Most people don't know how to select the material

2022-10-04
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Don't know how to select the material level instrument? It is enough to read this article

instruments play an important role in detecting, displaying, recording or controlling process parameters in the process of industrial production. The detection of technological production process is the basic means to understand and control industrial production. Only by accurately understanding the overall picture of technological production process at any time and controlling it, can we ensure the smooth production process and produce qualified products with high productivity and small consumption

type selection of level instruments

(I) general principles:

(1) we should deeply understand the process conditions, the nature of the measured medium, and the requirements of the measurement and control system, so as to fully evaluate the technical performance and economic effect of the instrument, so that it can play its due role in ensuring the stability of production, improving product quality, and increasing economic benefits

(2) differential pressure instrument, float type instrument and float type instrument shall be selected for liquid level and boundary volume surface measurement. When the requirements are not met, electric, resistive (electric contact), acoustic and other instruments can be selected to reduce the weight of the whole vehicle by 35% compared with 2015

the material level measurement should be selected according to the particle size of the material, the repose angle of the material, the conductivity of the material, the structural form of the silo and the measurement requirements

(3) the structure and material of the instrument should be selected according to the characteristics of the measured medium. The main factors considered are pressure, temperature, corrosion and conductivity; Whether there are polymerization, viscosity, precipitation, crystallization, conjunctiva, gasification, foaming and other phenomena; Density and density change; The amount of suspended solids in the liquid; The degree of liquid level disturbance and the particle size of solid materials

(4) the display mode and function of the instrument shall be determined according to the requirements of process operation and system composition. When signal transmission is required, instruments with analog signal output function or digital signal output function can be selected

(5) the instrument range shall be determined according to the actual display range or actual change range of the process object. Except for the level instrument used for volume measurement, the normal level should generally be about 50% of the instrument range

(6) the accuracy of the instrument should be selected according to the process requirements, but the accuracy grade of the level instrument for volume measurement should be above 0.5

(7) electronic level instruments used in explosive places such as combustible gases, vapors and combustible dust. The appropriate explosion-proof structure type or other protective measures shall be selected according to the determined dangerous place category and the dangerous degree of the measured medium

(8) for electronic level instruments used in places such as corrosive gases and harmful dust, appropriate enclosure protection type shall be selected according to the environmental conditions

(II) type selection of liquid level and interface measuring instruments

1. Differential pressure measuring instruments

(1) for continuous liquid level measurement, differential pressure instruments should be selected

for interface measurement, differential pressure instrument can be selected, but the total liquid level should always be higher than the upper pressure tap

(2) for high measurement accuracy requirements, the measurement system requires more complex accurate calculation, and when it is difficult for general analog instruments to achieve, the differential pressure intelligent transmission instrument can be selected, with an accuracy of more than 0.2

(3) when the liquid density changes significantly under normal working conditions, the differential pressure instrument should not be selected

(4) flat flange differential pressure instrument should be selected for corrosive liquid, crystalline liquid, viscous liquid, vaporizable liquid and liquid containing suspended solids

plug in flange differential pressure instrument should be selected for high crystalline liquid, high viscosity liquid, gelling liquid and precipitable liquid

for the liquid level of the measured medium above, if there is a large amount of condensate and sediment precipitation in the gas phase, or it is necessary to isolate the high-temperature liquid from the transmitter, or when replacing the measured medium, the measuring head needs to be strictly purified, the double flange differential pressure instrument can be selected

(5) when it is difficult to measure the liquid level of corrosive liquid, viscous liquid, crystalline liquid, molten liquid and sedimentary liquid with flange type differential pressure instrument, the method of blowing or flushing can be used, together with ordinary pressure gauge, pressure transmission instrument or differential pressure transmission instrument

(6) for objects that may condense in the gas phase, vaporize in the liquid phase, or have liquid separation in the gas phase at ambient temperature, when it is difficult to use flange differential pressure instruments and use ordinary differential pressure instruments for measurement, isolators, separators, vaporizers, balance vessels and other components should be set respectively according to the specific situation, or the measurement pipeline should be insulated and heat traced

(7) when measuring the liquid level of boiler drum with differential pressure instrument, the temperature compensated double chamber balance vessel should be used

(8) the positive and negative migration of differential pressure instruments should be considered when selecting the instrument range

2. Pontoon type measuring instrument

(1) for continuous measurement of liquid level with a measurement range of 2000mm and a specific density of 0.5-1.5, and continuous measurement of liquid interface with a measurement range of 1200mm and a specific density difference of 0.1-0.5, pontoon type instrument should be selected

pontoon instruments should be selected for vacuum objects and vaporizable liquids

pneumatic float type instrument should be selected for local liquid level indication or regulation

pontoon instruments must be used to clean liquids

(2) select the pontoon type instrument. When the accuracy requirements are high and the signal requires remote transmission, the force balance type should be selected; When the accuracy requirement is not high, and the local indication or adjustment, the displacement balance type can be selected

(3) for the liquid level measurement of open storage tank and open liquid storage tank, the inner pontoon should be selected; For liquid objects that are not crystallized or viscous at operating temperature, but may crystallize or viscous at ambient temperature, inner pontoons should also be selected. For process equipment that is not allowed to stop, the inner pontoon should not be selected, but the outer pontoon should be selected. For highly viscous, crystalline or high-temperature liquid objects, external floating Jane should not be selected

