1. General principles of automatic instrument selection
The general principles for the selection of testing instruments (components) and control valves are as follows:
1. Process conditions
The temperature, pressure, flow rate, viscosity, corrosiveness, toxicity, pulsation and other factors of the process are the main conditions for determining the selection of the instrument, which are related to the rationality of the instrument selection, the service life of the instrument and the fire, explosion-proof and security of the workshop. question.
2. Operational Importance
The importance of the parameters of each detection point in operation is the basis for the selection of the instrument’s indication, recording, accumulation, alarm, control, remote control and other functions. Generally speaking, variables that have little effect on the process but need to be monitored frequently can choose the indicator type; for important variables that need to know the changing trend frequently, the record type should be selected; and some variables that have a greater impact on the process need to be Variables that are monitored at any time should be controlled; for variables related to material balance and power consumption that require measurement or economic accounting, accumulation should be set; some variables that may affect production or safety should be set to alarm.
3. Economy and Uniformity
The selection of the instrument is also determined by the scale of investment. On the premise of meeting the requirements of technology and automatic control, necessary economic accounting should be carried out to obtain a suitable performance/price ratio.
In order to facilitate the maintenance and management of the instrument, the unity of the instrument should also be paid attention to when selecting the model. Try to choose products of the same series, the same specification and model and the same manufacturer.
4. Use and supply of instruments
The selected instrument should be a relatively mature product, and its performance has been proved reliable by on-site use; at the same time, it should be noted that the selected instrument should be in sufficient supply and will not affect the construction progress of the project.
Second, the selection of temperature instruments
<1> General principles
1. Unit and scale (scale)
The scale (scale) unit of a temperature instrument is unified in Celsius (°C).
2. Detect (measure) the insertion length of the component
The selection of the insertion length should be based on the principle that the detection (measurement) element is inserted into a representative position where the temperature of the measured medium is sensitive to change. However, in general, in order to facilitate interchangeability, the length of the first to second gears is often selected uniformly for the entire device.
When installing on flue, furnace and masonry equipment with thermal insulation materials, it should be selected according to actual needs.
The material of the protective cover of the detection (detection) element should not be lower than the material of the equipment or pipeline. If the protective sleeve of the shaped product is too thin or not resistant to corrosion (such as armored thermocouples), an additional protective sleeve should be added.
Temperature instruments, temperature switches, temperature detection (measurement) components and transmitters installed in inflammable and explosive places with live contacts should be explosion-proof.
<2> Selection of local temperature instrument
1. Accuracy class
General industrial thermometer: choose class 1.5 or class 1.
Precision measurement and laboratory thermometers: Class 0.5 or 0.25 should be selected.
2. Measuring range
The highest measured value is not greater than 90% of the upper limit of the measuring range of the instrument, and the normal measured value is about 1/2 of the upper limit of the measuring range of the instrument.
The measured value of the pressure thermometer should be between 1/2 and 3/4 of the upper limit of the measuring range of the instrument.
3. Bimetal thermometer
When meeting the requirements of measuring range, working pressure and accuracy, it should be preferred.
The diameter of the case is generally φ100mm. In places with poor lighting conditions, high positions and long viewing distances, φ150mm should be selected.
The connection method between the instrument shell and the protective tube should generally be a universal type, or an axial type or a radial type can be selected according to the principle of convenient observation.
4. Pressure thermometer
It is suitable for on-site or on-site panel display with low temperature below -80 ℃, unable to observe closely, with vibration and low accuracy requirements.
5. Glass thermometer
It is only used for special occasions with high measurement accuracy, small vibration, no mechanical damage and convenient observation. However, mercury-in-glass thermometers should not be used due to mercury hazards.
6. Base instrument
For on-site or on-site installation of measuring and control (adjustment) instruments, base-type temperature instruments should be used.
7. Temperature switch
It is suitable for occasions where contact signal output is required for temperature measurement.
<3> Selection of centralized temperature instrument
1. Detect (measure) components
(1) According to the temperature measurement range, select a thermocouple, thermal resistance or thermistor with the corresponding graduation number.
(2) Thermocouples are suitable for general occasions. Thermal resistances are suitable for vibration-free applications. Thermistors are suitable for occasions requiring fast measurement response.
(3) According to the requirements of the measurement object for the response speed, the detection (measurement) elements of the following time constants can be selected:
Thermocouple: 600s, 100s and 20s three levels;
Thermal resistance: 90～180s, 30～90s, 10～30s and <10s grade four;
Thermistor: <1s.
(4) According to the environmental conditions of use, select the junction box according to the following principles:
Ordinary type: places with better conditions;
Splash-proof, waterproof: wet or open-air places;
Explosion-proof: flammable and explosive places;
Socket type: for special occasions only.
(5) In general, the threaded connection method can be used, and the flange connection method should be used for the following occasions:
Installation on equipment, lined piping and non-ferrous metal piping;
Crystallization, scarring, clogging and highly corrosive media:
Flammable, explosive and highly toxic media.
