1. Correct selection of pressure gauge The correct s […]
1. Correct selection of pressure gauge
The correct selection of pressure detection instruments mainly includes determining the type, range, range, accuracy and sensitivity of the instrument, external dimensions, and whether it needs remote transmission and other functions, such as indication, recording, adjustment, alarm, etc.
The main basis for selection:
1. The measurement requirements of the production process, including range and accuracy. In the case of static test (or slow change), the maximum value of the pressure to be measured is specified to be two-thirds of the full scale value of the pressure gauge; in the case of pulsating (fluctuating) pressure, the maximum value of the pressure to be measured is selected One half of the full scale value of the pressure gauge.
The accuracy grades of commonly used pressure measuring instruments are 0.4, 1.0, 1.5 and 2.5, etc., which should be selected from the production process accuracy requirements and the most economical point of view. The maximum allowable error of the instrument is the product of the range of the instrument and the percentage of accuracy grade. If the error value exceeds the accuracy required by the process, a pressure instrument with a higher accuracy level must be replaced.
2. The properties of the measured medium, such as state (gas, liquid), temperature, viscosity, corrosiveness, degree of contamination, degree of flammability and explosiveness, etc. Such as oxygen meter, acetylene meter, with "no oil" mark, dedicated to special medium corrosion-resistant pressure gauge, high-temperature pressure gauge, diaphragm pressure gauge, etc.
3. Environmental conditions on site, such as ambient temperature, corrosion, vibration, humidity, etc. Such as shock-proof pressure gauges used for vibrating environmental conditions.
4. Suitable for observation by staff. Choose meters with different gauge diameters (outer dimensions) according to the location and lighting conditions of the detection meters.
2. Verification and calibration of pressure gauges
Before the instrument is used, it must be verified and calibrated. Long-term use of meters should also be regularly verified, and the cycle should be determined by the frequency and focus of use. When the meter has a long-distance transmission system and a secondary meter, it should be verified and calibrated together with the secondary meter.
Third, the correct installation of the pressure gauge and related matters
Performing pressure detection actually requires a measurement system to achieve it. In order to achieve accurate measurement, in addition to correct selection and verification (calibration) of the instrument, attention must be paid to the correct installation of the entire system. If only the meter itself is accurate, its displayed value cannot fully represent the actual parameters of the measured medium, because the error of the measurement system is not equal to the error of the meter.
The correct installation of the system includes the opening position of the pressure tap, the reasonable laying of the connecting pipe and the correct installation position of the instrument.
1. Selection of the position of the pressure port
(1) Avoid the area where the pipeline is bent, bifurcated and the stream forms vortex.
(2) When there are protruding objects in the pipeline (such as temperature measurement components), the pressure tap should be taken in front of it.
(3) When pressure must be taken near the regulating valve, if the pressure tap is in front, the distance from the valve should not be less than 2 times the pipe diameter; if the pressure tap is behind, the distance from the valve should not be less than 3 times Pipe diameter. (4) For a wide container, the pressure tap should be in an area where the fluid flows smoothly and there is no vortex. In short, the position of the pressure tap determined in the process flow should be able to ensure that the selected process parameters are measured.
2. Laying of connecting duct
The horizontal section of the connecting pipe should have a certain slope to facilitate the removal of condensed liquid or gas. When the measured medium is a gas, the pipe should be tilted towards the pressure tap; when the measured medium is a liquid, the pipe should be tilted towards the pressure measuring instrument; when the measured parameter is a smaller differential pressure value, it should be tilted The degree can be slightly larger. In addition, if the duct is turning up and down, according to the medium in the duct, a device for draining condensed liquid should be installed at the lowest point or an exhaust device at the highest point to ensure that the cause will not be caused in the duct for a long time. Accumulation of condensed liquid or gas affects the accuracy of measurement. Condensed liquid or gas should be discharged regularly.
3. Precautions for the installation and use of pressure measuring instruments
(1) The instrument should be installed perpendicular to the horizontal plane;
(2) The measuring point of the instrument and the installation of the instrument are at the same horizontal position, whether to consider the correction of the additional height error;
(3) The distance between the installation location of the instrument and the measuring point should be as short as possible to avoid slow indication;
(4) Ensure the tightness, and there should be no leakage, especially for flammable and explosive gas media and toxic and harmful media.
Additional devices should be added when using the instrument in the following situations, but additional errors should not be generated, otherwise corrections should be considered.
(1) In order to ensure that the instrument is not affected by the corrosion of the measured medium or the viscosity or crystallization, an isolation device should be installed;
(2) In order to ensure that the meter is not affected by the rapid change of the measured medium or the pulsating pressure, a buffer is installed. Especially when the pressure increases sharply and the pressure drops sharply, it is easy to damage the pressure gauges, or even break the spring tube and cause leakage;
(3) In order to ensure that the instrument is not affected by vibration, the pressure gauge should be equipped with a vibration reduction device and a fixing device;
(4) In order to ensure that the meter is not affected by the high temperature of the measured medium, a liquid-filled elbow device should be installed;
(5) It is strictly forbidden to use special instruments for other purposes. It is also strictly forbidden to measure without special reliable devices, and it is strictly prohibited to use general pressure gauges for pressure measurement of special media;
(6) For newly purchased pressure measuring instruments, before installation and use, measurement verification must be carried out to prevent vibration, damage or other factors from destroying the accuracy of the pressure instruments during transportation.