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Reference design of intelligent pressure transmitterSmart pressure transmitters are composed ofsmart sensorsand smart electronic boards, the smart sensor part includes:capacitive sensor, measuring membrane detection circuit, temperature sensor and temperature compensation circuit, etc.;the electronic board part includes: microcomputer controller and peripheral circuit, which completes the conversion of pressure signal to 4～20mA dc. It is used to measure the pressure of liquid, gas or steam, and then transforms thepressuresignal into a 4～20mA DC signal output. The smart pressure transmitters produced by Sisen Automation can communicate withHART hand operator, and can be set and adjusted through it, and can be monitored or connected with the host computer to form a field monitoring system. It is widely used in the measurement and control system of industrial pipelines with weak corrosiveness of liquid, gas and steam. The current intelligent transmitters are generally mixed intelligent transmitters with both digital and analog signals. The communication with the DCS is mainly based on the transmission of 4-20mA analog signals, and then the digital signal is superimposed on it to set the zero point, range and calibrate, configure and diagnose the transmitter remotely. It is only designed to be compatible with the existing DCS, and it is a transitional product of the true full digital intelligent transmitter (such as the field bus type intelligent transmitter). Compared with the conventional analog transmitter, the intelligent transmitter has the following characteristics in application: Wide range of measurement, large range ratio Smart transmitters generally have a wide range of quantities, a large range ratio, from 30:1 to 100:1, and some even reach 400:1. The range ratio of a transmitter refers to the ratio of the maximum measuring range (URV) to the minimum measuring range (LRV) [1]. The biggest advantage of the range of smart transmitters compared to conventional analog transmitters is that it can reduce the inventory, and the range can be changed at any time without replacing the meter, without worrying about the range exceeding the limit. For example, Zhongyuan Daxhua Group Co., Ltd. has added 3 sets of melamine equipment, and the intermediate by-product CO2 needs to be returned to the medium-pressure absorption tower of the urea device for re-absorption, and the ammonia entering the medium-pressure absorption tower needs to be greatly increased to absorb CO2, so the range of the instrument used to measure ammonia needs to be expanded from the original 10 t/h to 30t/h. At this time, it is very convenient to use smart transmitters, and the range can be adjusted online, without the need to replace the instrument. If it is a conventional analog transmitter, a new meter with a larger range needs to be replaced. Simple and convenient maintenance Communication with the smart transducer can be done manually with a hand-held communication device, either at the terminal block of the field transducer or at the terminal block of the control cabinet in the main control room. This allows the instrument maintenance personnel to perform tasks such as range modification, parameter setting, and instrument maintenance on the transducer in a relatively safer and more environmentally friendly control room. An instrument in the urea unit of the Zhongyuan Dahua Group Co., Ltd. that measures the liquid level of an ammonia tank always indicated incorrectly during the startup phase because of the different density of ammonia during the startup phase compared to the normal production phase, causing inaccurate instrument readings. The only solution was to set a measurement range for the transducer during startup and then set another measurement range during normal production. Additionally, during startup, it is easy to leak harmful gases such as ammonia at the site, and the instrument maintenance personnel had to wear gas masks every time to deal with it. If the original analog instrument is replaced with a smart transducer, the above problems can be solved simultaneously. In the control room, the instrument parameters can be modified, ensuring the personal safety of the personnel. It is important to note that the hand-held communication device cannot be directly connected to the power supply of the instrument control circuit to provide power to the transducer, as the power supply has a low impedance. The hand-held communication device cannot read information between its two ends. Generally, the loop resistance should be between 250 and 600 Ω. If the resistance is too small, communication cannot be established, and if the resistance is too large, the transducer will work abnormally. Have high and low pressure side conversion function Smart transmitters generally have the function of converting the high and low pressure sides of the sensor, which can be adjusted by a hand-held communicator. In production, we have encountered such a problem. Since the instrument has been running for many years, the pressure guide pipe connected to the transmitter needs to be replaced due to leakage. After replacement, it was found that the positive and negative phase pressure guide pipes were connected in reverse (because the pressure guide pipe is longer and there is a heating pipe together, it is easy to connect the positive and negative phases in reverse). If the two pressure guide pipes are exchanged, the workload is very large. However, smart transmitters have the function of converting the high and low pressure sides. Change the parameter "D45: H/L SWAP" in the instrument from NOROMAL (right high pressure, left low pressure) to REVERSE (right low pressure, left high pressure), and the instrument is normal after re-configuration, which saves a lot of time and effort. Smart transducerfixed output function This function is more convenient to use under the following conditions: ① When the transmitter fails, in order to ensure the safety of production equipment and personal safety, or to ensure the quality of the product, it is necessary to keep the output of the transmitter at a fixed value. ② After the annual overhaul, since all the links in the instrument control circuit have been overhauled, it is necessary to calibrate the entire circuit before startup. At this time, the hand operator can be used to use the fixed output function of the intelligent transmitter. Find the "OUTPUTSET (output setting)" item in the menu. Generally, any value between 3.8 and 21.6 mA can be set. Check whether the entire circuit where it is located is normal. |