1 surge formation process
When the intake air amount is too small, the compressor discharge pressure is reduced. If the pipe network capacity is large and the pressure is high, the pipe network pressure is greater than the exhaust pressure, so that the compressor discharge amount is further reduced as a result of the chain reaction. The displacement of the compressor is zero, even negative (the pipe network is reversed to the compressor), but the pipe network pressure is not maintained. As the pipe network is exhausted to the process, its pressure is also decreasing. When it is lower than the discharge pressure of the compressor, the compressor will exhaust to the pipe network, so that the pressure of the pipe network will rise. If the pipe network resistance is constant, the cycle will be repeated repeatedly, and the airflow will suddenly become small and small. Suddenly out of the surge.
2 The basic principles of the anti-surge control system
It can be seen from the surge forming process that under a certain exhaust pressure, the compressor flow can be prevented from being too small, and surge can be avoided. However, the resistance of the pipe network is certain, so it is generally used to open the anti-surge return valve to increase the compressor flow. As shown in Figure 1, since the theoretical calculation of the surge line and the detection of the actual surge line are difficult to be accurate, for safety reasons, an anti-surge line is placed before the surge line in practice, leaving 5% between the two lines. ~10% safety margin. When the actual working point is in the safe area below the anti-surge line, the anti-surge valve is closed. If it is above the anti-surge line, open the anti-surge valve. In order to reduce the waste caused by the backflow, the anti-surge valve should be as small as possible when opening and eliminating surge.
3 anti-surge control program
Anti-surge control scheme P2=f(ΔP1, T1), as shown in 2, the calculation function of the inlet differential pressure proportional to the inlet flow rate corrected by the inlet temperature T1 as the anti-surge line, and the outlet pressure P2 as the compressor The actual running work point. When the actual pressure value is less than f(△P1, T1), the working point is at point A, indicating that the compressor is in the normal working condition range, the anti-surge system does not adjust, and the anti-surge valve is closed.
When the actual pressure value is greater than f(â–³P1, T1), the working point is at point B, indicating that the compressor operating condition point is in the surge zone, and the anti-surge valve should be opened at an appropriate angle for anti-surge regulation.
4 Auxiliary function of anti-surge control scheme In order to improve the adaptability of anti-surge control scheme, it is necessary to add the following auxiliary functions to the scheme to ensure the adaptability of the scheme to changes in process conditions and environmental conditions.
4.1 Temperature compensation function The characteristic curve of the compressor may change with the change of the ambient temperature, especially when the temperature rises, causing the surge line to move forward. If the anti-surge line position is fixed, the surge line is made. The safety margin between the anti-surge line and the anti-surge line is reduced, and even the actual surge line is crossed over the anti-surge line, and the protection of the anti-surge line is lost.
To this end, the temperature compensation function should be set to correct the anti-surge line so that the anti-surge line can automatically move with temperature changes. When the temperature is high, the anti-surge line moves to the right. When the temperature is low, the anti-surge line Move to the left to maintain a certain safety margin between the surge line and the surge line.
4.2 Anti-surge valve fast opening and slow closing function In order to prevent the anti-surge system from repeatedly adjusting the anti-surge valve oscillation, affecting the stability of the compressor outlet, it is necessary to increase the anti-surge valve's quick opening and closing function. When surge occurs, it can respond quickly and quickly open the anti-surge valve to prevent danger.
In the process of closing the valve, it is slowly closed to prevent oscillation.
4.3 anti-surge line automatic movement function Long-term operation caused by compressor movement, static blade erosion, increased dirt and other reasons, will cause changes in compressor surge characteristics. This change tends to cause the actual surge line to move to the lower right, resulting in a reduction in the safe separation distance between the surge line and the set anti-surge line, so that the anti-surge line does not effectively prevent the compressor from entering the breath. Vibration zone. In order to effectively solve this problem, an anti-surge line automatic movement function is required in the anti-surge control system, and the function is that if the compressor surges, the anti-surge line automatically moves away from the surge line by a certain distance. Use this line as a new anti-surge line, which increases the safety gap between the anti-surge line and the surge line. If a similar surge occurs again, the anti-surge line will move forward again.
In order not to reduce the effective operating range too much, the maximum forward movement is 10 times, and the forward movement distance generally takes 5% to 10% of the safety margin. 4.4 Dynamic adjustment function When the actual working point of the compressor quickly approaches the surge line, If the anti-surge control system is set to adjust the original PI, it may not effectively prevent the compressor from entering the surge zone. In this case, the dynamic adjustment function must be set to speed up the adjustment.
