure is too high. At this time, the signal output to BOV is FIC1044.op+PIC1044.op. When the motor allows loading signal "OFF" (stop) or air separation failure, M-BOVRAMP is forced to output 100, GI1046 valve is fully opened. The climbing time of M-BOVRAMP is 12 minutes, from 100 to 0. When starting, because the flow is zero, in order to keep FIC1044 in manual state, first use CL language to delay 5 seconds, when FI1044.pv≤FIC1044.sp, FIC1044 is put into cascade. HC1046 is used for pressure relief and unloading by operators during stand-alone test and normal parking. Manually, GI1046 can only be opened up, not down. PLUS is the adder module. HI SELECT is a high selection logic block, which selects high opening and outputs to GI1046.

The logic control of the inlet guide vane and the logic control of the vent valve make the operating conditions of the compressor well controlled, no matter what the compression ratio of the compressor is, as long as the suction flow of the compressor is greater than the surge flow, it can be The precursors of surge phenomenon can be predicted and judged quickly and accurately, and controlled, and surge phenomenon can be completely avoided. The stable operation of the compressor can be ensured.

references:   

1.TPS System LCN Maintenance 2000,8 Honeywell,US

2. The computer control system of 2004,3,1 Tsinghua University Press published: Wang Jinbiao, eds.