Introduction
In industrial production, instruments play a key role in detecting, displaying, recording, or controlling process parameters. Process monitoring is a basic means to understand and control production. Accurate, continuous awareness of the process state and appropriate control are required to ensure smooth production and to produce qualified products with high productivity and low consumption. This article summarizes selection guidelines for automation instruments, temperature instruments, pressure instruments, flowmeters, and level instruments.
1. General Principles for Selecting Automation Instruments
Selection principles for sensing elements, instruments, and control valves are as follows.
1.1 Process conditions
Process factors such as temperature, pressure, flow rate, viscosity, corrosiveness, toxicity, and pulsation determine instrument selection. These factors affect suitability, service life, and safety measures such as fire protection and explosion protection.
1.2 Operational importance
The operational importance of each measurement point determines whether an instrument should provide indication, recording, totalizing, alarm, control, or remote control functions. Variables that have minor process impact but require frequent monitoring may use indicators. Important variables that need trend analysis should use recorders. Variables that significantly affect the process and require continuous monitoring should have control. Variables used for material or energy accounting should include totalizing. Variables that may affect production or safety should include alarms.
1.3 Economy and standardization
Selection depends on investment scale. Within the constraints of process and control requirements, perform economic assessment to achieve an appropriate performance/price ratio. For easier maintenance and asset management, prefer uniformity: choose instruments from the same series, model, and manufacturer when practical.
1.4 Availability and maturity
Prefer proven, mature products with reliable field performance. Ensure supply availability so procurement will not delay project execution.
2. Temperature Instruments Selection
2.1 General principles
Unit and scale. Use Celsius degrees (°C) as the unit for temperature instrument scales.
Insertion length of sensing element. Choose insertion length so the sensing element is placed where measured temperature is representative and responsive. For interchangeability, standardize on one or two nominal lengths. For duct, furnace, or insulated equipment installations, select insertion length according to actual needs.
The sensing protection sheath material should not be inferior to the equipment or pipe material. If a standard protection sheath is too thin or not corrosion resistant, add a protective tube.
In hazardous areas, local instruments with live contacts, temperature switches, sensing elements, and transmitters must be explosion-proof type.
2.2 Local temperature instruments
Accuracy classes. General industrial thermometers: class 1.5 or class 1. Precision and laboratory thermometers: class 0.5 or 0.25.
Measurement range. The maximum measured value should not exceed 90% of the instrument upper range; normal reading should be near half of the upper range. For pressure-type thermometers, the measured value should be between 1/2 and 3/4 of the instrument upper range.
Bimetal thermometers. Prefer bimetal thermometers when they meet the range, working pressure, and accuracy requirements. Case diameter is typically 100 mm; use 150 mm for poor lighting, high mounting, or long viewing distance. The connection between case and protection tube should generally be universal; choose axial or radial when observation convenience dictates.
Pressure-type thermometers. Suitable for temperatures below ?80°C, inaccessible locations, vibrating environments, or when lower accuracy is acceptable and local or panel display is required.
Glass thermometers. Use only in special situations with high accuracy requirements, little vibration, no mechanical damage risk, and convenient observation. Mercury glass thermometers are discouraged due to mercury hazards.
Head-mounted instrument assemblies. For local or local-panel measurement and control, prefer head-mounted temperature instruments.
Temperature switches. Use where temperature measurement needs contact output.
2.3 Centralized temperature measurement
Sensing elements. Select thermocouples, resistance thermometers, or thermistors according to the temperature range. Thermocouples are suitable for general situations; resistance thermometers are suitable where no vibration exists; thermistors suit fast-response requirements.
Choose time constants based on response needs: thermocouples: 600 s, 100 s, 20 s; resistance thermometers: 90–180 s, 30–90 s, 10–30 s, <10 s; thermistors: <1 s.
Junction box selection by environment: standard for good conditions; splashproof or waterproof for humid or outdoor locations; explosion-proof for flammable/explosive areas; socket type only for special cases.
