Pressure transmitters and differential pressure transmitters are measured by pressure and differential pressure (the difference between the two pressures), but they can be measured indirectly. For example, a pressure transmitter can measure the pressure in the device in addition to the 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/differential pressure transmitters can be considered, that is, one lower limit of measurement, one upper limit is measured, and their output signals are subtracted, and the liquid level can be measured. The differential pressure transmitter is generally used. It can also be used to measure the density of a medium if the liquid level and pressure in the container are constant. The measurement range of the pressure transmitter can be made very wide, starting from absolute pressure 0 to one hundred megapascals (general).
In addition to measuring the differential pressure of the two measured pressures, the differential pressure transmitter can also measure the flow of the medium fluid with various throttling elements, and can directly measure the liquid level of the pressurized container and the liquid level of the atmospheric pressure container. As well as pressure and negative pressure.
The structure made by the pressure/differential pressure transmitter is divided into a common type and an isolated type. The measuring diaphragm of the ordinary pressure/differential pressure transmitter is one, which directly senses the pressure or differential pressure of the measured medium; the isolated measuring capsule receives the pressure of a stable liquid (generally silicone oil). The stabilizing liquid is sealed in the middle of the two diaphragms, and the diaphragm directly receiving the pressure to be measured is an outer diaphragm, and the diaphragm of the original common diaphragm is an inner diaphragm, when the outer diaphragm receives a pressure signal. The pressure of the outer membrane is transmitted to the ordinary capsule by the transmission of the silicone oil, so that the pressure felt by the outer membrane can be measured.
The isolated pressure/differential pressure transmitter is mainly designed and used for special measured media. If the measured media leaves the device, it will crystallize. If the normal pressure/differential pressure transmitter needs to remove the media, it will The pressure guiding tube bellows chamber is blocked and it is not working properly, so the isolation type must be selected. The isolated transmitter is usually mounted in a blue-type installation, that is, the opening of the device under test so that the sensor diaphragm is part of the device wall after the transmitter is installed, so that it does not take out the measured medium, and generally does not cause Crystallization and clogging.
When the measured medium requires a higher crystallization temperature, a structure in which the diaphragm protrudes can be selected, so that the sensing diaphragm can be inserted into the device, so that the temperature of the sensed medium is not lowered, so that the measurement is guaranteed. That is, the plug-in flange transmitter is selected.
The isolated transmitter has a remote transmission type and an integral type. The remote transmission type is a reinforced capillary connection between the outer membrane box and the measuring membrane box. The general capillary tube is 3 to 5 meters, so that the outer membrane box is mounted on the device, and the inner membrane box and the transmitter can be installed in a maintenance bracket. The other form is that the outer membrane box and the transmitter are integrally formed by the flange on the device. For the isolated pressure transmitter, it can also be made into a threaded connection type, that is, the outer membrane box or the outer elastic element can be placed in front of the mounting thread. As long as the internal threaded boss is welded to the device under test, the transmitter can be directly Screwed onto the device for easy installation.
The isolated pressure/differential pressure transmitter is complex to manufacture and has high material requirements, so its price is usually 3 to 4 times that of the normal type.
2 Selection principle
The selection of the pressure/differential pressure transmitter is mainly based on: the nature of the measured medium is subject to the standard, to save money, easy to install and maintain as a reference. If the measured medium is high viscosity, easy to crystallize, and strongly corroded, an isolated transmitter must be used.
When selecting the type, consider the corrosion of the membrane medium by the fluid medium to be tested. Be sure to select the material of the membrane box. Otherwise, the outer membrane will be corroded in a short time after use, and the blue will be corroded and damaged. Personal accidents, so the choice of membrane material is very important. The diaphragm of the transmitter is made of ordinary stainless steel, 304 stainless steel, 316/316L stainless steel, tantalum material, etc.
In the selection, the temperature of the measured medium should be taken into consideration. If the temperature is high, it should reach 200 °C ~ 400 °C, and the high temperature type should be used. Otherwise, the silicone oil will vaporize and expand, making the measurement inaccurate.
When selecting the type, the working pressure level of the equipment should be considered. The pressure level of the transmitter must be consistent with the application. From an economic point of view, the material of the outer membrane box and the insert part is more important, and it is necessary to choose the right one, but the connection of the flange can reduce the material requirements, such as carbon steel, chrome plating, etc., which will save a lot of money.
The isolated pressure transmitter is available in a threaded connection, which saves money and is easy to install.
For the selection of common pressure and differential pressure transmitters, the corrosiveness of the tested medium should also be considered, but the temperature of the medium used can be disregarded because the common type is drawn into the meter and the temperature is long during working. Normal temperature, but the normal type of maintenance is larger than the isolation type. The first is the insulation problem. When the temperature is below zero, the pressure guiding tube will freeze. The transmitter will not work or even be damaged. This will increase the heat tracing and incubator.
From an economic point of view, when a transmitter is selected, a normal type transmitter can be used as long as it is not a crystallization medium, and a low-pressure crystallizable medium can also be added with a purge medium for indirect measurement (as long as the process allows the use of a purge liquid or Gas), the application of ordinary transmitter is to require maintenance personnel to carry out regular inspection, including whether the various pressure guiding tubes leak, whether the cleaning 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. Investment will be very economical. Pay attention to the combination of hardware maintenance and soft maintenance during maintenance.
From the selection range of the selected transmitter, the general transmitter has a certain range adjustable range, and the range of the range to be used is set in the range of 1/4~3/4 of its range, so the accuracy will be guaranteed. It is even more important for differential pressure transmitters. In practice, some applications (level measurement) need to migrate the measurement range of the transmitter, calculate the measurement range and migration according to the installation location of the site, and migrate with positive and negative migration.
