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​Learn more about pressure!
Nov 28, 2018


Learn more about pressure!



 

First, the feeling of pressure


         pressure is one of the five parameters in our measurement. Do you understand some basic knowledge of pressure? In the petrochemical production process, some equipment pressure is several hundred times higher than atmospheric pressure, while some equipment pressure is lower than atmospheric pressure. For example, the pressure in the ammonia production process of the fertilizer plant is as high as 32 MPa, and hydrogen and nitrogen can be synthesized into ammonia only under such pressure conditions. The magnitude of the pressure is determined by the force acting perpendicular to the unit area. The legal unit of measurement for pressure on the project is Pa, kPa, MPa, and its illegal unit of measurement is kgf/cm2, mmH20, mmHg, and so on. Pressure measurement is divided into gauge pressure, absolute pressure, multiple shifts or vacuum. The corresponding instruments for detecting pressure in the industry include pressure gauges, vacuum gauges, and pressure vacuum gauges. Most of the instruments that detect pressure on the project are gauge pressure. However, in the engineering adjustment and calculation of the instrument, most of the absolute pressure is used. According to the actual situation, it should be clear what pressure is. Gauge pressure is the difference between absolute pressure and atmospheric pressure, that is, the absolute pressure is equal to the gauge pressure plus atmospheric pressure. Engineering often encounters the measurement of vacuum, that is, negative pressure, which is lower than atmospheric pressure.


Here is a brief introduction to the difference between absolute pressure and gauge pressure.

 Absolute pressure (PJ): refers to the total pressure acting on the surface area of the object. The zero point is based on absolute vacuum, also known as total pressure or full pressure, and the symbol is PABS.

Atmospheric pressure (PD): The earth's surface is covered with a thick atmosphere of air. The objects in the atmosphere are subject to the pressure generated by the impact of air molecules. This pressure is called atmospheric pressure. One standard atmospheric pressure = 0.1 MPa = 1 bar.

Gauge pressure (PB): refers to the total pressure acting on the surface area of an object. The zero point is referenced to the local atmospheric pressure.

Three relationships: PB=PJ-PD

For ordinary industrial pressure gauges, the gauge pressure value is measured, that is, the pressure difference between absolute pressure and atmospheric pressure.

Vacuum degree: When the absolute pressure is lower than the local atmospheric pressure, the gauge pressure is negative pressure, and the negative pressure can be expressed by vacuum degree. The absolute value of negative pressure is called vacuum degree.

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The pressure measuring instruments are divided into the following four categories according to their working principles.

(1) The liquid column pressure gauge converts the measured pressure into a liquid column height difference for measurement.

(2) The elastic pressure gauge measures the displacement of the measured pressure into the elastic deformation of the elastic member.

(3) The electric pressure gauge converts the measured pressure into various electric quantities for measurement.

(4) The plug gauge is used to measure the measured pressure into the weight of the balance weight applied to the piston.

The accuracy grade of the pressure gauge is divided into two types: standard pressure gauge and work pressure gauge. The standard pressure gauge is divided into 0.25, 0.4, and 0.6, and the working pressure gauge is divided into 1.0, 1.5, 2.5, and 4.0.


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Second, the conversion of pressure units


The legal unit of measurement for pressure is Pascal, the symbol is Pa, and the conversion of statutory and non-statutory pressure measurement units is shown in Table 12-12.



Table 12-12 Pressure unit conversion

Pressure unitAtmAtMillimetre mercuryMillimeter water columnPascal
Standard atmospheric pressureEngineering atmospheric pressuremmHgmmH20Pa
mmHgkgfem2


Standard atmospheric pressure11.033760103361.013×105
Engineering atmospheric pressure0.9681735.56100009.80665×104

Millimetre mercury

0.001321.36×10—3113.6133.322
Millimeter water column9.67×10-59.9963×10-57.353×l0-219.80665
Pascal9.869×10—61.0197×10-S7.5006×lO-S0.101971



Third, the selection, calibration, installation of pressure gauge


(1) Principles of selection of pressure gauges

1) Requirements for pressure measurement in the production process. The pressure of the process is high and the accuracy of the measurement is required. Especially for places with explosion hazard, the range of pressure measurement should be strictly selected and whether additional equipment is required.