Connection method: threaded connections are typical; use flanged connections for installation on equipment, lined pipes, nonferrous pipes, or for media prone to crystallization, fouling, blockage, strong corrosion, or for flammable, explosive, or highly toxic media.
Special sensing elements: for >870°C, hydrogen-rich reducing gases (>5% H2), inert gases, or vacuum use tungsten-rhenium thermocouples or gas-flushed thermal sensors; for external surface or rotating surfaces use surface or armored thermocouples/resistance thermometers; for abrasive media use wear-resistant thermocouples; for multi-point measurement in one sheath use multi-point thermocouples; to save expensive sheath materials or increase response speed use armored thermocouples when bending or compact installation is required.
Transmitters. Use transmitters when interfacing to standard signal display instruments or control systems, typically 4–20 mA. Prefer integrated sensor-transmitter assemblies when they meet design requirements.
Display instruments. Single-point indication: use general indicators. Multi-point: use digital indicators. For historical data review, use recorders. Alarms: use indicators or recorders with contact outputs. Multi-point recording: use mid-size recorders (for example, 30-point).
Accessories. When multiple points share one display, use reliable switch selectors. For thermocouple measurements below 1600°C where cold junction temperature variation affects accuracy and the display lacks automatic cold junction compensation, use a cold-junction compensator.
Compensating cable selection. Choose compensation wire or cable compatible with thermocouple type, thermocouple grade, and environment. Select ordinary grade for ?20 to +100°C service and heat-resistant grade for ?40 to +250°C. Use shielded compensation cable where intermittent electric heating or strong electric/magnetic fields exist. Determine conductor cross-section from loop resistance over cable length and allowable input resistance of the display, transmitter, or computer interface.
3. Pressure Instruments Selection
3.1 Pressure gauge selection
Choose by environment and media:
- In highly corrosive atmosphere, dusty or washdown environments, prefer sealed full-plastic pressure gauges.
- For dilute nitric, acetic, ammonia, and other corrosive media use acid-resistant gauges, ammonia gauges, or stainless steel diaphragm gauges.
- For strong corrosives, solids-containing, or viscous media, use diaphragm pressure gauges; select diaphragm material according to media.
- Crystallizing or highly viscous media: diaphragm gauges.
- High mechanical vibration: use vibration-resistant gauges or marine-type gauges.
- In hazardous explosive atmospheres requiring electrical contact signals, use explosion-proof contact pressure gauges.
- Some media require dedicated gauges: ammonia, oxygen, hydrogen, chlorine, acetylene, hydrogen sulfide, caustic solutions, etc.; use gauges designed for those gases.
Accuracy classes. General pressure gauges and diaphragm/ capsule gauges: class 1.5 or 2.5. Precision and calibration gauges: class 0.4, 0.25, or 0.16.
Case sizes. For pipeline and equipment mounting: nominal diameters 100 mm or 150 mm. For pneumatic instrument lines and auxiliaries: 60 mm. For low-light, high-mount, or hard-to-observe locations: 200 mm or 250 mm.
Range selection. For stable pressure: normal operating pressure should be between one-third and two-thirds of the upper range. For pulsating pressures, normal operating pressure should be between one-third and one-half of upper range. For medium and high pressures (>4 MPa), normal operating pressure should not exceed half of the upper range.
Units and scale. Use SI units: pascal (Pa), kilopascal (kPa), and megapascal (MPa). For international projects or imported instruments, international or relevant national standards may be used.
3.2 Transmitters and sensors
Use transmitters when transmitting standard signals such as 4–20 mA. In flammable or explosive atmospheres use pneumatic transmitters or explosion-proof electrical transmitters. For crystallizing, fouling, blocking, viscous, or corrosive media use flange-mounted transmitters and select wetted materials per media. Where environment is benign and accuracy/reliability needs are low, remote mechanical pressure gauges, resistive or inductive remote gauges, or Hall-effect pressure transmitters may be acceptable. For very small pressures (<500 Pa), use micro-differential pressure transmitters.
3.3 Installation accessories
- For steam and media above 60°C use siphon or U-shaped pigtail loops.
