Field instrument selection, have to prevent 16 misunderstandings! With the improvement of the level of automation in various industries, the requirements for technology are getting higher and higher, so the selection of on-site instruments is put forward more stringent requirements. 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, so the appropriate field instrument selection has an important position in chemical production. So how to do a good job of on-site instrument selection and avoid misunderstandings, today Xiaobian will take you to understand.

Automation instruments can be simply divided into the following categories: detection instruments, display instruments, control instruments, actuators. Among them, the detection instrument and the actuator, as a field instrument, are the basic components of the automation system, but also an essential part, and its importance is self-evident in the production. Whether the selection of on-site instruments is appropriate is directly related to the stability and safety of chemical production, so avoid on-site instrument selection errors can not be ignored.

Field instruments are divided into two main categories: detection instruments and actuators. The detection instrument includes: temperature detection instrument, pressure detection instrument, flow detection instrument, level detection instrument, composition analysis instrument. The actuator includes: pneumatic actuator, electro-pneumatic converter, valve positioner, electric actuator. Each type of field instrument can be classified according to different measurement principles.

Flow meter as the name of the idea is to measure the flow of the flow measurement is to study the science of material change, mass tautometry is the basic law of the development of things, so its measurement object is not limited to the traditional sense of pipeline liquid, where the need to master the change of the flow measurement problems. Flow, pressure and temperature are listed as the three major detection parameters. For a certain fluid, as long as the three parameters are known, the energy can be calculated, and the three parameters must be detected in the measurement of energy conversion. Energy conversion is the basis of all production processes and scientific experiments, so flow and pressure, temperature meters are the most widely used.

In the flow meter selection, we often fall into such a misunderstanding.

Blind superstitious import

A large number of people think that imported ones are definitely the best. The author engaged in flow meters for more than 10 years of experience to prove that the domestic is the most cost-effective (except for the flow meter that cannot be produced in China); In addition to paying expensive money when purchasing their products, it is difficult to guarantee the after-sales service in the future (a small accessory has to let you wait for a month, but also have to pay dozens of times the domestic price).

What is expensive is good

There are two reasons for the expensive flow meter: one is the manufacturer's pricing method; The other is because the total amount of production is too small, and the income can only be increased by increasing the profit of a single unit.

Blind faith in a flowmeter manufacturer

Some people in the purchase of a manufacturer of a flow meter, that both sides have established a certain reputation, but they do not know that the flow meter is also a large family composed of nearly 100 kinds of products, no domestic flowmeter manufacturers dare to say that we can produce any flowmeter. This can be reflected in their price and future after-sales service.

Blind selection of new flowmeters

Don't be fooled by the manufacturer's salesman, they are often experimenting with you. Perhaps the verification before the factory is qualified, but is it reliable for long-term operation? Can you really meet your requirements? After all, the upgrade of the flow meter is not as fast as the mobile phone and TV, and the old one can stand the test of the market!

Selection based on pipe only

This is the most likely situation, the result is often not to achieve the ideal measurement effect. It is not the quality of the flow meter itself, the flow meter is also a measuring instrument, it has its own measuring range.

Not selected according to the type of media

Although this situation is not many, but the author has indeed seen the use of vortex flowmeters to measure mud.

What kind of flow meter can measure

Some irresponsible manufacturers and salesmen will say to you that our flowmeter can measure anything, if you hear this sentence you can "please" him out of your office, because he is talking nonsense to you!

Installation requirements are not met

For example, many flowmeters require adequate front and rear straight pipe segments. Please read the instruction manual carefully before installing the flowmeter to avoid unnecessary trouble.

No parameters are set after installation

Most of them will appear in battery-powered flowmeters, the setting parameters are set by the manufacturer, and they can be used when installed, which may cause great metering errors and unnecessary losses.

Maintenance is not performed on time

This will cause the accuracy of the flow meter to decline, so that the manufacturer can tell you how to maintain, just like the car has to be sent to the after-sales service center after a period of time.

