Selection principle of pressure gauge
The selection of pressure gauge should be based on the production requirements of the process, and make a specific analysis for the specific situation. Under the premise of meeting the process requirements, it should be considered comprehensively and comprehensively in accordance with the principle of economy. Generally, the following aspects should be considered:
1. Selection of type
The selection of instrument type must meet the requirements of process production. For example, do you need remote transmission,
Automatically record or alarm; whether the nature of the medium to be measured (such as the temperature of the measured medium, the size of the viscosity, the corrosiveness, the degree of dirt, whether it is flammable or explosive, etc.) has any special requirements for the instrument, and the on-site environmental conditions (such as humidity, humidity, Temperature, magnetic field strength, vibration, etc.) requirements for the type of instrument, etc. Therefore, the correct selection of the instrument type according to the process requirements is an important prerequisite for ensuring the normal operation of the instrument and safe production.
For example, the spring tube of the common pressure gauge mostly uses copper alloy (high pressure uses alloy steel), while the material of the spring tube of the ammonia pressure gauge uses carbon steel (or stainless steel), and copper alloy is not allowed. Because ammonia reacts with copper chemically and will explode, ordinary pressure gauges cannot be used for ammonia pressure measurement.
Oxygen pressure gauges can be identical to ordinary pressure gauges in structure and material, except that oxygen pressure gauges must be oil-free. Because oil enters the human oxygen system, it is easy to cause an explosion. When the oxygen pressure gauge used is calibrated, oil cannot be used as the working medium like an ordinary pressure gauge, and the oxygen pressure gauge should strictly avoid contact with oil pollution during storage. If you must use an existing oil gauge to measure the oxygen pressure, you must repeatedly clean with carbon tetrachloride before use, and carefully check until there is no oil pollution.
2. Determination of the measurement range
In order to ensure that the elastic element can work reliably within the safe range of elastic deformation, the pressure gauge is selected
During the process, there must be enough room according to the measured pressure and the speed of the pressure change. Therefore, the upper limit of the pressure gauge should be higher than the maximum pressure value possible in the process of production. According to the "Technical Regulations for Automatic Design of Chemical Industry", when measuring stable pressure, the maximum working pressure should not exceed 2/3 of the upper measurement limit; when measuring pulsating pressure, the maximum working pressure should not exceed 1/2 of the upper measurement limit; When measuring high pressure, the maximum working pressure should not exceed 3/5 of the upper measurement limit. Generally, the minimum value of the measured pressure should not be less than 1/3 of the upper limit value of the instrument. So as to ensure the linear relationship between the output of the instrument and the input volume. Improve the accuracy and sensitivity of the measurement results of the instrument.
After calculating the upper and lower limits of the instrument based on the maximum and minimum values of the measured parameters, this value cannot be used directly as the measurement range of the instrument. When we choose the upper limit of the scale of the instrument, we should choose it from the standard series prescribed by the state. China's pressure gauge measurement range standard series are: -0.1-0.06,0.15;
0-1,1.6,2.5,4,6,10X10"MPa (where n is a natural integer. It can be positive or negative).
3. Selection of accuracy grade
According to the maximum absolute error allowed by the production process and the selected maximum range of the instrument, calculate the maximum allowable error of the instrument, and determine the accuracy of the instrument in the accuracy level specified by the country. Generally speaking, the more precise the selected instrument, the more accurate and reliable the measurement result. However, it cannot be considered that the higher the accuracy of the selected instrument, the better, because the more precise instruments are generally more expensive, and the operation and maintenance are more complicated. Therefore, under the premise of meeting the technological requirements, it is necessary to select instruments with low accuracy, low cost and durability.
Selection examples:
1. When used for measuring special media such as viscous or acid-base, diaphragm pressure gauge, stainless steel spring tube, stainless steel movement, stainless steel shell or bakelite shell should be used.
According to the measured medium, the pressure gauge should have the specified color code and indicate the name of the special medium. The oxygen gauge must be marked with the word "oil-free" in red. The dark green underline color code for hydrogen and ammonia Yellow underline color coding and so on.
2. When installing against the wall, a pressure gauge with an edge should be selected; when directly installed on the pipeline, a pressure gauge without an edge should be selected; when used for direct gas measurement, a pressure gauge with a safety hole behind the case should be selected. For consideration of pressure measurement position and easy observation and management, the diameter of the case should be selected.
1) For a pressure gauge whose range is installed on a boiler or a pressure vessel, the maximum range (limit value of the scale on the dial) should be compatible with the working pressure of the equipment. The range of the pressure gauge is generally 1.5-3 times the working pressure of the equipment, preferably 2 times. If the range of the selected pressure gauge is too large, due to the pressure gauge of the same accuracy, the larger the range, the greater the absolute value of the allowable error and the deviation of the naked eye observation, which will affect the accuracy of the pressure reading; otherwise, if the pressure gauge is selected If the measuring range is too small, the working pressure of the equipment is equal to or close to the scale limit of the pressure gauge, which will cause the elastic element in the pressure gauge to be in the maximum deformation state for a long time, and it is easy to produce permanent deformation, causing the error of the pressure gauge to increase and the service life to decrease. In addition, the range of the pressure gauge is too small. In case of overpressure operation, the pointer crosses the maximum range and approaches zero, which causes the operator to have an illusion and cause a greater accident. Therefore, the pressure range of the pressure gauge should not exceed 60~70% of the scale limit.
2) The accuracy of the working pressure gauge is expressed as a percentage of the allowable error in the limit value of the dial scale. The accuracy level is generally marked on the dial. When selecting a pressure gauge, the accuracy should be determined according to the pressure level of the equipment and the actual work needs.
3) Dial diameter In order to enable the operator to accurately see the pressure value, the dial diameter of the pressure gauge should not be too small. If the pressure gauge is installed higher or farther from the post, the dial diameter should be increased.
4) If the medium used for pressure gauge measurement is corrosive, different elastic element materials must be selected according to the specific temperature, concentration and other parameters of the corrosive medium, otherwise the intended purpose cannot be achieved.
5) Emphasis on daily use and maintenance, regular inspection, cleaning and record of usage.
6) The general verification period for pressure gauges is half a year. Compulsory verification is a legal measure to ensure the reliable technical performance of pressure gauges, accurate transfer of values, and effective guarantee of safe production.