- For liquefiable gases where the take-off point is above the instrument, use separators.
- For dusty gases use filters.
- For pulsating pressures use dampers or buffers.
- When ambient temperature is near or below the medium freezing point, use insulation or trace heating.
- Use instrument protection enclosures for outdoor-installed switches and transmitters or for installations in atmospheres with severe corrosion, heavy dust, or other harmful conditions.
4. Flowmeter Selection
4.1 General principles
Scale selection. Scales should follow instrument modularity. If non-integer readings occur, select integer scales for easier interpretation.
Root-of-pressure differential scale:
- Maximum flow should not exceed 95% of full scale.
- Normal flow should be 70%–85% of full scale.
- Minimum flow should be at least 30% of full scale.
Linear scale:
- Maximum flow not above 90% of full scale.
- Normal flow 50%–70% of full scale.
- Minimum flow at least 10% of full scale.
Accuracy. Flowmeters used for energy metering should meet applicable regulations. Typical accuracy recommendations:
- Fuel in/out plant settlement: ±0.1%
- Workshop process technical-economic analysis: ±0.5%–2%
- Industrial and domestic water measurement: ±2.5%
- Steam measurement (including superheated and saturated): ±2.5%
- Natural gas and town gas: ±2.0%
- Oil for key equipment/process control: ±1.5%
- Other energy-bearing media for process control (compressed air, oxygen, nitrogen, hydrogen, water): ±2%
Units. Volumetric: m3/h, L/h. Mass flow: kg/h, t/h. Gas volume at standard conditions: Nm3/h (0°C, 0.1013 MPa).
4.2 Primary choices for common fluids, liquids, and steam
1) Differential-pressure flowmeters
Throttle devices. For general fluids, use standard orifice plates or standard nozzles complying with national standard GB2624-81 or ISO 5167-1980. When newer national standards apply, follow them.
Non-standard throttle devices: choose venturi tubes when low pressure loss and accurate measurement are required, for clean fluids, pipe diameters 100–800 mm, and pressures up to 1.0 MPa. Use double-orifice plates for clean fluids with Reynolds number between 3,000 and 300,000. Quarter-circle nozzles are suitable for clean fluids with Reynolds number 200–100,000. Eccentric orifice plates are for dirty media that may deposit solids and for horizontal or inclined pipe runs.
Tap locations. Standardize tap methods across a project. Common methods are corner taps or flange taps; use radial taps or other methods when required by conditions and accuracy.
Differential pressure transmitter range. Select based on calculations. Typical ranges: low: 6 kPa, 10 kPa; medium: 16 kPa, 25 kPa; high: 40 kPa, 60 kPa.
Accuracy improvements. For fluids with large temperature or pressure fluctuations, apply temperature/pressure compensation. Where straight-run length is insufficient or swirl exists, add flow conditioners of appropriate diameter.
Special differential meters: steam flowmeters for saturated steam when required accuracy is not higher than 2.5% and totalizing is needed; integral orifice meters for small clean fluid flows with DN < 50 mm and range ratio ≤ 3:1; steam temperature limit for integral orifice is 120°C.
2) Area-type flowmeters
Use rotameters when accuracy within 1.5% and range ratio ≤ 10:1; glass rotameters for small flows of clean, transparent, non-toxic fluids at pressure <1 MPa and temperature <100°C. Metal tube rotameters suit vaporizable, condensing, toxic, flammable, non-magnetic, fiber- and wear-free fluids compatible with stainless steel.
Special metal tube rotameters include jacketed versions for media prone to vaporization or crystallization, and corrosion-resistant designs for corrosive media.
Installation. Rotameters require vertical orientation with upward flow, minimal vibration, easy observation and maintenance, upstream and downstream isolation valves, and bypass valves. For dirty media, install an upstream filter.
3) Velocity-type flowmeters
Target meters for viscous liquids with some solids, when accuracy not higher than 1.5% and range ratio ≤ 3:1. Typically installed in horizontal pipelines with upstream straight lengths 15–40 D and downstream 5 D.
