With the improvement of the automation level in various industries, the requirements for technology are getting higher and higher, so more stringent requirements have been put forward for the selection of field instruments. Correct and reasonable on-site meter selection can not only reduce the probability of field instrument failure, but also improve the safety factor of chemical production, improve the economic efficiency of construction companies, so the appropriate field instrument selection has an important position in chemical production. Then how to do a good job site instrument selection, to avoid misunderstanding, today Xiaobian take everyone together to find out.
Automation instruments can be simply divided into the following categories: Test instruments, display instruments, control instruments, actuators. Among them, detection instruments and actuators as field instruments are the basic components of an automation system, and they are also indispensable parts, and their importance is self-evident in production. The appropriate selection of on-site instrumentation 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 fall into two main categories: test 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 classified according to different measurement principles.
The name of the flow meter is to measure the flow rate.
Flow measurement is the science to study the change of the quality of objects. The law of mutual change of quality is the basic law of the development of the relationship between things. Therefore, the object of measurement is not limited to the pipeline liquid in the traditional sense, and there is a problem of flow measurement wherever quantitative change is needed. Flow, pressure, and temperature are listed as three major test parameters. For a certain fluid, as long as these three parameters are known, the energy it has can be calculated, and 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, pressure, and temperature instruments are the most widely used.
In the selection of flow meters, we often fall into this misunderstanding.
Blind superstition imported
A large part of people think that imports are definitely the best. The author has been working on flow meters for more than a decade and it has been verified that domestically produced products are the most cost-effective (except flowmeters that cannot be produced in China); the imported instruments use expensive after-sales services to purchase their products. It's also hard to guarantee (a gadget has to wait for one month, and you have to pay several times the domestic price)
Superstitious is good
There are two reasons why you are expensive in a flow meter: one is the manufacturer's pricing method; the other is that because the total production is too small, you can only increase profits by increasing the profit of a single unit.
Blind superstition one flow meter manufacturer
Some people have purchased a flow meter from a manufacturer. They believe that both sides have established a certain reputation basis, but everyone knows that flow meters are also a large family of nearly 100 products. There is no domestic flow meter manufacturer who dares to say that we What flow meter can produce. This can be reflected both in their selling price and in the future after-sales service.
Blindly selecting a new flow meter
Don't be fooled by factory salesmen. They are often trying to get you to experiment. Maybe the factory calibration is acceptable, but is the long-term operation reliable? Can you really meet your requirements? After all, the upgrade of flowmeters is not as fast as mobile phones and TVs. The old one is a market tester!
Only based on pipeline selection
This is the easiest situation to occur, and the result is often that the desired measurement results cannot be achieved. It is not the quality of the flowmeter itself. The flowmeter is also a measuring instrument. It has its own measurement range.
Not based on media type selection
Although there are not many such cases, I have seen vortex flowmeters to measure the mud.
What kind of flow meter can measure flow
Some irresponsible manufacturers and salesmen will tell you that our flow meter can measure anything. 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 flowmeters require sufficient forward and backward pipe sections. Please read the instruction manual carefully before installing the flowmeter to avoid unnecessary trouble.
No setting of parameters after installation
Most will appear in the battery-powered flowmeter, set the parameters are set by the factory, and can be used to install, this may cause great measurement errors and unnecessary losses.
Not maintained on time
This will cause the accuracy of the flowmeter to drop, allowing the manufacturer to tell you how it should be maintained, just as if the car had been delivered to the after-sales service center for a while.
Pressure gauges are widely used in chemical, petroleum, metallurgy, mining, and power production. They are the most commonly used measuring instruments for displaying and controlling pressure. They are like people's eyes and play an important role in the production process.
In the selection of pressure gauges, we often fall into this misunderstanding.
Selection of pressure gauges: ordinary pressure gauges are used irrespective of media
The spring tube of an ordinary pressure gauge is copper. If used on a corrosive medium, 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 of them were qualified. The remaining pressure gauges were scrapped due to corrosion or damage to the spring tubes. After the use of stainless steel pressure gauge, there is no pressure gauge due to corrosion and scrapped.
Frequently used pressure gauges for pressure changes
In some compressor or pump outlet pressure changes frequently, the use of a large range of pressure gauges to prevent damage to the gauge, in fact, this method does not help. The transmission principle of the pressure gauge is that the deformation of the spring tube is engaged by the sector gear and the cylindrical gear, and the pointer is rotated by the oil filament. If the pointer of the pressure gauge rotates at a certain angle frequently, the gear of this angle will wear and cause the movement damage. A liquefied petroleum gas company's pressure gauge is always in a position where the gear is worn. After replacing it with a pressure gauge, the effect is obvious.
In addition, the pressure gauge range selection in order to ensure that the elastic element can work reliably in the elastic deformation of the safe range, the pressure gauge range selection not only according to the size of the measured pressure but also consider the speed of the measured pressure change, the range required Leave enough room. When measuring the stable pressure, the maximum working pressure cannot exceed 2/3 of the measuring range; when measuring the pulsating pressure, the maximum working pressure cannot exceed 1/2 of the measuring range; when measuring the high pressure, the maximum working pressure cannot exceed 3/5 of the measuring range. In order to ensure measurement accuracy, the minimum working pressure to be measured must not be less than 1/3 of the range.
