With the improvement of automation level in various industries, the requirements for technology are becoming higher and higher, so stricter requirements are put forward for the selection of field instruments. Correct and reasonable field instrument selection can not only reduce the probability of field instrument failure, but also improve the safety factor of chemical production and improve the economic benefits of construction enterprises. Therefore, appropriate field instrument selection plays an important role in chemical production. So how to do a good job in selecting field instruments and avoiding misunderstandings? Today, I will take you to understand.

Automation instruments can be simply divided into the following categories: detection instruments, display instruments, control instruments, and actuators. Among them, detection instruments and actuators, as field instruments, are basic components and indispensable parts of automation systems, and their importance is self-evident in production. Whether the selection of field instruments is appropriate or not is directly related to the stability and safety of chemical production, so avoiding errors in the selection of field instruments cannot be ignored.

Field instruments are divided into two categories: detection instruments and actuators. The detection instruments include temperature detection instruments, pressure detection instruments, flow detection instruments, liquid level detection instruments, and component analysis instruments. Actuators include: pneumatic actuators, electro-pneumatic converters, valve positioners, and electric actuators. Each type of field instrument can be subdivided and classified according to different measurement principles.

Flowmeter, as the name suggests, measures flow.

Flow measurement is the science of studying mass change, and the law of mass reciprocity is the basic law of the development of things. Therefore, its measurement object is no longer limited to the traditional sense of pipeline liquid, and wherever it is necessary to master quantitative change, there is a problem of flow measurement. Flow, pressure, and temperature are listed as three major detection parameters. For a given fluid, the energy it has can be calculated by knowing these three parameters, which must be detected in the measurement of energy conversion. Energy conversion is the foundation of all production processes and scientific experiments, so flow, pressure, and temperature instruments are the most widely used.

When selecting flow meters, we often fall into such a misunderstanding.

Blindly superstitious about imports

A large proportion of people believe that imported goods are definitely the best. The author's experience in working with flow meters for more than ten years has confirmed that domestic ones have the highest cost performance ratio (except for those that cannot be produced domestically); In addition to paying a high price when purchasing their products, it is difficult to ensure after-sales service in the future when selecting imported instruments (every small accessory has to keep you waiting for a month, and you also have to pay dozens of times the domestic price)

Superstition is good if it's expensive

There are two reasons for being expensive in flow meters: one is the manufacturer's pricing method; The other reason is that the total production volume is too small and can only be increased by increasing the profits of a single unit.

Blind superstition - flowmeter manufacturer

After purchasing a flowmeter from a manufacturer, some people believe that both parties have established a certain reputation foundation. However, flow meters are also a large family composed of nearly 100 products. No flowmeter manufacturer in China dares to say to the outside that we can produce any flowmeter. This can be reflected in their selling prices and future after-sales services.

Blind selection of new flow meters

Don't be fooled by the salespeople from the manufacturer, they are often experimenting with you. Perhaps the factory calibration is qualified, but is the long-term operation reliable? Can I really meet your requirements? After all, the upgrading of flow meters is not as fast as mobile phones and televisions. Older ones can withstand the market test!

Only based on pipeline selection

This is the most likely situation to occur, resulting in the inability to achieve ideal measurement results. "It is not a quality issue with the flow meter itself. The flow meter is also a measuring instrument that has its own measurement range.".

Selection not based on media type

Although this situation does not occur in many cases, the author has indeed seen the use of a vortex flowmeter to measure mud.

A flowmeter can measure any flow rate

Some irresponsible manufacturers and salespeople will tell you that our flowmeter can measure everything. If you hear this sentence, you can "please" him out of your office because he is talking nonsense to you!

Failure to install in accordance with installation requirements

For example, many flowmeters require sufficient front and rear straight pipe sections. Please read the operating instructions carefully before installing the flowmeter to avoid unnecessary trouble.

Do not set parameters after installation

Most of them occur in battery powered flowmeters. The setting parameters are set by the manufacturer and can be used after installation. This may cause significant metering errors and unnecessary losses.

Failure to perform maintenance on time

This can lead to a decrease in the accuracy of the flowmeter, allowing the manufacturer to tell you how to maintain it, just like having to send a car to an after-sales service center for maintenance after driving for a period of time.

