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Application And Selection Of Pressure Transmitter And Differential Pressure Transmitter
Jan 07, 2019


Application and selection of pressure transmitter and differential pressure transmitter


1 Overview

At present, smart transmitters are quite popular, and the applications are also the most extensive and common. It is characterized by high precision, large adjustable range, and very convenient adjustment and good stability. Among the various types of instruments, the transmitter is roughly divided into a pressure transmitter and a differential pressure transmitter. Transmitters are commonly used to measure pressure, differential pressure, vacuum, level, flow and density. The transmitter has two-wire system and four-wire system. Two-wire transmitters are especially numerous; there are intelligent and non-intelligent points, and there are more intelligent transmitters; there are pneumatic and electric parts, and most of the electric transmitters In addition, according to the application, there are intrinsic safety type and explosion-proof type; according to the application conditions, the main types of transmitters are as follows: low (micro) pressure / low differential pressure transmitter; medium / medium differential pressure Transmitter; high voltage / high differential pressure transmitter; absolute / vacuum / negative pressure differential pressure transmitter; high temperature / pressure, differential pressure transmitter; corrosion / pressure, differential pressure transmitter; easy to crystal / Pressure, differential pressure transmitter.

 

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2. Introduction to pressure/differential pressure transmitter

From a physics point of view, the pressure on any object should consist of two parts: atmospheric pressure and the pressure of the measured medium (generally referred to as gauge pressure). The sum of the pressures acting on the measured object is called absolute pressure.

 

The instrument that measures absolute pressure is called an absolute pressure gauge. For ordinary industrial pressure gauges, the gauge pressure value is measured, that is, the pressure difference between absolute pressure and atmospheric pressure. When the absolute pressure is greater than the atmospheric pressure value, the measured gauge pressure value is called positive gauge pressure; when the absolute pressure is less than the atmospheric pressure value, the measured gauge pressure value is negative value, which is called negative gauge pressure, that is, vacuum degree. The meter that measures the degree of vacuum is called a vacuum gauge.

 

In addition to measuring the differential pressure of the two measured pressures, the differential pressure transmitter can also be used with other various throttling meters to measure the flow rate. It can directly measure the liquid level of the pressurized container and the liquid level and pressure of the atmospheric pressure vessel. And negative pressure.

 

The production of pressure and differential pressure transmitters can be divided into general and isolated types. The measuring diaphragm of the common pressure and differential pressure transmitter is one, which can directly feel the pressure and differential pressure of the measured medium; and the measuring diaphragm of the isolated pressure and differential pressure transmitter receives a kind of The pressure of the stabilizing fluid (usually silicone oil), which is sealed between the two diaphragms, and the diaphragm that receives the measured pressure is the outer diaphragm. The diaphragm of the original common diaphragm is an inner membrane. When the pressure signal is received on the outer membrane, the pressure of the outer membrane is transmitted to the ordinary capsule through the transmission of the silicone oil, and the feeling of the outer membrane is measured. Subject to the pressure. Isolated pressure and differential pressure transmitters are mainly used for special measured media. If the measured media leaves the device, it will crystallize. However, the use of common pressure and differential pressure transmitters requires the removal of media, which will lead to the introduction. Blockage of the pressure tube and bellows chambers prevents the equipment from functioning properly, so isolated pressure and differential pressure transmitters must be used. Isolated pressure and differential pressure transmitters are usually flanged, that is, the flange is opened and flanged on the device under test. After the transmitter is installed, its sensing diaphragm is part of the equipment wall so that it does not need to be removed. The medium is measured so that it does not cause clogging of the crystal. When the crystallization temperature of the measured medium is high, the plug-in flange transmitter is used, and the diaphragm protruding structure can be selected, so that the sensing diaphragm can be inserted into the device, so that the sensing is obtained. The temperature of the measured medium will not decrease, so that the measurement is guaranteed.