(4) when the liquid disturbance in the container is large, the inner pontoon instrument should be equipped with a stable sleeve to prevent the disturbance

(5) the electric float instrument is used in the situation where the measured liquid level fluctuates frequently, and its output signal should be equipped with a damper

3. Float type measuring instrument

(1) for the continuous measurement and volume measurement of the clean liquid level of large storage tanks, as well as the positional measurement of the clean liquid level and interface of various storage tanks, in a highly urbanized country such as Singapore, the use of float type instrument degradable composite materials is expected to change the treatment of fractures

(2) for dirty liquids and frozen liquids at ambient temperature, float type instruments should not be used. For continuous measurement and multi-point measurement of viscous liquid, float type instrument should not be used

(3) when the float type measuring instrument is used for interface measurement, the specific density of the two liquids should be constant, and the specific density difference should not be less than 0.2

(4) when the internal float type liquid level instrument is used for the liquid level measurement of large storage tanks, in order to prevent the float from drifting, it should be equipped with guiding facilities; In order to prevent the float from being affected by the liquid level disturbance, a stable sleeve should be installed

(5) the liquid level or volume of large storage tanks shall be measured continuously. For single storage tanks or multiple storage tanks with high measurement accuracy, photoconductive liquid level meters should be selected; Steel band float level gauge can be selected for single storage tank with general requirements for measurement accuracy. For single storage tank or multiple storage tanks that require high-precision continuous measurement of liquid level, interface, volume and quality, storage tank measurement system shall be selected

(6) magnetic float level gauge should be selected for multi-point measurement of liquid level in open storage tank and open liquid storage tank, as well as multi-point measurement of corrosive, toxic and other dangerous liquids

(7) lever float level controller should be selected for position measurement of viscous liquid

4. Capacitive measuring instrument

(1) for continuous and positional liquid level measurement of corrosive liquid, sedimentary fluid and other chemical process media, capacitive liquid level meter should be selected

when used for interface measurement, the electrical properties of the two liquids must meet the technical requirements of the product

(2) the specific model, electrode structure type and electrode material of the capacitance liquid level meter should be determined according to the electrical performance of the measured medium, the material of the container and other factors

(3) for non viscous non-conductive liquid, shaft sleeve electrode can be used; For non viscous conductive liquid, sleeve electrode can be used; For non-conductive liquids that are easy to stick, bare electrodes can be used, and the electrode surface should choose materials with low affinity with the measured liquid or adopt automatic cleaning measures

(4) capacitance liquid level meter cannot be used for continuous measurement of conductive liquid level that is easy to stick

(5) capacitive measuring instruments are vulnerable to electromagnetic interference, so shielded cables should be selected or other anti electromagnetic interference measures should be taken

(6) the capacitance liquid level gauge used for position measurement should be horizontally installed; The capacitance liquid level gauge used for continuous measurement should be of vertical installation type

5. Resistance (electric contact) measuring instrument

(1) for the positional measurement of corrosive conductive liquid level and the interface between conductive liquid and non-conductive liquid, resistance (electric contact) instrument can be selected

(2) for the conductive liquid that is easy to scale the electrode and the electrolytic phenomenon of the process medium between the electrodes, it is generally not suitable to choose resistance (electric contact) instruments. For liquids that are non-conductive and easy to adhere to electrodes, resistance (electrical contact) instruments shall not be selected

6. Static pressure measuring instrument

(1) for the continuous measurement of the liquid level of water supply pools, wells and reservoirs with a depth of 5m-100m, static pressure instruments should be selected

for continuous measurement of liquid level in pressureless vessels, static pressure instruments can be selected

(2) under normal working conditions, when the liquid density changes significantly, static pressure instruments should not be selected

7. Acoustic measuring instrument

(1) for the continuous measurement and position measurement of corrosive liquid, highly viscous liquid, toxic liquid and other liquid levels that are difficult to be measured by ordinary level instruments, acoustic measuring instruments should be selected

(2) the specific model and structural type of acoustic instrument shall be determined according to the characteristics of the measured medium and other factors

(3) acoustic instruments must be used for liquid level measurement of containers that can reflect and transmit sound waves, not for vacuum containers. It is not suitable for liquids containing bubbles and liquids containing solid particles

(4) for containers with internal obstacles that affect the propagation of sound waves, acoustic instruments should not be used

(5) for the acoustic instrument that continuously measures the liquid level, if the temperature and composition of the measured liquid change significantly, it should be considered to compensate for the change of sound wave propagation speed to improve the measurement accuracy

(6) shielded cables should be used for cables between detectors and converters, or anti electromagnetic interference measures should be considered

8. Microwave measuring instrument

(1) for the continuous measurement of the liquid level of corrosive liquids, high viscosity liquids and toxic liquids in large fixed roof tanks and floating roof tanks, which are difficult to be measured with high precision by ordinary liquid level instruments, microwave measuring instruments should be selected

the measurement method of microwave measuring instrument adopts microwave continuous scanning within a specific frequency range. When the distance between the liquid level and the antenna changes, the frequency difference between the sensing signal and the reflected signal is proportional to the distance between the liquid level and the antenna. Therefore, the liquid level can be converted by measuring the frequency difference

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