Type introduction and maintenance of pressure gauge
What is a pressure gauge? What are the principles of pressure gauge selection? What are the precautions for using pressure gauges?
There are many types of pressure gauges, which not only have general (ordinary) pointer indicating type, but also digital type; not only regular type, but also special type; not only contact type, but also remote transmission type; not only vibration-resistant type, but also seismic resistance Type; not only diaphragm type, but also corrosion-resistant type.
The pressure gauge series is complete. It has not only conventional series, but also digital series; not only common media application series, but also special media application series; not only switching signal series, but also remote transmission signal series, etc., they are derived from practical needs and have formed Complete series. Pressure instruments are common measuring instruments widely used in various fields. It can intuitively display the pressure change of each process link, insight into the conditions in the product or medium process, form a monitoring of the safety trends in the production operation process, and build a rapid and reliable safety guarantee through automatic interlocking or sensing devices. Personal and property security played an important role.
Pressure gauges can be found everywhere in areas such as heat pipe networks, oil and gas transmission, water and gas supply systems, vehicle repair and maintenance plants and shops. Especially in the industrial process control and technical measurement process, the mechanical pressure gauge has been more and more widely used due to its high mechanical strength and convenient production due to the elastic sensitive element of the mechanical pressure gauge.
From the perspective of installation structure, there are direct installation type, embedded type and convex type, of which embedded type is divided into radial embedded type and axial embedded type, convex type also has radial convex type and axial type Protruding type. Direct installation type is divided into radial direct installation type and axial direct installation type. The radial direct installation type is the basic installation type. Generally, when the installation structure type is not specified, it refers to the radial direct installation type. The axial direct installation type considers the stability of its own support, and is generally only used on pressure gauges with a nominal diameter of less than 150mm. The so-called flush-mounted and convex-mounted pressure gauges are the pressure gauges with edges (mounting rings) that we often say. Axial embedded type means that the axial front side and radial embedded type refer to the radial front side, and the radial convex type (also called wall mounted type) refers to the radial rear side pressure gauge.
From the perspective of volume and range, the positive pressure volume is divided into micro-pressure range, low-pressure range, medium-pressure range, high-pressure range, and ultra-high-pressure range. It also subdivides several measurement ranges (meter range); in the negative pressure range (vacuum), there are three types of negative pressure (vacuum gauge); the pressure gauge of the positive pressure and the negative pressure is a cross-domain pressure table. Its standard name is pressure vacuum gauge, and it is also called vacuum pressure gauge. It can measure not only positive pressure but also negative pressure.
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 media to be measured (such as the temperature of the measured medium, viscosity, corrosion, dirtiness, flammability and explosiveness, 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), but 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 Chemical Automatic Control Design", 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 troublesome. 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.
Pressure gauge use and maintenance
1) The instrument must be vertical: a 17mm wrench should be used for tightening during installation, and the watch case should not be twisted strongly; collision should be avoided during transportation.
2) The instrument should be used at ambient temperature of -25~55℃.
3) The working environment vibration frequency is <25Hz, and the amplitude is not more than 1mm.
4) When the ambient temperature is too high during use, the indicator value of the instrument does not return to zero or the value is out of tolerance. You can cut the sealing rubber plug on the upper part of the case to make the inner cavity of the instrument open to the atmosphere.
5) The use range of the meter should be between 1/3 and 2/3 of the upper limit.
6) Isolation devices should be added to the measurement of corrosive media, media that may crystallize, and media with high viscosity.
7) The meter should be verified frequently (at least once every three months). If a fault is found, it should be repaired in time.
Note on the use of pressure gauge:
1. The meter must be vertical: a 17mm wrench should be used for tightening during installation, and the watch case should not be twisted strongly; collision should be avoided during transportation;
2. The meter should be used in the surrounding environment with a temperature of -25~55℃;
3. Use the working environment vibration frequency <25HZ, the amplitude is not greater than 1mm;
4. Because the ambient temperature is too high during use, the indicator value of the meter does not return to zero or the indication value is out of tolerance. You can cut the upper sealing rubber plug of the watch case to make the inner cavity of the meter open to the atmosphere;
5. The use range of the instrument should be between 1/3 and 2/3 of the upper limit;
6. Isolation devices should be added when measuring corrosive media, media that may crystallize, and media with high viscosity;
7. The meter should be verified frequently (at least once every three months), and if a fault is found, it should be repaired in time;
8. If the instrument is found to be invalid or damaged due to poor manufacturing quality within half a year from the date of delivery, the company is responsible for repair or replacement;
9. For instruments that measure corrosive media, the requirements should be indicated when ordering.
