1.ECS-700 is one of the DCS series. The system has the characteristics of flexible configuration, perfect control function, convenient data processing, centralized display operation, friendly man-machine interface, simple installation and specification, convenient debugging, strong versatility and reliable operation. . At present, it has been widely used in petroleum, chemical, machinery and other manufacturing industries.
The effective control of the air compressor system not only saves energy, but also improves the service life of the equipment, which is of great significance for the entire air separation process. The DCS system generally adopts a dual redundant control unit. When a certain control unit fails, the relevant redundant real unit will switch to the working unit without any disturbance, which can ensure the safe and reliable operation of the air compressor system. The use of the WedField ECS-700 system also provides the ability for multiple people to work together at the same time, allowing multiple engineers to manage a project, further improving control efficiency.
2 air compressor system
2.1 Air compressor system introduction
The air compressor system compresses the air to provide raw air with different pressure levels for subsequent equipment. For the air compressor system, the most basic control includes the air compressor motor start/stop control and the constant pressure control or constant current control of the outlet pressure. The specific use of constant current control or constant voltage control depends on the process selection, large air separation. The system generally uses constant voltage control. The rest of the control, such as air compressor automatic loading/unloading, anti-surge control, motor overload protection control, air compressor start and accident interlock protection control, are all designed for the safe operation of the unit.
The objects to be controlled in the air compressor system are two valves: the inlet guide vane and the anti-surge valve.
2.2 Air compressor system control requirements
For the control of air compressors and systems, it is generally necessary to consider the following aspects of control:
(1) Constant pressure control The function of the raw material air compressor is to meet the process requirements to deliver a constant pressure airflow to provide raw air for the expander, main heat exchanger and fractionation tower.
(2) Anti-surge control Surge condition is an abnormal working condition of the air compressor, which is one of the unique working conditions of the centrifugal compressor and is very destructive. Once the air compressor surges, it will directly damage the air compressor equipment, which will cause air separation parking. In general, the anti-surge control of the compressor is an anti-surge valve.
(3) Fault interlock logic control In order to avoid damage to the equipment, as long as one of the monitored parameters reaches the interlock value during the operation of the air compressor, the air compressor must be stopped. This is the basic of the air compressor interlock control. Claim.
(4) Auxiliary safety control In order to ensure the safety of each device in the system, it is also necessary to add safe auxiliary control, mainly including protection control of high voltage protection and motor overload.
1 High-pressure protection control, the air compressor outlet pressure is too high, which will affect the normal operation of subsequent equipment. Therefore, it is necessary to set a safe protection range for the outlet pressure of the air compressor. The adjustment of the opening of the inlet guide vane does not achieve the purpose. The protection regulator is used together with the anti-surge regulator to adjust the opening of the vent valve to protect the outlet pressure within the set range. 2 Motor overload protection control, adjust the guide vane to reduce the load through the motor overload protection controller to prevent the air compressor motor from overloading.
3 air compressor system control program
3.1 Anti-surge control of air compressor
The outlet flow of the air compressor does not match the outlet pressure, that is, when the flow rate is low or the pressure is high, the air compressor will surge. The occurrence of surge will have serious consequences: significantly worse compressor performance, increased noise and increased vibration of the entire unit. When the operating point of the air compressor is about to enter the surge zone, lowering the outlet pressure or increasing the outlet flow can prevent the air compressor from operating in a surge state.
It can also be seen that if the inlet vane opening degree is constant, the relationship between the air compressor flow rate and the exhaust pressure is a fixed curve. When the exhaust pressure increases, the air compressor operating point moves along the fixed curve, and the flow value is easily moved to the surge region; if the flow rate is reduced, the air compressor operating point also moves along the fixed curve, and the exhaust pressure ratio is also Extremely easy to move to the surge zone. Anti-surge control requires a combination of pressure (pressure ratio) and flow to form a set point. The set value is automatically changed by changing the flow rate as the above parameters change. In order to avoid surge caused by large pressure and small flow, the opening of the inlet guide vane can be adjusted to maintain the pressure and flow rate of the air compressor within a stable working range. The control of the vane adopts the air compressor outlet pressure to control the inlet guide vane opening degree during constant pressure control. In some air compressors, the vane is controlled by the motor overload protection, generally adopting the motor overload protection loop output and the outlet pressure control loop output. The high selection value controls the vane opening by feedback.
