With the improvement of the automation level of various industries, the requirements for technology are getting higher and higher, so the stricter requirements for the selection of field instruments are put forward. Correct and reasonable on-site instrument selection can not only reduce the probability of failure of field instruments, but also improve the safety factor of chemical production and improve the economic benefits of construction enterprises. Therefore, suitable field instrument selection plays an important role in chemical production. Then how to do a good job of field instrument selection, to avoid misunderstanding, today Xiaobian bring everyone to understand.
Automated instruments can be easily divided into the following categories: instrumentation, display instrumentation, control instrumentation, and actuators. Among them, the instrumentation and actuator as the field instrument are the essential components of the automation system, and are also an indispensable part. The importance of it is self-evident in production. Whether the selection of field instruments is appropriate is directly related to the stability and safety of chemical production, so avoiding the misunderstanding of on-site instrument selection cannot be ignored.
Field instruments are divided into two categories: instrumentation and actuators. The detection instruments include: temperature detection instrument, pressure detection instrument, flow detection instrument, liquid level detection instrument, and component analysis instrument. Actuators include: Pneumatic actuators, electro-pneumatic converters, valve positioners, electric actuators. Each type of field instrument can be classified according to the measurement principle.
The flow meter is as the name suggests to measure the flow.
Flow measurement is the science of material quality change. The law of quality mutual change is the basic law of the development of the relationship between things. Therefore, the measurement object is not limited to the pipeline liquid in the traditional sense. The flow measurement problem is wherever there is a change in volume. Flow, pressure and temperature are listed as three major test parameters. For a certain fluid, as long as you know these three parameters, you can calculate the energy it has. These three parameters must be detected in the energy conversion measurement. Energy conversion is the basis of all production processes and scientific experiments, so flow is the most widely used for pressure and temperature instruments.
When selecting flow meters, we often fall into such a misunderstanding.
Blindly superstitious imported
A large number of people think that imports are definitely the best. The author's experience in flow meter for more than ten years has confirmed that the domestically produced is the most cost-effective (except for the flowmeters that cannot be produced domestically); the imported instruments are not only expensive but also expensive after the purchase of their products. It's also hard to guarantee (a small accessory has to wait for a month, and you have to pay tens of times the domestic price)
Superstitious is good
There are two reasons for the expensive in the flow meter: one is the pricing method of the manufacturer; the Other is because the total production is too small, and can only increase the profit by increasing the single profit.
Blind superstition a flowmeter manufacturer
Some people have purchased a flowmeter from a manufacturer and believe that both parties have established a certain reputation. They do not know that the flow meter is also a large family of nearly one hundred products. No domestic flowmeter manufacturer dares to say us. What flow meters can be produced. This can be reflected in their selling price and future after-sales service.
Blind selection of new flowmeters
Don't be fooled by the factory clerk, they are often taking experiments with you. Maybe the calibration before leaving the factory is qualified, but is it reliable for long-term operation? Can you really meet your requirements? After all, the upgrade of the flowmeter is not as fast as mobile phones and TVs, and the old ones can stand the test of the market!
Only based on pipeline selection
This is the most prone situation, and the result is often not the ideal measurement. Not the quality of the flow meter itself, the flow meter is also a measuring instrument, it has its own measurement range.
No selection based on media type
Although this situation does not appear a lot, but I have seen the use of vortex flowmeter to measure mud.
A flow meter can measure what flow rate
Some irresponsible manufacturers and salesmen will tell you what our flowmeter can measure. If you hear this sentence, you can "please" him out of your office because he is talking nonsense to you!
Not installed according to installation requirements
For example, many flow meters require sufficient front and rear straight sections. 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 can be used when installed. This may cause large measurement errors and unnecessary losses.
Not performing maintenance on time
This will cause the accuracy of the flowmeter to drop, allowing the manufacturer to tell you how to maintain it, just like the car has to be sent to the after-sales service center for a period of time.
