Instruments: [Weekly Question] Discussion question for Week 11 of 2011: Maximum reward of 50

Hehe, I’ll be in charge of preparing one plate. I have been working in the instrumentation field for over ten years. The more time passes, the more I realize there’s an ever-increasing amount of things to learn. I work on front-line maintenance in a factory that deals with natural gas chemicals, with synthetic ammonia being the main product. Usually there isn’t much to do; the most common task seems to be replacing pressure gauges. Another point is that major overhauls involve a lot of work, and they are also opportunities to learn skills (I would like to remind newcomers that if there’s no chance to work on technical upgrades, then by participating actively in major overhauls, one can learn a great deal). There are many areas where instruments and processes are involved, so special care must be taken. Be cautious; it is essential to inform the process team in advance, and only after getting their approval can maintenance work proceed! The control valves used in our factory are made by Wuzhong; they rarely malfunction. The transmitters used are FOXBORO ones. The amount of maintenance required on a regular basis isn’t very high. As for components like orifice plates and thermocouples (resistive types), they’re even less likely to get damaged; only the components on the unit itself are somewhat more prone to damage!

This is information I found online, which I’d like to share with everyone. 1. Basic analysis steps for faults in field instrument systems. The measurement parameters of field instruments generally fall into four categories: temperature, pressure, flow rate, and liquid level. Now, based on different measurement parameters, we will analyze where the faults in various field instruments lie. 1. First, before analyzing faults in on-site instruments, it is necessary to have a thorough understanding of the production process, manufacturing methods, and conditions related to the instrument system in question. It is also important to know the design scheme and intent behind the instrument system, as well as its structure, characteristics, performance, and parameter requirements. 2. Before analyzing and checking for faults in the on-site instrument system, it is necessary to obtain information from the operators on site regarding the production load and changes in the parameters of the raw materials. The recording curves of the faulty instruments should also be examined, and a comprehensive analysis conducted to determine the cause of the instrument failure. 3. If the instrument’s recorded curve is a straight line (a line with no changes at all is called a straight line), or if the curve, which was originally fluctuating, suddenly becomes a straight line ； The fault is likely in the instrumentation system. Since most of the current recording instruments are DCS computer systems, they have extremely high sensitivity, allowing for a very accurate detection of changes in parameters. At this point, the process parameters can be manually adjusted to observe the changes in the curve. If there is no change, it is almost certain that there is a problem with the instrumentation system ； If there are normal variations, it can be generally concluded that there is no major issue with the instrumentation system. 4. When the process parameters are changed, it is observed that the recorded curve experiences a sudden change or jumps to its maximum or minimum value; faults in this case are often related to the instrumentation system. 5. Before the malfunction occurred, the recorded curves from the instruments remained normal. After fluctuations appeared, these curves became completely irregular, making it difficult to control the system; in some cases, even manual operation proved ineffective. In such situations, the malfunction may be caused by the process operating system. 6. When it is found that the instruments displayed on the DCS are abnormal, one can go to the site to check the readings of the same physical instruments; if there is a large difference between them, it is likely that there is a fault in the instrument system. In summary, when analyzing the causes of faults in field instruments, special attention should be paid to changes in the characteristics of the controlled process and control valves, as these can all be factors leading to failures in the field instrument system. Therefore, we must consider and carefully analyze both the field instrument system and the process operating system to determine the root cause. II. Steps for Fault Analysis of Instrument Control Systems for the Four Major Measurement Parameters 1. Steps for Fault Analysis of Temperature Control Instrument Systems When analyzing faults in temperature control instrument systems, two points should be noted first: the instruments in such systems are mostly electric instruments used for measurement, indication, and control ； The measurements taken by the instruments in this system are often significantly delayed. (1) The indicated value of the temperature instrument system suddenly changes to the maximum or minimum; this is generally due to a fault in the instrument system. Due to the significant measurement lag in the temperature instrument system, sudden changes do not occur. At this time, the causes of failure are usually broken thermocouples, thermal resistors, compensation wires, or a malfunctioning transmitter amplifier. (2) Rapid oscillations are observed in the indications of the temperature control instrument system, which is usually caused by improper adjustment of the PID control parameters. (3) Significant and slow fluctuations in the indications of the temperature control instrument system are likely to be caused by changes in process operations. If there are no such changes in the process operations at that time, it is likely to be a fault within the instrument control system itself. (4) Fault analysis steps for the temperature control system itself: Check whether the input signal to the control valve changes; if the input signal does not change, the control valve operates, and there is a leak in the diaphragm of the control valve ； Check whether the input signal to the control valve actuator has changed; if the input signal remains unchanged while the output signal does change, there is a fault with the actuator ； Check whether there is any change in the positioner’s input signal; then check if there is any change in the regulator’s output. If the regulator’s input remains unchanged while its output does change, this indicates a fault within the regulator itself. 2. Steps for fault analysis of pressure control instrument systems: (1) When the readings of the instruments in the pressure control system exhibit rapid oscillations, first check whether there have been any changes in the process operations; such changes are usually caused by issues with the process operations or by improper setting of the PID parameters of the regulator. (2) If the instruments in the pressure control system show no response, and the pressure reading remains unchanged despite changes in process operations, the fault is usually located in the pressure measurement system. First, check whether there is any blockage in the pressure sensing conduit system; if not, then examine whether there are any changes in the output of the pressure transmitter. If there are such changes, the fault lies in the controller’s measurement and indication system. 3. Steps for analyzing faults in the flow control instrument system: (1) When the indication value of the flow control instrument system reaches its minimum, first check the field measurement instruments; if they are functioning properly, then the fault lies with the display instrument. When the readings on the field monitoring instruments are also at their minimum, check the opening degree of the control valve. If the opening degree of the control valve is zero, it is usually due to a fault between the control valve and the regulator. When the readings from the on-site measuring instruments indicate a minimum value and the opening degree of the control valve is normal, the possible causes of the fault are insufficient system pressure, blockages in the system pipelines, the pump not being able to deliver sufficient flow, crystallization of the medium, or improper operation. In the case of instrument-related malfunctions, the possible causes include: for orifice plate differential pressure flowmeters, it could be that the positive-pressure pressure guiding pipe is blocked ； Leakage in the positive pressure chamber of the differential pressure transmitter ； Mechanical flow meters suffer from issues such as gear jamming or clogged filters. (2) When the indicated value of the flow control instrument system reaches its maximum, the measuring instruments often also indicate a maximum value. At this point, the control valve can be manually adjusted to open wider or narrower; if the flow rate can be reduced, it is generally due to process operation issues. If the flow rate fails to decrease, it is caused by a problem in the instrument system; check whether the control valve in the flow control instrument system is operating ； Check whether the instrument pressure measurement system is functioning normally ； Check whether the instrument signal transmission system is functioning properly.

