The function and working principle of the bourdon tube type pressure gauge are introduced, and the correct selection and installation methods of the pressure gauge and the precautions for use are pointed out.
The bourdon tube type pressure gauge is mainly used for the measurement of liquid, gas and steam pressure, and has been widely used because of its wide measurement range, simple structure, sturdy and durable, easy to carry and safe and reliable use. Its working principle is that under the action of pressure, the elastic deformation of the bourdon tube causes the displacement of the tube end. The displacement is amplified by the mechanical transmission mechanism and transmitted to the indicating device. out the measured pressure value. Pressure gauges have become an indispensable measuring instrument for detection, measurement, recording and control in various industrial applications, such as locomotive and vehicle pressure detection, boiler pressure detection, oxygen and acetylene pressure detection for welding and cutting, process assembly pressure detection, etc. However, in daily work, it is found that some engineers cannot correctly choose and use the pressure gauge, which affects the accuracy of pressure measurement, brings hidden dangers to safety production, and also causes waste of resources. This article introduces the correct selection of bourdon tube type pressure gauges and precautions for use, in order to provide some reference opinions for measurement personnel.
How to make a correct choice of pressure gauge?
1. The diameter of the pressure gauge shell is 40mm, 60mm, 100mm, 150mm, 200mm and 250mm. The diameter of the case can be selected according to the pressure measurement position and the needs of easy observation and management.
2. There are three types of pressure gauge bourdontubes: tin phosphor bronze bourdontubes, chrome vanadium steel bourdon tubes and stainless steel bourdon tubes. When the measured pressure is lower than 10MPa, copper is used, and when the pressure is higher than 10MPa, steel is generally used.
3. The size of the upper limit value of the pressure gauge is designed according to the conditions of the outer size, stiffness and nonlinearity of the bourdon tube. The measurement upper limit of the pressure gauge has five series, such as 1×10n, 1.6×10n, 2.5×10n, 4×10n, 6×10n, etc. n is a positive integer, negative number or zero. When selecting the upper limit of measurement, it should generally be greater than 1/3 of the maximum operating pressure, leaving a certain margin to ensure the safety and reliability of the pressure gauge and prolong its service life.
4. When selecting the operating range of the pressure gauge, it is appropriate to choose between 1/3 and 2/3 of the range of the gauge, because this operating range has high accuracy and can be used under two kinds of loads: stable and fluctuating. The pressure gauge is lower than 1/3 of the range, and the accuracy is low, so it is not suitable for use. The maximum range of use shall not exceed 3/4 of the range.5. The accuracy level of the pressure gauge reflects the degree of accuracy between the indicated value and the real value in the comparison between the tested gauge and the precision gauge. It is equal to the percentage of the absolute value of the maximum basic error and the upper limit of the measurement, which is determined according to the size of the error generated in the calibration. The pressure gauge accuracy grades are divided into 4 grades: 1, 1.6, 2.5 and 4. According to the requirements of the process, the accuracy level should be selected according to the relative allowable basic error required by the minimum value of the measured pressure. It is not allowed to one-sidedly emphasize high accuracy and cause waste. On the contrary, if the error is large, the measurement will be inaccurate.
① Accuracy is selected according to the requirements of measuring the minimum pressure relative to the basic error.
Accuracy level≤(measured pressure minimum value / measurement upper limit) × minimum allowable error value of measured pressure
Example: For a pressure gauge of 2.5MPa, the minimum value of the measured pressure is not less than 0.8MPa. To ensure that the relative allowable basic error does not exceed ±4%, the accuracy level should be selected as 1.28%, but in practice, the first level of pressure should be selected Table, because the accuracy value can only be taken small, not large.
② According to the absolute allowable basic error requirements, select the accuracy.
Accuracy level = (absolutely allowable basic error / measurement upper limit) × 100%
Example: A 1.6MPa pressure gauge can be used up to 1.1MPa, and the absolute allowable error is required to be no more than 0.024MPa. According to the calculation, a 1.6-level pressure gauge should be selected.
6. The selection of the pressure gauge should also consider the physical and chemical properties of the measured medium, so as to avoid the corrosive effect of the medium on the instrument. When used to measure special media such as viscous or acid and alkali, stainless steel bourdontube, stainless steel movement and casing should be selected.