Pressure gauges are widely used in the production of chemical, petroleum, metallurgy, mining, electric power, etc. It is the most commonly used measuring instrument for displaying and controlling pressure. It is like a human eye and plays a huge role in the production process of enterprises.
When selecting pressure gauges, we often fall into such a misunderstanding.
Selection of pressure gauge: ordinary pressure gauge is used regardless of medium
The spring tube of the ordinary pressure gauge is copper. If it is used on corrosive media, the service life of the pressure gauge will be greatly reduced. A chemical company sent 15 pressure gauges at a time. After inspection, repair and verification, only 3 were qualified. The rest of the pressure gauges were scrapped due to corrosion or damage of the spring tubes. After the selection of the stainless steel pressure gauge, there is no phenomenon that the pressure gauge is scrapped due to corrosion.
Pressure gauges with frequent pressure changes
In some compressors or pumps, the outlet pressure changes frequently, and a large-scale pressure gauge is used to prevent the damage of 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 with the cylindrical gear through the sector gear and the needle is rotated by the oil wire. If the pointer of the pressure gauge is frequently rotated at a certain angle, the gear of this angle is worn and the movement is damaged. The position of the damaged pressure gauge of a liquefied petroleum gas company is always gear wear, and after replacing it with a shockproof pressure gauge, the effect is obvious.
In addition, the selection of the pressure gauge range in order to ensure that the elastic component can work reliably within the safe range of elastic deformation, the selection of the pressure gauge range depends not only on the magnitude of the measured pressure but also on the speed of the measured pressure. Leave enough room. When measuring the steady pressure, the maximum working pressure should not exceed 2/3 of the range; when measuring the pulsating pressure, the maximum working pressure should not exceed 1/2 of the range; when measuring the high pressure, the maximum working pressure should not exceed 3/5 of the range. In order to ensure measurement accuracy, the minimum working pressure to be measured cannot be less than 1/3 of the range.
The frequency converter is a control device that converts the power frequency electricity into other frequency power. With the continuous development and improvement of frequency conversion technology, the functions and advantages of frequency converters are constantly improved. Its energy-saving effects, soft-start functions, strong speed control functions, and good protection functions are among the advanced control technologies in the development of enterprises. Get a wide range of applications.
In order to save power, choose inverter
Many manufacturers and salesmen boast of how high the power-saving rate of the inverter is, and the users believe that it is true. In order to save power, the inverter is used at a high cost, but the result is disappointing. The power saving after using the inverter is determined by the type of load it drives. For the fan and pump load, the energy saving effect is remarkable after the inverter is selected, and for the constant power load and the constant torque load, the energy saving effect is much worse, and even the power cannot be saved.
Determine the inverter selection by the rated power of the motor nameplate
There is a certain theoretical basis for selecting the frequency converter according to the rated power. However, in many actual situations, the motor running richness is too large, or the motor is overloaded, so that the inverter selection is too large, resulting in economic waste, or frequency conversion. The selection of the device is too small, causing damage to the motor or the inverter to explode. The easiest way to estimate is that the inverter selection is based on 1.1 times the maximum operating current when the motor is running stably. If the mechanical equipment is of heavy duty type, the inverter needs to be amplified for the first gear.
A cryogenic valve is a valve that can be used under low temperature conditions. A valve that operates at temperatures below -40 °C is often referred to as a cryogenic valve. Cryogenic valves are one of the most important equipments in the petrochemical, air separation, natural gas and other industries. The quality of the products determines whether they can be produced safely, economically and continuously. With the development of modern technology, the use of cryogenic valves is becoming more and more widespread, and the demand is also increasing.
The valve material is low temperature steel and it is directly considered as a low temperature valve.
In fact, it is only a semi-finished product of a cryogenic valve, because it has not been subjected to cryogenic cryogenic treatment. It can also be said that cryogenic cryogenic treatment is the most important aspect of cryogenic valves. The key to cryogenic valves is cryogenic treatment, so as to ensure that the parameters of the cryogenic valves are met. Requirements, especially the expansion coefficient, will not cause various valve jams in use. Sometimes considering the price issue, we must also consider the quality of the valve. After all, cryogenic valves are special valves.