(6) Thermocouples and thermal resistances used in special occasions:
In the case of reducing gas, inert gas and vacuum where the temperature is higher than 870℃ and the hydrogen content is more than 5%, tungsten-rhenium thermocouple or blowing thermocouple is selected;
The surface temperature of the equipment, the outer wall of the pipeline and the rotating body, select the surface or armored thermocouple and thermal resistance;
For medium containing hard solid particles, wear-resistant thermocouple is selected;
In the protection casing of the same detection (measurement) element, when multi-point temperature measurement is required, multi-point (branch) thermocouples are selected;
In order to save special protection tube materials (such as tantalum), improve the response speed or require the detection (measurement) component to be bent and installed, an armored thermocouple can be selected.
2. Transmitter
Transmitters are selected for the measurement or control system matched with the standard signal display instrument.
In the case of meeting the design requirements, it is recommended to select a transmitter that integrates measurement and transmission.
3. Display instrument
(1) A general indicator should be used for single-point display, a digital indicator should be used for multi-point display, and a general recorder should be used if historical data is required to be consulted.
(2) For the signal alarm system, an indicator or recorder with contact signal output should be selected.
(3) A medium-sized recorder (such as a 30-point recorder) should be used for multi-point recording.
4. Selection of auxiliary equipment
(1) When multiple points share one display instrument, a switch with reliable quality should be selected.
(2) Thermocouples are used to measure the temperature below 1600°C. When the temperature change of the cold junction makes the measurement system unable to meet the accuracy requirements, and the supporting display instrument has no automatic cold junction temperature compensation function, the cold junction temperature automatic compensator should be selected.
(3) Compensation wire
a. According to the number of thermocouples, the graduation number and the environmental conditions of use, the compensation wire or compensation cable that meets the requirements should be selected.
b. Select different levels of compensation wires or compensation cables according to the ambient temperature:
-20～+100℃ choose ordinary grade;
-40 ~ +250 ℃ select heat-resistant grade.
c. In places with intermittent electric heating or strong electricity and magnetic fields, shielded compensation wires or shielded compensation cables should be used.
d. The cross-sectional area of the compensation wire should be determined according to the reciprocating resistance value of its laying length and the external resistance allowed by the supporting display instrument, transmitter or computer interface.
3. Selection of pressure instruments
<1> Selection of pressure gauge
1. Select according to the use environment and the nature of the measuring medium
(1) In harsh environments such as strong atmospheric corrosiveness, a lot of dust and easy spraying of liquids, closed-type all-plastic pressure gauges should be used.
(2) For dilute nitric acid, acetic acid, ammonia and other general corrosive media, acid-resistant pressure gauges, ammonia pressure gauges or stainless steel diaphragm pressure gauges should be used.
(3) Dilute hydrochloric acid, hydrochloric acid gas, heavy oil and similar media with strong corrosiveness, solid particles, viscous liquid, etc., should use diaphragm pressure gauge or diaphragm pressure gauge. The material of the diaphragm or diaphragm must be selected according to the characteristics of the measuring medium.
(4) For media such as crystallization, scarring and high viscosity, a diaphragm pressure gauge should be used.
(5) In the case of strong mechanical vibration, a shock-resistant pressure gauge or a marine pressure gauge should be used.
(6) In flammable and explosive occasions, if electrical contact signals are required, an explosion-proof electrical contact pressure gauge should be used.
(7) Special pressure gauges should be used for the following measuring media:
Gas ammonia, liquid ammonia: ammonia pressure gauge, vacuum gauge, pressure vacuum gauge;
Oxygen: Oxygen pressure gauge;
Hydrogen: Hydrogen pressure gauge;
Chlorine: chlorine-resistant pressure gauge, pressure vacuum gauge;
Acetylene: Acetylene pressure gauge;
Hydrogen sulfide: sulfur-resistant pressure gauge;
Lye: alkali-resistant pressure gauge, pressure vacuum gauge.
2. the choice of accuracy level
(1) The pressure gauges, diaphragm pressure gauges and diaphragm pressure gauges used for general measurement should be grade 1.5 or 2.5.
(2) Pressure gauges for precision measurement and calibration should be graded 0.4, 0.25 or 0.16.
3. Selection of external dimensions
(1) The pressure gauge installed on the pipeline and equipment has a nominal diameter of φ100mm or φ150mm.
(2) The pressure gauge installed on the instrument pneumatic pipeline and its auxiliary equipment has a nominal diameter of φ60mm.
(3) For pressure gauges installed in places with low illumination, high position and difficult observation of indication values, the nominal diameter is φ200mm or φ250mm.
4. Selection of measuring range
(1) When measuring stable pressure, the normal operating pressure value should be 2/3 to 1/3 of the upper limit of the measuring range of the instrument.
