ABB LS25 laser analyzer troubleshooting



ABB LS25 laser analyzer troubleshooting

Introduction: "Normal display" of in-situ laser analyzer does not mean "reliable measurement"

The ABB AO2000-LS25 belongs to the tunable diode laser absorption spectroscopy (TDLAS) in-situ gas analyzer, which is installed in a pair of transmitting and receiving units to measure the average gas concentration along the optical path. It is commonly used in chimneys, pipelines, process chambers and other sites, and has the characteristics of fast response, no need for sampling pretreatment, and can measure gases such as O ₂, CO, CO ₂, CH ₄, H ₂ O, NH ∝, HCl, HF, H ₂ S. When on-site engineers encounter "concentration drift", "low transmission alarm", "laser alignment error", and "even more inaccurate after calibration", if they only focus on the concentration values in the software, it is easy to overlook one fact: the measurement reliability of the laser analyzer is determined by the optical path, blowing, temperature and pressure compensation, optical path parameters, and calibration status.

The LS25 consists of three independent units: a transmitting unit, a receiving unit, and a power supply unit. The launch unit contains a temperature controlled diode laser, collimating optics, and main electronics; The receiving unit includes a focusing lens, a photodetector, and receiving electronics; The power unit converts 100-240V AC to 24V DC and can also directly supply 24V DC. The protection level of the transmitting and receiving unit is IP66, and the standard optical window can withstand a maximum absolute pressure of 5 bar. A dedicated receiving cable is used to connect the transmission and reception, and the received signal is amplified and sent back to the transmission unit. The transmission unit also supplies power to the reception unit. Understanding the structure of "three units+opposing light paths" is the starting point for troubleshooting.

First check installation and alignment: The optical path is off center, and even if the software parameters are correct, it won't work

The installation requirements for LS25 are stricter than those for general point analyzers. It is recommended to measure a straight pipe section with at least 5 times the diameter of the flue upstream and at least 2 times the diameter of the flue downstream. The transmitting and receiving units should be easy for personnel to stand in front of the unit and adjust the fixing bolts with two M16 wrenches; The receiving unit should leave at least 1m of free space outward from the flue flange. Two pairs of perforations with a diameter of at least 50mm are required on both sides of the flue. The standard flange is DN50/PN10, with an inner diameter of 50mm and an outer diameter of 165mm. The flange can be directly welded to the process pipeline or equipped with a valve for easy safety maintenance.

The initial angle of the flange should meet 90 °± 1.5 °, and the deviation of the centerline of the flanges on both sides should be less than the specified value: DN50 flange δ min is 40mm, DN80 is 55mm, otherwise inserting the pipe will block the laser beam. After installation, the drift between the laser beam and the central axis of the receiving unit caused by temperature or vibration is allowed to be ± 0.3 °. If it exceeds this range, low transmission or alignment errors may occur.

When troubleshooting on site, first check whether the transmitting and receiving units have been displaced by external forces, whether the flange bolts are loose, and whether the insertion pipe has accumulated dust and deformed. If the LCD alternately displays "Laser line up error" and "Low transmission", do not rush to change the concentration parameter. Instead, align the red laser with the fixture again: first loosen the corresponding installation nut, remove the adapter ring, install the red laser fixture on the emitting side, find the light spot on the receiving side target, adjust the M16 adjustment screw on the emitting side to center the light spot, and tighten the locking screw; Then switch the red laser fixture to the receiving side and adjust the receiving side in reverse to center the light spot on the emitting side. After rough alignment, use a floating input multimeter to measure the alignment voltage, and repeatedly adjust the transmitting and receiving sides to the maximum. Align the voltage from 0V during 0% transmission to approximately -3V during 100% transmission. The receiving and transmitting sides should alternate to maximize until the reading no longer increases, then lock and confirm that there is no change.

The key to this step is' mechanical first, software later '. Many cases of 'low concentration' are actually due to the receiving end only receiving a portion of the light spot, weak second harmonic signals, unstable line width measurement, and inability to recover even after the instrument automatically tracks the line position. Cleaning the window and realigning it is more effective than recalibrating it at this time.

