According to the application, there are intrinsically safe and flameproof types; the main types of transmitters according to the application conditions are as follows:

* Low (micro) pressure/low differential pressure transmitter;

* Medium pressure/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 selection of the transmitter is usually considered according to the installation conditions, environmental conditions, instrument performance, economy, and application medium. In practical application, it is divided into direct measurement and indirect measurement; its uses include process measurement, process control, and device interlocking. Common transmitters include ordinary pressure transmitters, differential pressure transmitters, single flange transmitters, double flange transmitters, plug-in flange transmitters, etc.

Pressure transmitters and differential pressure transmitters measure pressure and the difference between two pressures in noun, but there are many parameters that they measure indirectly. Such as a pressure transmitter, in addition to measuring pressure, can also measure the liquid level in the equipment. When measuring the liquid level in a normal pressure vessel, a pressure transformer is required. When measuring the liquid level of a pressure vessel, two pressure transmitters can be used, that is, one for measuring the lower limit and one for measuring the upper limit. Their output signals can be subtracted to measure the liquid level. Generally, a differential pressure transmitter is 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 the pressure transmitter can be made very wide, starting from absolute pressure 0 to 100MPa (general situation).

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 rate, and can directly measure the liquid level of the pressure container and the liquid level of the normal pressure container as well as the pressure and Negative pressure.

Production

There are common types and isolation types in terms of the structure of pressure and differential pressure transmitters. There is one common type of measuring capsule, which directly senses the pressure and differential pressure of the measured medium; the isolated type of measuring capsule receives the pressure of a stabilizing fluid (usually silicone oil), and this stabilizing fluid is Sealed between two diaphragms, the diaphragm receiving the measured pressure is the outer diaphragm. The diaphragm of the original ordinary diaphragm is the inner diaphragm. When the outer diaphragm receives the pressure signal, the pressure of the outer diaphragm is transmitted to the ordinary diaphragm through the transmission of silicone oil, and the pressure of the outer diaphragm is measured. Feel the pressure.

The isolated type transmitter is mainly used for the special measured medium. If the measured medium leaves the equipment, it will produce crystallization, while the ordinary type transmitter needs to take out the medium, which will block the pressure guiding tube and the bellows chamber to make it Can not work normally, so must choose the isolation type. The isolation type is usually made into a flange installation, that is, the opening on the device under test is added with a flange so that after the transmitter is installed, its sensing diaphragm is a part of the device wall so that it will not take out the measured medium, and generally will not cause crystallization blockage.

When the measured medium requires a high crystallization temperature, the structure with a protruding diaphragm can be selected so that the sensing diaphragm can be inserted into the device so that the temperature of the sensed medium will not decrease, so the measurement is guaranteed. That is, the plug-in flange transmitter is selected.

The isolated transmitter has a remote type and an integrated type. The remote transmission type means that the outer membrane box and the measuring membrane box are connected by a reinforced capillary tube. Generally, the capillary is 3 to 5 meters. In this way, the outer membrane box is installed on the equipment, and the inner membrane box and the transmitter can be installed on a bracket for easy maintenance. ; Another form is that the outer membrane box and the transmitter are integrated and directly installed 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 diaphragm box or the outer elastic element can be in front of the mounting thread, and the transmitter can be directly connected as long as the internal thread boss is welded on the device under test. Screws onto the unit for easy installation.

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

Selection principles

The main basis for the selection of pressure/differential pressure transmitters is to take the property index of the measured medium as the standard and save money and facilitate installation and maintenance as a reference. If the measured medium is highly viscous, easy to crystallize, and strongly corroded, an isolated transmitter must be selected.

When selecting the model, it is necessary to consider the corrosion of its medium on the metal of the bellows, 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, causing equipment and personal accidents. , so the material selection is very important. The membrane box of the transmitter is made of ordinary stainless steel, 304 stainless steel, 316L stainless steel, tantalum membrane box, etc.

When selecting the type, the temperature of the medium to be measured should be considered. If the temperature is high, generally 200 ° C ~ 400 ° C, the high-temperature type should be selected, otherwise, the silicone oil will vaporize and expand, making the measurement inaccurate.

When selecting the type, the working pressure rating of the equipment should be considered, and the pressure rating of the transmitter must match the application. From an economic point of view, the material of the outer membrane box and the insertion part is more suitable, but the connecting flange can be made of carbon steel or chrome-plated, which will save a lot of money.

It is best to use a threaded connection for the isolation pressure transmitter, which saves money and is convenient for installation.

For the selection of ordinary 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 ordinary pressure changer is induced into the meter, and the long-term working temperature is normal temperature. However, the amount of maintenance used by the ordinary type is larger than that of the isolated type. The first is the problem of heat preservation. In the winter in the north, the pressure guide tube will freeze, and the transmitter will not work or even be damaged. This requires the addition of heat tracing and insulation boxes.

From an economic point of view, when selecting a transmitter, as long as it is not an easy-to-crystallization medium, an ordinary transmitter can be used, and for a low-pressure easy-to-crystallization medium, a purge medium can also be added for indirect measurement (as long as the process allows it to use a purge liquid or Gas), the application of ordinary transmitters requires maintenance personnel to conduct more regular inspections, including whether various pressure guiding tubes are leaking, whether the purge medium is normal, whether the insulation is good, etc. As long as the maintenance is good, a large number of ordinary transmitters are used once Sexual investment will save a lot.

In terms of the measurement range of the selected transmitter, the general transmitter has a certain range of adjustable range. It is best to set the range of use at 1/4~3/4 of its range so that accuracy will be guaranteed. It is even more important for differential pressure transmitters. In practice, some applications (liquid level measurement) need to migrate the measurement range of the transmitter and calculate the measurement range and migration amount according to the on-site installation position. Migration can be divided into positive migration and negative migration.