7. When measuring the pressure of special medium, special instrument should be used. Such as oxygen meter (oxygen meter must be marked with the words "no oil" in red), ammonia meter, etc. According to the different media measured by the pressure gauge, the pressure gauge is marked with the specified color code, and the name of the measured medium is indicated, see Table 1.
|Measuring Medium||Color Code|
|Other flammable gases||Red|
|Other inert gases or liquids||Black|
Table 1: Color codes of pressure gauges corresponding to different measuring media
8. After selecting the pressure gauge, the relevant metrological verification department is also required to carry out verification, and the pressure gauge that has passed the verification can be used, and the verification period shall not exceed half a year.
Precautions for installation and use of pressure gauges
For pressure detection, a measurement system is actually required to achieve it. To achieve accurate measurement, in addition to the correct selection and verification of the instrument, attention must also be paid to the correct installation of the entire system.
1. When in use, the pressure gauge should be installed vertically in a position that is easy to observe and maintain. When installing against a wall, choose a pressure gauge with an edge; when installing directly on a pipeline, choose a pressure gauge without an edge; when directly measuring gas, choose a pressure gauge with a safety hole behind the case.
2. When the vertical distance between the installation position of the pressure gauge and the pressure measuring point is very large, the liquid column difference should be corrected. The correction value is equal to the pressure value generated by the liquid column in the conduit of the vertical height difference between the instrument and the pressure measuring point. When the meter is higher than the pressure measuring point, add this correction value; on the contrary, when the meter is lower than the pressure measuring point, this value should be subtracted.
3. The minimum distance between the installation place of the pressure gauge and the pressure measuring point should be kept to avoid the delay of the indication of the gauge. Generally, the length of the connecting pipe should not exceed 50 m, and the inner diameter of the connecting pipe should not be too small.
4. The pressure gauge should be used within the range of the measured medium and ambient temperature from -40 ℃ to 60 ℃. When the ambient temperature exceeds (20±5)℃, in addition to the basic allowable error of the instrument itself, there are additional errors of temperature. The allowable total error of the indication value should be calculated as follows:
△=±[ !+K·P·(t- t0) ]
△——The allowable total error of the indication value
!——The allowable basic error (according to the range
K——pressure temperature coefficient, K=0.04 %/℃
P——range of pressure gauge
t——ambient temperature (- 40～60℃)
t0——when t is high At 25 °C, t0 is 25 °C; when t is lower than 15 °C, t0 is 15 °C.
Example: What is the total error of the indication value of a 1.6-grade (0～10) MPa pressure gauge when the ambient temperature is 38 ℃?
Calculation: △=±[ !+K·P·(t- t0) ] = ± [ ( 1.6%×10) + 0.04%×10×( 38- 25℃) = ±0.212MPa
Therefore, when the ambient temperature of the instrument is 38℃, the allowable total error of its indication value is 0.212MPa.
5. When measuring the pressure of condensable high-temperature liquid, the pressure gauge should be equipped with a high-temperature medium protection device to ensure that the medium entering the bourdon tube of the pressure gauge is condensed liquid, so as to protect the bourdon tube from the direct action of high-temperature medium. Install a two-way valve and a U-shaped condenser or annular pipe between the pressure measuring point and the instrument.
6. When measuring the air pressure with a large number of solid fine particles, the pressure gauge should be installed in the place with the lowest concentration of fine particles for measurement, and it should be installed above the pipeline on a horizontal pipeline; on a vertical pipeline, the medium should flow downward. place to install.
7. When measuring pulsating pressure, buffer devices and dampers should be installed in front of the pressure gauge to reduce the swing of the instrument pointer, improve the service life of the instrument, and facilitate reading.
8. When measuring the pressure of corrosive medium, in order to prevent the pressure gauge from being corroded, an isolation device should be installed in front of the pressure gauge.
9. When measuring the pressure of fluid with high viscosity, a diaphragm pressure gauge with flange can be used or an isolator with a closing device can be installed in front of the pressure gauge.
10. When the pressure gauge is used in dusty occasions, a dust cover should be installed outside the gauge.
11. It is strictly forbidden to use special instruments for other purposes, and it is also strictly forbidden to measure on devices without special reliability. It is also strictly forbidden to use pressure gauges for pressure measurement of special media.
With the rapid development of industrial production science and technology, the requirements for safety in industrial sites are also getting higher and higher. Only by mastering the correct selection and installation methods of pressure gauges and precautions for use, on-site measurement personnel can eliminate the hidden dangers of safety production and truly play a role in providing the basic guarantee of measurement technology for industrial safety production.