(2) When measuring the pulsating pressure (such as the pressure at the outlet of the pump, compressor and fan), the normal operating pressure value should be 1/2 to 1/3 of the upper limit of the measuring range of the instrument.
(3) When measuring high and medium pressure (greater than 4MPa), the normal operating pressure value should not exceed 1/2 of the upper limit of the measuring range of the instrument.
5. Unit and scale (scale)
(1) All pressure instruments shall use legal measurement units. Namely: Pa (Pa), kilopascal (kPa) and megapascal (MPa).
(2) For foreign-related design projects and imported instruments, international general standards or corresponding national standards may be adopted.
<2> Selection of transmitter and sensor
(1) When transmitting with standard signal (4~20mA), the transmitter should be selected.
(2) In flammable and explosive situations, pneumatic transmitters or explosion-proof electric transmitters should be used.
(3) For crystallization, scarring, clogging, viscous and corrosive media, flange-type transmitters should be used. The material in direct contact with the medium must be selected according to the characteristics of the medium.
(4) For occasions where the use environment is good and the measurement accuracy and reliability are not high, resistance type, inductance type remote pressure gauge or Hall pressure transmitter can be selected.
(5) When measuring tiny pressure (less than 500Pa), a differential pressure transmitter can be selected.
<3> Selection of installation accessories
(1) When measuring water vapor and media with a temperature greater than 60 °C, a spiral or U-shaped elbow should be used.
(2) When measuring easily liquefied gas, if the pressure point is higher than the meter, a separator should be used.
(3) When measuring dust-containing gas, a dust collector should be selected.
(4) When measuring pulsating pressure, dampers or buffers should be used.
(5) When the ambient temperature is close to or lower than the freezing point or freezing point of the measuring medium, adiabatic or heat tracing measures should be taken.
(6) The instrument protection (temperature) box should be selected in the following occasions.
Pressure switches and transmitters for outdoor installation.
Pressure switches and transmitters installed in workshops with severe atmospheric corrosion, dust and other harmful substances.
Fourth, the selection of flow meters
<1> General principles
1. Scale selection
The scale of the instrument should meet the requirements of the scale modulus of the instrument. When the scale reading is not an integer, it is convenient to convert the reading, and it can also be selected according to the integer.
(1) Square root scale range
The maximum flow does not exceed 95% of full scale;
Normal flow is 70% to 85% of full scale;
The minimum flow is not less than 30% of full scale.
(2) Linear scale range
The maximum flow does not exceed 90% of full scale;
Normal flow is 50% to 70% of full scale;
The minimum flow is not less than 10% of full scale.
2. Instrument accuracy
The flowmeter used for energy measurement shall comply with the provisions of the General Rules for the Equipping and Management of Enterprise Energy Measurement Instruments (Trial).
(1) For the measurement of fuel inbound and outbound settlement, ±0.1%;
(2) Measurement for technical and economic analysis of workshop teams and technological processes, ±0.5% to 2%;
(3) For industrial and civil water measurement, ±2.5%;
(4) For steam metering including superheated steam and saturated steam, ±2.5%;
(5) For the measurement of natural gas, gas and domestic gas, ±2.0%;
(6) Measurement of oil used for key energy-consuming equipment and process control, ±1.5%;
(7) Measurement of other energetic working fluids (such as compressed air, oxygen, nitrogen, hydrogen, water, etc.) used for process control, ±2%.
3. Flow unit
The volume flow is m3/h, l/h;
Mass flow in kg/h, t/h;
In the standard state, the gas volume flow rate is Nm3/h (0°C, 0.1013MPa)
<2> Selection of general fluid, liquid and steam flow measuring instruments
1. Differential pressure flowmeter
(1) Throttle device
①Standard throttling device
For the flow measurement of general fluids, standard throttling devices (standard orifice plates, standard nozzles) should be used. The selection of standard throttling device must comply with the provisions of GB2624-8l or the international standard ISO 5167-1980. If there are new national standard regulations, the new regulations should be implemented.
②Non-standard throttling device
Those who meet the following conditions can choose a Venturi tube:
Accurate measurements at low pressure losses are required;
The measured medium is clean gas or liquid;
The inner diameter of the pipe is in the range of 100-800mm;
The fluid pressure is within 1.0MPa.
If the following conditions are met, a double orifice plate can be used:
The measured medium is clean gas and liquid;
The Reynolds number is greater than (equal to) 3000 and less than (equal to)) 300000.
Those who meet the following conditions can choose 1/4 round nozzle:
The measured medium is clean gas and liquid;
The Reynolds number is greater than 200 and less than 100,000.
If the following conditions are met, the round hole plate can be selected:
Dirty media (such as blast furnace gas, mud, etc.) that may produce sediment before and after the orifice plate;
Must have horizontal or sloping pipes.
③Selection of pressure taking method
It should be considered that the whole project should adopt a unified pressure taking method as far as possible.
