ABB LS25 laser analyzer troubleshooting

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.