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Application and type selection of pressure/differential pressure transmitter

1 overview

among various types of instrument communication speed meters, the transmitter is the most widely used and common. The transmitter is generally divided into pressure transmitter and differential pressure transmitter. Transmitters are often used to measure pressure, differential pressure, vacuum, liquid level, flow and density. There are two-wire and four wire transmitters, especially two-wire transmitters; There are intelligent and non intelligent, and there are more and more intelligent transmitters; There are pneumatic and electric, and most of them are electric transmitters; In addition, according to the application occasions, there are intrinsically safe and flameproof types; 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 pressure/high differential pressure transmitter

absolute pressure/vacuum/negative pressure differential pressure transmitter

high temperature/pressure, differential pressure transmitter

corrosion resistant/pressure, differential pressure transmitter

easy crystallization/pressure, differential pressure transmitter

the type selection of transmitter is usually considered according to installation conditions, environmental conditions, instrument performance, economy and application media. In practice, it can be divided into direct measurement and indirect measurement; Its uses include process measurement, process control and device interlocking. Common transmitters include ordinary pressure transmitter, differential pressure transmitter, single flange transmitter, double flange transmitter, plug-in flange transmitter, etc

pressure transmitter and differential pressure transmitter measure pressure and the difference between two pressures in terms of terms, but they indirectly measure many parameters. For example, the pressure transmitter can measure the liquid level in the equipment in addition to measuring the pressure. When measuring the liquid level in atmospheric vessels, a pressure transformer is needed. When measuring the liquid level of pressure vessels, two pressure transformers can be used, one for the lower limit of measurement and one for the upper limit of measurement. Their output signals can be subtracted to measure the liquid level. Generally, differential pressure transmitters are used. It can also be used to measure the density of the medium when the liquid level and pressure in the container remain unchanged. The measurement range of pressure transmitter can be very wide, from absolute pressure 0 to 100MPa (generally)

2 introduction of pressure/differential pressure transmitter

in addition to measuring the differential pressure value of the two measured pressures, the differential pressure transmitter can also cooperate with various throttling elements to measure the flow, and can directly measure the liquid level of pressure vessels and atmospheric vessels, as well as the pressure and negative pressure

2.1 production

from the structure of pressure and differential pressure transmitter, it can be divided into ordinary type and isolated type. The ordinary measuring capsule is one, which directly senses the pressure and differential pressure of the measured medium; The isolated measuring capsule receives the pressure of a stabilizing fluid (usually silicone oil), which is sealed between the two diaphragms and receives more gears to be measured, so that the diaphragm with limited force of component size is the outer diaphragm. The diaphragm of the original ordinary capsule is the inner membrane. When the outer membrane receives the pressure signal, the pressure of the outer membrane is transmitted to the ordinary capsule intact through the transmission of silicone oil, and the pressure felt by the outer membrane is measured

the isolated transmitter is mainly used for special measured medium. For example, the measured medium will produce crystallization after leaving the equipment, while the ordinary transmitter needs to take out the medium, which will block the impulse pipe and capsule chamber, making it unable to work normally, so the isolated transmitter must be selected. The isolation type is usually installed as a flange, that is, an additive flange is opened on the tested equipment, so that the induction diaphragm of the transmitter is a part of the equipment wall after installation, so that it will not take out the measured medium and generally will not cause crystallization blockage

when the required crystallization temperature of the measured medium is high, the structure of protruding the diaphragm can be selected, so that the sensing diaphragm can be inserted into the equipment, so that the induced medium temperature will not decrease. In this way, the measurement is guaranteed, that is, the plug-in flange transmitter is selected

the isolated transmitter has remote transmission type and integrated type. The remote transmission type means that the outer capsule and the measuring capsule are connected by a strengthened capillary, generally the capillary is 3~5 meters, so that the outer capsule is installed on the equipment, and the inner capsule and transmitter can be installed on the support convenient for maintenance; Another form is that the outer capsule is integrated with the transmitter and installed directly on the equipment by the flange. For the isolated pressure transmitter, it can also be made into a threaded connection type, that is, the outer capsule or outer elastic element can be installed in front of the thread. As long as the internal thread boss is welded on the tested equipment, the transmitter can be directly screwed to the equipment, which is very convenient for installation

the production of isolated pressure/differential pressure transmitter is complex and the material requirements are high, so its price is usually three times that of ordinary type

2.2 selection principle

the main basis for the selection of pressure/differential pressure transmitter is: the property index of the measured medium shall prevail, and the reference is to save money and facilitate installation and maintenance. If the measured medium is high viscosity, easy to crystallize and strong corrosion, the isolated transmitter must be selected

the corrosion of its medium to the metal of the capsule should be considered when selecting the type. The material of the capsule must be selected properly, otherwise the outer membrane will be corroded in a short time after use, and the flange will also be corroded, causing equipment and personal accidents, so the material selection is very important. The capsule materials of the transmitter include ordinary stainless steel, 304 stainless steel, 316L stainless steel, tantalum capsule materials, etc

the temperature of the measured medium should be considered when selecting the type. If the temperature is high, it is generally 200 ℃ ~400 ℃, the high temperature type should be selected, otherwise the silicone oil will produce vaporization and expansion, making the measurement inaccurate

