Smart sensors in chemical plants

General sensors can only be used as sensitive elements and must be equipped with conversion instruments to detect changes in physical quantities, chemical quantities, etc. With the development of microelectronics technology, intelligent sensors have emerged. Intelligent sensors use ultra-large-scale integrated circuits and use embedded software to coordinate internal operations. On the basis of completing nonlinear compensation, zero point error, temperature compensation, fault diagnosis, etc. of input signals, they can also complete the control of industrial processes, making the functions of the control system further decentralized. Intelligent sensors integrate all functions of sensors and intelligent instruments and some control functions, with high linearity and low temperature drift, reducing the complexity of the system and simplifying the system structure. The characteristics are as follows: ● Artificial intelligence to a certain extent is a combination of hardware and software, which can realize learning functions and better reflect the role of instruments in control systems. According to different measurement requirements, appropriate solutions can be selected, and information can be comprehensively processed to predict the system status. ● Multi-sensitivity function integrates the original scattered and independent single-sensitivity sensors into sensors with multi-sensitivity functions, which can measure multiple physical quantities and chemical quantities at the same time and fully reflect the comprehensive information of the measured quantity. ● High precision and wide measuring range. The influence of the measured change on the characteristics of the detection element can be detected at any time, and various operations can be completed. The output signal is more accurate. At the same time, its range ratio can reach 100:1, up to 400:1. One smart sensor can cope with a wide measurement range, which is particularly suitable for control occasions requiring a large range ratio. ● The communication function can use a standardized bus interface for information exchange, which is one of the key features of smart sensors. The emergence of smart sensors changes the processing of complex signals from centralized to decentralized, which can ensure the quality of data processing, improve anti-interference performance, and reduce the cost of the system. It enables sensors to develop from single function and single detection to multi-function and multi-variable detection, and from passive signal conversion to active control and active information processing. It also enables sensors to develop from isolated components to systematization and networking. Smart Sensors and Instrument Engineering in Chemical Plants Figure 1 is a schematic diagram of an instrument engineering project in a chemical plant. The liquid enters the chemical tank through the valve positioner. An electromagnetic flowmeter is installed in the inlet pipeline to control the instantaneous flow and cumulative flow; a pressure sensor is installed at the bottom of the chemical tank to detect the liquid level. Since this project is located at a chemical site, intrinsically safe instruments are needed, data exchange is required, and high control accuracy is required.

Figure 1. Schematic diagram of an instrumentation project in a chemical plant

According to the project needs and the actual situation of Siemens PLC control, the control system of the chemical plant instrumentation project can be constructed as shown in Figure 2.

Figure 2. Control system construction

In this control system, the DP/PA coupler is the junction and physical connector of the two buses PROFIBUS-DP and PROFIBUS-PA. The DP/PA coupler can be used alone or in conjunction with DP Link. In this project, it can be used alone.

There are two types of DP/PA couplers, namely Exi DP/PA coupler and FDC 157-0 DP/PA coupler. The former has a maximum output current of 110mA and is used in explosion-proof occasions; the latter has a maximum output current of 1000mA. Since the control cabinet is installed in the central control room, only sensors and actuators are installed on site, and intrinsically safe field instruments are required. Therefore, FDC 157-0 DP/P coupler is selected from an economic point of view.

Pressure sensor selection SITRANS P DSⅢ is an intelligent digital pressure transmitter with HART, PROFIBUS PA or FF bus communication, and has the characteristics of key operation, diagnostic function and high precision (as shown in Figure 3). This intelligent pressure transmitter is mainly used for pressure, differential pressure, absolute pressure and liquid level measurement, and has special safety standards.

Figure 3. Pressure sensor SITRANS P DSIII

Advanced diagnostic functions of SITRANS P intelligent pressure transmitters include:

● Dual time registers can set reminder signals when preventive maintenance or calibration is required.

● Three adjustable limit values, which means that preset alarms or temperature monitoring can be implemented.

● Over-limit recording: Uses non-recoverable instantaneous actual values ​​to record the maximum/minimum pressure, sensor temperature and electronic unit temperature. The recorded information also includes the status of SITRANS P, process and ambient temperature.