Pressure gauges are widely used in the production of chemical, petroleum, metallurgy, mining, electric power, and other industries. They are the most commonly used measuring instruments for displaying and controlling pressure. They are like human eyes and play a significant role in the production process of enterprises.

When selecting pressure instruments, we often fall into such a misunderstanding.

Selection of pressure gauge: select ordinary pressure gauge regardless of medium

The spring tube of an ordinary pressure gauge is made of copper. If used in corrosive media, the service life of the pressure gauge will be greatly reduced. A chemical enterprise sent 15 pressure gauges at a time. After inspection, repair, and verification, only 3 were qualified, while the remaining pressure gauges were discarded due to corrosion or damage to the spring tube. After selecting a stainless steel pressure gauge, there was no phenomenon that the pressure gauge was scrapped due to corrosion.

Select a large range pressure gauge where pressure changes frequently

In some compressors or pumps where the outlet pressure changes frequently, selecting a pressure gauge with a large range to prevent damage to the pressure gauge does not actually help. The transmission principle of the pressure gauge is that the deformation of the spring tube is caused by the meshing of the sector gear and the cylindrical gear, and the pointer is rotated by the oil wire. If the pointer of the pressure gauge rotates frequently at a certain angle, the gear at that angle will be worn, causing damage to the movement core. The damaged position of the pressure gauge of a liquefied petroleum gas company is always gear wear. After replacing it with a shockproof pressure gauge, the effect is obvious.

In addition, in order to ensure that the elastic elements can work reliably within the safe range of elastic deformation, the selection of the pressure gauge range should not only be based on the magnitude of the measured pressure, but also consider the speed of the measured pressure change, and its range should have sufficient leeway. When measuring stable pressure, the maximum working pressure cannot exceed 2/3 of the range; When measuring pulsating pressure, the maximum working pressure should not exceed 1/2 of the range; When measuring high pressure, the maximum working pressure cannot exceed 3/5 of the range. In order to ensure measurement accuracy, the minimum working pressure measured must not be lower than 1/3 of the range.

A frequency converter is a control device that converts power frequency electricity into other frequency electricity. With the continuous development and improvement of frequency conversion technology, the functions and advantages of frequency converters continue to improve. Its energy-saving effect, soft start function, strong speed regulation function, good protection function, and other advanced control technologies have been widely used in the development of enterprises.

Select frequency converter to save power

Many manufacturers and salespeople boast about how high the power saving rate of frequency converters is, and users also believe it. They choose frequency converters at high cost simply to save power, but the results are greatly disappointed. Whether a frequency converter can save power after use depends on the type of load it drives. For fan and pump loads, the energy-saving effect is significant after selecting frequency converters, while for constant power and torque loads, the energy-saving effect is much worse, even unable to save electricity.

Frequency converter selection is determined by the rated power on the motor nameplate

There is a certain theoretical basis for selecting frequency converters based on their rated power. However, in many field situations, the motor operation margin is too large, or the motor operates under overload. Therefore, the type selection of frequency converters is either too large, resulting in economic waste, or too small, resulting in electrical damage or frequency converter explosion. The simplest estimation method is to select a frequency converter based on 1.1 times the maximum operating current of the motor during stable operation. If the mechanical equipment is of heavy-duty type, the frequency converter also needs to be used at a larger scale.

Low temperature valve refers to a valve that can be used under low temperature conditions. Generally, a valve with a working temperature below - 40 ℃ is referred to as a low temperature valve. Cryogenic valves are one of the indispensable and important equipment in petrochemical, air separation, natural gas and other industries, and their quality determines whether they can be safely, economically, and continuously produced. With the development of modern science and technology, low-temperature valves are increasingly widely used and in increasing demand.

If the valve material is low-temperature steel, it is directly considered as a low-temperature valve

In fact, it is only a semi-finished product of a low-temperature valve, because it has not undergone low-temperature cryogenic treatment, which can be said that low-temperature cryogenic treatment is the top priority of low-temperature valves. The key to low-temperature valves is to conduct cryogenic treatment, so as to ensure that all parameters of the low-temperature valve meet the requirements, especially the expansion coefficient, so as not to cause various valve stuck conditions in use. Sometimes when considering the price, it is also necessary to consider the quality of the valve. After all, cryogenic valves are special valves.