Blowing and window contamination: the first suspicion of low transmission alarm

LS25 uses positive pressure blowing to keep the window clean and prevent dust and particle deposition. The blowing air must be dry and clean, and it is recommended to use instrument air; When there is no instrument air, a separate fan is required. The typical blowing flow rate is about 20-50 l/min, depending on the process; The initial blowing speed inside the flange can be set at about 1/10 of the flue gas velocity. The air quality should meet ISO 8573.1 Class 2-3, filter to 1 μ m, and remove condensed water and oil mist. The residual oil mist at 21 ° C should not exceed 0.5mg/m ³. High temperature, high pressure, or O ₂ measurement may require nitrogen purging.

If blowing is also required inside the transmitting and receiving units, the direction should be as shown in the diagram, and the gas must be cleaner. It is recommended to use nitrogen with a flow rate of less than 0.5 l/min to avoid pressure accumulation inside the unit. Oil and water in the instrument air may permanently damage the internal optical components. For toxic, highly corrosive, or high-pressure applications, the first flange should use an isolation flange and be equipped with a shut-off valve; Before dismantling the isolation flange, it must be confirmed that the process has been shut down or the volume between valves has been thoroughly purged.

On site "Low transmission" warning usually starts with checking three things: whether the window has accumulated dust, whether the blowing is cut off, and whether the light path is offset. Windows can be cleaned with non abrasive cleaning agents or solvents; If the window has cracks or damage, it should be replaced. The window is installed in a wedge shape, and when replacing it, the direction must be consistent with the original direction, otherwise it will damage the alignment. If the transmission is still low after cleaning, check the alignment again; If the transmission returns to normal but the concentration is still abnormal, then enter the temperature pressure and optical path parameter investigation.

Temperature and pressure, optical path length, and average: unpredictable 'invisible parameters'

The concentration calculation of LS25 depends on gas pressure, temperature, and optical path length. The absorption line intensity and line width vary with pressure and temperature, and the instrument compensates with a correction factor that deviates from the standard atmosphere (1.013bar, 23 ° C); For relative units such as ppm,%, mg/Nm ³, temperature and pressure also directly participate in the conversion of the ideal gas law. Therefore, the pressure must be in bar absolute and the temperature in Celsius. Gauge pressure to absolute pressure: P (bara)=1.013+P (barg); The conversion of pressure gauge to bara: P (bara)=1.013+P (pressure gauge)/14.5. Fahrenheit to Celsius: T (C)=(T (F) -32)/1.8; Kelvin approximation: T (C)=T (K) -273.

The pressure/temperature input methods can be selected as Fixed, Current loop, Internal, Spectral, or Serial. Fixed is a manually fixed value; The current loop uses a 4-20mA external probe, and the corresponding values for PLC 4mA and 20mA need to be set; Internally, use the pressure sensor inside the transmitting unit to measure the ambient pressure, and use the average temperature sensor inside the transmitting/receiving unit to measure the temperature; Spectral is only applicable to instruments capable of spectral temperature measurement; Serial is input through RS-232 or Ethernet special data packets. If an external temperature and pressure probe is used on site and a "PLC T-read error" or "PLC P-read error" occurs, it indicates that the input current exceeds 23.7 mA or is below 0.3 mA. The probe wiring, power supply, and range should be checked; If it reports "T-read out of range" or "P-read out of range", it indicates that the current exceeds 22mA or is below 3.2mA, and the measurement may no longer be reliable. The sensor should be checked or a fixed value should be used instead.

The optical path parameters are divided into three segments: the optical path Lg passing through the gas, the optical path Lf passing through the flange, and the optical path Lru/tu passing through the interior of the transmitting and receiving units. Lg is usually equal to the diameter of the flue or the free space between the inserted pipes; When equipped with a measuring pool, it is equal to the pool length. If the measured gas also exists inside the flange or unit, the corresponding gas concentration needs to be set and compensated; If it does not exist, set the concentration to zero. Parameter setting errors can lead to concentration proportional deviation, especially when setting Lg as the distance from the flange tip to the flange tip, or omitting the internal optical path. The average frequency of concentration determines the smoothness of display and 4-20mA output, with an average time of Tav=N × Tprim; Line width averaging, line width adaptive averaging, and spectral temperature averaging can also affect weak signal stability. If there is a large fluctuation in concentration on site, do not blindly increase the average frequency. Instead, check the stability of the light path and purge first.