 

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Isolated pressure and differential pressure transmitters are available in both remote and integral versions. The teleport type is a reinforced capillary connection between the outer membrane cassette and the measuring membrane cassette. The general capillary length is 3 to 5 meters, so that the outer membrane cassette is mounted on the device, and the inner membrane cassette and the pressure and differential pressure transmitter can be installed. The bracket is easy to maintain; the integral type is to make the outer membrane box and the pressure and differential pressure transmitter as a whole and directly mounted on the device by the flange. For isolated pressure and differential pressure transmitters, it can also be made into a threaded connection type, ie the outer membrane box or the outer elastic element can be mounted on the front of the thread, as long as the internal threaded boss is welded to the device under test, the pressure can be applied. The differential pressure transmitter is screwed directly onto the unit for easy installation. The isolated pressure/differential pressure transmitter has a complicated manufacturing process and high material requirements, so its price is usually three times that of a normal pressure and differential pressure transmitter.

 

The pressure transmitter and the differential pressure transmitter are explained from the noun. The difference between the pressure and the two pressures is measured, but there are many parameters that are indirectly measured. For example, a pressure transmitter can measure the liquid level inside a container device in addition to measuring pressure. When measuring the liquid level in an atmospheric pressure vessel, a pressure transmitter is required. When measuring the liquid level of the pressurized container, two pressure transmitters, one measuring lower limit value and one measuring upper limit value, are used to subtract the output signals of the two pressure transmitters. The liquid level can be measured, and the differential pressure transmitter is generally used. It can also be used to measure the density of a medium if the level of liquid and pressure in the vessel equipment is constant. The measurement range of the pressure transmitter can be made very wide, and generally 100M Pa can be achieved from absolute pressure 0.

 

3. Selection principle and method of pressure/differential pressure transmitter

The selection of pressure and differential pressure transmitters is usually based on a combination of installation conditions, environmental conditions, instrument performance, economy and application media. In practical applications, it can be divided into direct measurement and indirect measurement; its uses include process measurement, process control and device interlocking. Common pressure and differential pressure transmitters include common pressure transmitters, differential pressure transmitters, single flange transmitters, dual flange transmitters, and plug-in flange transmitters. Pay full attention to the following points in the selection of pressure / differential pressure transmitter:

(1) Based on the nature of the measured medium, it is based on saving money and facilitating installation and maintenance. If the measured medium is high viscosity, easy to crystallize and corrode, then an isolated transmitter must be used.

(2) When selecting the type, the corrosion of the medium of the tested material on the metal of the bellows should also be considered. The membrane material must be selected. Otherwise, the outer membrane will be corroded and damaged in a short time after use. Lan is also corroded and damaged, causing equipment and personal accidents, so the choice of membrane material for pressure and differential pressure transmitters is especially important. The diaphragm of the transmitter is made of ordinary stainless steel, 304 stainless steel, 316L stainless steel, and diaphragm case material.

(3) The temperature of the measured medium should also be taken into account when selecting the type. If the temperature of the material medium is as high as 200 ° C ~ 400 ° C, the high temperature type pressure and differential pressure transmitter should be used, otherwise the silicone oil in the transmitter will Vaporization expansion occurs, making measurement inaccurate.

(4) The working pressure level of the equipment must also be taken into consideration when selecting the type. The pressure level of the pressure and differential pressure transmitter must be consistent with the application. From an economic point of view, the outer membrane box and the insert part are more suitable, but the connecting flange can be carbon steel or chrome-plated, which will save a lot of money. The isolated pressure transmitter is preferably selected in the form of a threaded connection, which saves money and is convenient to install.

(5) For the selection of common pressure and differential pressure transmitters, the corrosiveness of the measured medium should also be considered, but the medium temperature used can be ignored, because the ordinary type pressure is the pressure inside the meter, and the long-term working temperature It is normal temperature, but the maintenance type of the normal type is larger than that of the isolated type. The first is the insulation problem. In winter, the temperature in the north reaches zero, which will make the pressure tube freeze, which will cause the transmitter to work and even damage. This requires additional measures such as heat tracing and heat preservation.

(6) From an economic point of view, when the transmitter is selected, a normal type transmitter can be used as long as it is not a crystallization medium, and a low-pressure crystallization medium can also be added with a purge medium for indirect measurement (as long as the process allows blowing) Sweeping or gas), the application of ordinary transmitters requires maintenance personnel to carry out regular inspections, including whether the various pressure guiding tubes leak, whether the cleaning medium is normal, whether the insulation is good, and so on. As long as the maintenance is good, a large amount of one-time investment using a common type of transmitter will save a lot.