This is a rather tricky issue. For measuring pressure and flow, we use EJA transmitters manufactured by Chuan Yi; they operate quite stably. As for temperature measurement, most manufacturers can provide suitable thermocouples and resistance temperature detectors without any major problems. The main challenges lie in measuring the level and pressure of sulfur – crystallization often occurs, leading to blockages. For instance, at the top of the stripper column, we switched to using single-flange transmitters, which helps slow down the rate of crystallization. For level measurement, we employ float-type sensors. It’s crucial to ensure proper heat tracing and insulation; this also helps mitigate crystallization. There’s also a ratio analyzer; the NSL-880 instrument we use. The key to its stable operation is trace heating. If the heating is properly maintained, it’s generally fine to clean it just once every two weeks. However, if the heating isn’t controlled properly, sulfur tends to accumulate on the lenses in the measurement chamber.

For pressure transmitters, use Yokogawa EJA, Rosemount, and Yamateke; they require little maintenance. There have been cases of capillary leakage with Yamateke models. I’ve also used products from small domestic manufacturers; those with low pressure and stable operating conditions require little maintenance, but those with large fluctuations don’t work well and break down frequently. Using domestic thermal resistors and thermocouples generally poses no problems. There are too many problems with pressure gauges. Technically, they shouldn’t be that complicated, and there are many manufacturers of them. Yet most people don’t want to use them again after using them once. It’s better to go for well-known brands; even if they cost 3-4 times more, it’s still worth it. For oval flowmeters and electromagnetic flowmeters, both imported and domestic versions have been used; the domestic ones seem to have lower accuracy. For pneumatic diaphragm control valves, Samson is still the better choice. I have used domestic brands from Wuzhong Instrument Factory and Chuanyi, but their cost-performance ratio is much worse. Additionally, the quality of instrument air is very important; oil removal and drying must be done properly.

