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.