Not selected according to the tested medium

Cryogenic valves are mainly used to output liquid cryogenic media such as liquid oxygen, liquid nitrogen, liquefied natural gas, etc. Unqualified materials may cause external or internal leakage of the shell and sealing surface; The comprehensive mechanical properties, strength, and stiffness of components cannot meet the use requirements or even fracture. Therefore, in the process of developing, designing, and developing liquefied natural gas valves, material quality is the primary and key issue.

In addition to the above type selection errors, we also have many errors in the field instrument installation.

For example, in the process of valve installation, we are prone to fall into these nine misunderstandings.

Bolt too long

For bolts on valves, only one or two threads exceed the nut. Can reduce the risk of damage or corrosion. Why buy a bolt longer than you need? Typically, bolts are too long because someone doesn't have time to calculate the correct length, or individuals don't care what the end result looks like. This is a lazy project.

The control valve is not isolated separately

Although isolating the valve takes up valuable space, it is important to allow personnel to work on the valve when maintenance is required. If space is limited, and if the gate valve is considered too long, at least install a butterfly valve, which occupies almost no space. Always remember that it is easier to use them for maintenance and operations that must be carried out while standing on them, and to perform maintenance tasks more effectively.

No pressure gauge or device installed

Some utilities like to calibrate testers, and these facilities typically do a good job of providing their field personnel with connection testing equipment, but some devices even have interfaces for installing accessories. Although not specified, it is designed so that the actual pressure of the valve can be seen. Even with supervisory control and data acquisition (SCADA) and telemetry capabilities, it is so convenient for someone to stand next to the valve at a certain point and need to see what the pressure is.

Installation space is too small

If it is troublesome to install a valve station, it may involve excavation of concrete and other work. Do not try to reduce the installation space as much as possible to save that cost. Performing basic maintenance at a later stage will be very difficult. Also remember that tools can be very long, so space must be provided to allow for loosening of the bolts. "You also need some space, which allows you to add devices later.".

Do not consider later disassembly

Most of the time, installers understand that you cannot connect everything together in a concrete chamber without requiring some type of connection to remove components at some point in the future. If all components are tightly tightened without gaps, it is almost impossible to separate them. Whether it is a groove type coupling, flange joint, or pipe joint, it is necessary. In the future, it may sometimes be necessary to remove components, and although this is usually not a concern for installation contractors, it should be a concern for owners and engineers.

Horizontal installation of concentric reducer

This may be nitpicking, but it's also worth noting. The eccentric reducer can be installed horizontally. Concentric reducers are installed in vertical lines. In some applications, it is necessary to install an eccentric reducer on a horizontal line, but this problem usually involves cost: concentric reducers are cheap.

Valve wells that do not allow drainage

All the rooms are wet. Even during valve actuation, when air is discharged from the valve cover, water will fall on the floor at some point. Any one in industry has seen a flooded valve at any time, but there is really no excuse (unless, of course, the entire area is flooded, in which case you have a bigger problem). If it is not possible to install a drain pipe, use a simple drain pump, assuming there is a power supply. A float valve with an injector will effectively keep the chamber dry without power.

Do not expel air

When the pressure drops, air is expelled from the suspension and transferred into the pipeline, which can cause problems downstream of the valve. A simple vent valve will free up any air that may be present and will prevent downstream problems. The vent valve upstream of the control valve is also effective because air in the pilot line can cause instability. Why doesn't it remove air before it reaches the valve?

Spare tap

This may be a minor issue, but it is always helpful to have spare taps in the chambers upstream and downstream of the control valve. This setting provides convenience for future maintenance, whether it's connecting hoses, adding remote sensing to control valves, or adding pressure transmitters to SCADA. For the small cost of adding accessories during the design phase, it significantly increases future availability. This makes the maintenance task even more difficult because everything is covered with paint, so it is not possible to read the nameplate or make adjustments.

In general, with the rapid development of automatic control technology, more intelligent, more accurate, more reliable, and more stable automation products have been developed and applied. Doing a good job in instrument selection and avoiding falling into error in instrument selection is one of the important aspects of our instrument work.