1. System overview

The automatic control system of 70 m3/h heavy oil treatment unit matched with a gas turbine set of an electric power company is a set of heavy oil dewatering and desalting system. The heavy oil is used for combustion after being treated. The burning flue gas generates high temperature steam and generates electricity. The automatic control system mainly realizes the automatic control function of heating section, adding demineralizer and demulsifier section and desalination section.

The whole system consists of crude oil tank, heat exchanger, high temperature heater and two-stage desalination tank. Crude oil is first injected into the heat exchanger. The residual heat from the reflux of the crude oil is used to preheat the crude oil. The addition of demineralizer and demulsifier is convenient for the salt in the heavy oil. Separation and heavy oil separation, and then into the steam heater to further heat to reduce the viscosity of heavy oil, and finally in the desalination tank using electric field positive and negative stage will be the heavy oil in the positive and negative ions to both sides to achieve the purpose of desalination and water removal.

Heavy oil has a high viscosity at room temperature, so it needs to be heated to reduce its viscosity in order to facilitate salt precipitation from the heavy oil; in addition, because the oil molecules usually encapsulate salt ions, adding demineralizers and demulsifiers can effectively separate the two. In the desalination tank, the proportion of water is greater than the proportion of oil, water will sink into the bottom of the tank, salt is sucked to both sides by vertical electric field, part of the salt will be dissolved in water, together from the sewage valve in the form of sewage discharge. The treated heavy oil also has some residual heat. In order to improve the efficiency of the system, the waste heat is recovered and used to preheat the crude oil to be processed. This treatment can make the crude oil to be processed temperature higher at the inlet of the steam heater. The treated heavy oil needs to be tested before it enters the net oil tank. If the oil and water content reaches the industrial target, it can be put into the net oil tank, otherwise it will be recovered to the crude oil tank.

Heavy oil is injected with water to dissolve the salt ions that will be separated before entering the two-stage desalination tank. The injection of the primary desalination tank is partly from the purified water, the other from the sewage of the secondary desalination tank (because the quality of the secondary desalination sewage is better than that of the primary desalination sewage, so it can be recycled); the injection of the secondary desalination tank, because of the higher technical requirements, all comes from the purified water. The drainage of the primary desalination tank is all discharged from the sewage outlet because of its poor quality; the drainage of the secondary desalination tank is partly recovered for the injection of the primary desalination tank and partly discharged from the sewage outlet because of its good quality. These operations can be achieved by automatic control of water injection shut-off valve and drain valve.

Sampling test must be carried out before the basic treated heavy oil enters the clean oil tank. When the water cut is more than 0.5% measured by the water cut analyzer, the oil tank is automatically switched over to the crude oil tank and the oil purifying valve is closed. When the water cut is less than 0.5%, the clean oil tank can not be cut back automatically. After the content of sodium and potassium salt is qualified, the oil tank is switched over to the clean oil tank by manual instruction. The oil return valve is closed. The setting value of the analyzer can be adjusted manually.

Two, system requirements

1. the reliability of the control system is very high, and redundant structure design is adopted.

2. In order to avoid manual operation can not meet the requirements of precision, the control system has a one-button start-up function, high control accuracy requirements.

3. the system has good networking function and can be connected with the DCS system of gas turbine.

4. The selected control system adopts a general design scheme, which is universal, reliable and maintainable.

5. the control software has good flexibility, which is convenient for adjusting and modifying the control mode.

Three, system configuration and function realization

According to the control requirements of the system, the PLC-300 series controller of SIEMENS company and the remote I/O system of UniMAT ET200M slave station of Yiwei company are used to form the control station of the system, and the operation station of the system is built by WinCC man-machine interface software and upper computer unit.

In order to avoid the loss caused by the abnormal shutdown of the control system, redundant structure is used in the control system. Redundant systems are divided into software redundancy and hardware redundancy. Software redundancy is a low-cost solution for SIEMENS to achieve redundant functions. It can be used in control systems with low switching time requirements for master-standby systems. The biggest advantage of software redundancy relative to hardware redundancy is that it costs less. Although the redundant switching time is slightly longer than hardware redundancy (about 100 ms), it implements all the functions of CPU redundancy, power redundancy, communication redundancy, template hot-swap and so on. It is a good solution in the case of less strict switching time requirements.

1. The system adopts CPU315-2DP redundancy and CP343-1 industrial Ethernet synchronization to ensure high synchronization performance.

2. WinCC supervisory software connects with redundant CPU through industrial Ethernet. When the main CPU fails to switch to the standby CPU, WinCC can automatically switch to the standby CPU without human intervention.

According to the control needs, a key boot function is designed. When the external conditions have automatic working conditions, the automatic boot key can be clicked to automatically carry out heavy oil treatment without human intervention.

1. There are many equipments in the system which need to be adjusted by PID, such as heavy oil flow rate, liquid level of two-stage desalination tank, etc. After dynamic setting on the WinCC screen, the corresponding display frame of PID adjusting parameters can be seen by clicking on the device with the mouse, and the hand automatic switching, proportional gain, integral coefficient and differential coefficient of the device can be displayed in real time.

2. WinCC human-computer interface has rich drawing function and dynamic display function. It can express the complex process clearly and succinctly, and decompose the heavy oil treatment process into several relatively independent pictures. The control attributes and dynamic events on the screen are set intuitively and connected with the input and output points of the PLC equipment in real time, so that users can see the running status of the field equipment on the PC at any time.

3. The alarm record in WinCC software can set the alarm upper and lower limits of any digital or analog quantity. After configuring the alarm box on the screen, the alarm variable, alarm time and alarm information can be displayed. If necessary, the alarm sound can also be set through the global script, and the alarm sound effect can be eliminated after the alarm is confirmed.

4. WinCC software can archive the variables which need long-term observation trend, and observe the trend of some variables at any time when the system is running. It is convenient to compare and observe some of the same types of variables, and can also magnify the local observation. The trend chart has the function of dynamic change and real-time increase.

5. For the operator station and engineer station in the upper computer, the privilege protection should be set for the actions of their respective operating ranges in order to facilitate system security. For the operation of important equipment, a prompt window is set on the operation station, and only after the operator confirms, can the operation be realized. On-site more equipment, such as the main and backup water injection pumps, generally have automatic and manual switching, in order to facilitate when one of the equipment failures or maintenance when it is disconnected from the system.

6. main monitoring interface

                                             电脱盐图

                                             总览图

                                             泵阀总汇图

                                            油罐总览图

                                                 小锅炉图

Four use effect analysis

This control system achieves the expected purpose in heavy oil processing. Selecting soft redundant system according to the importance of heavy oil processing system in practice can realize the switching of main and standby systems when necessary, improve the stability of the system, and save the system cost to a certain extent. The system is in good condition after being put into operation.