Pressure gauge is widely used in chemical, petroleum, metallurgy, mining, electricity and other production, is the most commonly used for display and control of pressure measuring instruments, it is like the human eye, in the enterprise production process plays a huge role.

In the selection of pressure instruments, we often fall into such a misunderstanding.

Pressure gauge selection: do not consider the medium and choose ordinary pressure gauge

Ordinary pressure gauge spring tube is made of copper, such as the use of corrosive media will greatly reduce the service life of the pressure gauge. A chemical company sent 15 pressure gauges at a time, after inspection, repair and verification, only 3 are qualified, and the rest of the pressure gauges are scrapped because the spring tube is corroded or damaged. After the selection of stainless steel pressure gauge, there is no pressure gauge due to corrosion and scrap phenomenon.

Pressure gauges with a large range are used where the pressure changes frequently

In some compressors or pumps the outlet pressure changes frequently, the use of large range pressure gauge to prevent damage to the pressure gauge, in fact, this method does not help. The transmission principle of the pressure gauge is that the deformation of the spring tube is meshed between the sector gear and the cylindrical gear and the needle rotates through the oil wire. If the pointer of the pressure gauge rotates frequently at a certain Angle, the gear at this Angle will be worn and the movement will be damaged. The damaged position of the pressure gauge of a liquefied petroleum gas company is always gear wear, and the effect is obvious after replacing it with a shock-proof pressure gauge.

In addition, in order to ensure that the elastic element can work reliably within the safe range of elastic deformation, the selection of the measuring range of the pressure gauge should not only be based on the size of the measured pressure but also consider the speed of the measured pressure change, and the measuring range should leave enough room. When measuring stable pressure, the maximum working pressure should not exceed 2/3 of the measuring range; When measuring the pulsating pressure, the maximum working pressure should not exceed 1/2 of the measuring range; When measuring high pressure, the maximum working pressure should not exceed 3/5 of the range. In order to ensure the accuracy of the measurement, the minimum working pressure to be measured cannot be less than 1/3 of the measuring range.

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

In order to save electricity, choose a frequency converter

Many manufacturers and salesmen brag about how high the power saving rate of the inverter, and users also believe it, and choose the inverter at high cost simply to save electricity, but the result is disappointed. Whether the power can be saved after using the inverter is determined by the type of load it drives. For fan and pump loads, the energy-saving effect is significant after the selection of inverter, while for constant power load and constant torque load, the energy-saving effect is much worse, and even can not save electricity.

Inverter selection is determined by the rated power of the motor nameplate

The rated power to choose the inverter is a certain theoretical basis, but in many actual situations, the motor operation is too large, or the motor overload operation, so that the inverter selection is either too large, resulting in economic waste, or the inverter selection is too small, resulting in motor damage or frequency converter explosion. The easiest way to estimate is that the frequency converter selection is based on 1.1 times the maximum operating current of the motor when it is stable, and if the mechanical equipment is a heavy-duty type, the frequency converter also needs to be amplified for use.

Low-temperature valve refers to the valve that can be used in low-temperature conditions, and the valve with a working temperature below -40 ° C is usually called a low-temperature valve. Low-temperature valve is one of the indispensable important equipment in petrochemical, air separation, natural gas and other industries, and its quality determines whether it can be safely, economically and sustainably produced. With the development of modern science and technology, the use of low-temperature valves is more and more widespread, and the demand is also increasing.

