〠Abstract 】 I have analyzed and discussed the working principle, calibration, adjustment, fault handling and other aspects of the intelligent differential pressure transmitter for the reference of the relevant instrument calibration and maintenance personnel.
Foreword Differential pressure transmitters have a wide range of applications in the factory, mainly used to measure the differential pressure, pressure, level and density of liquids, gases or steam, and then convert it to 420mA. DC signal output. To ensure its normal operation and accuracy, regular inspections and calibrations are necessary.
First, the principle of differential pressure transmitter Differential pressure from the two side of the pressure tube directly on the transmitter sensor on both sides of the diaphragm, the diaphragm through the diaphragm to the measuring element, the measuring element will test The resulting differential pressure signal is converted to its corresponding electrical signal and passed to the converter. After amplification and other processing, it becomes standard electrical signal output. Differential pressure transmitter several application measurement methods: (1) combined with the throttling element, use the differential pressure value before and after the throttling element to measure the liquid flow; (2), the pressure difference between mJ and the liquid's own gravity , Measure the height of the liquid; (3) Directly measure the pressure difference of the liquid in different pipes and tanks.
=, Differential Pressure Transmitter Calibration Steps We know that differential pressure transmitters are connected to a pressure-transmitting tube in the application. Normally, the pressure tube and differential pressure transmitter's connector need to be disassembled. Then connect to the pressure source for calibration. This is very cumbersome, and the work and labor intensity is high. The most worrying factor is that when the joint is disassembled, the pressure guiding tube is torn off or leakage occurs. We know that no matter what type of differential pressure transmitter, the positive and negative pressure chambers have exhaust, drain valves or cocks; this provides us with the convenience of on-site calibration of differential pressure transmitters, that is, without dismantling The pressure tube can calibrate the differential pressure transmitter. When calibrating the differential pressure transmitter, first close the positive and negative valves of the three valve group, open the balancing valve, and then loosen the exhaust, drain valve, or tap to vent. Then replace the positive pressure chamber with a self-made connector. The exhaust, drain valve or cock, while the negative pressure chamber is kept open to allow it to open to the atmosphere. The pressure source is connected to the self-made connector through the rubber hose, the balance valve is closed, and the gas seal is checked. Then the ammeter (voltage meter) and the communicator are connected to the transmitter output circuit, and the calibration is started after the power is warmed up.
The intelligent differential pressure transmitter is calibrated with "set range", "heavy range", and "fine adjustment". The "set range" operation is mainly through the LRV. The URV's digital settings are used to complete the configuration work, while the "re-quantity range" operation requires the transmitter to be connected to a standard pressure source and guided by a series of commands, with the transmitter directly sensing the actual pressure and setting the values. The initial and final setting of the range depends directly on the actual pressure input value. However, to see that although the analog output of the transmitter is correctly related to the input value used, the digital reading of the process value shows a slightly different value, which can be calibrated by a fine-tuning item. Since each part needs to be adjusted separately and must be adjusted together, the actual calibration can be performed in the following steps: (1) First make a 4~0mA fine adjustment to correct the internal converter of the transmitter because it does not involve Sensing parts, no external pressure signal source; (2) Do a full fine-tuning again, so that 4-20mA, digital readings and the actual application of the pressure signal match, so the need for pressure signal source; (3) the final re-quantification process, By adjusting the analog output 4-20mA to the applied pressure signal source, the effect is exactly the same as that of the zero (z) and range (R) switches on the transmitter housing.
Discussion of the problem: Some people think that as long as the HART communicator can be used to change the smart transmitter range, zero point and span adjustment can be performed without the need for input pressure source, but this method can not be called calibration, only Can be called "setting range." True calibration requires a standard pressure source input to the transmitter. Since the scaler (LRV.URV) is not calibrated without the use of an etalon, ignoring the input part (input transducer pressure) for output adjustment (transducer's conversion circuit) is not the correct calibration. In addition, the relationship between the pressure and differential pressure detection components and the A/D conversion circuit and the current output is not equal, and the purpose of the calibration is to find out the relationship between the three. Emphasize that only when the input and output (pressure of the input transducer, A/D conversion circuit, loop current output circuit) are debugged together can it be called a true calibration.