(7) From the selection range of pressure and differential pressure transmitters, the general transmitter has a certain range adjustable range. It is best to set the range of the used range to 1/4 of the full scale of the transmitter. In the 3/4 segment, accuracy will be guaranteed, which is even more important for differential pressure transmitters. In practice, some applications (such as liquid level measurement) need to migrate the measurement range of the transmitter, calculate the measurement range and migration according to the site installation position, and migrate the positive and negative migration.

 

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(8) According to the design specifications, in the selection of engineering design, whether pneumatic or electric transmitters are used, according to their respective characteristics, should be based on the specific conditions of the equipment: such as the degree of centralized operation; whether with DCS Computer phase operation; response speed; economy; reliability and maintenance; safety (such as explosion-proof, power outage, gas source failure, etc.); environmental conditions and transmission distance. For comprehensive consideration and analysis. In general, the following conditions should be selected as pneumatic transmitters: the distance from the transmitter to the display adjustment meter is short, usually not more than 150m; the process materials are flammable and explosive medium and the relative humidity is very large. The occasion requires less investment in instruments; general small and medium-sized enterprises require easy maintenance and economical reliability; in large-scale devices mainly based on electric instruments, some on-site local adjustment loops are not required to be introduced into the central control room for centralized operation. The distance between the transmitter and the display adjustment unit is more than 150m; the large enterprise requires central control with high centralized management; the DCS computer is set for control and management; the response speed is fast; the information processing and operation For complex occasions, it is advisable to choose an electric transmitter. 

4.In the actual work, in the modern production equipment, they are mixed and selected according to their respective characteristics. Therefore, the following issues should be noted in the selection of pressure/differential pressure transmitters:

(1) Measurement range, required accuracy and measurement function: In order to ensure the accuracy of pressure measurement, the minimum pressure measurement value should be higher than 1/3 of the measurement range of the pressure gauge; for occasions requiring high distance measurement or high measurement accuracy Pressure sensor or pressure and differential pressure transmitter should be selected; when the measurement accuracy is not high, resistor or inductive, Hall effect remote pressure gauge can be selected; pneumatic base pressure indicating regulator is suitable for local pressure Indication adjustment

(2) The environment faced by the measuring instrument: such as the industrial environment of petrochemical industry, there are flammable (toxic) and explosive atmospheres, and there is a high ambient temperature; pressure transmitters and pressure switches should be in accordance with the explosion-proof requirements of the installation site. Reasonable choice.

(3) Physical and chemical properties and state of the measured medium: such as strong acid, strong alkali, viscous, easy to solidify crystallization and gasification;

(4) Changes in operating conditions: such as changes in medium temperature, pressure, and concentration. Sometimes it is necessary to take into account changes in the concentration and density of the gas and liquid phases from the start of the drive to the normal production of the parameters;

(5) The structure, shape and size of the container to be tested, equipment accessories in the container and various inlet and outlet pipe ports should be considered: such as towers, solution tanks, reactors, boiler steam drums, vertical tanks, spherical tanks, etc. ;

(6) There must be a unified consideration in the selection of engineering instruments: it is required to reduce the variety specifications as much as possible, and reduce spare parts for profit management;

(7) Actual process situation: It should be considered which type of equipment the object to be tested belongs to. Such as tanks, tanks, the volume of the tank is small, the measurement range is not too large, the tank volume is large, and the measurement range may be large;

(8) Also depends on the physical and chemical properties and cleanliness of the medium. The conventional differential pressure transmitter and float type liquid level transmitter are preferred, and the material of the contact medium part is also selected; for some suspensions, foam Single-flange differential pressure transmitters are available for the media. Some insertable double flange differential pressure transmitters that are easy to precipitate and easy to crystallize;

(9) For the liquid level of some high-viscosity media and the liquid level of high-pressure equipment, since the equipment cannot be opened, it can be measured by a radiation level gauge;

(10) Other requirements, such as environmental protection and sanitation requirements, specific requirements of process patents. Of course, in addition to measurement methods and technical problems, there are instrument investment problems.

 

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5. Conclusion

With the continuous development of intelligent pressure/differential pressure transmitter technology, it is also being widely used. Therefore, in summary, the selection of pressure gauges and pressure/pressure differential transmitters is technically feasible, economically reasonable, and convenient to manage.