Selection of Pressure Instruments【Forward】

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This post was last edited by RSchufaqi on 2017-9-27 08:25. 1. Selection of pressure gauges (1) Choose based on the operating environment and the properties of the medium to be measured. 1) In environments with high levels of atmospheric corrosion, a lot of dust, or exposure to splashing liquids, it is advisable to use sealed all-plastic pressure gauges. 2) For dilute nitric acid, acetic acid, ammonia, and other generally corrosive media, acid-resistant pressure gauges, ammonia pressure gauges, or stainless steel diaphragm pressure gauges should be used. 3) For media such as dilute hydrochloric acid, hydrogen chloride gas, heavy oils, and similar substances that are highly corrosive, contain solid particles, or are viscous liquids, diaphragm pressure gauges should be used. The material of its diaphragm must be selected based on the properties of the medium being measured. 4) For media such as those with crystallization, scarring, and high viscosity, diaphragm pressure gauges should be used. 5) In environments with strong mechanical vibrations, shock-resistant pressure gauges or marine pressure gauges should be used. 6) In flammable and explosive environments, where electrical contact signals are required, explosion-proof electrical contact pressure gauges should be used. 7) Specialized pressure gauges should be used for the following measurement media: ① Gaseous ammonia, liquid ammonia: Ammonia pressure gauges, vacuum gauges, pressure-vacuum gauges ； ②Oxygen: Oxygen pressure gauge ； ③Hydrogen: Hydrogen pressure gauge ； ④Chlorine: Chlorine-resistant pressure gauges, pressure-vacuum gauges ； ⑤Acetylene: Acetylene pressure gauge ； ⑥Hydrogen sulfide: sulfur-resistant pressure gauge ； ⑦Alkaline solutions: Alkali-resistant pressure gauges, pressure and vacuum gauges. (2) Selection of accuracy class: 1) For pressure gauges, diaphragm box pressure gauges, and diaphragm pressure gauges used for general measurements, a class of 1.5 or 2.5 should be selected. 2) Pressure gauges for precise measurement and calibration should be selected of class 0.4, class 0.25, or class 0.16. (3) Selection of external dimensions 1) The pressure gauges installed on pipes and equipment have a nominal diameter of φ100mm or φ150mm. 2) The pressure gauges installed on the instrument pneumatic pipelines and their auxiliary equipment have a nominal diameter of φ60mm. 3) Pressure gauges installed in areas with low illumination, at high elevations, or where it is difficult to observe the readings, have a nominal diameter of φ200mm or φ250mm. (4) Selection of measurement range 1) When measuring a stable pressure, the normal operating pressure value should be between 2/3 and 1/3 of the upper limit of the instrument’s measurement range. 2) When measuring pulsating pressure (such as the pressure at the outlet of pumps, compressors, and fans), the normal operating pressure value should be between 1/2 and 1/3 of the upper limit of the instrument’s measurement range. 3) When measuring high and medium pressures (greater than 4 MPa), the normal operating pressure value should not exceed 1/2 of the upper limit of the instrument’s measurement range. (5) Units and scales 1) Pressure instruments must use legally recognized measurement units. That is: Pa, kPa, and MPa. 2) For foreign-related design projects and the introduction of instruments, internationally recognized standards or corresponding **standards can be adopted. 2. Selection of transmitters and sensors (1) When transmitting with a standard signal (4–20 mA), a transmitter should be selected. (2) In flammable and explosive environments, pneumatic transmitters or explosion-proof electric transmitters should be used. (3) For crystalline, scaly, clogged, viscous, and corrosive media, flanged transmitters should be selected. The material in direct contact with the medium must be selected based on the properties of that medium. (4) In applications with favorable operating conditions and low requirements for measurement accuracy and reliability, resistive or inductive remote pressure gauges, or Hall pressure transducers can be used. (5) When measuring very low pressures (less than 500 Pa), a differential pressure transmitter can be used. 3. Selection of installation accessories (1) When measuring water vapor and media with a temperature above 60°C, spiral or U-shaped elbows should be used. (2) When measuring easily liquefiable gases, if the pressure tapping point is above the instrument, a separator should be used. (3) When measuring dust-containing gases, a dust collector should be used. (4) When measuring pulsating pressure, a damper or buffer should be used. (5) When the operating ambient temperature is close to or below the freezing point or solidification point of the measured medium, insulation or heating measures should be taken. (6) Instrument protection (temperature) boxes should be used in the following situations. 1) Pressure switches and transmitters installed outdoors. 2) Pressure switches and transmitters installed in factories with severe atmospheric corrosion, high dust levels, and other harmful substances.