● Simulation function: Enables the transmitter to check the electronic part and the entire circuit behind the sensor, for example, for system commissioning before starting. This provides important information: Is the transmitter electronic part working properly? At what pressure will an over-limit response occur? Is the control loop working properly? Are all display functions working?

● Simple plug-and-play function: In the field, if the transmitter fails in the electronics or measuring unit, only the faulty part can be replaced without recalibration.

● On-site adjustment is easier: DS III series intelligent transmitters have a unique local button. Through the local buttons, the transmitter can be configured with various functions, such as zero adjustment, setting of upper and lower limits of pressurized and non-pressurized measurement, unit selection, damping setting, output selection, etc. In addition, since the local buttons adopt the principle of magnetic induction, even in dangerous places, the parameters can be adjusted without opening the transmitter cover and using any special tools. In addition, the configuration buttons can be locked to avoid misoperation.

In the intrinsically safe type of flow sensor selection, the SITRANS FM series electromagnetic flowmeter has various ranges that meet ATEX and FM certification, including split and integrated installation. It has intrinsically safe input and output, complies with NAMUR NE 21, multi-information and multi-language display, real-time flow and cumulative forward and reverse flow, advanced self-diagnosis function and error record. In the chemical industry, there are often harsh environments, so a wide range of anti-corrosion materials are required. The lining, electrode and shell materials provided by Siemens SITRANS FM can withstand the corrosion of such harsh process media.

Figure 4: Pneumatic valve positioner

Selection of valve positioner The SIPART PS2 series valve positioner shown in Figure 4 has the following features:

● Automatic initialization. SIPART PS2 can be automatically initialized and set through a simple menu in a few minutes. In this process, the microprocessor can determine the zero point, end position, direction of movement and speed by itself. From these items, the minimum pulse time and dead zone can be determined and optimized control can be performed.

● Low gas consumption. Another significant feature of SIPART PS2 is its extremely low gas consumption. Traditional valve positioners will have a lot of instrument air emissions or leaks during operation, resulting in a large demand for instrument air and high operating costs. Due to the technology of the new piezoelectric valve, SIPART PS2 will not have any instrument air emissions when the valve is not moving, and will only use gas when it is most needed (i.e. when the valve is moving).

● Rich diagnostic functions. The new SIPART PS2 can provide more parameters and additional diagnostic information, including working time records, current temperature, minimum/maximum temperature, working time for each temperature segment, online control of valve seat (upper and lower stroke positions), monitoring or display of adjustable thresholds, etc.

After the valve positioner is selected, it is time to install and wire it. Figure 5 shows the instrument PA bus wiring.

Figure 5: Instrument PA bus wiring

Conclusion

With the continuous development of complex process industries, industrial process control has put forward newer and higher requirements for the collection, transmission and data conversion of a large number of field signals, as well as for accuracy, reliability, and integrated control.

● Intelligent sensors and fieldbus are two important parts of the intelligent sensor and instrument control system The

intelligent sensor and instrument control system uses the fieldbus to establish a highly reliable data communication line at the control site, realize data communication between each intelligent sensor and between the intelligent sensor and the main control machine, and turn a single scattered intelligent sensor into a network node. Data processing in intelligent sensors helps to reduce the workload of the main control station, so that a large amount of information can be processed on the spot, reducing the information round trip between the field instrument and the main control station, and reducing the requirements for network data communication capacity. The data pre-processed by the intelligent sensor is collected on the host through the fieldbus for more advanced processing (mainly system configuration, optimization, management, diagnosis, fault tolerance, etc.), so that the system can analyze and judge the controlled object from surface to point, and then from point to surface, and improve the reliability and fault tolerance of the system.

● Differences between smart sensors and general sensors

Compared with general sensors, smart sensors that use digital communication as a transmission method need to have the following functions:

1. Share a bus to transmit information, and have multiple computing, data processing and control functions, thereby reducing the burden on the host. Replace 4-20mA analog signal transmission, realize the digitization of transmission signals, and enhance the anti-interference ability of signals.

2. Use a unified network protocol to become a node of the field bus control system to realize information exchange between sensors and actuators. The system can verify, configure, and test it, thereby improving the reliability of the system. The interface is standardized and has a "plug and play" feature.

Smart sensors are the mainstream products of future industrial process control systems and will bring revolutionary changes to traditional control system structures and methods.