Four factors of pressure gauge selection
The selection of the pressure gauge should be based on the pressure measurement requirements of the process production process, combined with other aspects of the situation, to comprehensive consideration and specific analysis, generally consider the following issues: the choice of instrument type, the determination of the measurement range of the instrument, the instrument The selection of the accuracy level and the consideration of the use environment and media performance.
The range selection determines the meter range based on the magnitude of the measured pressure. For flexible pressure gauges, when measuring stable pressure, the maximum pressure value should not exceed 3/4 of full scale. When measuring fluctuation pressure, the maximum pressure value should not exceed 2/3 of full scale, and the lowest measured pressure value should not be low. In the full range of 1/3.
Accuracy selection, according to the maximum measurement error allowed by production, the accuracy level of the instrument is determined by economic and practical principles. General industrial pressure gauges of grade 1.5 or 2.5 are sufficient.
Use environment and media performance considerations
Environmental and medium performance considerations, such as high temperature, corrosion, humidity, vibration, etc.
The performance of the measured medium, such as temperature, corrosion, crystallization, flammability, explosive and so on. In this way, the type and model of the pressure gauge are determined.
Selection of the shape of the pressure gauge
Selection of the shape of the pressure gauge: The pressure gauge indicated on site is generally 100mm in diameter. In the case of high standard or poor lighting conditions, the surface diameter is 200~250mm, and the diameter of the panel is 150mm.
Pressure gauge installation three major considerations
The selection of the pressure measuring point should reflect the true size of the measured pressure.
1 Select the section of the pipe where the measured medium flows straight, and do not choose where the pipe turns, forks, dead ends or other places where vortices are easy to form.
2 When measuring the pressure of the flowing medium, the pressure point should be perpendicular to the flow direction. The inner wall of the end face of the pressure pipe should be flush with the inner wall of the production equipment, and there should be no protrusions or burrs.
3 When measuring the liquid (gas) body pressure, the pressure point should be below the pipe (upper part) so that no gas (liquid) body is accumulated in the pressure guiding pipe.
Pressure guiding pipe 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 50m to reduce the delay of pressure indication. If it exceeds 50m, a pressure gauge that can be transported over long distances should be used.
2 The horizontal installation of the pressure guiding tube should be guaranteed to have a slope of 1:10 to 1:20 to facilitate the discharge of liquid (or gas) accumulated therein.
3 When the measured medium is condensed or frozen, an insulated heat tracing line must be added.
4 A shut-off valve should be installed between the pressure port and the pressure gauge for use in maintenance pressure monitoring. The shut-off valve should be installed close to the pressure tap.
Pressure gauge installation
1 The pressure gauge should be installed in an area that is easy to observe and repair.
2 Installation location should be designed to avoid vibration and high temperature effects.
3 When measuring the steam pressure, a condensate tube should be installed to prevent the high temperature steam from directly contacting the load cell.
[Fig. 3-17(a)]; for the pressure measurement of corrosive medium, an isolation tank with a neutral medium should be added, and the figure (b) below shows that the density ρ2 of the measured medium is greater than and less than the density ρ1 of the spacer. Two situations.
1—pressure gauge; 2—cut valve; 3—condensation tube; 4—pressure vessel;
4 The connection of the pressure gauge should be based on the level of pressure to be measured and the nature of the medium, and the appropriate material should be selected as a gasket to prevent leakage.
5 When the measured pressure is small and the pressure gauge and the pressure tap are not at the same height, the measurement error caused by this height should be corrected by Δp=±Hρg. Where H is the height difference, ρ is the density of the medium in the pressure guiding tube, and g is the acceleration of gravity.
6 For safety reasons, in addition to venting holes, the pressure gauge for measuring high pressure should be placed on the wall or unmanned when installed to prevent accidents.
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