With the continuous advancement of automation across various industries, the demand for advanced technology has grown significantly. This has led to stricter requirements for the selection of field instruments. Proper and reasonable selection of on-site instruments not only reduces the likelihood of instrument failure but also enhances the safety of chemical production and boosts the economic efficiency of construction companies. Therefore, choosing the right field instruments plays a vital role in industrial operations. But how can we ensure proper selection and avoid common misconceptions? Let’s explore this together.
Automated instruments are typically categorized into four main groups: instrumentation, display instruments, control instruments, and actuators. Among these, instrumentation and actuators are essential components of the automation system and serve as critical parts in any process. Their importance cannot be overstated, as the correct selection directly impacts the stability and safety of chemical processes. Avoiding misunderstandings during the selection process is therefore crucial.
Field instruments can be broadly divided into two categories: measurement devices and actuators. Measurement instruments include temperature, pressure, flow, level, and component analysis instruments. Actuators, on the other hand, consist of pneumatic actuators, electro-pneumatic converters, valve positioners, and electric actuators. Each type of field instrument can be further classified based on its measurement principle.
Flow meters, as their name suggests, are used to measure fluid flow. Flow measurement is essentially the study of material quality changes. The law of mutual change in quality is fundamental to understanding relationships between things. Thus, the scope of flow measurement extends beyond traditional pipelines. It applies wherever there is a change in volume, making flow one of the three key parameters alongside pressure and temperature. Knowing these three values allows us to calculate the energy of a given fluid. These parameters must be measured in energy conversion processes, which form the basis of all industrial and scientific activities. Hence, flow measurement is widely applied in conjunction with pressure and temperature instruments.
When selecting flow meters, several common misconceptions often arise. One is the blind belief that imported instruments are always superior. While some high-end models may offer better performance, many domestic alternatives provide excellent value for money. Imported instruments tend to be expensive, both in purchase and maintenance, with long lead times and high costs for spare parts.
Another misconception is assuming that more expensive equipment is always better. Some manufacturers inflate prices due to low production volumes or poor cost management. Choosing a manufacturer based solely on brand reputation can also be misleading, as no single company produces all types of flow meters. Additionally, blindly adopting new models without considering long-term reliability is risky, as not all upgrades are necessary or beneficial.
Selecting a flow meter based only on pipeline size can lead to inaccurate measurements. Similarly, ignoring the type of medium being measured can cause serious issues—such as using a vortex flowmeter for sludge, which is not suitable. Some salespeople may claim a meter can measure anything, which is misleading. Proper installation and parameter settings are also critical, as improper setup can lead to errors. Regular maintenance is equally important to maintain accuracy and prevent failures.
Pressure gauges are widely used in chemical, petroleum, metallurgy, and power industries. They are essential for monitoring and controlling pressure, acting like the eyes of the system. However, selecting the wrong gauge can lead to problems. For example, using a standard copper-spring gauge in corrosive environments can result in rapid failure. Stainless steel gauges are more suitable in such cases.
Frequent pressure fluctuations can damage standard gauges. In such scenarios, shockproof models are recommended. When selecting a pressure gauge, it's important to consider both the pressure range and the nature of the load. For steady pressure, the maximum working pressure should not exceed 2/3 of the range; for pulsating pressure, it should not exceed 1/2; and for high-pressure applications, it should not exceed 3/5. Also, the minimum measurable pressure should not be less than 1/3 of the range to ensure accuracy.
Frequency converters are widely used in modern industries for energy savings, soft-start functions, and speed control. However, they are not always the best solution. For constant power or torque loads, the energy-saving benefits are minimal. Selecting a frequency converter based on the motor's rated power is a common practice, but real-world conditions may require adjustments. A safe approach is to choose an inverter with 1.1 times the motor's operating current, especially for heavy-duty applications.
Cryogenic valves are essential in industries like petrochemicals and natural gas. They must withstand extremely low temperatures, typically below -40°C. Simply using low-temperature steel does not make a valve cryogenic—cryogenic treatment is essential to ensure dimensional stability and prevent jamming. Material selection is critical, as unqualified materials can lead to leaks or structural failures.
In addition to selection mistakes, installation errors are also common. For instance, using overly long bolts, failing to isolate control valves, or neglecting pressure gauges can complicate future maintenance. Proper installation space, consideration for future disassembly, and the inclusion of drain systems and bleed valves are all important to ensure long-term reliability.
In summary, with the rapid development of automation technology, the need for accurate, reliable, and intelligent instruments is greater than ever. Making informed choices and avoiding common misconceptions is essential for successful industrial operations.
Encoding disk refers to a digital encoder that measures angular displacement. It has the advantages of strong resolving power, high measurement accuracy and reliable work. It is one of the most commonly used displacement sensors for measuring the position of the shaft rotation angle. The code disk is divided into two types: absolute encoder and incremental encoder. The former can directly give the digital code corresponding to the angular position; the latter uses a computing system to make the pulse increment generated by the rotating code disc against a certain reference number addition and subtraction. The metal encoding disk is directly engraved with through and unconnected lines, and it is not fragile, but because the metal is not easy to corrode but easy deform, the accuracy is limited, and its thermal stability is an order of magnitude worse than that of glass. It is mainly used in motor, hardware, electrical appliances, automobile and other fields.
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