Not selected according to the measured medium
The cryogenic valve is mainly used to output liquid cryogenic media such as liquid oxygen, liquid nitrogen, and liquefied natural gas. If the material is unqualified, it will cause leakage or leakage of the casing and sealing surface; the comprehensive mechanical properties, strength and rigidity of the parts can not meet the requirements of use or even break. Therefore, in the process of developing, designing and developing LNG valves Material is the primary key issue.
In addition to the above selection errors, we also have many misunderstandings in the field instrument installation.
For example, in the valve installation process, we are easy to fall into these nine misunderstandings.
Bolt too long
The bolt on the valve has only one or two threads over the nut. It can reduce the risk of damage or corrosion. Why buy a bolt longer than you need? Usually, the bolts are too long, because someone doesn't have time to calculate the correct length, or the individual doesn't care what the end result looks like. This is a lazy project.
Control valve is not isolated separately
Although the isolation valve takes up valuable space, it is important to allow personnel to work on the valve when maintenance is required. If the space is limited, if the gate valve is considered too long, at least the butterfly valve is installed, it takes up almost no space. Always remember that it is easier to work and perform maintenance tasks more effectively when you have to stand on top for maintenance and operation.
No pressure gauge or device installed
Some utilities like to calibrate the tester. These facilities usually provide a good connection test device for their field personnel, but some devices even have an interface to the accessory. 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, someone will be standing next to the valve at some point, and it is convenient to see what the pressure is.
Installation space is too small
If it is very troublesome to install a valve station, it may involve excavating concrete and so on. Do not try to save the cost by making it reduce the installation space as much as possible. Basic maintenance in the later stages will be very difficult. Also keep in mind that the tool can be very long, so space reservations must be set so that the bolts can be loosened. It also needs some space, which allows you to add devices later.
Not considering post-disassembly
Most of the time, the installer understands that you can't connect everything together in a concrete room without the need for some type of connection to remove parts at some point in the future. If all the parts are tightly tightened and there are no gaps, it is almost impossible to separate them. Whether it is a slot coupling, a flange joint or a pipe joint, it is necessary. In the future, it may sometimes be necessary to remove parts, and although this is usually not a concern for the installation contractor, it should be the concern of the owner and the engineer.
Concentric reducer horizontal installation
This may be nitpicking, but it is also worthy of attention. The eccentric reducer can be installed horizontally. Concentric reducers are mounted on vertical lines. In some applications it is necessary to install on a horizontal line, using an eccentric reducer, but this problem usually involves cost: concentric reducers are cheaper.
Valve well that does not allow drainage
All rooms are wet. Even when the valve is started, when the air is discharged from the bonnet, the water will fall on the floor at some point. Any one has seen a flooded valve at any time in the industry, but there is really no excuse (unless, of course, the entire area is submerged, in which case you have bigger problems). If the drain cannot be installed, use a simple drain pump, assuming a power source. In the absence of power, a float valve with an ejector will effectively keep the chamber dry.
Does not exclude air
When the pressure drops, air is expelled from the suspension and transferred to the pipe, which can cause problems downstream of the valve. A simple bleed valve will get rid of any air that may be present and will prevent downstream problems. The bleed valve upstream of the control valve is also effective because the air in the pilot line can cause instability. Why not remove the air before it reaches the valve?
Spare tap
This can be a minor problem, but it is always helpful to have spare taps in the chamber upstream and downstream of the control valve. This setting provides convenience for future maintenance, whether it is connecting hoses, adding remote sensing to the control valve or adding a pressure transmitter to SCADA. For the small cost of adding Accessories during the design phase, it significantly increases usability in the future. It makes the maintenance task more difficult because everything is covered by paint, so the nameplate cannot be read or adjusted.
In general, with the rapid development of self-control technology, more intelligent, more accurate, more reliable and more stable automation products have been developed and applied. It is important to select the instrument and avoid falling into the misunderstanding of instrument selection. One of the content.
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