Abstract: This paper introduces the design scheme, system composition, hardware design and software design of an oil storage and transportation monitoring system based on PLC. It elaborates on the characteristics of the intelligent electric valve controlled by the Pakscan master station controller and its communication method with the PLC . At the same time, a three-level fault display and alarm system is adopted to improve the reliability of the system.
Keywords: PLC; intelligent electric valve; master controller
Abstract: The paper introduces a design of oil storage and transportation the monitoring system of based on PLC, and what is the system made up of . Elaborates the characteristics of the dynamoelectric valve of intelligence used in the Pakscan main station controller and how to communicate with PLC. Introduce the 3 grade system of showing malfunction alarm to increase the dependability of the system at the same time.
Key words: PLC; Dynamoelectric valve of intelligence; main station controller
Programmable Logic Controllers (PLCs) are a new type of industrial controller developed based on microprocessor technology and integrating computer technology and automatic control technology. They are widely used in various fields of industrial control. Modern industrial production equipment contains many digital and analog control devices, such as motor start/stop, solenoid valve opening/closing, product counting, and temperature, pressure, and flow setting and control. Using PLCs to solve automatic control problems in industrial settings has become one of the most effective methods. This paper takes an oil storage and transportation monitoring system as an example, introducing the design scheme, system composition, control method, and communication network of a PLC-based oil storage and transportation monitoring system. This has practical significance for promoting the application of PLC technology.
1 Introduction
The various control valves used in oil storage tanks and pipelines in oil depots are essential field instruments in the oil storage and transportation process. Their level of intelligence, the amount of information they contain, and their fault diagnosis and tolerance capabilities directly affect the reliability, stability, and ease of use of the data acquisition and monitoring system. Typically, there are numerous and dispersed control valves in the depot area. Ordinary control valves contain limited information and involve extensive wiring, which complicates the design of the depot monitoring system to some extent. This system adopts intelligent electric valves and their master station controllers from Rotork, UK, along with a three-level fault display and alarm system. This greatly simplifies the complex design of the monitoring system, and, thanks to its rich diagnostic information and fault tolerance capabilities, improves the system's reliability.
2 Hardware Implementation of the Monitoring System
2.1 Rotork Intelligent Valve Control Equipment
Rotork's intelligent valve control system is a valve data acquisition, monitoring, and control system consisting of a master controller and connected field electric valves. The master controller can control up to 240 field control valves connected to a two-wire current loop, which can be up to 20 km long.
The field electric valves are highly intelligent, containing a wealth of data and diagnostic information. However, their most significant characteristic is that multiple intelligent valves are interconnected in a loop via only two wires. Ultimately, only the start and end wires connect to the master controller, and all valve information is transmitted to the master controller via these two wires. The field electric valves also feature line fault shielding. When an open circuit, short circuit, or ground fault occurs in the loop, the intelligent valve can shield the faulty end of the line, allowing the master controller to still communicate with all intelligent valves on the line unaffected, while simultaneously sending fault information to the master controller. The two-wire shielding principle is shown in Figure 1.
Figure 1 Fault shielding principle
Under normal operating conditions, the communication current signal flows out from port A of the master controller along one line of the loop and returns from port B via the same loop. At this time, the other line is redundant. When a fault occurs in one line, that faulty line is shielded by a valve, and the smart valves on both sides of the faulty line can communicate with the master controller through their respective loops. When two lines fail, the smart valves between the two faults are shielded, and the smart valves outside the two faulty lines can still communicate with the master controller through the two "arm-shaped" loops.
2.2 System Design
This system uses a PLC as the controller to perform real-time signal sampling, pulse accumulation, status judgment, and output control; a microcomputer is used as the host computer to store, analyze, process, display, and print the collected data, enabling real-time monitoring of the system. The most crucial aspects of this system are: firstly, the PLC accurately, reliably, and quickly transmits the collected data to the host computer; secondly, the host computer's control and scheduling commands must be accurately sent to the PLC. Data communication is key to accomplishing these two tasks. The master controller is a panel-mounted intelligent instrument consisting of a main CPU card, a loop communication card, a power supply, an LCD display, and a 16-button keypad. It has two fixed databases: one is the field unit database, responsible for receiving and recording data such as the address, torque, and opening degree of the intelligent valves from the two-wire loop, and controlling the valve movement according to read/write commands from the host computer. This database is logically divided into four areas, each recording data for 60 valves; the other database is the master controller status and self-diagnostic database, responsible for recording the relevant status of the communication protocol and issuing commands to the intelligent valves. The main controller's buttons and LCD display can read data such as the opening degree, torque, and address of the intelligent valve, control the opening and closing of the valve, receive alarm signals, and communicate with the PLC.