This is really a difficult question to answer. Our factory is engaged in chlor-alkali chemicals, as well as resins, chlorobenzene, and other products. Imported models such as EJA, ROSEMONT, and YAMATAKE are used; they usually come as complete systems. The DCS systems are mostly modified versions, and they are of a smaller scale. The instruments acquired through bidding are not the best quality – in fact, they’re not even among the better ones. Since the management considers them too expensive, one can only imagine what the quality is like. Regulating valves mainly use inferior domestic ones; I’ve never used those from Wuzhong, and it’s normal for them to fail. Given the poor quality of the tables, it’s only natural to deduct bonuses based on the manufacturing process. The instrument air occasionally contained oil and water; clogs in the positioners caused valve fluctuations, which also led to the deduction of bonuses. The environment in our factory is quite corrosive to the casings of instruments; usually, after a few years it’s no longer possible to unscrew the covers on these instruments. Therefore, after connecting the wires, we apply a layer of butter, but this doesn’t give very good results either. In the end, we have to cover them with plastic sheeting. I want to use an instrument protection box, but not all of them are suitable for use. For the instruments in operation, priority is given to those with interlocks and that are critical; as long as their appearance and parameters are normal, that’s sufficient. Spare copies of these instruments must also be available, so that they can be replaced immediately in case of problems – ensuring safe and uninterrupted production is of top priority. The secondary instruments are acceptable. Pay more attention to production and equipment conditions in daily operations, as there are too many issues related to instruments due to process problems. PH\ORP\COD generally need to be checked once a month. Perform on-site instrument repairs; be sure to obtain all necessary certifications—don’t find it too much of a hassle. For any unclear process media, be sure to ask the operators plenty of questions; don’t hesitate to be repetitive.

I have been working with instruments for 4 years; the first three years were in a privately-owned enterprise that was undergoing restructuring, yet it still operated on a state-owned enterprise model. There were many employees, and the tasks were highly specialized. However, everything was done with a focus on minimizing costs, which resulted in poor quality of instruments in use, high maintenance requirements, and little adherence to standards. There were also numerous safety hazards. I participated in several technical improvement projects, and various systems were gradually replaced with domestic alternatives; however, their operation was not very stable. I encountered many different types of systems, including those from Yokogawa, Siemens, Foxboro, Allen-Bradley, and Zhongkong. I have some understanding of field operations and control systems, but my practical skills and on-site experience are insufficient; I don’t know how to approach complex problems； I switched to a factory owned by a foreign company a year ago; all the instruments there are imported, and there are many standards to follow. As the factory is relatively small, it doesn’t specialize solely in instruments – it deals with various tasks related to instrumentation. The instruments used on site are mainly from Rosemount, Jamesbury, and Fisher, while the control systems are from AB, ensuring stable operation. However, I feel that there is still a lot to learn about instruments*; learning is endless.

My company is an American firm that produces flame retardants (bromine-based). The measuring instruments used include Rosemount models ranging from pressure electromagnetic types to mass and flange types; control valves are of the Festo brand, while weighing devices are from Toledo and Sartorius. The pressure gauge used to be from Brady (I later switched to Tiankang) because there were too many fakes. Since it’s a new project, I was involved in it from start to finish, which gave me a deep understanding of it. First and foremost, the project team needs to be professional; the design firm chosen should be reliable, and the construction company must be competent (as preliminary work such as underground piping systems, fire alarm controls, civil engineering foundations, and embedded pipelines all need to be done properly). Otherwise, the workload later on will be enormous.) At the same time, we also know that in project management, factors such as funds, time, and quality are interdependent. Set the closing time, and the rest can be adjusted accordingly. Later on, I carried out maintenance work. I believe the most important things are 1) understanding the manufacturing process and communicating more with the operators, 2) identifying the key points at which instruments need to be coordinated with production, and 3) paying attention to those elements that could trigger ESD interlocks. The other main aspects are work responsibility and dedication.

I work for a private enterprise in Shandong; I’ve been in this industry for 7 years. I have been involved in the instrument maintenance of ion-exchange membrane chlor-alkali and PVC plants, as well as in the project construction and maintenance of epichlorohydrin facilities. I’ve worked with CS3000, JX300, and ECS100 in DCS systems. Having gotten used to CS3000, I find this system to be stable and easy to configure. The transmitters are mainly Rosemount products; they have good quality and stable performance. However, it seems that their quality has declined somewhat in recent years. The most commonly used flowmeters are those from Rosemount in terms of mass flow measurement, as well as vortex and electromagnetic type flowmeters. E+H offers electromagnetic, vortex, and mass flowmeters, while Yokogawa provides electromagnetic, vortex, and rotor-type flowmeters. It seems that Rosemount’s mass flowmeters are the best, E+H’s electromagnetic flowmeters are excellent, and Rosemount’s vortex flowmeters are also good. However, the quality of Rosemount’s 8732E series of electromagnetic flowmeters is not satisfactory. Control valves are mainly produced by Fisher, Benshan, Shanghai ValveTech, Wuzhong, and Nanjing Automation. Those of imported origin are clearly of higher quality than domestic ones. Although the CV3000 series of control valves has been phased out by Wuzhong, it remains a good choice as long as the operating conditions are not too severe; it is inexpensive and still offers fairly good control accuracy. For level gauges, we have used Vega, Rosemount, and E+H radar sensors; K-TEK magnetostrictive sensors; Rosemount double-flange types; and servo sensors from Tokyo Keikaku ； The radar seems to be very useful from Rosenmount, and the servos from Tokyo Keikaku are also excellent. The thermometer is domestic, and the pressure gauge is also domestic ； There’s quite a bit of loss.