At present, smart transmitters have become quite popular. It is characterized by high precision, large adjustable range, and very convenient adjustment and good stability. It should be considered when selecting.
According to the design specifications, in the selection of engineering design, whether pneumatic or electric transmitters are used, because of their special features, should be comprehensively considered and analyzed according to the specific conditions of the device. The following points are available for selection when selecting:
Centralized operation and response speed;
Whether it is compatible with the operation of the DCS computer;
Economic, reliability and maintenance aspects;
Safety (explosion proof, power failure, gas source failure, etc.);
Environmental conditions and transmission distance. In general, the following conditions are preferred for the selection of pneumatic transmitters:
The distance from the transmitter to the display adjustment unit is short, usually not more than 150 meters;
Process materials are flammable and explosive medium and occasions with relatively high relative humidity;
An occasion where the instrument investment is small and the response speed is not fast;
Generally, small and medium-sized enterprises require easy maintenance and economical reliability;
In large installations dominated by electric meters, some on-site adjustment loops are not required to be centrally operated in the central control room. The following conditions are preferred for the selection of electric actuators:
The distance between the transmitter and the display adjustment unit is more than 150 meters;
Large enterprises require central control with a high degree of centralized management;
An object that is controlled and managed by a DCS computer;
Requires fast response, complex information processing and computational operations.
In practice, in modern production facilities, they are mixed and selected according to their respective characteristics.
3. Pressure transmitter selection
From a physics point of view, the pressure on any object includes both atmospheric pressure and the pressure of the measured medium (generally referred to as gauge pressure). The sum of the pressures of the two parts acting on the object to be measured is called the absolute pressure. P absolutely = P table + atmospheric pressure The instrument that measures absolute pressure is called 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.
(1) In order to ensure the accuracy of pressure measurement, the small pressure measurement value of Zui should be higher than 1/3 of the measurement range of the pressure gauge;
(2) For applications requiring high distance measurement or high measurement accuracy, pressure sensors or pressure transmitters （Pressure transmitters' catalog）should be used;
(3) When the measurement accuracy requirement is not high, a resistive or inductive, Hall effect remote transmission pressure transmitter can be selected;
(4) The pneumatic base type pressure indicating regulator is suitable for local pressure indication adjustment;
(5) Pressure transmitter and pressure switch should be reasonably selected according to the explosion-proof requirements of the installation site.
4. Selection of differential pressure transmitters Differential pressure transmitters are selected according to the following points:
(1) Measurement range, required accuracy and measurement function;
(2) The environment faced by the measuring instrument, such as the petrochemical industrial environment, has a hot (toxic) and explosive atmosphere, and has a high ambient temperature;
(3) Physicochemical properties and state of the measured medium, such as strong acid, strong alkali, viscous, easy to solidify crystallization and vaporization;
(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 nozzles should be considered, such as towers, solution tanks, reactors, boiler steam drums, vertical tanks, spherical tanks, etc.;
(6) Other requirements, such as environmental protection and sanitation;
(7) There must be a unified consideration in the selection of engineering instruments, and it is required to reduce the number of specifications as much as possible and reduce spare parts for profit management;
(8) Actual process conditions:
1 Consider which type of device the measured object belongs to. For tanks and 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;
2 depends on the physical and chemical properties and cleanliness of the medium, the conventional differential pressure transmitter and the float type liquid level transmitter, and also the material selection of the contact medium part;
3 For some suspensions, foams and other media, single-lange differential pressure transmitters are available. Some insertable double-blue differential pressure transmitters that are easy to precipitate and easy to crystallize;
4 The level of the high-viscosity medium and the liquid level of the high-pressure equipment, because the equipment can not be opened, can be measured by the radiation level gauge;
5 In addition to measurement methods and technical problems, there are also investment problems with instruments. In summary, the selection of the transmitter is technically feasible, economically reasonable, and convenient to manage.
5. Pressure and differential pressure transmitter installation instructions
A straight pipe section shall be ensured at the pressure point, and each side shall be larger than 5D (pipeline passage);
When taking pressure on the steam pipe, a pressure pipe should be installed on the side of the pipe;
The balance tank should be installed at the high point of the zui of the pressure pipe;
The drain pipe should be installed close to the transmitter pressure pipe connection;
The distance between the pressure point and the pipeline of the transmitter should be greater than 1 meter;
6. The installation position of the pressure transmitter on the process pipe is related to the measured medium.
Care should be taken to consider the following when installing:
1) Try to avoid direct contact between the transmitter and corrosive or overheated measured media.
2) Prevent the dross from depositing in the pressure guiding tube.
3) The pressure guiding tube should be as short as possible.
4) The liquid column pressure in the pressure guiding pipes on both sides should be balanced.
5) The pressure guiding tube should be installed in a place where the temperature gradient and temperature fluctuation are small. When measuring the liquid flow, the pressure receiving port should be opened on the side of the process pipe to avoid the sedimentation of the dross. At the same time, the transmitter should be installed beside or below the pressure tap to allow air bubbles to drain into the process piping.
When measuring the gas flow, the pressure tap should be open at the top or side of the process piping. And the transmitter should be placed next to or above the process piping so that the accumulated liquid can easily flow into the process piping.
Use a transmitter with a side relief valve in the pressure chamber. The pressure port should be open on the side of the process piping. For example, when the measured medium is liquid, the relief valve of the 3051 intelligent pressure transmitter should be installed on top to eliminate the gas infiltrated into the measured medium. When the measured medium is a gas, the bleed valve of the transmitter should be installed below to discharge the accumulated liquid. The pressure chamber is rotated 180. , you can change the bleed valve above it to the bottom. Due to the importance of its location, the sealing of the valve must be good, otherwise the unit vacuum will be reduced and the economy will be reduced.