Pressure transmitter is the most commonly used equipment in modern industrial control. Whether it can work normally affects the normal operation of industrial production. However, whether it is a domestic transmitter or an imported transmitter, it is inevitable that some faults will occur during the use process, and various faults may occur in the working environment, improper operation of the human body or the transmitter itself. Therefore, it is necessary to understand the common faults of the pressure transmitter and the daily maintenance methods! Today, Yikong Jun has compiled some common transmitter faults, solutions, daily maintenance and precautions for everyone. Let's take a look.
Pressure transmitters inevitably have a variety of faults during use, and there are many influencing factors, such as: filling liquid leakage between the diaphragm and the sensing element in the pressure transmitter, zero point and range shift, output is not Stabilization, etc. will cause the pressure transmitter to drop in accuracy or even damage; weather factors will also affect the transmitter, such as: lightning strikes will damage the circuit of the transmitter bellows, resulting in communication failure; wet environment will damage the line; Unreasonable selection of the transmitter range can cause irreversible deformation of the pressure sensitive component.
When there is a line fault, the computer displays an abnormal value, and the transmitter junction box is opened to check whether the line has a virtual connection, short circuit or disconnection. The fault is detected by measuring the power supply, shaking the insulation and measuring the resistance.
2. Frequency conversion interference
When wiring is performed, various signal lines interfere with each other, especially when the power line and the signal line are in the same pipeline, and the interference is more serious. In this case, the transmitter does not communicate or even causes an error. This type of erroneous operation can be avoided by increasing the distance between the instrument cable and the power cable tray.
3. Pressure pipe failure
There are three types of failures of the impulse piping, such as clogging of the pressure piping, air leakage of the pressure piping, and effusion of the pressure piping. The pressure clogging is generally caused by untimely discharge or dirty or sticky medium; The transmitter is connected to the power, the shut-off valve and other accessories are more, and the leakage point is increased; the effluent of the impulse tube is usually caused by the unreasonable pressure of the gas or the installation of the pressure tube, and the pressure of the pressure tube will affect measurement accuracy.
4. Electrical signal transmission failure
Pressure transmitters that are improperly used or maintained can easily cause transmission failure of electrical signals. For example, in order to save time and place the transmitter near the device under test, the signal transmission distance is too far, and the signal will be disturbed or attenuated. At this time, the cross-sectional area of the cable should be increased as needed.
1. The output signal is zero
When the pressure transmitter has zero pressure, it can be processed from the following aspects: first check whether there is pressure in the pipeline, whether the instrument is normally powered, and then check whether there is a reverse polarity of the power supply, and finally check the electrons. Circuit board, sensible film head, transmitter power supply voltage, etc.
2. Add pressure without reaction
If the pressure is not reactive, check whether the valve of the pressure tube is normal, whether the transmitter protection function jumper switch is normal, whether the pressure tube is blocked, check the white point and range of the white feeder, and replace the sensor head.
3. Pressure variable reading deviation
When the pressure transmitter appears to have a high or low pressure reading, first check the pressure piping for leaks. Check the valve on the pressure tube to fine tune the sensor. If there is still a problem, replace it with a new one. Sensor head.
4. Pressure variable reading is unstable
This problem can be checked by isolating external sources of interference, checking whether the pressure guiding tube is leaking, whether there is any debris in the pipeline, clothing or deformation when inspecting the diaphragm, and checking the pressure sensitive membrane head.
5. Pressure indication fluctuates greatly
Check the other parameters of the working condition to see if there is any change. If other parameters such as temperature, flow, and liquid level fluctuate, it is the pressure change caused by the change of working condition. At this time, the pressure transmitter should be no. If the other process conditions have not changed, but the pressure indication is fluctuating, then the pressure display shows no control, generally the regulating valve, check if the output of the positioner on the regulating valve is constant, not constant, then To deal with the valve problem; if it is constant, the reason is on the pressure transmitter.
At the site, the pressure transmitter is turned off and the discharge is turned on to see if the pressure transmitter shows zero. If the zero point is displayed, the measurement of this transmitter is no problem. Check the cable of the pressure transmitter to the DCS cabinet to see if the shielding of the branch cable and the main cable is intact. This is usually the case that the shield is broken or the cable is broken. The virtual connection at the joint and the exposed cable core are oxidized and cause poor contact. The shielding is not good, and there is a large amount of electromagnetic interference in the environment where it is located. Such fluctuations are also serious.
6. The pressure transmitter's meter is not displayed
In this case, the cable of the instrument is generally loose, or the cable is broken. This situation usually occurs, and different indications appear at each process point. We can judge the general situation based on these instructions. If it is not possible to determine the specific situation, the cable between the terminal cabinet and the field junction box and the transmitter should be checked point by point.
7. Pressure display does not change with process conditions
This situation is likely to be a blockage. It is necessary to check the place where the pressure transmitter is plugged between the pressure-receiving valves and then clean the blockage.
1. Tour inspection
Check the meter to indicate the situation, whether there is any abnormality, see if he is within the specified range; some transmitters do not have on-site instructions, go to the control room to see its secondary indication. If there is any debris around the instrument or if there is dust on the surface of the instrument, it should be removed and cleaned in time. There are false leakage and corrosion between the instrument and process interface, the pressure guiding tube and each valve.
2. Regular inspection
For some instruments that do not need to be inspected daily, it is necessary to check regularly. Regular zero check, because the transmitter has a secondary valve or three valve group, five valve group, so zero check is very convenient, does not require too much time. However, the transmitter used in the control system, regardless of the inspection time, still needs to change the automatic to manual control, so the zero return period of this instrument can be longer. Since these inspections require the removal of street inspection equipment, it is cumbersome, so if there is no abnormality, the inspection period can be longer.
Sewage, condensate and venting are carried out on a regular basis.
Periodically purge the pressure pipe of the easily blocked medium, and fill the isolation liquid.
Regularly check the transmitter parts intact, no serious corrosion, damage; nameplate, logo clear and correct; fasteners should not be loose, the connector is in good contact, the terminal wiring is firm.
Regular on-site measurement of the line, including whether the input and output circuits are intact, whether the line is disconnected, short-circuited, and whether the insulation is reliable.
The housing must be well grounded while the transmitter is running. The transmitter used to protect the system should have measures to prevent power failure, short circuit, or open circuit.
In the winter season, the instrument should be inspected for the source and pipeline insulation and heat tracing to avoid damage to the source pipeline or the measuring components of the transmitter.
(1) Prevent the transmitter from coming into contact with corrosive or overheated media;
(2) preventing the deposition of dross in the conduit;
(3) When measuring the liquid pressure, the pressure tap should be opened on the side of the process pipe to avoid sedimentation;
(4) When measuring the gas pressure, the pressure tap should be opened at the top of the process pipe, and the transmitter should also be installed in the upper part of the process pipe so that the accumulated liquid can be easily injected into the process pipe;
(5) The pressure guiding pipe should be installed in a place with small temperature fluctuations;
(6) When measuring steam or other high-temperature medium, it is necessary to add a condenser such as a buffer tube (coil), and the operating temperature of the transmitter should not exceed the limit;
(7) When freezing occurs in winter, the transmitter installed outside must adopt anti-freezing measures to prevent the liquid in the pressure inlet from expanding due to the volume of ice, resulting in damage;
(8) When measuring the liquid pressure, the installation position of the transmitter should avoid the impact of the liquid (water hammer phenomenon), so as to avoid the sensor over-voltage damage;
(9) When wiring, pass the cable through the waterproof joint (accessory) or the winding tube and tighten the sealing nut to prevent rainwater and the like from leaking into the transmitter housing through the cable.