At present, intelligent transmitters are quite popular. It is characterized by high precision, a large adjustable range, convenient adjustment, and good stability. More consideration should be given when selecting models.

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

* The degree of centralized operation;
* Whether to cooperate with the DCS computer;
* Responding speed; 
* Economical;
* 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 and adjustment instrument is relatively short, usually no more than 150m;
* The process material is a flammable and explosive medium and a place with high relative humidity;
* Requires less instrument investment;
* Generally small and medium-sized enterprises require easy maintenance, economical and reliability;

In large-scale installations dominated by electric instruments, some on-site regulation loops do not require centralized operation in the central control room.

It is advisable to choose an electric instrument under the following conditions:

* The distance between the transmitter and the display adjustment unit exceeds 150m;
* Large enterprises require central control with highly centralized management;
* Objects that are controlled and managed by DCS computers;
* Requires fast response speed, complex information processing, and calculation.

In practice, in modern production equipment, they are mixed and selected based on their respective characteristics.

Selection of differential pressure transmitter

The differential pressure transmitter is selected according to the following points:

(1) Measurement range, required accuracy, and measurement function;

(2) The environment that the measuring instrument faces, such as the industrial environment of the petrochemical industry, the existence of flammable (toxic) and explosive atmospheres, and the high ambient temperature; 

(3) The physical and chemical properties and state of the measured medium, such as strong acid, strong alkali, viscous, easy to solidify and crystallize, and gasification;

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

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

(6) Other requirements, such as environmental protection and hygiene requirements;

(7) The selection of engineering instruments should be considered uniformly, and it is required to reduce the varieties and specifications as much as possible and reduce spare parts to facilitate management;

(8) Specific requirements for process licensors.

(9) The actual process situation:

① Consider which type of equipment the object under test belongs to. Such as tanks and tanks, if the volume of the tank is small, the measurement range will not be too large, and if the volume of the tank is large, the measurement range may be large;

② Depending on the physical and chemical properties and cleanliness of the medium, conventional differential pressure transmitters and float level transmitters are preferred, and the material of the contact medium part should be selected;

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

④ For the liquid level of high-viscosity medium and the liquid level of high-pressure equipment, since the equipment cannot be opened, a radiation level gauge can be used to measure;

⑤ In addition to the measurement method and technical problems, there are also instrument investment problems.

In summary, the selection of the transmitter should be technically feasible, economically reasonable, and manageably convenient.

Selection method of pressure transmitter

From a physical point of view, the pressure on any object should include two parts: atmospheric pressure and the pressure of the measured medium (generally called gauge pressure). The sum of these two parts of pressure acting on the measured object is called absolute pressure.

P absolute = P gauge + atmospheric pressure

An instrument that measures absolute pressure is called an absolute pressure gauge. For ordinary industrial pressure gauges, gauge pressure values are measured, that is, the pressure difference between absolute pressure and atmospheric pressure. When the absolute pressure is greater than the atmospheric pressure, the measured gauge pressure is positive, which is called positive gauge pressure; when the absolute pressure is less than the atmospheric pressure, the measured gauge pressure is negative, which is called negative gauge pressure, that is, the degree of vacuum. Instruments that measure vacuum is called vacuum gauges.

① In order to ensure the pressure measurement accuracy, the minimum pressure measurement value should be higher than 1/3 of the pressure gauge measurement range;

② For occasions that require long-distance measurement or require high measurement accuracy, pressure sensors or pressure transmitters should be selected;

③ When the measurement accuracy is not high, you can choose resistance or inductance, Hall effect remote pressure gauge;

④ Pneumatic base pressure indicating regulator is suitable for local pressure indicating adjustment;

⑤ Pressure transmitters and pressure switches should be reasonably selected according to the explosion-proof requirements of the installation site.

Selection of pressure gauges

(1) Range selection

* When measuring stable pressure, the maximum range of the general pressure gauge is selected to be close to or greater than 1.5 times the normal pressure measurement value;
* When measuring pulsating pressure, the maximum range of the general pressure gauge is selected to be close to or greater than twice the normal pressure measurement value;
* When measuring the outlet pressure of the machine pump, the maximum range of the general pressure gauge is selected to be close to the maximum pressure value of the machine pump outlet;
* When measuring high pressure, the maximum range of the general pressure gauge should be selected to be greater than 1.7 times the maximum pressure measurement value;
* In order to ensure pressure measurement accuracy, the minimum pressure measurement value should be higher than 1/3 of the pressure gauge measurement range.

(2) Type selection

* When measuring pressure>0.4 MPa, a spring tube pressure gauge can be selected;
* When the pressure measurement is <0.04 MPa, bellows and bellows pressure gauges can be selected;
* For occasions that are viscous, easy to crystallize, corrosive, and contain solid particles, diaphragm pressure gauges or chemical sealing devices can be used;
* When measuring steam or medium pressure above 60°C, stainless steel pressure gauges should be selected or condensation rings should be installed;
* A damper or a shock-resistant pressure gauge should be added to the pulsation pressure measurement;
* A dust collector should be installed when measuring the gas-containing dust;
* When measuring the pressure of gas containing liquid, a gas-liquid separator should be installed;
* Special pressure gauges should be used to measure certain chemical media: ammonia pressure gauges are used for ammonia-containing media pressure measurements, oxygen pressure gauges are used for oxygen-containing pressure measurements, acetylene pressure gauges are used for acetylene pressure measurements, and sulfur-resistant pressure gauges are used for sulfur-containing media pressure. pressure gauge;
* High-pressure gauges (greater than 10MPa) should have pressure relief safety facilities.