What methods are there to troubleshoot instrument failures on the production site?

What methods are there to troubleshoot instrument failures on the production site?

Quickly eliminating instrument failures and potential hazards on the production site to maintain normal production is the responsibility of every instrument worker. The author shares here the experience of how instrument workers can improve their ability to troubleshoot instrument failures on the production site, hoping it will be helpful to everyone!

(1) Learn to See the Essence Through Phenomena

When an instrument shows abnormal phenomena, apart from the instrument itself being faulty, the influence of faults in other instruments in the system and human error in operation can also cause the instrument to malfunction. If one blindly disassembles and repairs the instrument without clarifying the cause of the fault, the situation is often made worse.

For instance, there was a boiler outlet steam flow meter. The indication and cumulative flow during the day and mid-shift were basically normal based on experience. However, during the night shift, when a certain section was not in production and the gas consumption should have been lower, the indication and cumulative flow were instead higher, and this happened every day. Some people wondered if the large temperature difference between day and night affected the performance of the instrument. After checking and calibrating the instrument and transmitter, they were found to be normal, and no obvious leakage was found during on-site inspection. No matter how hard they thought, they couldn't figure it out. Later, someone was arranged to observe on duty and found that the negative blowdown pipe of the instrument was warm. After checking, it was found that the blowdown valve was leaking. But why did it only affect the night shift? Because the gas consumption during the night shift decreased, the boiler load was lighter, and the steam supply pressure was nearly 0.4 MPa higher than that during the day shift. After the steam pressure increased, the leakage became larger, causing the instrument to show even higher readings. Since the leakage of the negative blowdown valve was not severe, it was not obvious when the steam pressure was low. Moreover, the blowdown pipe opening was connected to the blowdown trench, making it difficult to observe whether there was a leakage. In the process of troubleshooting, because the leakage, the essence of the problem, was not identified, a lot of time was wasted in vain. Therefore, learning to see the essence through phenomena is one of the skills that an instrument engineer should master.

(2) Pay attention to the interaction and influence among systems

Most automatic control systems are composed of multiple instruments. Therefore, it is necessary to pay attention to the interaction and influence among them. For instance, sometimes the instrument connected at the back may affect the one at the front. If the sensitivity of the electric actuator is too high, it will cause output oscillation. If the valve characteristics are poor, it will cause fluctuations in the valve position signal, which will affect other instruments or systems through feedback signals. In the field, if such influences are not observed carefully, they are often overlooked, leading to incorrect identification of faults.

(3) Do not overlook the influence of human factors

Misoperation, incorrect wiring or incorrect switch positions can also lead to incorrect fault diagnosis. If one does not observe carefully but assumes the instrument itself is faulty and starts to disassemble and repair it, they will take a detour in the maintenance process. In severe cases, they may even make the situation worse. For instance, a novice instrument technician, unfamiliar with the performance characteristics of a PID regulator, may choose inappropriate PID parameters, resulting in a large output even with a very small input signal. With many wires connected to the controller, operator, electric actuator, servo amplifier, valve position signal, etc., it is easy to make a mistake in wiring if one is not careful. Improper placement of the instrument's switch positions can also cause malfunctions, such as the forward and reverse action switch, manual and automatic conversion switch, etc. 

(4) Be good at learning and accumulating

Under the circumstances where on-site detection conditions are poor, higher requirements are placed on maintenance work. Therefore, it is necessary to combine the actual situation of the enterprise and quickly familiarize oneself with the enterprise's instruments and control systems to facilitate maintenance and fault handling. Therefore, one should learn and master the working principles, structures, and characteristics of instruments; understand the interrelationships of various parts of the enterprise's control system, the working principles of various control systems, and the characteristics of the controlled objects, etc.

When instruments and systems are not functioning normally, based on the knowledge learned and combined with the fault phenomena, and by connecting with relevant instruments for analysis, trace back to the source to find the true cause of the fault. If the faulty instrument has damaged other instruments, the fault cause should be identified first, and then other instruments should be repaired.

It is necessary to formulate and abide by operating procedures to prevent misoperation as much as possible. During maintenance, it is very necessary to develop the habit of checking in a reasonable, detailed, and item-by-item manner according to the procedures. Fault handling records should be kept well to accumulate experience, explore patterns, and master characteristics.

(5) Do a good job in preventive work to reduce potential faults

Some instruments may seem fine but could have hidden faults that are hard to detect, especially those related to the control system. If there are potential faults, the impact could be even greater. Therefore, it is very important to implement a regular inspection system during maintenance. At the same time, the pressure guiding tubes of the instruments should be drained regularly; the mechanical moving parts should be lubricated regularly; and the dirty parts should be cleaned regularly. This can significantly reduce potential faults.

After an instrument is repaired, it must be powered on for a trial run. If there are potential faults, they may gradually manifest themselves and should be eliminated as much as possible. Before connecting the instrument to the system, it is best to test its constant current performance, the impact of power voltage fluctuations, and the stability of the output value. For example, observe whether there is any drift or jitter in the output. If it exceeds the basic error, there may be a soft fault in the components, and further inspection should be carried out.

Pay attention to the maintenance process, such as poor contact and false soldering. For instance, some instruments work normally when placed horizontally but show changes when placed horizontally. Some instruments change their readings when the printed circuit board is pressed by hand. The possible reason is that there is a false soldering point. When removing screws, they must be placed properly and the quantity remembered. If not careful, the screws may fall into the instrument and roll around inside the casing, causing a short circuit at an unknown time. When installing transmitters and actuators on site, the covers must be sealed well to prevent dust and moisture from entering the instrument.

(6) When checking for instrument malfunctions, it is important to be good at identifying characteristic features. The prerequisite for grasping the characteristic features of instrument malfunctions is to be familiar with the normal characteristics of the instrument, so as to identify malfunctions. If an instrument is abnormal but shows the following manifestations, it basically does not have major faults: the controller output can change with the input signal, the transmitter output changes when the pressure guiding tube is drained, the actuator can track the feedback signal, and the display value of the instrument can change with the input signal, etc. Through mutual comparison, the one that is too abnormal may be the faulty instrument.

(7) Skills Required for Handling Instrument Faults

Mastering some basic skills is necessary, such as manually rotating the moving parts when power is off to check if there is any jamming or obstruction; measuring the voltage values of the circuits related to the measuring instruments to see if they deviate from the normal range; checking if the limit switch screws of the electric actuator are loose, etc. 

When a control system malfunctions, the situation is often urgent and requires quick repair. At this time, one should not panic or act blindly, otherwise it will be counterproductive. When multiple fault phenomena occur, one should learn to identify the characteristics, find the causes, and locate the parts. 

On the basis of being familiar with and mastering the composition and function of the enterprise's instruments and control systems, more attention should be paid to observing some weak links and instruments and systems prone to failure, especially the operating status of  gauge pressure transmitter. When dealing with faults, the true source of the fault should be found from the interconnection and mutual influence of the instruments - for example, when the reading of a capacitance type pressure transmitter is abnormal, it cannot be isolated for investigation, but the parameters of the upstream and downstream equipment should be comprehensively analyzed; when the data of multiple  smart pressure transmitter  deviate, it is necessary to check whether the system calibration is uniform. Only by accurately locating the root cause of problems with key instruments like pressure transmitter HART can blind maintenance be avoided; and only by clarifying the interlocking logic between EX pressure transmitter and other equipment can efficiency be maximized.