Turbine meters for clean gases and liquids with kinematic viscosity ≤ 5×10?6 m2/s, for accurate measurement and range ratio ≤ 10:1. Install horizontally so the line is full, with upstream filter, upstream straight-run ≥ 20 D, downstream ≥ 5 D.
Vortex flowmeters for large and medium flows of clean gases, steam, and liquids. Not suitable for low-velocity fluids or liquids with viscosity > 20×10?3 Pa·s. Upstream straight-run 15–40 D or ≥ 10 D when a flow conditioner is used; downstream ≥ 5 D.
Water meters for local cumulative water measurement when range ratio ≤ 30:1. Install in horizontal pipes with upstream straight-run ≥ 8 D and downstream ≥ 5 D.
4.3 Corrosive, conductive, or solids-laden media
1) Electromagnetic flowmeters
Suitable for liquids with conductivity > 10 μS/cm or homogeneous slurry flows. They offer corrosion and wear resistance, no pressure loss, and can measure strong acids, bases, salts, ammonia, slurries, pulp, etc. Mounting direction may be vertical, horizontal, or inclined; vertical installations must be upward flow. For horizontal mounting, ensure the pipe is always full and electrodes are on the same horizontal plane. Upstream straight-run 5–10 D, downstream 3–5 D, but recommendations vary by manufacturer. Keep transmitters away from strong magnetic fields.
4.4 High-viscosity fluids
1) Positive displacement meters
Elliptical gear meters for clean, high-viscosity liquids requiring good accuracy when range ratio < 10:1. Install horizontally with the dial vertical; provide upstream filter and isolation and bypass valves. Use micro elliptical gear meters for very low flows and deaerators for vaporizable media.
Worm wheel meters for clean fluids, especially lubricating oils, when high accuracy is needed. Install horizontally with bypass line and inlet filter.
Scoop-type meters for continuous closed-pipe measurement of oils and similar fluids where accurate totalizing is required. Ensure the line is full and the counter faces vertically.
2) Target meters may also be used for viscous fluids with few solids when accuracy ≤ 1.5% and range ratio ≤ 3:1.
4.5 Large-diameter pipelines
For large diameters where pressure loss significantly affects energy consumption, consider low-loss or insert-style technologies such as acoustic averaging tubes, insertion vortex or turbine meters, electromagnetic flowmeters, venturi tubes, or ultrasonic flowmeters.
1) Acoustic averaging tube meters for clean gases, steam, and low-viscosity liquids where pressure loss must be minimized. Install horizontally with upstream straight-run 6–24 D and downstream 3–4 D.
4.6 New technologies
1) Ultrasonic flowmeters
Suitable for any sound-conducting fluid. For strongly corrosive, nonconductive, flammable or radioactive media where contact measurement is not feasible, ultrasonic meters can be used.
2) Coriolis mass flowmeters
When direct, accurate mass flow measurement of liquids, high-density gases, or slurries is required, use mass flowmeters. They provide accurate mass flow independent of temperature, pressure, density, or viscosity changes, can be mounted in any orientation, and typically do not require straight-run lengths.
4.7 Solids flow measurement
Impulse or impact meters for free-falling solid and granular flows. Impact meters work for a wide range of particle sizes in dusty environments, provided individual particle weight is not more than about 5% of the designed impact plate weight. Free fall conditions and geometry must be calculated for correct installation.
Conveyor belt scales for measuring solids on belts, installed on conveyors that meet standard performance criteria. Mounting position relative to chute affects accuracy.
Dynamic weighbridges for continuous weighing of rail cars.
5. Level Instruments Selection
5.1 General principles
Understand process conditions, media properties, and measurement/control system requirements to evaluate technical and economic tradeoffs.
Liquid level and interface measurement commonly use differential-pressure, float, or displacer methods. When these are unsuitable, consider capacitance, resistive (electro-contact), ultrasonic, microwave, or other technologies. Choose solids level sensors according to particle size, repose angle, conductivity, silo geometry, and measurement needs.