Generally, the method of corner connection or flange pressure is adopted.
According to the conditions of use and measurement requirements, other pressure taking methods such as radial pressure taking can be used.
(2) Selection of differential pressure range of differential pressure transmitter
The selection of the differential pressure range should be determined according to the calculation. Generally, it should be selected according to the different working pressure of the fluid:
Low differential pressure: 6kPa, 10kPa;
Medium differential pressure: 16kPa, 25kPa;
High differential pressure: 40kPa, 60kPa.
(3) Measures to improve measurement accuracy
For fluids with large temperature and pressure fluctuations, temperature and pressure compensation measures should be considered;
When the length of the straight pipe section of the pipeline is insufficient or the swirling flow is generated in the pipeline, the fluid correction measures should be considered, and the rectifier of the corresponding pipe diameter should be selected.
(4) Special type differential pressure flowmeter
①Steam flowmeter
For the flow of saturated steam, when the required accuracy is not higher than 2.5, and it is calculated locally or remotely, a steam flow meter can be used.
②Built-in orifice flowmeter
For the micro flow measurement of clean liquid, steam and gas without suspended solids, when the range ratio is not greater than 3:1, the measurement accuracy is not high, and the diameter of the pipeline is less than 50mm, the built-in orifice flowmeter can be selected. When measuring steam, the steam temperature is not more than 120℃.
2. Area flowmeter
when to When the accuracy is not higher than 1.5 and the range ratio is not more than 10:1, the rotor flowmeter can be selected.
(1) Glass rotameter
Glass rotor flowmeter can be used for local indication of small and medium flow rate, small flow rate, pressure less than 1MPa, temperature lower than 100°C, clean and transparent, non-toxic, non-flammable and explosive, non-corrosive and non-adhering to glass.
(2) Metal tube rotameter
①Ordinary metal tube rotameter
It is easy to vaporize, easy to condense, toxic, flammable, explosive, and does not contain magnetic substances, fibers and abrasive substances, and it is non-corrosive to stainless steel (1Crl8Ni9Ti) for small and medium flow measurement of fluids. When local indication or remote signal transmission is required, Ordinary metal tube rotameter can be used.
②Special type metal tube rotameter
Jacketed Metal Tube Rotameter
When the measured medium is easy to crystallize or vaporize or has high viscosity, a jacketed metal tube rotameter can be selected. A heating or cooling medium is passed through the jacket.
Anti-corrosion metal tube rotameter
For flow measurement of corrosive medium, anti-corrosion metal tube rotor flowmeter can be used.
(3) Rotameter
Vertical installation is required, and the inclination is not more than 5°. The fluid should be from bottom to top, the installation position should be less vibrated, easy to observe and maintain, and upstream and downstream shut-off valves and bypass valves should be provided. For dirty media, a filter must be installed at the inlet of the flowmeter.
3. Velocity flowmeter
(1) Target flowmeter
For liquid flow measurement with high viscosity and a small amount of solid particles, when the accuracy is not higher than 1.5 and the range ratio is not more than 3:1, the target flowmeter can be used.
Target flowmeters are generally installed on horizontal pipes. The length of the front straight pipe section is 15-40D, and the length of the rear straight pipe section is 5D.
(2) Turbine flowmeter
For the flow measurement of clean gas and clean liquid with kinematic viscosity not greater than 5×10-6m2/s, turbine flowmeter can be used when more precise measurement is required and the range ratio is not greater than 10:1.
The turbine flowmeter should be installed on a horizontal pipeline to fill the entire pipeline with liquid, and set upstream and downstream stop valves and bypass valves, as well as a filter upstream and a discharge valve downstream.
The length of the straight pipe section: the upstream is not less than 20D, and the downstream is not less than 5D.
(3) Vortex flowmeter (Kaman vortex flowmeter or vortex flowmeter)
For large and medium flow measurement of clean gas, steam and liquid, vortex flowmeter can be selected. Vortex flowmeters should not be used for the measurement of low-speed fluids and liquids with a viscosity greater than 20×10-3pa·s. When selecting, the pipeline velocity should be checked.
The flowmeter has the characteristics of small pressure loss and easy installation.
Requirements for straight pipe sections: upstream is 15-40D (depending on the piping conditions); when adding a rectifier upstream, the upstream is not less than 10D; the downstream is at least 5D.
(4) Water meter
The flow rate of accumulated water on site, when the turndown ratio is less than 30:1, can use a water meter.
The water meter is installed on the horizontal pipeline, and the length of the straight pipe section is required to be no less than 8D upstream and no less than 5D downstream.
<3> Selection of corrosive, conductive or flow measuring instruments with solid particles
1. Electromagnetic flowmeter
It is used for flow measurement of liquid or uniform liquid-solid two-phase medium with conductivity greater than 10μS/cm. Has good corrosion resistance and wear resistance, no pressure loss. It can measure various media such as strong acid, strong alkali, salt, ammonia water, mud, ore pulp and paper pulp.
