With the development of the economy and the improvement of industrial automation, differential pressure transmitters have been widely used in steel manufacturing, food hygiene, chemical and chemical, paper and other enterprises. In the application process, the advantages of flow measurement are shown. The daily measurement methods of differential pressure transmitter are as follows:
1. By using the pressure difference to achieve the height of the measured liquid, the pressure difference is generated by the gravity of the liquid itself.
2. It is used in combination with other components to achieve the purpose of measuring the liquid flow. For example, it can be combined with the throttling element to measure the liquid flow rate by using the differential pressure value generated before and after the throttling element.
3. Directly measure the pressure difference between different pipes and tank liquids. In application, the application technology level of differential pressure transmitters is also constantly improving. Of course, in the application process, accidents may occur by chance. If the maintenance personnel cannot solve the problem in time due to insufficient level, etc., the production schedule and the normal use of the instrument will be affected to varying degrees. It can be seen that the role of on-site maintenance personnel is very important, which also provides on-site maintenance personnel with a stage to continuously improve their own technology.
Principle of differential pressure transmitter
Design principle: The result measured by the differential pressure transmitter is the pressure difference, ie ΔP=pgΔh. Since the oil tank is often cylindrical and the area S of the cross-section circle is constant, then the weight G=ΔP·S=pgΔh·S, S is constant, and G is proportional to ΔP. That is, as long as the ΔP value is accurately detected, it is inversely proportional to the height ΔP. When the temperature changes, although the volume of the oil expands or shrinks, the actual liquid level rises or falls, and the pressure on the detected side remains unchanged. of. If the user needs to display the actual liquid level, it can also be solved by introducing media temperature compensation.
Differential pressure transmitter working principle diagram
Working principle: a space is divided into two chambers by sensitive components (multi-purpose bellows), and when pressure is applied to the two chambers respectively, the pressure sensor generates displacement (or displacement tendency) under the joint pressure of both sides. This displacement is proportional to the pressure difference between the two chambers (differential pressure), which is converted into a standard signal output that reflects the magnitude of the differential pressure. In addition, in actual construction, the structure of the sensitive component, the form of the chamber, The way of displacement conversion, there are many formats of standard signals.
Differential pressure transmitter
The following points should be noted during the use of the differential pressure transmitter:
1. Do not apply voltage higher than 36V to the transmitter, resulting in damage to the transmitter.
2. The measured medium is not allowed to freeze. Otherwise, the sensor element isolation diaphragm will be damaged, resulting in damage to the differential pressure transmitter. If necessary, the differential pressure transmitter should be temperature-protected to prevent freezing.
3. Never touch the diaphragm with a hard object, resulting in permanent damage to the diaphragm.
4. When measuring steam or other high temperature media, use a heat pipe to connect the differential pressure transmitter to the pipe and use the pressure on the pipe to transfer it to the transformer. When the measured medium is water vapor, an appropriate amount of water should be injected into the heat pipe to prevent the superheated steam from directly contacting the differential pressure transmitter and damaging the sensor.
5. When measuring steam or other high temperature medium, the temperature should not exceed the limit temperature when the differential pressure transmitter is used. Above the limit temperature used by the differential pressure transmitter, a heat sink must be used.
6. During the pressure transmission process, it should be noted that the connection between the differential pressure transmitter and the heat pipe should not leak; before starting to use, if the valve is closed, the valve should be opened very carefully and slowly. In order to avoid direct impact on the sensor diaphragm by the side medium, the differential pressure sensor diaphragm is damaged; the pipeline must remain unblocked, and deposits in the pipeline may pop up and damage the sensor diaphragm.
Differential pressure transmitter fault analysis
1. Direct inspection method: Many faults seem complicated, but they can often be solved quickly by means of maintenance personnel's experience and through direct means such as listening and listening. For example, according to the status quo of the problem, the maintenance personnel can observe whether the equipment parts are damaged, and ask the on-site operators to operate the equipment before the failure (fire, smoke, odor, power supply change, lightning strike, moisture, misuse, mistake). Repair), smell the instrument for charred odor, touch the parts to loosen and so on.
