How to check the fault of the instrument? Seven steps to check the cause of instrument failure

In case of instrument failure, in order to quickly find out the cause of the failure, it is necessary to understand the production process flow and the structure, characteristics, performance and parameters of the instrument control system. On this premise, the following principles should be followed to find out the instrument failure.


1、 Ask the process first and then look at the instrument


Since the operators use the instrument directly, they should be asked about the situation before and after the instrument failure, to know whether the production of the front and rear sections is normal, and whether the process operation indicators have been adjusted or changed, so as to determine whether it is the process or the instrument. When asking the operator, observe the display changes of the instrument. For example, the recording curve of the instrument is always normal before the failure, but the recording curve fluctuates greatly afterwards, making it difficult to control the system, even manual operation. This may be the reason for process operation or equipment. If the flow and liquid level control system fluctuates greatly and no problem is found after checking the transmitter, manual operation can be used to see whether the flow or liquid level can be stabilized. If the fluctuation is still large, it may be the cause of the process. If the pressure fluctuates greatly, you can find the cause from the process, such as load change, adding or subtracting materials, switching the return valve, improper operation, etc., which will cause the change of the internal pressure of the equipment.


2、 Go to the control room before going to the site


When the instrument fails, the display condition of the instrument can be observed on the basis of asking the operator to roughly determine the problem. The terminal blocks can be measured with a digital multimeter. Through the measurement, the wiring from the measuring element to the terminal in the control room can be judged, such as whether there are open circuit, short circuit and grounding faults. If the thermocouple has thermal potential output, or use needle nose pliers to short-circuit the input end of the display instrument to see if the instrument can indicate room temperature. In case of thermal resistance, check whether the resistance value changes; Short circuit the input terminal of the display instrument with pointed nose pliers to see if the instrument indicates below zero, or disconnect the terminal wiring to see if it indicates maximum or overflow. If the transmitter sees whether it has current output, it is more convenient to judge the fault with HART manual operator. For the control system, check whether the positions of manual and automatic switches are placed correctly. If necessary, switch the operator to manual mode, and use manual operation to observe whether the actuator operates normally and whether there is valve position feedback signal. Measure whether there is interference voltage and its magnitude when interference is suspected.



3、 Look at the simple before the complex


First observe whether a single instrument is abnormal or multiple instruments are abnormal, and then check whether the power supply and power box of the instrument are normal and whether the fuse is broken; Check the relevant wiring to see if it has poor contact or short circuit, and if the switch position is correct. Observe whether the impulse pipe and valve have leakage. For the instrument measuring micro pressure, check whether the rubber pipe or plastic pipe connected with the impulse pipe falls off or leaks. If the pressure and flow instruments do not fluctuate and change, it is often caused by the blockage of the impulse pipe.


4、 Check the primary instrument first and then the secondary instrument


On the basis of observation in the control room, if there is a problem with the primary instrument on site, the field instrument can be checked, such as whether the thermocouple and thermal resistance terminal wiring are loose, whether there is water ingress, whether they are damaged, whether the actuator is stuck or lack of oil, and so on, the problem can be handled.


When the pressure and differential pressure transmitters are abnormal, blow off the blowdown first, purge the impulse piping, and check whether the three valve manifold and other valves are blocked or leaking. For the flow transmitter, close the positive and negative pressure valves of the three valve manifold, open the balance valve and observe whether the zero position of the transmitter is normal; Then, in the open meter state, quickly open and close the positive pipe blowdown valve to observe whether the output current changes in the increasing direction. Quickly open and close the negative pipe blowdown valve to observe whether the output current changes in the decreasing direction. If the current changes normally, there is no major problem with the transmitter.


For the liquid level transmitter, close the positive and negative pressure valves of your three valve manifold and open the balance valve to observe whether the output current of the transmitter is normal. This current is related to migration. If it is negative migration, the output current should be 20mA when the differential pressure is zero. If it is positive migration, the output current should be less than 4mA when the differential pressure is zero. For the transmitter with negative migration, under the open meter state, quickly open and close the positive pipe blowdown valve to observe that the output current should change in the decreasing direction. Quickly open and close the negative pipe blowdown valve to observe whether the output current changes in the increasing direction. If the current change is normal, there is no major problem with the transmitter. During the above inspection, open and close the blowdown valve to see that the output current changes, and then close the valve immediately. In this way, the condensate will not be discharged too much.


For the fault judgment of the secondary meter, the oil law can be followed. For example, the temperature parameter is later large, so the display value of the instrument is impossible to change suddenly. For example, the display changes to the maximum or minimum, and after the problem of the primary element is eliminated, it is usually the display fault. If the pressure indication does not fluctuate or changes slowly, excluding the plugging of impulse pipe and valve, it should be the problem of display instrument. If the flow recorder does not fluctuate approximately in a straight line, it may be that the instrument is faulty, because the fluctuation of flow parameters is relatively large, and more or less changes of parameters should be recorded on the recorder to reflect. If you have doubts about the display parameters of DCS or recorder, you can look at the conventional standby instrument or other instruments on site, such as the indicator head of the transmitter, to see how much the display difference between them is, so as to determine the fault. Check whether the controller is normal. You can manually change a given value to cause a new deviation, and then observe the change of the controller output current to judge whether the controller is normal.


5、 Look outside the instrument first and then inside


Follow the steps of “easy before difficult”, first check the external of the instrument, such as whether the power supply is normal; Check whether the impulse pipe has any leakage point, and observe whether the blowdown is smooth, so as to judge whether the impulse pipe or valve is blocked; Check the wire connection and terminal for looseness, corrosion, poor contact, etc. The thermocouple can also be short circuited, and the open terminal can be used for the thermal resistance to judge the fault position. Measure the voltage of instrument panel terminal block or instrument wiring terminal to judge the fault. On this basis, determine whether the instrument needs to be removed for treatment.


6、 See the light first and then the dark


When dealing with on-site instrument faults, first check the terminals and wiring in the instrument panel. If no problem is found, but it is still suspected that there is a problem with the conductor, then check the conductor in the cable tray and trench. For the sewage pipe leading into the trench, the final inspection shall also be carried out. If it is suspected that the protective sleeves of thermocouples and thermal resistors are damaged, they should also be removed for inspection after checking and confirming that there is no problem in other parts.


7、 Set software before hardware


With the extensive application of intelligent instruments, when dealing with field instrument faults, the idea can not be limited to the original method of dealing with analog instruments. The function of intelligent instrument hardware depends on software support. Without software, this kind of instrument cannot work. Therefore, when checking and processing intelligent instruments, we should first check whether the setting of the instrument is normal. The author once encountered the situation that the instrument could not work after power restoration due to lightning at night. Later, it was found that some of the setting parameters of the instrument had changed, and it would be normal to reset. I also encountered that after replacing the network card of the industrial computer, I could not connect to the network. I installed the driver and replaced the network card. After several hours, I found that the CMOS setting item of the computer had changed, and the reset was normal.

Post time: Jun-16-2022