The installation direction can be vertical, horizontal or inclined. When installing vertically, the liquid must be from bottom to top. For liquid-solid two-phase media, it is best to install vertically.
When installed on a horizontal pipe, the liquid should be filled with the pipe section, and the electrodes of the transmitter should be on the same horizontal plane; the length of the straight pipe section should be no less than 5-10D upstream and no less than 3-5D downstream or no requirement (manufacturer different, different requirements).
The transmitter should not be installed in places where the magnetic field strength is greater than 398A/m.
2. Non-standard throttling device see above
<four> selection of high viscosity fluid flow measuring instruments
1. Volumetric flowmeter
(1) Oval gear flowmeter
Clean, high-viscosity liquids require more accurate flow measurement. When the range ratio is less than 10:1, an oval gear flowmeter can be used.
The oval gear flowmeter should be installed on the horizontal pipeline, and the indicator dial surface should be in the vertical plane; the upstream and downstream shut-off valves and bypass valves should be provided. A filter should be installed upstream.
For micro flow, a micro oval gear flowmeter can be used.
When measuring all kinds of easily gasified media, an air eliminator should be added.
(2) Waistwheel flowmeter
For clean gas or liquid, especially lubricating oil, flow measurement that requires high accuracy, waist wheel flowmeter is optional.
The flowmeter should be installed horizontally, with a bypass pipeline and a filter installed at the inlet end.
(3) Scraper flowmeter
Continuous measurement of liquid flow in closed pipelines, especially accurate measurement of various oil products, scraper flowmeter can be selected.
The installation of the scraper flowmeter should fill the pipeline with fluid, and it should be installed horizontally so that the number of the counter is in the vertical direction.
When measuring various oil products and requiring accurate measurement, an air eliminator should be added.
2. Target flowmeter
For liquid flow measurement with high viscosity and a small amount of solid particles, when the accuracy is not higher than 1.5 and the range ratio is not more than 3:1, the target flowmeter can be used.
Target flowmeters are generally installed on horizontal pipes. The length of the front straight pipe section is 15-40D, and the length of the rear straight pipe section is 5D.
<5> Selection of large diameter flow measuring instruments
When the pipe diameter is large, the pressure loss has a significant impact on the energy consumption. Conventional flowmeters are expensive. When the pressure loss is large, flute-shaped uniform velocity tubes, plug-in vortex streets, plug-in turbines, electromagnetic flowmeters, venturi tubes, and ultrasonic flowmeters can be selected according to the situation.
1, flute uniform velocity tube flowmeter
For flow measurement of clean gas, steam, and clean liquid with a viscosity less than 0.3Pa·s, when the pressure loss is required to be small, the flute uniform velocity tube flowmeter can be selected.
The flute-shaped uniform velocity pipe is installed on the horizontal pipeline, and the length of the straight pipe section: the upstream is not less than 6-24D, and the downstream is not less than 3-4D.
2. Insertion turbine flowmeter, insertion vortex flowmeter, electromagnetic flowmeter, Venturi tube
See above.
<6> Selection of new flow measuring instruments
1. Ultrasonic flowmeter
Ultrasonic flowmeters can be used for all sound-conducting fluids. In addition to general media, for media that work under harsh conditions such as strong corrosiveness, non-conductivity, flammable and explosive, and radioactivity, when contact measurement cannot be used, it can be used. Ultrasonic flow meter.
2. Mass flow meter
When it is necessary to directly and accurately measure the mass flow of liquids, high-density gases and slurries, mass flow meters can be used.
Mass flow meters provide accurate and reliable mass flow data independent of changes in fluid temperature, pressure, density or viscosity.
Mass flow meters can be installed in any direction without straight pipe runs.
<7> Selection of powder and block solid flow measuring instruments
1. Impulse flowmeter
For the flow measurement of free-falling powder particles and block solids, when the material is required to be closed and conveyed, an impulse flowmeter should be used; the impulse flowmeter is suitable for various bulk materials of any particle size, and can be accurate even in the case of a lot of dust Measured, but the weight of the bulk material shall not be greater than 5% of the weight of the predetermined punching plate.
The installation of the impulse flowmeter requires that the material must be guaranteed to fall freely, and no external force should act on the measured object. There are certain requirements for the installation angle of the punching plate, the angle and height between the feeding port and the punching plate, and have a certain relationship with the selection of the range. It should be calculated before selection.
2. Electronic belt scale
Solids flow measurement for belt conveyors, mounted on belt conveyors with standard performance. The installation requirements of the weighing frame are strict. The position of the weighing frame on the belt and the distance from the blanking port will affect the measurement accuracy. The installation position should be selected.
3. Track scale
For continuous automatic weighing of railway freight cars, dynamic track scales should be selected.
