Pressure gauge: An instrument used to measure atmospheric pressure less than or greater than atmospheric pressure.
Pressure: actually refers to the physical pressure, that is, the amount of pressure on a unit area.
Absolute pressure: The pressure that is higher than absolute pressure zero based on absolute pressure zero.
Positive pressure: The pressure higher than the atmospheric pressure, based on the atmospheric pressure.
Negative pressure (vacuum): based on atmospheric pressure, the pressure below atmospheric pressure.
Differential pressure: The difference between two pressures.
Gauge pressure: based on atmospheric pressure, greater than or less than atmospheric pressure.
1, indicating error
Within the measuring range, the error of the indicating value shall not be greater than the allowable error specified.
2. Return error
Within the measurement range, the return error shall not be greater than the absolute value of the allowable error.
3, tap displacement
After tapping the case, the change of the pointer value should not be greater than 1/2 of the allowable absolute value of the error.
4, pointer deflection stationarity
In the measuring range, the pointer deflection should be stable, no jumping and jamming phenomenon.
Two, pressure gauge accuracy level knowledge introduction
Pressure gauge accuracy grade is short for pressure gauge accuracy grade or accuracy grade, and its applicable standards are
GB/T1226-2001 "General pressure gauge", and JJG52-1999 "spring tube type general pressure gauge, pressure vacuum gauge and vacuum gauge".
The accuracy level of the pressure gauge is expressed as a percentage of the allowable error in the gauge range, and is generally divided into
0.5, 1, 1.5, 2, 2.5, 3, 4 seven grades (no level 3 and 4 on the boiler), the smaller the value, the higher the accuracy. For example, the dial range of 0~ 2.5MPa precision 2.5mpa pressure gauge, its pointer pressure value and the actual pressure value of the measured medium allowed error, not more than 2.5mpax 2.5%=± 0.0625MPa: when the pressure gauge indicates 0.8MPa, The actual gas size is between 0.7375~0.862 5MPa.
It can be seen that the actual error of the pressure gauge is not only related to the accuracy, but also related to the size of the range of the pressure gauge.
At the same time, the higher the accuracy (that is, the smaller the number), the smaller the allowable error of the pressure gauge. With the same accuracy, the greater the range, the greater the error of the pressure gauge.
Nominal diameter of housing (mm accuracy class
40; 60 2.5; 4.0
10. 1.6; 2.5
150; 200; 250 1.0; 1.6
Note: The 1.5 pressure gauge in use allows the error to be calculated according to the 1.6 level, and the accuracy level can not be changed.
Third, the expression and classification of pressure
There are two ways to express pressure: one is the pressure expressed by absolute vacuum as a benchmark, which is called absolute pressure; The other is the pressure expressed by atmospheric pressure as a benchmark, which is called relative pressure. Since the pressure measured by most pressure gauges is relative pressure, the relative pressure is also called gauge pressure. When the absolute pressure is less than the atmospheric pressure, the absolute pressure in the container can be expressed by the value of less than one atmosphere. It is called "vacuum degree". Their relationship is as follows:
Absolute pressure = atmospheric pressure + relative pressure
Vacuum = atmospheric calorific force - absolute bed force
China's legal pressure unit is Pa(N/㎡), called PASCAL, referred to as Pa. Because this unit is too small, it is often used in 10^6 times the unit MPa(mpa).
Pressure gauge according to its measuring accuracy, can be divided into precision pressure gauge, general pressure gauge. Precision pressure gauge measurement accuracy levels are 0.1, 0.16, 0.25, 0.4: General pressure gauge measurement accuracy levels are 1.0, 1.6, 2.5, 4.0.
Pressure gauges can be classified in a variety of ways according to the benchmark of their indicated pressure:
1, according to the measurement accuracy of pressure gauge, can be divided into precision pressure gauge (GB/T1227) and general pressure gauge (GB/T1226);
2, according to the pressure gauge pressure benchmark is different, divided into general pressure gauge, absolute pressure gauge, differential pressure gauge;
3. According to the measuring range of the pressure gauge, it is divided into vacuum meter, pressure vacuum meter, micro pressure meter, low pressure meter, medium pressure meter and high pressure meter;
4, according to the composition of pressure gauge, divided into liquid column type, electronic type and mechanical type.