Calibration: Proportional calibration and global calibration cannot be mixed

LS25 is calibrated with certified gas at the factory and usually does not require calibration upon receipt. After running for a period of time, laser aging may cause drift. It is recommended to verify annually with standard gas and calibration pool. Calibration is not recommended when the reading deviation is less than 2-3%, as this is close to the absolute uncertainty of the calibration gas itself; The instrument accuracy is about 1% of the reading (when the temperature and pressure settings are correct). The second harmonic technology itself has no baseline, and the zero point does not require calibration; If zero is forcibly marked, the software will report an error. Calibration is only allowed if the amplitude of the absorbed signal is higher than the preset level (usually 20 times the 1m optical path detection limit).

PROPORTIONAL only adjusts the proportionality constant between the peak amplitude of the absorption line and the concentration, without requiring special gas parameters or signal quality, and can be adjusted as needed on the process gas. It is possible to calibrate a specific gas separately, or use "Adjust" to automatically adjust other gases according to the proportion of the already calibrated gas. Global calibration simultaneously adjusts line width parameters and proportional constants, requiring stable and controllable reference gas, calibration pool, and certification mixture gas. Typically, nitrogen gas is used to prepare the measured gas, and dry air can be used for O ₂; Global calibration requires accurate pressure and temperature input into the calibration pool, and should be performed by experienced personnel after major repairs. Do not perform global calibration on process gases, and do not perform global calibration at low concentrations, short optical paths, or poor signal-to-noise ratios. Global calibration should only be considered after replacing the laser module, motherboard, receiver board, or adjusting the laser temperature, modulation amplitude, phase, and current.

When calibrating the pool connection, if there is flow in the gas path, the pool pressure is higher than the ambient pressure, and the pressure difference depends on the flow rate and the outlet pipe diameter and length. It is best to use an absolute pressure sensor to measure the pressure near the pool; If only environmental pressure can be measured, the airflow can be turned off and stabilized for 1 minute before calibration. Reactive and adsorbent gases such as HF, NH3, H ₂ O, HCl, H ₂ S should be treated with stainless steel tanks (PTFE for HF) and PTFE short tubes. The flow rate can be increased to 5-7 l/min, and the attenuation should be observed by turning off the flow after the concentration stabilizes; Rapid decay indicates that the pool needs to be passivated. After calibration, the settings file and reading file should be saved for easy tracking.

Quick troubleshooting of LCD error codes

The LCD of LS25 is the first diagnostic window on site. The common information and processing methods are as follows:

Low transmission: Transmission below the reliable measurement limit. First, clean the window, check for blowing, and realign it; Do not change the concentration parameters first.

Laser line up error: The laser did not hit the detector. Check for window contamination, light path obstruction, and alignment of transmission/reception.

PLC T-read error/PLC P-read error: 4-20mA temperature and voltage input current>23.7mA or<0.3mA. Check probe wiring, power supply, and range; Fixed values can be temporarily used instead.

T-read out of range/P-read out of range (warning): Current>22mA or<3.2mA, temperature and voltage may be incorrect. Check the sensor or change the fixed value.

P/T out of range (warning): Temperature and pressure compensation exceeds the acceptable range, and concentration accuracy may decrease. If the input value is correct, it indicates that the instrument is operating outside the specifications.

Ex read out of range (warning): The input current of the external gas sensor exceeds the range, check the Extra/Low current input.

Voltage supply failure: The power supply of the transmitting unit exceeds the range of 18-36V. Check the voltage drop of the power supply and cables; Repeatedly contacting the service.

Low/High laser temp (error): Laser temperature adjustment failed, instrument enters FAULT and automatically restarts after one hour. Check whether the launch unit is overheated, has heat dissipation and ventilation; Repeatedly checking electrical contacts or hardware.

Low/High box temp: The internal temperature of the transmitting or receiving unit exceeds the limit. Check the environmental temperature, thermal radiation, ventilation, and add insulation if necessary.

