Application of Intrinsically Safe Explosion proof Weighing Sensor

After the rise of the industrial revolution, various industries have developed rapidly. The weighing and measurement of petroleum, chemical, coal, arms and other industrial sectors in the process of product production, storage and transportation, and flammable and explosive substances such as hazardous gases or dust often bring huge security risks to the industry. Therefore, explosion-proof electronic scales must be selected, To ensure the life safety of staff and the property interests of enterprises to the greatest extent. This paper introduces the overview of explosion-proof load cells, the characteristics of explosion-proof load cells, precautions about explosion-proof load cells, and the application of explosion-proof load cells to strengthen people's understanding of explosion-proof load cells, so as to enhance the safety guarantee of various industrial departments in the process of selecting and using load cells

Explosion proof load cell type:

1. Intrinsically safe explosion-proof load cell

Intrinsically safe explosion-proof load cell is composed of load cell body (including elastic element, resistance strain gauge, bridge circuit and protective housing), lower pressure pad, upper pressure head and cable. It is mainly the bridge circuit part that may become the ignition source, and it is manufactured in strict accordance with GB3836.4-2000 National Standard for Intrinsically Safe Electrical Equipment. Make the maximum input voltage Ui and the maximum output voltage U0 less than 30VDC; The maximum input current Ii and maximum output current I0 are both less than 40mADC; The maximum internal capacitance Ci (including cable part) is less than 20nF; The maximum internal inductance Li ≈ 0 can avoid explosion of explosive gas mixture caused by electric spark and thermal effect.

Intrinsically safe load cells are also divided into class ia and class ib. Class ia load cell can be used in Zone 0, i.e. gas medium, with continuous hazard greater than 1000 hours/year; Class IB load cell can be used in Zone 1, i.e., gas medium, with a risk of 10~1000 hours/year.

For example, the explosion-proof sign of the spoke intrinsically safe explosion-proof weighing sensor with welded seal of upper and lower diaphragms is "Exib Ⅱ BT4", that is, the explosion-proof type is Class ib intrinsically safe, the gas grade is Class IIB (dimethyl ether, gas, cyclopropane), the ignition temperature is Class T4 (160 ℃ ＞ T ≥ 135 ℃, the maximum surface temperature is 135 ℃), and it is applicable to explosion-proof electronic scales in flammable and explosive hazardous areas Zone 1 and Zone 2. The explosion-proof measures for such intrinsically safe load cells are as follows: the design of elastic elements and upper and lower diaphragms shall meet the technical requirements of GB4942.2 National Standard of Degrees of Protection for Enclosures of Low Voltage Electrical Apparatus; Eight single shear resistance strain gauges with a resistance of 350 Ω are used, and every two resistance strain gauges are connected in series as a bridge arm to form a Wheatstone full bridge circuit. The excitation voltage of the bridge is DC 10V, and the working current is not greater than 15mA. Under the specified test conditions, the spark test is carried out on the bridge circuit, and the minimum ignition current, minimum ignition voltage, minimum ignition current ratio and the allowable current value of the conductor all meet the requirements of the intrinsically safe circuit. The load cell with the above measures can reach the intrinsically safe explosion-proof grade of ia and ib.

Minimum ignition current: the minimum current that can ignite. If the current decreases by 5%, it is the non ignition current.

Minimum ignition voltage: the minimum voltage at which ignition can occur. A 5% reduction in this voltage is the non ignition voltage.

Minimum ignition current ratio: for DC 24 V, 95 mH inductive circuit, spark ignition test is conducted, and the ratio of minimum ignition current of various gas mixtures to minimum ignition current of methane and air mixtures.

The cable of explosion-proof weighing sensor is an important part of intrinsically safe circuit system, and its performance directly affects the safety of explosion-proof electronic scale. Therefore, in addition to the conventional technical performance requirements of the weighing sensor for the cable, the cable selection must also meet the explosion-proof performance requirements, mainly including the distributed capacitance, inductance and the ratio of inductance to resistance of the cable. Since these are energy storage elements, cables with small electrical capacity and inductance should be selected as far as possible. Copper core stranded wire is generally selected, and the sectional area of each core wire is not less than 0.5mm2; The dielectric strength shall be able to withstand 2 times the rated voltage of intrinsically safe circuit, but not less than 500V withstand voltage test.

2. Explosion proof load cell

Explosion proof load cell is to install an explosion-proof enclosure outside the load cell body, so that the load cell with bridge circuit is in the inner cavity of the explosion-proof enclosure, and only one inner cavity is allowed to be set. The length and clearance of enclosure flameproof joint surface shall fully comply with GB3836.2-2000 National Standard for Flameproof Electrical Equipment.

The flameproof enclosure varies according to the structure of the elastic element of the load cell. The flameproof enclosure of the cylindrical and cylindrical load cells is of cylindrical structure; The flameproof housing of the spoke type, axisymmetric type and bending ring type load cells is a flange plate structure composed of a circular body of elastic elements and upper and lower welded diaphragm protective covers connected with the body; The explosion-proof enclosure of double hole parallel beam type, central shear beam type and cantilever shear beam type load cells is rectangular box type structure or U-shaped section long groove type structure.

The protection grade of the above enclosures shall meet the technical requirements of the Enclosure Protection Grade of Low Voltage Electrical Apparatus. All fasteners shall be provided with anti loosening measures. If the aluminum alloy shell is used, its magnesium content shall not be greater than 0.5%, the tensile strength shall not be less than 12kg/mm2, the roughness of the flameproof surface shall be better than Ra1.6, and the inner surface of the shell shall be painted (such as epoxy enamel) to extinguish the arc.

As long as the structural design and material selection of the flameproof enclosure of the load cell are reasonable, the length and gap of the flameproof joint surface meet the requirements of the national standard for flameproof electrical equipment, and the load cell can meet the level d standard for flameproof electrical equipment.

3. Increased safety load cell

Increased safety load cell refers to a load cell that will not produce arc, spark or may ignite explosive mixture under normal working conditions, and is resistant to certain high temperature (generally 60 ℃~70 ℃). In structural design and manufacturing process, measures shall be taken as far as possible to improve reliability and safety, so as to avoid electric arc, spark and other phenomena under normal and approved overload conditions. The explosion-proof sign is "Exe Ⅱ BT6", that is, the explosion-proof type is Class e increased safety type, the gas grade is Class IIB (dimethyl ether, gas, cyclopropane), and the ignition temperature is Class T6 (100 ℃ ＞ T ≥ 85 ℃, the maximum surface temperature is 85 ℃). For example, the steel double hole parallel beam load cell with 0.2mm thick stainless steel bellows welded and sealed can meet the explosion-proof requirements of Class e increased safety type. The explosion-proof measures to be taken are as follows: the outgoing conductor shall be compressed with a clamp, and the cable connector shall be a sealing ring type anti loose clamp to prevent looseness and falling off, so as to ensure that the cable is firm and sealed, and the gland and the sealing ring shall be used to compress back and forth. The sealing ring shall be made of silicone rubber, and its hardness shall meet the requirements of Shore 45-55. Sealing and clamping structures of external cables can only be used after passing the clamping effect test, rubber aging test and damp heat test.