In addition to the inlet guide vanes, the air compressor anti-surge control object is mainly to control the anti-surge valve. When the pressure or flow in the air compressor is too large, the pressure can be released by the quick opening of the anti-surge valve. The DCS air compressor interlocking program can set the relevant parameters of the anti-surge circuit to the interlocking condition and control the opening and closing of the air compressor anti-surge valve.
PIC_11 is the outlet pressure value of the air compressor, FIC_11Q is the set appropriate safety pressure value, and RS is the air compressor stop signal (using the logic value). When the outlet pressure of the air compressor is greater than the set suitable pressure value, that is, when the working point enters between the safety line and the surge line during the operation of the unit, the anti-surge valve loses power, and the inlet guide vanes are slowly closed to a certain angle (about For 10% of full opening, the compressor does not stop, and the air compressor system works at a small load. When the parking condition is satisfied, the air compressor stops, the anti-surge valve loses power, and it opens quickly, and the inlet guide vanes are slowly closed. When the air compressor is parked, the anti-surge valve is generally required to be from (1 to 2) s. The high-performance valve is about 0.5s from fully closed to fully open. The control system adopts the ECS-700 system of Zhejiang Zhongkong, which supports a high-speed scanning cycle of 20ms. It is feasible to properly adjust the distance between the control line and the surge line (such as 5%).
3.2 Variable flow control of air compressor system
In today's large air separation process, it is generally possible to change at (75 to 105)% load, which requires the air compressor load to change automatically. Within the surge control range, a cascade adjustment loop is provided to control the opening of the inlet guide vanes. The set value of the cascade is calculated by the program.
CALCU is a program calculation block, and the internal program is compiled according to the specific situation. For the air separation system, the programming is generally converted from oxygen production, and the additional amount is programmed when considering the molecular sieve system switching. The output CAV is used as an external setting of the PID regulator, and then the PID operation is performed to control the vane valve.
3.3 Interlocking control of air compressor system
The interlocking of air compressors is a very important chain, involving the safety of equipment in air compressors. There are many factors that cause the air compressor to interlock and stop. For example, the motor drive shaft bearing temperature, the motor drive shaft reverse bearing temperature, the first to fourth stage bearing temperature, the large gear bearing temperature, etc. are greater than the set value, one to four Whether the vibration amplitude of the x and y axes is too high (more than 82.5), etc., if any one of these values ​​is abnormal, the air compressor system will be interlocked and stopped. When the air compressor system fails due to human factors, the air compressor can be stopped by the emergency stop button.
3.4 Implementation of DCS Control
(1) Creation of the system engineering to be carried out. Create a new project and add the configuration server controller, operation node, and operation domain.
New engineering project:
Created by: Database: Path: Air compressor system zxl D:\SUPCON_PROJECT\Database.ini D:\SUPCON_PROJECT\Air compressor system OK Cancel VFS ysBuilder - Air compressor system file (F) Edit (E) View (V) Help (H) Air compressor system configuration server global default configuration control domain configuration operation domain configuration engineer zxl operation domain
(2) Configure the controlled object and log in to the configuration software as shown.
Log in to the zxl air compressor system engineer: Password: Database: Project: D:\SUPCON_PROJECT\Database.ini OK Cancel
(3) Open the controller from the configuration server, add the bit number to be measured to the bit number table, and program the user program. Then open the operating domain from the configuration server, add the operation team, perform flowchart editing, alarm settings, and so on.
(4) After the flow chart is completed, enter the monitoring software to implement real-time monitoring of the on-site controlled system. The monitoring system sets three levels of authority, namely operator level, engineer level and administrator level. Each person corresponds to a unique user name and password. The levels are as follows: Operator level: You can view the relevant records on the monitor screen to confirm the alarm.
Engineer level: At the operator level, variable input and remote control are also available.
Administrator level: At the engineer level, user management is also possible.
4 Conclusion
By analyzing the characteristics of the air compressor system control, the ECS-700 configuration software is used for configuration, real-time monitoring of the important parameters of the air compressor system is controlled, and the control of the air compressor system becomes more intuitive and effective. .
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