Figure 2 Design diagram of oil storage and transportation monitoring system
2.3 Monitoring System Structure
Figure 3. Structural block diagram of the on-site intelligent electric valve monitoring system
The system's control section utilizes an HBR dual-hot-standby PLC system from GE Fanuc, USA, which controls the opening, stopping, and closing of 140 intelligent valves (IQ actuators). The supervisory control station can monitor the valve position feedback status, valve position value, and alarm signals of each intelligent valve, and can also perform valve opening, stopping, and closing operations.
The Pakscan IIE master controller communicates with the PLC via the Modbus protocol and is connected through an RS-485 interface on port 1. Under normal operation, the master PLC and master controller operate, while the slave PLC and backup controller maintain synchronization with the master PLC and master controller, respectively. The intelligent valve transmits data to the master controller, which reads the data from the master controller via the RS-485 interface and issues commands to it. The master controller then executes the commands, driving the intelligent valve to operate accordingly. If the master PLC or master controller fails, the system automatically switches to the slave PLC or backup controller, respectively.
Because the system uses the Modbus communication protocol, one PLC can connect to multiple Pakscan IIE master controllers. Therefore, if there are many smart valves on site, the system can be easily expanded and the wiring is simple.
3 Software Design
3.1 Communication Program Design
The PLC uses a Modbus RTU master communication module. The Pakscan IIE master controller is a remote terminal unit that acts as a Modbus slave. The PLC's CPU controls the read and write operations of the Pakscan IIE master controller through the Modbus RTU master communication module, which is referred to as the Modbus host. The system uses a single Modbus host two-wire communication method, which can connect up to 32 Pakscan IIE master controllers.
The main communication module program consists of three parts: initializing the communication module; reading and writing Modbus/RTU data; and monitoring the communication status.
The initialization of the communication module mainly involves configuring the parameters of three initialization control blocks: the Slave Control Block (SCB), the Information Control Block (MCB), and the Communication Requirement Parameter Block (COM_REQ). The SCB is a 15-register-long data block that defines parameters such as the model, number, and status of the Slaves it communicates with; one SCB block is required for each Slave. The MCB is a 6-register-long data block that defines the command information that the Master requires each Slave to execute, including parameters such as command type, RTU reference address offset, PLC reference address offset, and master number; one MCB block is required for each command type. The COM_REQ is a 17-register-long data block that defines the communication method, port control words, and status parameters for monitoring the SCB and MCB; one COM_REQ block is required for each port. All these initialization parameters are loaded into the RTU master communication module during the first scan cycle of the PLC's power-on or cold start initialization. Afterward, the RTU master communication module is responsible for communicating with the PakscanIIE master controller, while the PLC exchanges data with the RTU master communication module.
Programming for reading and writing Modbus/RTU data and monitoring communication status is relatively simple; it only requires reading and writing the corresponding PLC parameter addresses defined during initialization.
3.2 Monitoring Software Design
The supervisory control station (SCADA) can accurately monitor and control all information and equipment involved in the storage and transportation process. Through programming, configuration, and connection, it can vividly reflect the actual process flow, display dynamic data, set PID control parameters and process parameters, and view historical trends and alarm reports.
Rotork's field-operated electric valves are installed on the process pipelines. Each valve can be manually started, stopped, and closed via a button, and its status and position at any given time are displayed. A well-designed human-machine interface makes operation simple and intuitive.
3.3 Fault Alarm System Design
In the design of the automatic control system, we designed a three-level fault display and alarm system. Level 1 is installed on the control cabinet panels at the control site, using indicator lights to indicate normal operation and fault status. When the equipment is operating normally, the corresponding indicator light illuminates; when a fault occurs, the indicator light flashes at a frequency of 1Hz. To prevent indicator light bulbs from malfunctioning and failing to accurately reflect the equipment's operating status, a fault reset/lamp test button is specifically provided. Pressing this button continuously for 3 seconds at any time during system operation should illuminate all indicator lights. If any indicator light remains off, it indicates that the indicator light is faulty and should be replaced immediately. After resetting the button, the indicator lights will continue to display the equipment's operating status as before. Level 2 fault display is located on the large-screen monitor in the central control room. When equipment malfunctions, the fault type is displayed in text, the corresponding equipment on the process flow diagram flashes, and the fault is recorded in the historical event table. Level 3 fault display is located in the signal box in the central control room. When equipment malfunctions, the signal box will alert personnel with audible and visual alarms, allowing for timely fault handling. When handling faults, they are further classified. Some faults require the system to stop operating, while others have little impact on the system's operation and can be operated with the faults. The faults can be eliminated during operation, which greatly reduces the downtime of the entire system and improves the reliability of the system.
4. Conclusion
The integration of Rotork's intelligent valve control equipment with PLCs makes the wiring of the tank farm storage and transportation monitoring system simple and convenient to control. The redundancy of the PLC and the dual backup of the Pakscan IIE master station controller ensure the high reliability of the system and also improve the automation level of the control system.
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