Fifth, the selection of the level instrument
<1> General principles
(1) It is necessary to deeply understand the process conditions, the properties of the measured medium, and the requirements of the measurement control system in order to fully evaluate the technical performance and economic effects of the instrument, so as to ensure stable production, improve product quality, and increase economic benefits. play its due role.
(2) Differential pressure type instruments, float type instruments and float type instruments should be used for liquid level and interface measurement. When the requirements are not met, capacitive, resistive (electrical contact), and sonic instruments can be used.
The material surface measurement should be selected according to the particle size of the material, the angle of repose of the material, the electrical conductivity of the material, the structure 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 to consider are pressure, temperature, corrosiveness, electrical conductivity; whether there are phenomena such as polymerization, viscosity, precipitation, crystallization, conjunctiva, gasification, foaming, etc.; density and density changes; the amount of suspended solids in the liquid; The degree of surface disturbance and the particle size of the solid material.
(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 measuring range of the instrument should be determined according to the actual display range or actual variation range of the process object. In addition to the level meter for volume measurement, the normal level should generally be about 50% of the meter range.
(6) The accuracy of the instrument should be selected according to the process requirements, but the level of the level instrument used for volume measurement should be above 0.5.
(7) Electronic level instruments used in explosive hazardous places such as combustible gas, steam and combustible dust. The appropriate explosion-proof structure type should be selected or other protective measures should be taken according to the determined hazardous location category and the hazard degree of the measured medium.
(8) For electronic level instruments used in places such as corrosive gases and harmful dusts, the appropriate enclosure protection type should be selected according to the environmental conditions of use.
<2> Selection of liquid level and interface measuring instruments
1. Differential pressure measuring instrument
(1) For the continuous measurement of the liquid level, a differential pressure instrument should be selected.
For interface measurement, a differential pressure instrument can be selected, but it is required that the total liquid level should always be higher than the upper pressure port.
(2) For high requirements on measurement accuracy, the measurement system needs more complex precise operations, and when the general analog instrument is difficult to achieve, the differential pressure intelligent transmission instrument can be selected, and its accuracy is above 0.2.
(3) When the liquid density changes significantly under normal working conditions, it is not appropriate to use a differential pressure instrument.
(4) Flat flange differential pressure instruments should be used for corrosive liquids, crystalline liquids, viscous liquids, easily vaporized liquids, and liquids containing suspended solids.
High-crystalline liquid, high-viscosity liquid, gelatinous liquid, and precipitation liquid should use the plug-in flange differential pressure instrument.
If there is a large amount of condensate and sediment on the liquid level of the measured medium above, or if the high-temperature liquid needs to be isolated from the transmitter, or when the measured medium needs to be replaced, the measuring head needs to be strictly purified, the double flange type can be selected. Differential pressure gauge.
(5) When it is difficult to measure the liquid level of corrosive liquids, viscous liquids, crystalline liquids, molten liquids and precipitating liquids with a flanged differential pressure instrument, the method of blowing air or flushing liquid can be used, in conjunction with ordinary Pressure gauge, pressure transmitter instrument or differential pressure transmitter instrument for measurement.
(6) At ambient temperature, the gas phase may condense, the liquid phase may be vaporized, or the gas phase may have liquid separation, when it is difficult to use a flanged differential pressure instrument and an ordinary differential pressure instrument is used for measurement, it should be determined according to the specific situation. Set up isolators, separators, vaporizers, balance vessels and other components, or heat and trace the measurement pipeline.
(7) When measuring the liquid level of the boiler drum with a differential pressure instrument, a temperature-compensated double-chamber balance container should be used.
(8) The positive and negative migration of differential pressure instruments should be considered when selecting the instrument range.
2. Buoy measuring instrument
(1) For the continuous measurement of the liquid level within the measurement range of 2000mm and the specific density of 0.5 to 1.5, and the continuous measurement of the liquid interface with the measurement range of within 1200mm and the specific density difference of 0.1 to 0.5, the buoy-type instrument should be used.
For vacuum objects and liquids that are easy to vaporize, float type instruments should be used.
Pneumatic float-type instruments should be used for on-site liquid level indication or adjustment.
Displacement meters must be used for cleaning liquids.
(2) Select the buoy-type instrument. When the accuracy requirement is high and the signal requires remote transmission, the force balance type should be selected; when the accuracy requirement is not high and local indication or adjustment is required, the displacement balance type can be selected.
(3) For the liquid level measurement of open storage tanks and open liquid storage tanks, the inner buoy should be selected; for liquid objects that do not crystallize and are not viscous at the operating temperature, but may crystallize or stick to the ambient temperature, also Inner buoys should be used. For process equipment that is not allowed to stop, the inner buoy should not be used, but the outer buoy should be used. For highly viscous, crystalline or high-temperature liquid objects, external floats should not be used.
(4) When the internal buoy instrument has large liquid disturbance in the container, a stable casing to prevent disturbance should be installed.