Third, on the adjustment after several suggestions After the end of the adjustment work, the exhaust, drain valve and cocks should be rotated back to the original position, and should be wrapped with raw tape, tightening to ensure no leakage, but before tightening The exhaust and discharge of positive and negative pressure chambers should be performed first. At this point, process pressure can also be used to perform simple static pressure error checking of the transmitter.
four. Differential Pressure Transmitter Fault Diagnosis In the measurement process, the differential pressure transmitter often has some failures, and the timely determination and analysis of the failures is crucial to the ongoing production. Based on our experience in routine maintenance, we summarized some judgment analysis methods and analysis procedures.
(1) Investigation method: Review the fire, smoke, odor, power supply changes, lightning, wetness, misoperation, and mis-maintenance prior to failure.
(2) Intuitive method: to observe the external damage of the circuit, the leakage of the pressure guiding tube, the overheating of the circuit, and the status of the power switch.
(3) Detection method: Short-circuit detection: In the case of ensuring safety, the relevant part of the circuit is directly shorted, such as: the output value of the differential transmitter is too small, the pressure guiding tube can be disconnected, directly from the pressure valve The differential pressure signal is directly led to the two sides of the differential pressure transmitter, and the output of the transmitter is observed to judge the connectivity of the pressure-reducing pipeline. The disconnection detection: separates the suspected faulty part from other parts. Check whether the fault disappears. If it disappears, locate the fault. Otherwise, you can search the next step. For example, the smart differential pressure transmitter cannot communicate with Hart remotely, and the power supply can be disconnected from the meter body, and the power supply can be used on site. The method communicates with power to the transmitter to see if the cable is overlaid with an electromagnetic signal of about 2 kHz to interfere with the communication; replacement detection: replace the part that is suspected to be defective and determine the location of the fault. Such as: suspect that the transmitter circuit board failure, you can temporarily replace a block to determine the cause; segment detection: the measurement loop is divided into several parts, such as: power supply, signal output, signal transmission, signal detection, according to sub-section Inspection, from simple to complex, from the table and the inside, narrows the scope and finds out the fault location.
Fives. Differential Pressure Transmitter Migration Failure Analysis When using differential pressure transmitters to measure the liquid level in industrial production, if the differential pressure transmitter's positive and negative pressure chambers are at the same level as the container's pressure level, it is not Need to migrate. In practical applications, measuring instruments may not necessarily be on the same level as the pressure pickup point due to considerations such as the installation location of the equipment and the ease of maintenance; if the measured medium is a highly corrosive or heavy viscosity liquid, It is not possible to directly introduce the medium into the pressure gauge. The spacer tank must be installed and the spacer fluid must be used to transmit the pressure signal to prevent the instrument under test from being corroded. At this time, the influence of the liquid column of the medium and the spacer liquid on the reading of the pressure gauge should be considered. In order to correctly indicate the height of the liquid level, the differential pressure transmitter must do some technical processing—that is, migration. Migration is divided into no migration, negative migration and positive migration.
Positive migration failure: To determine whether the differential pressure transmitter being migrated is accurate during field use, the positive and negative pressure measurement chambers of the three-pressure differential pressure transmitter group should be closed first, and the balance valve and metering blanking plug should be opened. The meter output should be less than 4mA at this time. If the output is not lower than 4mA, it may be a positive pressure chamber lead or a three-valve block. Second, close the pressure chamber pressure point, open the vent switch, then the output should be 4mA. If the output is lower than 4mA, the migration may become smaller or the zero may be lower. If the spacer fluid is filled, the spacer fluid may not be filled or leaked from the side. If the output is higher than 4mA, the migration amount becomes larger or smaller. High.
Negative transfer failure: to judge whether the negative transfer differential pressure transmitter is accurate during field use, first close the positive and negative pressure measurement chambers of the differential pressure transmitter three valve group, open the balance valve and the instrument vent plug, and the instrument The output should be 20mA. Secondly, close the positive and negative pressure chamber to take pressure point, open the vent switch, this time, the instrument output should be 4mA, if not 20mA or 4mA, you should check the positive and negative pressure chamber lead is blocked, the migration amount is changed, zero position Whether it is accurate, whether the spacer fluid is lost or not.
Concluding remark The accurate control of the liquid level is the prerequisite for the stable operation of the production equipment. Only by mastering the working principle and failure analysis method of the differential pressure transmitter measuring liquid can it be applied flexibly in practical applications and timely and accurately handle the failure of the field instrument. , Improve the accuracy of meter measurement, reduce the daily maintenance of the meter, improve the working efficiency of the meter maintenance personnel.
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