Pressure instruments are widely used in various fields of the national economy. In the modern industrial production process, pressure is a very important parameter. Pressure, temperature, and flow are collectively referred to as the three major elements of industrial automation control. Therefore, in order to ensure the reasonable and reliable application of the pressure transmitter, it must be reasonably selected and installed correctly. Today, the editor brings you relevant information on the selection and installation of pressure transmitters.
Main selection basis
The selection of pressure/differential pressure transmission transmitter is mainly based on the measured medium, temperature environment, accuracy level, installation and maintenance cost, etc. After determining the manufacturer and model of the used transmitter, the transmission is determined according to the use requirements The range of the device. When selecting the range, comprehensive consideration should be given to the principles of up and down adjustable range, highest accuracy, and suitable price.
1. Measuring medium
If you are measuring a relatively clean fluid, you can directly use a standard pressure transmitter. If the measured medium is high-viscosity, easy to crystallize and strong corrosion, the isolated transmitter must be selected. When selecting the type, it is necessary to consider the corrosion of its medium to 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 to cause equipment and Personal accident. The diaphragm of the transmitter is made of ordinary stainless steel, 304 stainless steel, 316L stainless steel, and tantalum diaphragm.
In terms of the measurement range of the transmitter, generally the transmitter has a certain range adjustable range. It is best to set the used range range to 1/4~3/4 of its range, so that the accuracy will be guaranteed. It is even more important for differential pressure transmitters. In practice, some applications need to migrate the measuring range of the transmitter. At this time, the measuring range and migration amount need to be calculated according to the on-site installation location, and then positive or negative migration is performed.
3. Accuracy level
Each type of measuring meter will have accuracy errors, but because the accuracy grades marked by various countries are different, for example, the accuracy of China and the United States is the most linear part of the sensor, which is what we usually call the measurement range 10 The accuracy is between% and 90%; the accuracy of the European standard is the worst part of linearity, which is what we usually call the measurement range of 0% to 10%, and the accuracy between 90% and 100%. If the accuracy of the European standard is 1%, the accuracy of the Chinese standard is 0.5%.
4. Wetted material
The material of the contact medium part of the general pressure transmitter is 316 stainless steel. If the measuring medium is not corrosive to 316 stainless steel, then basically all pressure transmitters are suitable for measuring the pressure of the medium; if the measuring medium is 316 Stainless steel is corrosive, so it is necessary to use a chemical seal, which can not only measure the pressure of the medium, but also effectively prevent the contact between the medium and the wetted part of the pressure transmitter, thereby protecting the pressure transmitter and prolonging its service life The role of.
5. Output signal
At present, due to the needs of various acquisitions, there are many kinds of output signals of pressure transmitters on the market, mainly 4~20mA, 0~20mA, 0~10V, 0~5V, etc. The more commonly used ones are 4~20mA and 0 There are two types of ~10V, and only 4~20mA signal is two-wire system (excluding grounding and shielding wires), and the others are three-wire system.
6. Medium temperature
Since the signal of the pressure transmitter is partially converted by the electronic circuit, in general, the temperature of the measuring medium of the pressure transmitter is -30 to +100 degrees. If the temperature is too high, the condensate bend is generally used to cool the medium.
After determining the above parameters, it is necessary to confirm the process connection interface of the pressure transmitter and the power supply voltage of the pressure transmitter; if it is used in special occasions, also consider the explosion-proof and protection level.
Preparation before installation
1. Check the equipment: Since the equipment provided is different from the design supplier and the model, it is necessary to determine the transmitter corresponding to each tag according to the range, the design and installation method and the material required by the process medium.
2. Determine the installation location: Various series of pressure transmitters should adopt a waterproof and dust-proof structure and can be installed in any place. However, considering the convenience of daily operation and maintenance, extending the service life, and ensuring reliability, the installation location has the following requirements:
(1) There is enough working space around, and the distance from adjacent objects (in any direction) is greater than 0.5m;
(2) There is no serious corrosive gas around;
(3) Free from the surrounding heat radiation and direct sunlight;
(4) To prevent interference to the output due to the vibration of the transmitter and the pressure guiding tube (capillary tube), the transmitter should be installed in a vibration-free place.
Pressure transmitter installation requirements
1. Installation of pneumatic pressure transmitter
(1) The pressure transmitter should be installed on a vertical or horizontal pipeline without strong vibration.