(5) The electric displacer meter is used for occasions where the measured liquid level fluctuates frequently, and the output signal should be damped.
3. Float measuring instrument
(1) For the continuous measurement and volume measurement of the cleaning liquid level of large storage tanks, as well as the positional measurement of the liquid level and interface of various storage tank cleaning liquids, float type instruments should be selected.
(2) Dirty liquids and liquids frozen at ambient temperature should not be used with float-type instruments. For continuous measurement and multi-point measurement of viscous liquid, it is also not suitable to use a float type instrument.
(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 liquid level measurement in large storage tanks, in order to prevent the float from drifting, guide facilities should be provided; in order to prevent the float from being affected by the liquid level disturbance, a stable casing should be installed.
(5) Continuous measurement of liquid level or volume of liquid in large storage tanks. For single storage tanks or multiple storage tanks that require high measurement accuracy, light-guided liquid level meters should be used; for single storage tanks with general measurement accuracy requirements, steel With float level gauge. For single storage tanks or multiple storage tanks that require high-precision continuous measurement of liquid level, interface, volume and mass, the storage tank measurement system should be selected.
(6) Multi-point measurement of liquid level in open storage tanks and open liquid storage tanks, as well as multi-point measurement of corrosive, toxic and other dangerous liquids, should use magnetic float type liquid level gauges.
(7) For the level measurement of viscous liquids, a lever type float level controller should be used.
4. Capacitive measuring instrument
(1) For the continuous measurement and level measurement of corrosive liquids, precipitating fluids and other chemical process media, capacitive liquid level meters 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 properties of the measured medium, the material of the container and other factors.
(3) For non-viscous non-conductive liquids, shaft-sleeve electrodes can be used; for non-viscous conductive liquids, sleeve-type electrodes can be used; for viscous non-conductive liquids, bare electrodes can be used , the electrode surface should choose a material with low affinity with the liquid to be tested or adopt automatic cleaning measures.
(4) Capacitance level gauge cannot be used for continuous measurement of viscous conductive liquid level.
(5) Capacitive measuring instruments are susceptible to electromagnetic interference, and shielded cables should be used, or other anti-electromagnetic interference measures should be taken.
(6) Capacitance liquid level gauges used for position measurement should be installed horizontally; capacitance liquid level meters used for continuous measurement should be installed vertically.
5. Resistive (electrical contact) measuring instrument
(1) For the level measurement of corrosive conductive liquids, as well as the interface measurement of conductive liquids and non-conductive liquids, the Use resistive (electrical contact) meters.
(2) For conductive liquids that easily foul electrodes and electrolysis of process medium between electrodes, resistance type (electrical contact type) meters are generally not suitable. For liquids that are non-conductive and easy to adhere to electrodes, resistive (electrical contact) meters should not be used.
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 to 100m, static pressure instruments should be selected.
For continuous measurement of liquid level in unpressurized vessels, hydrostatic instruments can be selected.
(2) Under normal working conditions, when the liquid density changes significantly, it is not suitable to use a static pressure instrument.
7. Sonic measuring instrument
(1) For continuous measurement and level measurement of corrosive liquids, high viscous liquids, toxic liquids and other liquid levels that are difficult to measure by ordinary level instruments, acoustic wave type measurement instruments should be used.
(2) The specific model and structure of the sonic instrument should be determined according to the characteristics of the measured medium and other factors.
(3) Sonic instruments must be used for liquid level measurement in containers that can reflect and transmit sound waves, and cannot be used in vacuum containers. Not suitable for liquids containing bubbles and liquids containing solid particles.
(4) Acoustic instruments should not be used for containers with internal obstacles that affect the propagation of sound waves.
(5) For the acoustic wave instrument that continuously measures the liquid level, if the temperature and composition of the liquid to be measured change significantly, compensation for the change of the acoustic wave propagation speed should be considered to improve the measurement accuracy.
(6) The cable between the detector and the converter should be shielded, or measures to prevent electromagnetic interference 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 that are difficult to measure with high precision by ordinary liquid level instruments, microwave measuring instruments should be used.
The measurement method of microwave measuring instrument adopts microwave continuous scanning in a specific frequency range. When the distance between the liquid level and the antenna changes, a frequency difference is generated between the sensing signal and the reflected signal, and the frequency difference is related to the distance between the liquid level and the antenna. Proportional, so the difference in measurement frequency can be converted to obtain the liquid level.
(2) The structure and material of the antenna should be determined according to the characteristics of the measured medium, the pressure in the storage tank and other factors.
(3) For storage tanks with internal obstacles that affect microwave propagation, microwave instruments should not be used.
(4) When the density of water vapor and hydrocarbon vapor in the tank has a significant change under normal working conditions, compensation for the change of microwave propagation speed should be considered; for boiling or disturbed liquid level, reducing diameter should be considered. The static pipe of the horn and other compensation measures to improve the measurement accuracy.