Instrument construction and materials must suit media properties: pressure, temperature, corrosiveness, electrical properties, tendency to polymerize, form deposits, crystallize, foam, vaporize, or entrain suspended solids; also consider density and density variation, surface agitation, and solids particle size.
Display and signal functions should match operational and system requirements. For signal transmission choose instruments with analog or digital outputs as needed.
Range should cover actual variation; normal level should be near 50% of instrument range for most instruments except those used for volume metering. Accuracy depends on process needs; for volumetric level meters, aim for 0.5% or better.
For electronic level instruments in explosive atmospheres or areas with corrosive gases or harmful dust, select appropriate explosion-proof housings or protective measures according to the hazard classification and media.
5.2 Liquid level and interface measurement
1) Differential-pressure instruments
Prefer differential-pressure instruments for continuous liquid level measurement. They can be used for interface measurement if the total liquid level remains above the upper pressure tap. For high-accuracy or complex computation needs, consider intelligent differential-pressure transmitters with 0.2% accuracy or better.
Avoid differential-pressure instruments when liquid density varies significantly under normal conditions. For corrosive, crystallizing, viscous, volatile, or suspended-solids-bearing liquids, use flange-sealed differential-pressure instruments; for highly crystallizing, viscous, or sedimenting liquids, consider insertion-flange instruments. When vapor condensation or the need to isolate high-temperature liquid from the transmitter exists, use double-flange assemblies. In some cases use purge or flush methods with ordinary pressure gauges or transmitters.
For boiler drum level measurement use temperature-compensated dual-chamber equalizing vessels.
Consider zero and span shifts when selecting range.
2) Float-operated level gauges
Use for continuous measurement up to 2,000 mm range for liquids with specific gravity 0.5–1.5, and for interface measurement up to 1,200 mm range when density difference is 0.1–0.5. Suitable for vacuum or volatile liquids. For local indication or control, pneumatic float gauges are appropriate. Float gauges require clean liquids. For high remote-signal accuracy use force-balance types; for local indication or control with lower accuracy, use displacement-balance types.
Internal floats suit open tanks and basins, and cases where liquid may crystallize or become viscous at ambient temperature but not at operating temperature. For processes that cannot be stopped, avoid internal floats; use external floats. For highly viscous, crystallizing, or high-temperature liquids, external floats are not appropriate. Add stilling tubes to reduce agitation effects. For electronic float types in fluctuating level environments, add damping to output signals.
3) Displacer (plunger) level sensors
Use for large tanks with clean liquids for continuous measurement and volumetric metering. Do not use for heavily fouled liquids or liquids that freeze at ambient temperature. For interface measurement the two liquids should have fairly constant densities and a density difference of at least 0.2. For internal displacers add guides and stilling tubes to prevent float drift and agitation effects. For high-accuracy single or multiple tank level, interface, and volume measurement, use dedicated tank gauging systems such as guided-wave radar or complete tank measurement systems.
4) Capacitance level gauges
Use for corrosive, sedimenting, or other chemical process liquids. For interface measurement the electrical properties of both liquids must meet instrument requirements. Electrode design and material depend on media electrical properties and vessel materials. For nonadhesive nonconductive liquids use sleeve electrodes; for nonadhesive conductive liquids use coaxial electrodes; for sticky nonconductive liquids use exposed electrodes with low-affinity materials or automatic cleaning. Capacitance sensors are not suitable for sticky conductive liquids for continuous measurement. Because they are sensitive to electromagnetic interference, use shielded cable or other EMI mitigation. For point measurement use horizontal installation; for continuous measurement use vertical installation.
5) Resistive (electro-contact) probes
Use for point level measurement of conductive liquids or those with some moisture content, such as coal or coke. Ensure electrode-to-ground resistance meets product specifications. Avoid when electrodes foul easily or electrolysis occurs between electrodes; do not use for nonconductive, strongly adhesive liquids.