the working pressure grade of the equipment should be considered when selecting the type, and the pressure grade of the transmitter must be consistent with the application. From an economic point of view, the material of the outer capsule and the inserted part is more suitable, but the connecting flange can be carbon steel and chromium plating, which will save a lot of money

it is better to select the threaded connection form for the isolated pressure transmitter, which saves money and is convenient for installation

for the selection of ordinary pressure and differential pressure transmitters, the corrosion of the measured medium should also be considered, but the temperature of the medium used can be ignored, because the ordinary pressure transformer is introduced into the meter, and the long-term working temperature is normal temperature, but the maintenance of the ordinary type is greater than that of the isolated type. The first is the problem of heat preservation. In northern winter, when it is below zero, the impulse pipe will freeze, and the transmitter will not work or even be damaged, so it is necessary to add heat tracing and incubator

from an economic point of view, when selecting transmitters, ordinary transmitters can be used as long as they are not easy to crystallize media, and for low-pressure easy to crystallize media, purging media can also be added for indirect measurement (as long as the process allows the use of purging liquid or gas). The application of ordinary transmitters requires maintenance personnel to conduct regular inspections, including whether all kinds of impulse pipes leak, whether the purging medium is normal, whether the insulation is good, etc, As long as the maintenance is good, the one-time investment of using a large number of ordinary transmitters will be saved a lot

in terms of the measurement range of the transmitter, generally, the transmitter has a certain range that can be adjusted. It is best to set the range used at 1/4~3/4 of its range, so that the accuracy can be guaranteed, which is more important for micro differential pressure transmitter. In practice, some applications (liquid level measurement) need to migrate the measurement range of the transmitter. The measurement range and migration amount are calculated according to the on-site installation position. Migration can be divided into positive migration and negative migration

at present, intelligent transmitter has been quite popular. Its characteristics are high precision, large adjustable range, convenient adjustment and good stability. It should be considered when selecting type. Chuanyi Yokogawa EJA, Beijing Far East 1751, and Li Ning, Dean of the energy College of Hony Xiamen University, told Weill that the ST3000 and ark800 series are very reliable

according to the design specifications, whether to use pneumatic transmitter or electric transmitter in the engineering design and selection should be comprehensively considered and analyzed according to the specific conditions of the device because of their respective advantages. The following points can be used for reference:

centralized operation degree

whether it operates and cooperates with DCS computer

response speed


reliability and use and maintenance

safety (explosion-proof, power failure, gas source failure, etc.)

environmental conditions and transmission distance

generally speaking, the following conditions are suitable for the selection of pneumatic instruments:

the distance from the transmitter to the display regulating instrument is short, and it is usually not more than 150m

process materials are flammable and explosive media and occasions with high relative humidity

less instrument investment is required

general small and medium-sized enterprises require easy maintenance, economy and reliability

in large-scale devices dominated by electric instruments, some on-site local control circuits are not required to be introduced into the central control room for centralized operation

the following conditions are suitable for selecting electric instruments:

the distance between the transmitter and the display adjustment unit exceeds 150m

large enterprises require highly centralized central control

set the objects controlled and managed by DCS computer

occasions requiring fast response and complex information processing and calculation

in practice, in modern production devices, they are mixed and selected by giving full play to their respective characteristics

3 selection of differential pressure transmitter

the selection of differential pressure transmitter is based on the following points:

(1) measurement range, required accuracy and measurement function

(2) the environment faced by the measuring instrument, such as the industrial environment of petrochemical industry, the existence of flammable (toxic) and explosive dangerous atmosphere, and high ambient temperature

(3) physical and chemical properties and states of the measured medium, such as strong acid, strong alkali, viscosity, easy solidification, crystallization and gasification

(4) changes in operating conditions, such as changes in medium temperature, pressure and concentration. Sometimes, it is necessary to consider the changes of gas and liquid phase concentration and density from start-up to normal production

(5) the structure, shape, size, equipment accessories in the container and various inlet and outlet material nozzles of the tested object container should be considered, such as towers, solution tanks, reactors, boiler drums, vertical tanks, spherical tanks, etc

(6) other requirements, such as environmental protection and hygiene

(7) the selection of engineering instruments should be considered in a unified way. It is required to reduce the variety and specification as much as possible and reduce spare parts for management

(8) specific requirements of process patentees

(9) actual process conditions:

① consider what kind of equipment the tested object belongs to. Such as tanks, tanks, the tank volume is small, the measurement range will not be too large, and the tank volume is large, the measurement range may be large

② depending on the physicochemical properties and cleanliness of the medium, the conventional differential pressure transmitter and pontoon level transmitter are preferred, and the materials of the parts contacting the medium are also selected

③ single flange differential pressure transmitter can be used for some suspended solids, foam and other media. Some easy to precipitate and crystallize use plug-in double flange differential pressure transmitter

④ for the liquid level of high viscosity medium and high-pressure equipment, since the equipment cannot be perforated, the radioactive liquid level gauge can be used to measure

⑤ in addition to the problems of measurement methods and technology, there is also the problem of instrument investment

to sum up, the type selection of transmitter should be technically feasible, economically reasonable and conveniently managed

4 selection method of pressure transmitter

from a physical point of view, the pressure on any object should include large

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