9. Nuclear radiation measuring instrument
(1) For the non-contact continuous measurement and level measurement of the liquid level of high temperature, high pressure, high viscosity, strong corrosion, explosive and toxic media, when it is difficult to use other liquid level instruments to meet the measurement requirements, the nuclear radiation type instrument can be selected. .
(2) The intensity of the radiation source should be selected according to the measurement requirements. At the same time, after the radiation passes through the measured object, the radiation dose at the work site should be as small as possible, and the safety dose standard should comply with the current “Radiation Protection Regulations” (GB8703-88). ), otherwise, protective measures such as isolation shielding should be fully considered.
(3) The type of radiation source should be selected according to the measurement requirements and the characteristics of the measured object, such as the density of the measured medium, the geometric shape of the container, the material and the wall thickness. When the intensity of the radiation source is required to be small, radium (Re) can be used; when the intensity of the radiation source is required to be large, cesium 137 (Csl37) can be used; when the thick-walled container requires strong penetration ability, cobalt 60 (Co60 ).
(4) In order to avoid the measurement error caused by the decay of the radiation source, improve the stability of the operation and reduce the number of calibrations, the measuring instrument should be able to compensate for the decay.
10. Laser measuring instrument
(1) For the continuous measurement of the liquid level of containers with complex structures or mechanical obstacles, and containers that are difficult to install according to conventional methods, laser measuring instruments should be selected.
(2) For completely transparent liquids without reflection, laser measuring instruments cannot be used.
<three> material surface measuring instrument selection
1. Capacitive measuring instrument
(1) For granular materials and powdered and granular materials, such as coal, plastic monomer, fertilizer, sand, etc., for continuous measurement and position measurement, capacitive measuring instruments should be used.
(2) The extension cable of the detector should be shielded cable, or measures to prevent electromagnetic interference should be considered.
2. Sonic measuring instrument
(1) For the level measurement of granular material surfaces with a particle size of less than 10mm in silos and hoppers with no vibration or small vibration, a tuning fork level meter can be selected.
(2) For the level measurement of powder and granular materials with a particle size of less than 5mm, a sound-blocking ultrasonic level meter should be used.
(3) For continuous measurement and level measurement of micropowder materials, reflective ultrasonic level gauges should be used. The reflective ultrasonic level gauge is not suitable for the level measurement of dust-filled bins and hoppers, nor for level measurement with uneven surfaces.
3. Resistive (electrical contact) measuring instrument
(1) For granular and powdery materials with good or poor electrical conductivity, but containing moisture, such as coal, coke and other material surface level measurement, resistance measuring instruments can be used.
(2) The value of the electrode-to-ground resistance specified by the product must be met to ensure the reliability and sensitivity of the measurement.
4. Microwave measuring instrument
(1) For the level measurement and continuous measurement of block and granular materials with high temperature, high adhesion, high corrosiveness and high toxicity, microwave measuring instruments should be used.
(2) It is not suitable for level measurement with uneven surface.
5. Nuclear radiation measuring instrument
(1) For the level measurement and continuous measurement of bulk, granular and powder-granular materials with high temperature, high pressure, high adhesion, high corrosiveness and high toxicity, nuclear radiation measuring instruments can be selected.
(2) Other requirements shall comply with the aforementioned provisions.
6. Laser measuring instrument
(1) For containers with complex structures or mechanical obstacles, and for continuous measurement of the material surface of containers that are difficult to install by conventional methods, laser measuring instruments should be used.
(2) For completely transparent materials without reflection, laser measuring instruments cannot be used.
7. Anti-rotation measuring instrument
(1) For silos and hoppers with low pressure and no pulsating pressure, for the positional measurement of granular and powder granular materials with a specific density of more than 0.2, a resistance-rotating measuring instrument can be used.
(2) The size of the rotor should be selected according to the specific density of the material.
(3) In order to avoid the malfunction of the instrument caused by the material hitting the rotor, a protective plate should be set above the rotor.
8. Diaphragm measuring instrument
(1) For the positional measurement of granular or powder granular materials in silos and hoppers, diaphragm measuring instruments can be selected.
(2) Since the action of the diaphragm is easily affected by the adhesion of particles and the influence of the flow pressure of the particles, it cannot be used in applications with high precision requirements.
9. Heavy hammer measuring instrument
(1) For large-scale silos, bulk warehouses, and open or closed pressure-free containers with large material level height and wide variation range, the material surface of bulk, granular and powder-granular materials with little adhesion should be continuously measured at regular intervals. Use a heavy-hammer measuring instrument.
(2) The form of the heavy hammer should be selected according to the particle size, dry humidity and other factors of the material.
(3) For the material level measurement of bins and containers with serious dust diffusion, a heavy-hammer measuring instrument with an air blowing device should be used.
Post time: Nov-21-2022