4.5 Manual, semi-automatic, automatic function The anti-surge control system should have manual and automatic switching functions. The manual mode is that when the actual working point is on the right side of the anti-surge line, the switch of the anti-surge valve can be artificially controlled. When the actual working point is on the left side of the anti-surge line, the valve position of the artificial anti-surge valve is invalid. The semi-automatic mode is that the actual output of the anti-surge valve takes the high value of manual and automatic output, that is, the artificial input valve position can only be greater than the automatic output, otherwise the artificial input valve position does not work. The automatic mode is when the actual working point enters the left side of the anti-surge line. In the automatic position, the PID automatically adjusts the valve opening. When the actual working point enters the right side of the anti-surge line, the PID automatically adjusts the closing valve when in the automatic position.
4.6 manual override function
5 Anti-surge control scheme in the application of the waste acid regeneration unit compressor. The 10,000-ton/year waste acid regeneration unit of the Haihua Refinery Huizhou Refinery Branch introduced the US MECS process package, which uses the waste sulfuric acid produced by alkylation as raw material. And add a small amount of hydrogen sulfide gas to produce 99.2% of fresh sulfuric acid for alkylation.
The DA100-21310 sulfur dioxide main compressor unit has two sets (one open and one standby), one set of lubricating oil system (shared one high-position fuel tank), and the dry-type laminated coupling is used between the main and auxiliary machines. Two compressors share an anti-surge valve.
The factory selects the TRIGANT controller for the anti-surge control system of the waste acid regeneration unit compressor, adopts the triple redundant fault-tolerant (TMR) microprocessor hardware technology and the TRISTATION1131 software, and the monitoring system adopts the INTOUCH software.
The control system has complete data acquisition, process monitoring, safety interlock and unit integrated control functions. The whole system consists of a control station, an operation station, an engineering station and an auxiliary operation station.
TRIDANT provides fault tolerance through Triple Module Redundancy (TMR). This system consists of three identical system channels. Each system channel executes the control program independently and works in parallel on the other two channels. The hardware decision mechanism then diagnoses and diagnoses all digital inputs and outputs from the field. The analog input is processed as a median.
TRISTATION 1131 is an easy-to-use programming software generated on the WINDOWS interface. There is a complete set of programming tools that can be used to develop various control software.
Wonderware's InTouch visualization software is a powerful and intuitive Human Machine Interface (HMI) for industrial automation, process control, management monitoring and data acquisition. InTouchHMI enables users to view and control industrial production processes in real time, while providing engineers with an easy-to-use development environment and extensive capabilities that enable engineers to quickly build, test and deploy powerful automation applications to connect and deliver real-time applications. information. InTouch software is an open, scalable human-machine interface that provides flexibility for customer application design while providing connectivity to a wide range of automation devices in the industry.
Anti-surge control is a protection for the compressor unit. It opens the anti-surge valve in advance before the unit's working point enters the surge zone, increasing the flow rate and returning the working point to normal. The performance curve of the anti-surge control system of the compressor of the waste acid regeneration device. The system adopts dynamic anti-surge technology, which can dynamically adjust the anti-surge line according to the operating state of the unit, and control the anti-surge valve to make the unit run safer and reliable. The abscissa of the coordinate system is ΔP1, the ordinate of the coordinate system is Pd/Ps. ΔP1 is the inlet differential pressure, Ps is the inlet pressure, and Pd is the outlet pressure. The three fold lines are the surge line, the anti-surge line, and the preset anti-surge line. When the working point enters the left side of the surge line, it indicates that the unit has entered the surge zone. The system should quickly open the anti-surge valve in advance, and this will not happen if the system is used normally. The preset anti-surge line is a factory anti-surge line provided by the manufacturer. When the unit is in the actual running process, if the working point enters the left side of the surge line and is in the automatic position, in order to make the unit control safer in the future, the anti-surge line will automatically pan to the right, up to ten times, and the screen count shows the number of corrections. . When this phenomenon does not occur, the actual anti-surge line overlaps with the expected anti-surge line. The performance curve combines unit performance and anti-surge control. The working point enters the left side of the actual anti-surge line. When in the automatic position, the PID automatically adjusts to open the valve. The working point enters the right side of the actual anti-surge line. In the automatic position, the PID automatically adjusts the closing valve. Anti-surge adjustment methods are manual, semi-automatic and fully automatic. The actual operation of the past year has verified the effectiveness and reliability of this anti-surge control scheme.
6 Conclusion
The anti-surge control scheme adopts an algorithm for real-time monitoring and early prediction of working conditions, and takes preventive measures in advance according to the predicted results, ensuring the efficiency of anti-surge control, achieving the purpose of saving energy and improving efficiency, and prolonging the The service life of the unit ensures safe, stable and long-term operation of the unit. At the same time, the TRIDANT system with friendly working interface and strong network function is realized, which makes the implemented application system incorporate more humanized design concepts, which is convenient for application and promotion.
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