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

In fact, it is only a semi-finished low-temperature valve, because it has not been low-temperature cryogenic treatment, it can be said that low-temperature cryogenic treatment is the top priority of low-temperature valve, the key to low-temperature valve is cryogenic treatment, so as to ensure that the various parameters of the low-temperature valve meet the requirements, especially the expansion coefficient, will not lead to various valve stuck in use. Sometimes in considering the price of the problem, but also consider the quality of the valve. After all, the low-temperature valve is a special valve. The low-temperature valve is mainly used for the output of liquid low-temperature media such as liquid oxygen, liquid nitrogen, liquefied natural gas, etc. The material is not qualified, will cause the shell and sealing surface leakage or leakage; The comprehensive mechanical properties, strength and steel of the parts can not meet the requirements of use or even fracture, therefore, in the development, design and development of liquefied natural gas valves, the material is the primary key issue.

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

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

Bolt too long

The bolt on the valve, only one or two threads over the nut can be. Reduces the risk of damage or corrosion. Why buy a bolt that is longer than you need? Often, bolts are too long because someone doesn't have time to calculate the correct length, or the individual simply doesn't care what the end result looks like. This is lazy engineering.

Control valves are not isolated

While isolating valves takes up valuable space, it is important to be able to allow personnel to work on the valve when maintenance is needed. If space is limited, if the gate valve is considered too long, at least install the butterfly valve, which hardly takes up any space. Always remember that for those who have to stand on them for maintenance and operations, it is easier to work with them and perform maintenance tasks more efficiently.

No pressure gauge or device is installed

Some utilities like to calibrate testers, and these facilities often do a good job of providing their field personnel with connections to inspection equipment, but some devices even have interfaces for mounting accessories. Although it is not specified, it is designed so that the actual pressure of the valve can be seen. Even with supervised control and data acquisition (SCADA) and telemetry capabilities, someone at some point will be standing next to the valve and needs to see what the pressure is, and that's so convenient.

Installation space is too small

If installing a valve station is cumbersome, which may involve digging concrete, etc., don't try to save that bit of money by making it as little installation space as possible. It will be very difficult to carry out basic maintenance at a later stage. Also keep in mind that the tool can be very long, so you must set the space to reserve space so that the bolt can be loosened. It also requires some space, which allows you to add devices later.

Later disassembly is not considered

Most of the time, installers understand that you can't connect everything together in one concrete chamber without needing some type of connection to remove parts at some future time. If all the pieces are tightly tightened and there are no gaps, pulling them apart is almost impossible. Whether it is a trough coupling, flange joint or pipe joint, it is necessary. In the future, parts may sometimes need to be removed, and while this is not usually a concern for the installation contractor, it should be a concern for the owner and engineer.

Concentric reducer installed horizontally

This may be nitpicking, but it is worth paying attention to. Eccentric reducer can be mounted horizontally. Concentric reducers are mounted in vertical lines. In some applications it is necessary to install on a horizontal line, to use an eccentric reducer, but this problem usually involves cost: concentric reducer is cheap.

Valve Wells that do not allow drainage

All the rooms are wet. Even during valve start-up, when air is discharged from the valve cover, water will fall on the floor at some point. Anyone in industry at any time has seen a flooded valve, but there's really no excuse (unless, of course, the entire area is flooded, in which case you have bigger problems). If it is not possible to install a drain pipe, use a simple drain pump, assuming a power supply is available. In the absence of power, a float valve with an injector will effectively keep the chamber dry.

Air not excluded

When the pressure drops, air is expelled from the suspension and transferred into the pipe, which can cause problems downstream of the valve. A simple bleeder valve will get rid of any air that may be present and will prevent downstream problems. A bleed valve upstream of the control valve is also effective, as air in the guide line can cause instability. Why not remove the air before it reaches the valve?

Spare tap

This may be a minor problem, but it is always helpful to have spare taps in the chambers upstream and downstream of the control valve. This setup makes it easy 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 at the design stage, it significantly increases usability in the future. Making maintenance tasks more difficult because everything is covered in paint, so nameplates can't be read or adjusted.

In general, with the rapid development of automatic control technology, more intelligent, more accurate, more reliable, more stable automation products are developed and applied, do a good job of instrument selection, to avoid falling into the error of instrument selection, is one of the important contents of our instrument work.