Fourth, the structure and working principle of pressure gauge
The open-type spring tube pressure gauge is composed of five main parts, such as the case part, the pointer, the dial part, the spring bend, the transmission mechanism part and the pipe joint. When the spring tube is subjected to the medium pressure, its active end is stretched outward, the transmission mechanism drives the pointer to rotate, and the pressure of the medium is indicated on the dial
The sealed spring pressure gauge is also composed of six main parts, such as the shell part, pointer, dial, spring tube, bend tube and transmission room moving mechanism, etc. The inner cavity of the spring tube is a closed form, the external pressure acts on the outside of the spring tube to deform the spring tube, and the transmission mechanism drives the pointer to rotate indicating the environmental pressure.
The number of amps in the circle below the dial of the pressure gauge is the precision grade mark of the pressure gauge. Indicates that the basic error of the table does not exceed a percentage of full scale. The smaller the number, the higher the precision.
The pressure value in the measuring range is displayed by the pointer, and the indicating range of the dial is generally made of 270 degrees.
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5. Installation and setting of pressure gauge
(1) It should be set in a place that is easy to observe and clean, and should avoid being affected by adverse factors such as radiant heat, freezing or vibration;
(2) The pressure gauge should be installed vertically, and when the installation position is higher, it can be slightly tilted forward for easy observation, but the tilt Angle shall not exceed 30 degrees;
(3) In one case, the pressure gauge should be equipped with a buffer bend to ease the compressed air directly into the spring bend, but also can accumulate condensate;
(4) A three-way cock or needle valve should be installed between the pressure gauge and the buffer bend to replace and verify the pressure gauge.
Six, pressure gauge selection and use of precautions.
Pressure instrument is a common measuring instrument, widely used in various fields. It can intuitively show the pressure changes in each process, insight into the formation of conditions in the product or medium process, monitor the safety trend in the production and operation process, and through the automatic chain or sensing device, build a rapid and reliable safety guarantee, to prevent accidents, protect personal and property safety has played an important role, known as the "eye" of safety.
The maximum range of the pressure gauge installed on the boiler and pressure vessel (the scale limit value on the dial) should be adapted to the working pressure of the equipment. The measuring range of the pressure gauge is generally 1.5 to 3 times the working pressure of the equipment, and it is best to take 2 times. If the selected pressure gauge range is too large, due to the same precision of the pressure gauge, 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; Conversely, if the selected pressure. If the gauge range is too small and the working pressure of the equipment is equal to or close to the scale limit of the pressure gauge, the elastic element in the pressure gauge will be in the maximum deformation state for a long time, which is easy to produce permanent deformation, causing the error of the pressure gauge to increase and the service life to decrease. In addition, the measuring range of the pressure gauge is too small, the overpressure operation, the pointer over the maximum measuring range is close to zero, and the operator has an illusion, resulting in greater accidents. Therefore, the pressure range of the pressure gauge should not exceed 60 to 70% of the scale limit.
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Pressure unit conversion table
Seven, commonly used pressure units and conversion
PASCAL Pa(N/m2); Kilogram force per square centimeter Kgf/cm2; bar;
mbar mbar; Standard atmospheric pressure atm; Tortorr; inH2O:
Mm of mercury mmHg; Pounds per square inch PSI(lb/in2)
1Mpa≈10Kg/cm 1Bar1Kq/cm
1Mpa=1000Kpa 1Bar=100Kpa
1Mpa=10000mBar 1Bar=1000mBar
1Mpa=7500mmHg 1Bar=750mmHg
In order to facilitate memory, it can be simplified as follows:
1atm=0.1MPa=100KPa= 1kg =1bar=10 meters of water column =14.5PSI
1KPa=0.01 kg =0.01bar
10 mbar = = = 0.3 inHg = 7.5 7.5 mmHg torr
=100mmH2O=4inH2O
Eight, spring tube pressure gauge and installation points
At present, the most widely used local pressure gauge in the production site is the mechanical pressure gauge, the mechanical pressure gauge according to the different elastic sensitive elements are mainly divided into spring tube (bourdon tube) pressure gauge, diaphragm pressure gauge, diaphragm pressure gauge and bellows pressure gauge, this paper mainly takes the spring tube pressure gauge as an example.