6) Hydrostatic (pressure) level sensors
For water depths of 5 m to 100 m such as wells, water tanks, and reservoirs, use hydrostatic sensors. Suitable for free-surface vessels without pressure. Avoid when liquid density varies significantly under normal operation.
7) Ultrasonic level sensors
Use for corrosive, high-viscosity, or toxic liquids where other level sensors perform poorly. Select model based on media properties. Ultrasonic sensors require reflective and sound-propagating surfaces and are not suitable for vacuum vessels. Avoid for liquids with significant entrained gas or solid particles. For vessels with obstructions affecting sound propagation do not use ultrasonic. For continuous measurement where temperature or composition changes affect sound speed, provide compensation. Use shielded cable between sensor and transmitter to reduce EMI.
8) Microwave (radar) level sensors
For high-accuracy measurement in large fixed-roof or floating-roof tanks containing corrosive, viscous, or toxic liquids, use microwave radar sensors. The sensor scans microwaves and measures frequency shift between transmitted and reflected signals proportional to distance. Antenna design and materials depend on media and tank pressure. Avoid if internal obstacles affect microwave propagation. For varying vapor densities, provide propagation speed compensation; for boiling or agitated surfaces, consider compensation measures such as stilling probes to improve accuracy.
9) Nuclear level gauges
Use non-contact nuclear gauges when other technologies cannot meet measurement requirements for high temperature, high pressure, high viscosity, corrosive, explosive, or toxic fluids. Select radiation source strength to minimize workplace dose while meeting measurement needs and comply with radiation protection standards; apply shielding and isolation as necessary. Choose isotope type based on penetration requirements: radium for low strength, cesium-137 for moderate, cobalt-60 for high penetration. To minimize errors from source decay, instruments should compensate for source decay to reduce calibration frequency and improve stability.
10) Laser level sensors
Use for tanks with complex geometry or obstructed access where conventional installation is difficult. Avoid for completely transparent, nonreflective liquids.
5.3 Solids level and bulk material measurement
1) Capacitance sensors
Use for granular or powdered materials such as coal, polymer beads, fertilizers, sand, etc., for continuous or point level measurement. Use shielded cable for detector extensions to mitigate EMI.
2) Acoustic and ultrasonic sensors
Tuning fork (vibrating fork) point level detectors suit bins or hoppers with little vibration and particle sizes under 10 mm. Ultrasonic blockage sensors suit powders with particle sizes under 5 mm for point level detection. Reflective ultrasonic sensors are suitable for fine powders for continuous and point measurement but are not suitable for dust-clouded environments or uneven surfaces.
3) Resistive (electro-contact) sensors
For granular and powdered materials with reasonable conductivity or moisture content such as coal and coke, resistive point sensors can be used, provided electrode-to-ground resistance meets specifications.
4) Microwave sensors
Use microwave sensors for high-temperature, adhesive, corrosive, or toxic bulk materials, both for point and continuous level measurement. Not suitable for uneven surfaces.
5) Nuclear sensors
For high-temperature, high-pressure, adhesive, corrosive, or toxic bulk materials where other methods fail, nuclear sensors are an option; follow the same radiation safety and selection considerations as for liquid nuclear gauges.
6) Laser sensors
Use for containers with complex structures or difficult installations. Avoid for fully transparent nonreflective solids.
7) Rotational paddle (rotor) sensors
For nonpressurized silos and hoppers with discharged material specific gravity ≥ 0.2, rotational paddle point sensors are suitable. Select paddle size by bulk density, and install a protection plate above the paddle to prevent impact-induced false trips.
8) Diaphragm switches
Diaphragm point sensors can be used for particulate materials in silos and hoppers. Because diaphragms are sensitive to dust adhesion and pressure from flowing material, avoid for high-accuracy applications.
9) Weight-and-fall (counterweight) measures
For large silos and warehouses with wide level ranges, or open or closed vessels, use counterweight-based timed continuous measurement for blocky or granular, nonadhesive bulk materials. Choose counterweight form based on particle size and moisture. In heavily dusty environments use counterweight devices with purge capability.