I work in the automation industry; I’ve worked at many small factories. Although I’m not very familiar with the processes at each factory, I have considerable experience with various types of instruments. Here is my basic understanding of different types of instruments: 1. Temperature instruments: There are many types, including those based on thermal expansion, bimetallic elements, thermal resistors, and thermocouples. For temperatures below 400°C, I prefer to use thermal resistors. In situations where there is stirring or significant vibration in the pipes, I opt for armored instruments with an additional protective cover. K-type thermocouples are commonly used at 400~800℃. Pipes below 32 need to be expanded; the on-site guidelines suggest using bimetallic materials for temperatures below 200 degrees, while temperature-sensitive materials are recommended for temperatures above 200 degrees. 2. Pressure types: For EJA and Rosemount, people talk about the level of accuracy required, but in practice there isn’t really a need for an accuracy of 0.075. There are pressure types for anti-corrosion purposes, ones specific for hydrogen, ones specific for oxygen, and so on; attention must be paid to these in the field. For example, oxygen must be kept away from oil. 3. Flow-related issues: This is the most complex and troublesome aspect. To understand it in depth, one must study fluid mechanics thoroughly. The most commonly used ones I have are orifice plates, vortex flow meters, and electromagnetic types. For orifice plates, care must be taken to prevent freezing and clogging; for vortex flow meters, attention should be paid to shock protection; as for electromagnetic flow meters, the electrical conductivity of the medium is an important factor to consider. If what’s needed is an accurate measurement of the cumulative flow rate on-site, I recommend using a water meter – it’s quite accurate, has low installation requirements, though its size is a bit large! 4. Level detection types: There are too many of these types to list them all; they include magnetic flap switches, magnetostrictive sensors, float balls, capacitive sensors, radio frequency admittance sensors, servo systems, differential pressure sensors, electric float balls, electric floats, radar, ultrasonic sensors, X-ray sensors – there are many, many of them. There are also digital types: tuning forks, rotary resistive types, radio frequency admittance, and so on. What needs to be paid attention to together is: the temperature and pressure of the medium, the process connections, whether crystallization occurs, and its viscosity; once these are properly controlled, it can generally be used. 5. Analytical instruments: I haven’t dealt with many water analysis instruments; mainly those for measuring pH and conductivity. Generally speaking, analytical instruments should not be treated in the same way as conventional instruments. It’s remarkable when things that require complex processes in a laboratory can be converted into on-site, real-time measurements. This requires regular maintenance, including checking whether the key components of these analytical instruments are aging, damaged, or have reached the end of their useful life. It’s not like pressure-level types, where the quality doesn’t decline much over a year even after installation. Let’s talk about the above first; those of us interested in the instrumentation industry can continue the discussion.

I have been working at this starch sugar production plant for three years, and I am responsible for everything related to instrument selection as well as their maintenance. When selecting instruments for the workshop, relatively high-quality ones were chosen: flow meters from Kolon, level gauges from Yokogawa, and control valves from Wuzhong. Also, platinum thermoresistors are used quite frequently. It seems that the valve that requires the most maintenance is likely the control valve in Wuzhong. I’m not sure whether it’s an issue with our air supply or with these valves themselves; the gaskets of those pressure reducing valves tend to break easily, and the replacement parts are also hard to find. Furthermore, it is likely that the materials used for the valve seat and valve core were not carefully selected based on the properties of those materials at the time of design, which led to quite severe erosion. For the faults that occur regularly, I usually first check whether it is just one instrument that is faulty or if many instruments are affected. If it is only a single instrument that is malfunctioning, then I analyze the specific situation in question. If there are problems over a large area, analyze the fault symptoms. Look for reasons related to the PLC cabinet; for example, sometimes a large number of pumps cannot be controlled from the control room, and upon investigation it is found that the issue is caused by unstable voltage supply to the motors. The flow meter in one of the cabinets did not show any readings at all, although it appeared to be functioning normally on site; it was found that the 24V power supply within that control cabinet was not working properly. If there is a problem with a certain instrument, I usually start by dividing the inspection into two parts, using the terminals in the PLC cabinet as a boundary. If the signals entering the PLC cabinet are normal, then I examine the interior of the PLC cabinet or the program ； If the signals entering the PLC cabinet are abnormal, first check the issues with the field instruments. If the problems with the field instruments have been ruled out and the issue still persists, then examine the interior of the PLC cabinet. I have not been working for long, and I don’t have much experience; these are some of the insights I’ve gained over the past few years. I hope it will be useful to everyone.