The spring tube pressure gauge has many varieties and specifications, and has the characteristics of simple structure, convenient use, high precision, durable and cheap, so it is widely used in various pressure measurement occasions in industrial production. Spring tube pressure gauge is also called wave tube pressure gauge, which is divided into C-type tube, coil spring tube, spiral tube and other types. Its structure is shown in the figure.
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Spring tube pressure gauge structure diagram
The elastic deformation of the spring tube under the pressure of the medium in the inner cavity is transformed into the elastic displacement of the free end of the spring tube by using its elastic characteristics, and then amplified by the gear transmission mechanism of the pressure gauge, the pressure gauge will show the relative value of the atmospheric pressure (or high or low). The pressure value in the measuring range is displayed by the pointer, and the measuring range of the spring tube is generally 0.1MPa ~ 250MPa.
For the installation of pressure gauges should pay attention to the following matters:
1. The installation position of the pressure source component should be selected in a place where the medium flow rate is stable.
2. When measuring the pressure of turbidity media such as dust, solid particles or sediment, the source parts should be mounted upward at an Angle.
3. When the pressure source component is installed on the horizontal and inclined process pipeline, the orientation of the pressure inlet shall comply with the following provisions:
(1) When measuring gas pressure, in the upper half of the process pipe.
(2) When measuring liquid pressure, the lower half of the process pipe is within the range of 0 ~ 45 degrees Angle between the horizontal center line of the process pipe.
(3) When measuring steam pressure, the upper and lower half of the process pipe and the horizontal center line of the process pipe are within the range of 0 ~ 45 degrees.
Nine, pressure gauge verification cycle
Pressure gauges are divided into mandatory verification and non-mandatory verification, not all pressure gauges are mandatory verification. In 2019, the General Administration of Market Supervision issued the "Announcement of the Catalogue of Measuring Instruments for Compulsory Management" (No. 48 of 2019), and the 21st and 22nd pressure gauges for safety protection in the catalogue are shown in the figure that require compulsory inspection
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Pressure gauges that require mandatory inspection
For non-compulsory verification of pressure gauges, it is also necessary to regularly verify, but the verification cycle is determined by the enterprise itself.
For chemical enterprises, on October 10, 1988, the Ministry of Chemical Industry and the State Bureau of Technical Supervision jointly issued the "Measures for the Management of Chemical Industry Measuring Instruments Classification" (trial), and the measuring instruments are managed according to the three levels of A, B and C. In accordance with this method, the appendix "Chemical Production Enterprise Measuring Instruments Classification Management Catalogue" in the chemical enterprise pressure gauge classification and verification cycle is as follows:
Grade A: Pressure gauge for safety protection, verified at least once every six months.
Grade B: Pressure gauge used for process control, the verification period is the maintenance period of the device.
Grade C: Pressure gauge for monitoring, verification period of 1-2 years; Monitoring pressure gauge, do not need periodic verification, once the failure is replaced.
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Pressure gauge is important, but it is not universal, only the pressure displayed by the pressure gauge as a judge of the pressure of equipment, pipelines, etc., is not safe. On September 17, 2023, the accident of 10 dead and 3 injured in Ordos Yiding Ecological Agriculture Development Co., Ltd. is a typical accident case. Through this accident, it also reminds us that we need to strengthen the daily maintenance and regular testing of monitoring and measuring instruments such as pressure, liquid level and temperature to ensure that they are in good condition, which is also one of the important contents of the integrity of the equipment.