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Five steps to complete pressure transmitter selection and installation
Dec 21, 2018


Five steps to complete pressure transmitter selection and installation


Pressure instruments and meters have a very wide range of applications in all areas of the national economy. Pressure is a very important parameter in modern industrial production. Pressure, temperature and flow are collectively referred to as the three elements of industrial automation control. Therefore, in order to ensure the reasonable and reliable application of the pressure transmitter, it must be properly selected and properly installed. Today, Yikong Jun brings you relevant information on the selection and installation of pressure transmitters.


Main selection basis


 

Five steps to complete pressure transmitter selection and installation

The selection of pressure/differential pressure transmitteris mainly based on the measured medium, temperature environment, accuracy level, installation and maintenance cost, etc. After determining the manufacturer and model of the transmitter used, the transmission is determined according to the requirements of use. The range of the device. The range selection should be based on the principle that the range is adjustable up and down, the highest precision, and the price is suitable.

1.Measuring medium

If you are measuring a relatively clean fluid, use a standard pressure transmitter directly. If the measured medium is high viscosity, easy to crystallize and corrode, then an isolated transmitter must be used. When selecting the type, consider the corrosion of the medium to the metal of the bellows. Be sure to choose the material of the bellows. Otherwise, the outer membrane will be corroded after a short time, and the flange will be corroded and damaged. Personal accidents. The diaphragm of the transmitter is made of ordinary stainless steel, 304 stainless steel, 316L stainless steel, and diaphragm case material.

2. Range

From the selection range of the selected transmitter, the general transmitter has a certain range adjustable range. It is better to set the range of the 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 measurement range of the transmitter. In this case, the measurement range and migration amount should be calculated according to the site installation position, and then the positive or negative migration should be performed.

3. Accuracy level

Each meter will have accuracy errors, but the accuracy level is different for each country. For example, the accuracy of the Chinese and American standards is the best part of the linearity of the sensor, which is what we usually call the measurement range. Accuracy between % and 90%; and European standard accuracy is the worst part of linearity, which is what we usually call 0% to 10% of the measurement range, and between 90% and 100%. If the accuracy of the European standard is 1%, the accuracy of the standard in China is 0.5%.

 

Five steps to complete pressure transmitter selection and installation

4. Wetted material

The contact medium part of the general pressure transmitter is made of 316 stainless steel. If the measuring medium is not corrosive to 316 stainless steel, then basically all pressure transmitters are suitable for measuring the medium pressure; if measuring medium pair 316 Stainless steel is corrosive, so it is necessary to use a chemical seal, which not only can measure the pressure of the medium, but also effectively prevent the contact between the medium and the pressure transmitter, thus protecting the pressure transmitter and prolonging the service life. The role.

5. Output signal

At present, due to various collection needs, there are many kinds of output signals of pressure transmitters on the market, mainly 4~20mA, 0~20mA, 0~10V, 0~5V, etc., 4~20mA and 0 are commonly used. ~10V two, and only 4~20mA signal is two-wire system (excluding grounding and shielding line), 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 pressure transmitter is -30 to +100 degrees. If the temperature is too high, a condensation bend is generally used to cool the medium.

7. Other

After determining these parameters, the process connection interface of the pressure transmitter and the supply voltage of the pressure transmitter should also be confirmed; if it is used in special occasions, the explosion protection and the degree of protection should be considered.


Preparation before installation


 

Five steps to complete pressure transmitter selection and installation

1. Checking equipment: Since the supply equipment is different from the design supplier and model, it is necessary to determine the transmitter corresponding to each bit number according to the measuring range and design and installation method and the material required by the process medium.

2. Determine the installation position: Various series of pressure transmitters should adopt waterproof and dustproof structure and can be installed in any place. However, from the aspects of facilitating daily operation and maintenance, prolonging the service life, and ensuring reliability, the installation position has the following requirements:

(1) There is enough working space around, and the distance from any adjacent object (in any direction) is greater than 0.5m;

(2) There is no serious corrosive gas around;

(3) Free from ambient heat radiation and direct sunlight;

(4) Prevent the output from being disturbed by the vibration of the transmitter and the pressure guiding tube (capillary). The transmitter should be installed in a vibration-free place.


