Low (micro) pressure/low differential pressure transmitter;
Medium/medium pressure differential pressure transmitter;
High pressure/differential pressure transmitter;
Absolute pressure/vacuum/negative pressure differential pressure transmitter;
High temperature/pressure and differential pressure transmitter;
Corrosion-resistant/pressure, differential pressure transmitter;
Easy crystallization/pressure differential pressure transmitter.
The selection of transmitter is usually based on installation conditions, environmental conditions, instrument performance, economy and application medium. In practical application, it can be divided into direct measurement and indirect measurement; Its applications include process measurement, process control and equipment interlocking. Common transmitters include ordinary pressure transmitter, differential pressure transmitter, single flange transmitter, double flange transmitter and plug-in flange transmitter.
Pressure transmitter and differential pressure transmitter only measure the pressure and the difference between the two pressures in the nominal sense, but they indirectly measure many parameters. For example, in addition to pressure, the pressure transmitter can also measure the liquid level in the equipment. When measuring the liquid level in the atmospheric container, a pressure transformer is needed. When measuring the liquid level of a pressure vessel, two pressure transformers can be used, one for the lower limit and the other for the upper limit. Their output signals can be subtracted to measure the liquid level, and generally a differential pressure transmitter is used. It can also be used to measure the density of medium under the condition of constant liquid level and pressure in the container. The range of pressure transmitter can be very wide, from absolute pressure 0 to 100MPa (general).
Selection principle:
The selection of pressure/differential pressure transmitter is mainly based on the performance index of the measured medium and the reference of saving money and facilitating installation and maintenance. If the measured medium has high viscosity, easy crystallization and strong corrosiveness, an isolated transmitter must be selected.
When selecting materials, the corrosion of the media to the metal of the bellows must be considered, and the material of the bellows must be selected, otherwise the outer diaphragm will be corroded in a short time after use, and the flange will also be corroded, resulting in equipment and personal accidents, so the selection of materials is very important. The capsule materials of the transmitter include ordinary stainless steel, 304 stainless steel, 3 16L stainless steel and tantalum capsule materials.
When selecting the type, the temperature of the measured medium should be considered. If the temperature is high, generally 200℃~400℃, the high temperature type should be selected, otherwise the silicone oil will vaporize and expand, resulting in inaccurate measurement.
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 situation. From the economic point of view, the materials of the outer membrane box and the insertion part are more suitable, but the connecting flange can be made of carbon steel and chromium plating, which will save a lot of money.
The selection of isolated pressure transmitter is best in the form of threaded connection, which saves money and is convenient for installation.
For the selection of common pressure and differential pressure transmitters, the corrosiveness of the measured medium should also be considered, but the temperature of the medium used can be ignored, because the common pressure transformer introduces pressure into the instrument and the long-term working temperature is normal temperature, but the maintenance amount of the common type is greater than that of the isolated type. First of all, the problem of heat preservation. In winter in the north, when it is below zero, the pressure guide pipe will freeze, and the transmitter will not work or even be damaged, so it is necessary to increase the heat tracing incubator.
From the economic point of view, when selecting a transmitter, you can use a common transmitter as long as it is not easy to crystallize, or you can add a purging medium for indirect measurement of low-pressure crystallization medium (as long as the process allows the use of purging liquid or gas). The application of ordinary transmitters requires regular inspection by maintenance personnel, including whether all kinds of pressure guide pipes are leaking, whether the purging medium is normal and whether the heat preservation is good. As long as it is well maintained, using ordinary launch opportunities can save a lot of one-time investment.
Generally speaking, the range of the transmitter is adjustable, so it is better to set the range in the range of 1/4~3/4, so that the accuracy will be guaranteed, which is more important for the differential pressure transmitter. In practical application, some applications (liquid level measurement) need to transfer the range of the transmitter, and the range and transfer amount are calculated according to the installation position on site. There are both positive and negative migration.
Select the differential pressure transmitter according to the following points:
1, measuring range, required accuracy and measuring function;
2. The environment faced by measuring instruments, such as the industrial environment of petrochemical industry, has a flammable (toxic) and explosive atmosphere with high ambient temperature;
3. Physical and chemical properties and state of the medium to be tested, such as strong acid, strong alkali, viscosity, easy solidification, easy crystallization, easy gasification, etc.
4. Changes in operating conditions, such as temperature, pressure and medium concentration. Sometimes, it is necessary to consider the change of gas-liquid concentration and density from start-up to normal production;
5. The structure, shape, size, equipment accessories and various inlet and outlet nozzles of the tested container, such as tower, solution tank, reactor, boiler drum, vertical tank and spherical tank, should be considered.
6. Other requirements, such as environmental protection and health requirements;
7, the selection of engineering instruments should have a unified consideration, requirements as far as possible to reduce varieties and specifications, reduce spare parts, easy to manage;
8. Specific requirements of the process patentee.
9, the actual process:
1) Consider what kind of equipment the tested object belongs to. Such as tanks, tanks, tanks are small, the measuring range will not be too large, tanks are large, and the measuring range may be large;
2) According to the physical and chemical properties and cleanliness of the medium, the conventional differential pressure transmitter and float level transmitter are preferred, and the materials contacting the medium are selected;
3) For some suspended matter, foam and other media, a single flange differential pressure transmitter can be used. Some are easy to precipitate and crystallize with plug-in double flange differential pressure transmitter;
4) For the liquid level of high-viscosity medium and high-pressure equipment, radioactive liquid level meter can be selected for measurement because the equipment cannot be drilled;
5) In addition to measurement methods and technical problems, there is also the problem of instrument investment.
To sum up, the selection of transmitter should be technically feasible, economical and reasonable, and easy to manage.