2. Gradually check the method: After the fault occurs, the maintenance personnel found that there are many factors that can cause the fault to occur. At this time, the maintenance personnel need to adopt an appropriate method to carry out the gradual troubleshooting of the fault, and the most likely faulty parts and Other components are isolated and diagnosed. If the problem is found, it can be further repaired. If the fault is not part of the component, the next suspected part is inquired. For example, the intelligent differential pressure transmitter can not normally Hart remote communication, the power supply can be disconnected from the watch body, and the differential pressure transmitter is energized for communication by means of the field plus power supply to check whether the cable is superimposed on the electromagnetic signal surface interference. communication.
3. Replacement part method: This method is to remove the suspected faulty parts, replace them with good parts, and check whether the instrument is running normally. If you suspect that the differential pressure transmitter board has failed, you can temporarily replace it to determine the cause.
Common fault analysis
There are many types of faults in the differential pressure transmission path, such as wear of parts, lack of lubricant, failure of the pressure guiding tube, etc. The following typical faults of the typical pressure guiding tube are analyzed in all the following faults. Through the analysis of this fault, To further explain the troubleshooting methods and techniques.
problem:
The example of positive pressure tube leakage is used to analyze and analyze the fault phenomenon of leakage of the pressure guiding tube. The flow measurement method of the purified air main pipeline of a heating furnace instrument control valve of a certain unit is: orifice plate + differential pressure transmitter. The wind flow rate during the normal production of the device is basically stable, but in the later production process, the wind flow rate is found to be much lower than the normal position.
Inspection process: After direct observation, it was found that the signals of all circuits were in good condition and the secondary instrument was in normal operation. After the differential pressure transmitter was sent to the calibration room for calibration, no abnormalities were found. Based on the above basic characteristics, the maintenance personnel initially suspected that the pressure guiding tube may have fallen. After carefully checking the pressure guiding tube, it was found that the positive pressure guiding tube leaked, because the welding was not well sealed. Therefore, the welding of the pressure guiding tube is repaired by a professional welding tool, and the problem is solved. Finally, the flow measurement is restored to normal.
Analysis of the failure of the positive pressure guiding tube leakage: The phenomenon of leakage of the positive pressure guiding tube is: the output of the differential pressure transmitter drops, rises and remains unchanged, and the body analysis is as follows: When the flow rate rises, the differential pressure transmitter (transmission) The device itself performs the influence of the output signal output, and sets the original flow rate to F1, PI=P1+-P1-, FI=K, F'I is the output value of the transmitter before the change; the actual flow after the increase is F2 , (ie: F2>FI), F'2=K, F'2 is the output value of the differential pressure transmitter after the flow and increase. Due to the increase of the flow rate, the static pressure of the pipeline increases to P0, and as the flow rate increases, The actual pressure tube is reduced to PO', and the positive pressure tube is reduced to Ps.
Then: P2+=P1++P0-Ps, P2-=P1-+PO-PO port
P2=P2+-P2-=P1+ (P0 port-PS)
When: P0â–¡=Ps positive pressure conduit leaks, and when the flow rate rises, the differential pressure transmitter output does not change.
When: P0â–¡>Ps positive pressure conduit leaks, and when the flow rate rises, the differential pressure transmitter output increases.
When: POâ–¡ When the flow rate drops, the effect of the output of the differential pressure transmitter (the transmitter itself is output signal), Let the actual flow after the fall be F2, namely: F2 As the flow rate decreases, the static pressure of the pipeline decreases by P0. At the same time, the static pressure of the negative pressure tube increases PO' due to the decrease of the fluid flow rate, and the pressure drop of the positive pressure tube decreases to Ps. Then: P2+=P1+-PO-Ps, P2-=P1--PO+PO port P2=P2+-P2-=P1-(Ps+P0') F'2=K=K is: When the flow rate drops, the differential pressure transmitter output is always smaller than the actual flow.
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