EEPROM error: Internal memory failure. Upload backup settings file; Repeatedly contacting the service.

Self test fail: Start self-test failed. Restart; Record all LCD information, download the Error log and System log, and contact the service.

Low/High line position, No absorption line: Spectral abnormality, possibly due to unknown interference gas, laser temperature/wavelength error, emission unit overheating, or laser malfunction. Verify by continuously running the calibration pool and appropriate concentration gas for more than 70 minutes; Still not disappearing or with obvious calibration errors, contact the service.

Saturation of direct/Saturation of harp. (Error): Direct signal or second harmonic saturation. When the concentration is too high or the optical path is too long, shorten the measurement optical path and reduce the receiving gain; Covering the receiving window and still reporting an error may indicate an electronic malfunction.

Communication and Explosion proof Version: Don't mistake 'unable to connect' for instrument failure

The front panel service port of LS25 is isolated RS-232, and the service software serial port defaults to 9600, 8 data bits, no verification, and 1 stop bit; The default IP for Ethernet is usually 192.168.1.237, with port 5001. You can change the IP, subnet mask, and gateway in the TCP/IP menu. Note that the LS25 Ethernet port will be turned off when using the service interface, and both should not be occupied simultaneously. When connecting to AO2000, the software versions 3.0.7/4.0.1 and ≥ 5.0.0 have different interfaces: for ≥ 5.0.0, it is recommended to connect X09 to the factory network and X08 to the analyzer; A single analyzer should be connected directly with a crossover cable, and multiple analyzers should be connected with switches. The IP address of the analyzer should be on the same network segment as X08 and unique; LS25 has been added to the "Setup system modules" of AO2000, with a 14 digit serial number and the first three digits must be 008, which is different for each unit. If AO2000 cannot read LS25, first check the network cable, switch, IP network segment, and port, and then check the AO2000 module serial number and IP settings.

The explosion-proof version should be separately noted. Class I Division 2 version is CSA certified T4, and the power box cannot be installed in Div.2. It must be placed in a non hazardous area or replaced with a suitable Div.2 power supply; Cable joints (Myers Hubs) and sealants shall be constructed in accordance with the manufacturer's and NEC/local specifications, and unused joints must be sealed; The service PC must be placed in a secure area. ATEX Zone 2 version is Ex nA nC [op is Ga] IIC T5 Gc, -20 ° C ≤ Ta ≤+55 ° C; receiving and main power cable connectors M20 (outer diameter 7-12mm), network and service connectors M16 (outer diameter 3.5-8mm); Unused connectors are sealed according to local standards; Wipe with a damp cloth before maintenance to avoid static electricity. Regardless of the version, the transmission/reception cable must not be replaced or changed in length beyond 20m without authorization, otherwise it may affect calibration and accuracy; The maximum length of the receiving cable is 150m, the maximum length of the power cable is 100m, and the service PC cable can be extended from 3m to about 10m.

Maintenance and span/zero point inspection: Move troubleshooting to daily operations

LS25 has no moving parts and does not require consumables, resulting in low maintenance. Most applications have an acceptable maintenance interval of three months or more. Regularly check the optical transmission in daily life, clean the window, check blowing and alignment when the transmission drops; It is recommended to introduce a sufficiently high concentration gas every three months for at least 10 minutes to confirm the response when testing the "zero gas" application; Calibration should be verified every 3-12 months, depending on accuracy requirements. Save the settings and measurement data after each calibration check for easy traceability in the future.

Span inspection can be performed using either a flow pool or an internal sealed pool. When the circulation pool is used for non corrosive gases, the 100mm space inside the receiving unit is utilized, while for corrosive gases, an external circulation pool is used; The span mode first stores the normal measurement absorption signal and continuously deducts it during inspection, requiring stable process signals; The default concentration is calculated based on a 1m optical path length, for example, a 1% span gas will display 0.1%. The internal sealed pool is moved into the optical path by a servo motor, including zero and span checks. The default span is 75-80% of the full range, and the entire process takes about 15 seconds. Span/zero point inspection is not a "recalibration", but a verification; If the inspection deviation is large, then determine whether proportional calibration or global calibration is needed.