2) The nature of the measured medium. For example, the temperature of the measured medium, the viscosity, the corrosiveness, the dirtyness, the dirty condition, whether it is flammable or explosive.

3) Environmental conditions. For example, high temperature, corrosion, humidity, vibration, and the like. In addition, for the elastic pressure gauge, in order to ensure the reliable operation of the pressure gauge in the safe range of the elastic deformation, it is necessary to leave sufficient range for the selection of the pressure gauge to prevent the elastic element from being damaged during the elastic deformation. The following rules should be observed when selecting the pressure gauge: the maximum pressure value should not exceed 3/4 of the full scale; when the measured pressure fluctuates greatly, the maximum pressure value should not exceed 2/3 of the full scale; to ensure Measurement accuracy, the minimum value of the measured pressure should not be lower than 1/3 of the full scale

(2) Reasons for the error in the checksum of the pressure gauge

The pressure gauge calibration content is indication error, variation, and linearity. In addition, there are adjustments to zero, end point, and nonlinearity. The standard instruments required for pressure gauge adjustment are piston pressure gauges and standard pressure gauges of class 0.5 or higher. Digital pressure gauges are also required to verify other pressure gauges.

The main cause of the error in the elastic pressure gauge is the change in the mass of the elastic member, the friction, wear, deformation, and clearance of the transmission-amplifying mechanism.

1) Elastic hysteresis of the component When the measured pressure returns to the original value, the deformation cannot be completely restored to the original value, as shown in Figure 12-19. This elastic hysteresis is similar to hysteresis.

2) Elastic decay of the component During the use of the pressure gauge, the indication error gradually increases. The reason for the indication error is that the elastic component is of poor quality during the heat treatment. This error can be adjusted in the periodic verification.

3) Temperature influence of components When the higher measured medium is in direct contact with the elastic element or is affected by the thermal radiation of other equipment, the indication value of the elastic element is high. The reason for the elastic deformation is that, in addition to the internal pressure of the component material, the elastic modulus of the metal material changes in the opposite direction to the temperature, that is, the thermometer has a high modulus of elasticity. This results in a decrease in the indication value during the detection process. In addition, the position of the pressure tapping point and the installation of the pressure guiding tube do not meet the specified standards, which will bring measurement errors.

The basic requirements for pressure gauge installation are as follows.

1) The pressure measuring point selects the correct pressure measuring point according to the demand of the measured medium, inserts into the inner end surface of the pressure measuring tube of the production equipment, and the inner wall of the contact with the process equipment should be flush, no protrusions or burrs are allowed, so as to avoid Affect the correct acquisition of static pressure.

2) The choice of installation location avoids the effects of vibration and high temperatures.

3) Anti-temperature, anti-corrosion, anti-freeze and anti-blocking measures. For example, when detecting steam pressure, condensate crab should be installed to prevent high-temperature steam from directly contacting the load cell and damaging the load cell. For corrosive measured media, Isolation tanks filled with neutral media should be installed. Generally speaking, for different measured media, such as high temperature, low temperature, corrosion, dirt, crystallization, sedimentation, viscosity, etc., the pressure measuring device should take corresponding protective measures.

4) When it is convenient to install the pressure gauge, it should be considered that the inspection is convenient in the future. A pressure shut-off valve should be installed between the pressure pressure port and the pressure gauge. The shut-off valve is installed near the pressure tap. If it is necessary to adjust the field, the measured medium is dirty, and the shut-off valve is changed to a three-way valve.

5) Installation diagram The pressure piping should not be too long, otherwise the measurement will be delayed. The installation of the pressure gauge is shown in Figure 12-20. In the case shown in (c), the pressure is higher than the actual pressure in the pipe. The reading should be subtracted from the pressure gauge to the pipe pressure Vl. The static pressure of this liquid column.

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