The frequency converter is a control device that converts power frequency to other frequencies. With the constant development and perfection of frequency conversion technology, the functions and advantages of frequency converters have been continuously improved. Its advanced energy-saving effects, soft-start functions, strong speed control functions, and good protection functions are among the development of enterprises. Widely used.
Use frequency converter for power saving
Many manufacturers and salespeople boasted how high the energy-saving rate of the inverter was, and users believed it was true. Simply choosing the inverter for high cost and saving power, the result was disappointing. Whether or not the power can be saved after using the inverter is determined by the type of load it drives. For fans and pump loads, the energy saving effect is significant after the inverter is selected. However, for constant power loads and constant torque loads, the energy saving effect is much poorer, and even power saving is not possible.
Determine the inverter selection by the motor nameplate rated power
There is a certain theoretical basis for choosing the inverter with rated power, but in many actual field conditions, the motor is running too rich, or the motor is overloaded, so that the inverter selection is either too large, causing economic waste, or frequency conversion The selector is too small, causing damage to the motor or the inverter crashing. The simplest method of estimation 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 a heavy load type, the inverter needs to be used in one step.
Cryogenic valves are valves that can be used under low temperature conditions. Valves that operate below -40°C are commonly referred to as cryogenic valves. Cryogenic valves are one of the indispensable important equipments in petrochemical industry, air separation, natural gas and other industries. Their quality determines whether they can produce safely, economically, and continuously. With the development of modern science and technology, the use of cryogenic valves is getting wider and wider, and the demand is increasing.
The material of the valve is cryogenic steel, which is directly considered as cryogenic valve
In fact, it is only a semi-finished valve, because it has not been cryogenic cryogenic treatment, it can be said that cryogenic cryogenic treatment is the most important thing for cryogenic valves, cryogenic valve is the key to cryogenic treatment, so as to ensure that all parameters of the cryogenic valve achieve The requirements, especially the coefficient of expansion, do not lead to various valve stuck conditions during use. Sometimes considering the issue of price, we must also consider the quality of the valve. After all, cryogenic valves are special valves.
Not selected based on measured media
Cryogenic valves are mainly used for the output of liquid cryogenic media such as liquid oxygen, liquid nitrogen, and liquefied natural gas. If the material is not qualified, it will cause external leakage or internal leakage of the housing and the sealing surface; the comprehensive mechanical properties, strength, and strength of the components cannot meet the requirements for use or even fracture, therefore, during the development, design, and development of liquefied natural gas valves, Material is the primary key issue.
In addition to the above misunderstandings, we also have many misunderstandings in the field instrument installation.
For example, in the valve installation process, we easily fall into these nine errors.
Bolt too long
Bolts on the valve have only one or two threads over the nut. Can reduce the risk of damage or corrosion. Why buy a bolt longer than you need? Usually, the bolt is too long because someone has no time to calculate the correct length, or the individual does not care what the end result is. 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 space is limited, if the gate valve is considered to be too long, at least the butterfly valve is installed, it takes up almost no space. Always keep in mind that it is easier to work and perform maintenance tasks more efficiently when you must stand on top of maintenance and operations.
No pressure gauge or device installed
Some utilities like to calibrate testers. These facilities usually provide a good connection test equipment for their on-site personnel, but some devices even install accessories on the interface. 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 at a certain point will be standing next to the valve and need to see what the pressure is, and that is so convenient.
Installation space is too small
If it is troublesome to install a valve station, it may involve the work of excavating concrete, etc. Do not try to save that cost by making it possible to reduce the installation space as much as possible. It will be very difficult to carry out basic maintenance later. Also keep in mind that tools can be long, so space must be set so that bolts can be loosened. You also need some space, which allows you to add devices later.
Do not consider late disassembly
Most of the time, the installer knows that you can't connect everything in a concrete room without the need for some sort of connection to remove parts at some point in the future. If all parts are tightened tightly and there is no gap, it is almost impossible to separate them. Whether grooved couplings, flanged joints or pipe joints are all necessary. In the future, it may sometimes be necessary to remove the components, and while this is generally not a concern of the installation contractor, it should be of concern to the owner and the engineer.
Concentric reducer horizontal installation
This may be nitpicking, but it is also worthy of attention. Eccentric reducers can be installed horizontally. Concentric reducers are installed in the vertical line. In some applications it is necessary to install on a horizontal line and use an eccentric reducer, but this problem usually involves costs: Concentric reducers are cheaper.
Drained valve wells are not allowed
All rooms are wet. Even when air is discharged from the valve cover during valve activation, the water will fall on the floor at a certain 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 you cannot install a drain, use a simple drain pump, assuming power. In the absence of power, a float valve with an injector will effectively keep the chamber dry.
Do not rule out air
When the pressure drops, air is drained from the suspension and is transferred into the pipeline, which will cause problems downstream of the valve. A simple air release 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 may cause instability. Why does it not remove 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 chamber upstream and downstream of the control valve. This setup facilitates future maintenance, whether 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 the availability in the future. Makes maintenance tasks more difficult because everything is covered by paint and it is not possible to read nameplates or make adjustments.
In general, with the rapid development of automatic control technology, smarter, more accurate, more reliable, more stable automation products have been developed and applied. It is important for us to do a good job in instrument selection and avoid falling into misunderstanding of instrument selection. One of the content.
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