(2) The surrounding environment should not contain impurities that are corrosive to carbon steel, copper and alloys.
(3) To facilitate maintenance and zero adjustment, sufficient space should be reserved on the top and side of the equipment.
(4) The pressure transmitter and the pressure measurement location should be as close as possible, generally no more than 5m.
(5) When the measured pressure is zero after installation, the output pressure of the pressure transmitter should be 20kPa, otherwise adjust.
(6) The air source used by the pressure transmitter should be filtered pure compressed air.
2. Installation requirements of electric pressure transmitter
(1) Installation requirements when measuring gas pressure
A. When measuring gas pressure, the pressure relief point of the pressure measurement pipeline must be selected in the upper half of the pipeline to prevent liquid from accumulating in the pressure measurement pipeline.
B. A flush valve should be installed in the front of the pressure transmitter to prevent liquid or dirt from entering the transmitter.
C. When installing a pressure transmitter on the pipeline throttling device, the pressure point must be in the area within 45° of the horizontal direction from the vertical.
(2) Installation requirements when measuring steam pressure
When measuring steam pressure, the pressure point must be installed in the area within 45° from the horizontal line to the vertical line. Install a gas collector at the highest point of the pressure point and discharge gas regularly to ensure the accuracy of the transmitter.
(3) Installation requirements when measuring liquid pressure
When measuring the liquid pressure, the pressure point must be installed in the area within 45° of the horizontal direction from the vertical line.
(4) Installation requirements when measuring the pressure of corrosive media When measuring the pressure of corrosive media, an isolation device should be installed in front of the pressure transmitter and an isolation liquid should be injected into it.
Correct installation of pressure transmitter
1. Selection of pressure point
(1) The selected pressure measuring point should reflect the true size of the measured pressure. The pressure point should be selected in the pipe section where the measured medium flows in a straight line, not in the pipe bends, bifurcations, dead angles and places where vortexes are easily formed.
(2) For gaseous media, a small amount of condensate in the gas should be able to smoothly flow back to the process pipeline, so the pressure point should be in the upper half of the pipeline; for liquid-liquid media, a small amount of gas precipitated in the liquid should be able to return to the pipeline , At the same time, prevent impurities and particles in the pipeline from entering the measuring pipeline and instrument, so the pressure point should be in the lower half of the pipeline, but not at the bottom of the pipeline.
2. Laying of pressure guiding tube
(1) The thickness of the pressure guiding tube should be appropriate, generally the inner diameter is 6~10mm, the length should be as short as possible, and the longest should not exceed 50 meters;
(2) When the pressure guiding tube is installed horizontally, an inclination of 1:10~1:20 should be maintained to facilitate the discharge of the liquid (or gas) accumulated in the tube;
(3) A shut-off valve should be installed between the pressure port and the pressure gauge to facilitate the maintenance of the pressure transmitter.
3. Determination of the installation point
(1) The transmitter should be installed where it is easy to observe and repair.
(2) The installation site should avoid the influence of vibration and high temperature.
(3) When detecting liquids and vapors with a temperature higher than 60℃, the source parts of the pressure transmitter installed on the spot should be equipped with an annular bend or a U-shaped condensation bend.
Calibration of pressure transmitter
Using the transmitter for a period of time may lead to changes in measurement performance, zero drift, and increased measurement errors. Therefore, it is necessary to perform regular calibration. The accuracy of the calibrator used for calibration should be 3 of the required calibration accuracy. Times more. Use the pressure calibrator to input 0%, 25%, 50%, 75%, 100% range standard pressure values to the transmitter in the forward and reverse directions to check whether the current output signal is within the allowable accuracy range. If it is out of tolerance, it needs to be adjusted. The adjustment method is as follows (4~20mA output):
(1) Use a pressure calibrator to input 4mA point standard pressure, adjust the ZERO (zero point) potentiometer to make the 4mA current output within the accuracy range;
(2) Use the pressure calibrator to input the standard pressure of 20mA point, and adjust the SPAN (range) potentiometer to make the 20mA current output within the accuracy range;
(3) ZERO and SPAN will restrain each other, repeat steps (1) and (2) until the current output of 4mA point and 20mA point meets the accuracy requirements at the same time, and then check whether the current output of 25%, 50%, 75% of the range point meets the accuracy requirements, If the requirements are met, the transmitter calibration is qualified.