The refrigeration section of the compressed air refrigeration system consists of a pressurized turboexpander. When the supercharger is at a certain inlet pressure and speed, when the inlet flow of the supercharger is less than a certain value, the machine will surge, and the pressure will fluctuate greatly, causing strong vibration of the equipment, resulting in lower efficiency of the supercharger. In severe cases, equipment damage can occur.
Surge conditions are related to the characteristics of the pipe network. The larger the pipe network capacity, the larger the surge amplitude and the lower the frequency; the smaller the pipe network capacity, the opposite. The original anti-surge control adopts the fixed limit anti-surge method. According to the characteristic curve of the supercharger, the fixed flow value is selected as the supercharger surge control line, but the following defects exist: the fixed limit anti-surge method is to ensure the unit is available. The safety under the worst working conditions is expected, so the safety area is large, resulting in a decrease in system efficiency. The energy consumption of the supercharger is too large, and the production equipment that needs to change the load of the supercharger is not economical. Therefore, the author uses a new variable limit anti-surge system to design multiple anti-surge methods, which not only solves the surge problem, but also improves the system efficiency.
1 Surge various control lines 1.1 Surge characteristic curve and empirical formula For example, the characteristic curve of the supercharger means: the absolute ratio of the outlet absolute pressure to the inlet absolute pressure (or compression ratio) and the inlet volume flow of the supercharger The relationship curve.
The characteristic curve of the supercharger is connected to the highest point of the characteristic curve of the supercharger at different speeds. The obtained curve is the surge characteristic curve, and the left part is called the surge zone, such as the shaded part.
For the surge limit line, mathematical expressions can be obtained by theoretical derivation. This surge limit line can be similar to a parabolic equation near the military automation, and its empirical formula is: where: Q is the volume flow of the inlet of the supercharger; T is the absolute temperature of the inlet of the supercharger; Pi, P2 are the supercharged respectively Machine inlet and outlet pressure; K, "all constants, supplied by the supercharger manufacturer.
1.2 The surge limit line approximates the surge characteristic curve to a straight line, that is, the flow rate on the surge limit vibration limit line is the surge flow of the supercharger.
The main point of control of the various control lines of the supercharger is to calculate the booster surge approximation approximately equal to the surge flow value divided by the actual flow value. That is: when the actual flow value of the supercharger is less than the surge flow value, the supercharger will surge; when <1, the actual flow value of the supercharger is greater than the surge flow value, the supercharger will not surge. .
1.3 Surge Control Line To prevent the occurrence of surge, the control response must be made before the operating point reaches the surge limit line to increase the flow of the booster. Therefore, a surge control line SCL is set on the right side of the line, for example.
Control line property function.
1.4 The cycle trip line is used as the limit. If the operating point crosses this limit, the anti-surge valve will be opened quickly to avoid surge.
Sexual function.
1.5 Safety line safety line (SOL) specifies an operating limit. If the operating point crosses this limit, it indicates that the supercharger is experiencing surge and should quickly stop the surge.
2 anti-surge control program for the original surge control defects, the author uses a new variable limit anti-surge system. The surge control part is composed of computer measurement and control software, intelligent control instrument and return valve. The pressure, flow and other signals on the site are sent to a 420 mA current signal by a pressure transmitter and a flow meter, and transmitted to a smart meter of the lower control level. The operator can obtain various test data through the measurement and control software to understand the anti-surge control work and realize anti-surge control.
2.1 Anti-surge design principle Anti-surge design principle is: 1) By adjusting the opening degree of the return valve and the working condition of the supercharger, the anti-surge offset (Dev) is as close as possible to zero, so that the supercharged function Produces maximum efficiency; 2) according to different control lines set, when the operating point crosses different control lines to generate different control responses; 3) combines closed-loop PI control and open-loop control; 4) selects reactive-sensitive transmitters And respond to the action of the rapid adjustment valve to improve the response speed of the anti-surge control system; 5) design software protection, hardware protection and manual shut-off valve multiple anti-surge method to improve the system anti-surge ability.
2.2 Supercharger return valve If the system is equipped with a return valve in the supercharger. The valve has the following purposes: 1) Pressure regulation: keep the pressure of the booster outlet constant, the opening or closing of the valve can reduce or increase the pressure; 2) Anti-surge: the inlet flow is close to or reaches the asthma When the flow rate is increased, the valve will automatically open to increase the inlet flow and prevent the occurrence of surge. 3) Opening the large return valve can increase the load of the booster and slow down the expander.
Inhaling the booster to the extruder, the booster return valve 3, the new variable limit anti-surge system computer monitors the inlet pressure, the outlet pressure and the inlet flow in real time, and calculates the operating point position. According to the operating point, the different control Zhang Zhongnan, etc.: Anti-surge method of compressed air refrigeration measurement and control system inlet flow / (kg / s) test results The refrigeration part of the test process is: 1) quickly open the nozzle of the turbo expander, this When the import flow rate and export pressure rise rapidly. Then slowly open the nozzle and the parameters slowly rise to a steady state. The curve seen in this process 2) slowly closes the return valve. During this process, the inlet flow rate drops and the outlet pressure rises. See curve SC. The last operating point is stabilized at point C.
It can be seen that the final stable point (7 points is near the surge control line, at this time the booster exerts maximum efficiency. Surging is easy in the middle S-C process, the process is stable, the parameters are within the error tolerance, accurate The ground reaches the set value and can be in a stable state for a long time, and the surge is effectively controlled.
6 Conclusion Through the debugging application, the system runs stably and reliably, and realizes the anti-surge method, which can provide certain solutions to related problems.
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