Pressure transmitter installation requirements


 

Five steps to complete pressure transmitter selection and installation

1. Installation of pneumatic pressure transmitter

(1) The pressure transmitter should be installed on a vertical or horizontal pipe without strong vibration.

(2) The surrounding environment should not contain impurities that corrode carbon steel, copper and alloys.

(3) In order to facilitate maintenance and zero adjustment, sufficient space should be reserved above and on the side of the equipment.

(4) Pressure transmitter and pressure measurement location should be as close as possible, generally no more than 5m.

(5) When the measured pressure is zero after installation, the pressure transmitter output pressure should be 20kPa, otherwise it will be adjusted.

(6) The air source used in the pressure transmitter should be pure compressed air after filtration.

2. Installation requirements for electric pressure transmitter

(1) Installation requirements when measuring gas pressure

A When measuring the gas pressure, the pressure-removing point of the pressure-measuring pipe shall be selected in the upper part of the pipe to prevent the liquid from accumulating in the pressure-measuring pipe.

B Flush the valve at the front of the pressure transmitter to prevent liquid or dirt from entering the transmitter.

C When installing a pressure transmitter on a pipe throttling device, the pressure tapping point must be within 45° of the horizontal direction from the vertical.

(2) Installation requirements when measuring steam pressure

The pressure point at which the steam pressure is measured must be installed in an area that is within 45° from the horizontal line to the vertical line. The gas collector is installed at the highest point of the pressure measurement point and the gas is periodically discharged to ensure accurate measurement of the transmitter.

(3) Installation requirements when measuring liquid pressure

When measuring the liquid pressure, the pressure point must be installed in an area that is within 45 degrees of the horizontal direction from the vertical line.

(4) Installation requirements when measuring corrosive medium pressure When measuring corrosive medium pressure, an isolation device should be installed in front of the pressure transmitter and a spacer fluid should be injected therein.


Correct installation of the pressure transmitter


 

Five steps to complete pressure transmitter selection and installation

1. Selection of pressure points

(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 straight, not in the pipeline elbow, bifurcation, dead angle and the place where the vortex is easy to form.

(2) For the gas medium, a small amount of condensate in the gas should flow smoothly back to the process pipe, so the pressure point should be in the upper part of the pipe; for the liquid liquid medium, a small amount of gas precipitated in the liquid should be returned to the pipe. At the same time, prevent impurities and particles in the pipeline from entering the measuring pipeline and the instrument, so the pressure point should be in the lower half of the pipeline, but not at the bottom of the pipeline.

2. Pressure guiding tube laying

(1) The thickness of the pressure guiding tube should be suitable, generally the inner diameter is 6~10mm, the length should be as short as possible, and the longest time should not exceed 50 meters;

(2) The horizontal direction of the pressure guiding pipe should be maintained at a slope of 1:10~1:20 to facilitate the discharge of liquid (or gas) accumulated in the pipe;

(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 in an area that is easy to observe and repair.

(2) The installation location should be designed to avoid the effects of vibration and high temperature.

(3) When detecting liquids and vapors with a temperature higher than 60 °C, the source parts of the pressure transmitter installed locally should be equipped with a circular bend or a U-shaped condensation bend.


Pressure transmitter calibration


Five steps to complete pressure transmitter selection and installation

 

When the transmitter is used for a period of time, it may cause a change in metering performance, a zero drift, and an increase in measurement error. Therefore, it is necessary to perform periodic calibration. The accuracy of the calibrator used for calibration should be the required calibration accuracy. More than double. Use the pressure calibrator to input the standard pressure values of 0%, 25%, 50%, 75%, and 100% of the range forward and reverse to the transmitter to check whether the current output signal is within the allowable accuracy range. If it is too poor, it needs to be adjusted. The adjustment method is as follows (4~20mA output):

(1) Input the standard pressure of 4mA point with the pressure calibrator, adjust the ZERO (zero point) potentiometer, so that the 4mA current and current output is within the accuracy range;

(2) Input the standard pressure of 20mA point with the pressure calibrator, adjust the SPAN (range) potentiometer, so that the 20mA current output is within the accuracy range;

(3) ZERO and SPAN will be mutually restrained. Repeat steps (1) and (2) until the current output of 4mA point and 20mA point meets the accuracy requirement, and then check whether the current output of 25%, 50%, 75% of the range point meets the accuracy requirement. If the requirements are met, the transmitter is calibrated.