1 Mathematical model establishment

Early performance control and anti-surge control systems for centrifugal compressors were relatively simple analog control systems with poor control performance. The general form is: in order to maintain a constant compressor outlet pressure, a pressure control regulator is arranged on the outlet line, which senses the change of the compressor network pressure when the compressor operating conditions change, and transmits the signal to the transmitter through the transmitter. The governor of the steam turbine to adjust the speed of the unit accordingly. The anti-asthmatic control system usually adopts a single-parameter control system. The anti-asthmatic circuit controls the bypass valve by the flow indicating controller. When the pipe network flow is less than or equal to the minimum flow limit, the bypass valve is opened to return some of the gas to the compressor inlet line or Venting increases the flow through the compressor and prevents surges from occurring. There are two problems with this type of anti-surge system.

One is not economic. Because there is only one minimum flow limit specified, there is no consideration of different surge limit flow rates at different speeds, so that some of the gas does not need to be recirculated during the anti-surge control implementation to bypass the backflow and cause energy waste.

Second, because the control loop is a simple analog loop, there are many factors that cannot be considered. The anti-asthmatic control quality is not good enough to prevent asthma most effectively.

In response to the above problems, the author designed a digital direct anti-surge control system to replace the original anti-surge control system with reference to the new development of anti-asthmatic control technology. Among them, a plurality of control loops are included, and factors that may occur in the operation of the compressor are considered more, so that the safety and reliability of anti-surge can be improved. The use of a multi-parameter control system reduces energy waste. Compared with the single parameter control system, the energy saving effect is obvious and the anti-asthmatic control quality is improved. The designed anti-surge control system mainly includes: variable flow limit, working point movement rate and three basic control loops of quick opening valve. The variable flow limit control loop is an incremental PID loop, and the operating point shift rate control loop is a differential loop. This is a more complex system including the compressor and its driver, which can be simplified to the numerical model shown in Figure 1.

The entire system can be roughly divided into three parts:

(1) The compressor part, including the inlet and outlet gas volume links;

(2) a steam turbine section, including a governor and an amplifying actuator;

(3) Anti-asthmatic return valve part, including anti-surge control circuit and computer system. In order to adjust the compressor speed n, the compressor discharge pressure pd signal is connected to the turbine governor; to control the anti-surge return valve to open and close, the compressor exhaust flow qc signal and the relevant inlet and outlet pressures required for calculation The p1, pd, temperature T1, and Td signals are connected to the computer. Corresponding equations can be derived for each step to perform calculations in the simulation calculations. In the figure, qd and qh respectively represent the gas volume of the compressor, that is, the exhaust pipe network flow rate and the return flow rate; qa is the compressor intake air flow rate; X and Z are the displacement output signals of the governor and the actuator, respectively.

1.1 Turbine-compressor rotor equation

1.2 governor equation

1.3 spool valve oil motivation equation

1.4 Compressor compression equation

1.5 compressor characteristic equation

1.6 Compressor outlet and inlet volume equation

1.7 Intake pipe gas state parameter equation

1.8 bypass return valve equation

1.9 anti-asthmatic control equation

Generally, the fan theorem is often used to derive the anti-asthmatic equation, and the error is large. Based on the similarity principle, this paper uses the following parameters to sort the compressor characteristic line and derive the anti-asthmatic control equation according to the actual characteristic line.

In order to overcome the shortcomings of the standard incremental PID control loop, a non-standard incremental PID control loop <3> is used, such as introducing an integral separation measure to avoid possible integral saturation; for example, at the PID regulator output An inertial link is connected in series to become an incremental PID control loop that is not fully differentiated.

2 examples

In the equation discussed above, the number of variables is one more than the number of equations.

When solving the equation, one of the variables is given. Usually, given the change of the flow rate of the compressor network, such as Xqd, other variables can be solved.

In addition to the ordinary differential equations, there are algebraic equations in the above equations. In this paper, the R2K method is used to solve the ordinary differential equations, and the algebraic equations are calculated by the general substitution method.

As an example, numerical calculation analysis is only performed here for the variable flow control loop. In order to obtain the performance control of the compressor unit and the numerical results of the anti-surge digital control system, a series of Xqd is set for calculation. The reference working condition takes the normal working condition of the compressor. Some calculation results are shown in Figures 2 to 4. They clearly show the whole. The dynamic change process of the parameters related to the anti-asthmatic control system. In Figure 2, 3, the change of Xqd has not reached the set value of the protection curve, and the anti-surge valve is not opened. Only when the pipe network flow Xqd (= -30%) is reduced beyond the set value of the guard line (set value Xqc =-0.24%), the anti-surge bypass valve is opened (Fig. 4), so that part of the flow is returned to the compressor inlet (Xqh 6%), and the flow rate through the compressor is set to the flow line set by the protection line. The compressor does not appear. Surge. Through the calculation, the change process of the parameters of each link of the system can be obtained. Figures 2 to 4 show the parameters of the different pipe network flow changes by taking the compressor parameters such as Xpd, Xn, Xqc and Xqh (after the bypass valve is opened) as an example. The changing characteristics, the stability process, the control link response, etc. The results show that the pressure and speed changes are stable and the flow rate changes slowly and the anti-surge circuit is delayed by about 8 s, which is consistent with the characteristics of the designed unit and the bypass valve and its circuit structure.

3 conclusions

The results of the above examples show that the model is used to successfully simulate the anti-asthmatic control process, and the dynamic changes such as compressor outlet pressure, flow rate and speed parameter are calculated to provide a basis for analyzing the anti-surge control system. In addition, the author also successfully carried out the digital simulation test of each anti-asthmatic circuit by using the above model, evaluating the design quality of the control system and analyzing the influence of various design parameters of the control system on the control quality. All the research results show that the proposed model and method can reflect the essence of the anti-asthmatic digital control system, and simulate the control process of each anti-asthmatic control loop, and evaluate the design of the anti-asthmatic control system and select the optimization parameters.

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