ABB LS4000 laser analyzer

Engineering Practice of Installing Alignment and Purging Systems

LS4000 is an optical instrument that has strict requirements for mechanical installation accuracy. If installed improperly, even the best algorithm cannot compensate for signal loss.

1. Flange parallelism: uncompromising 1.5 ° red line

The data sheet clearly requires that the flanges of the transmitting and receiving units must be parallel within 1.5 °. Due to the straight propagation of the optical path, if there is an angular deviation between the flanges on both sides, the laser beam hitting the detector at the receiving end will cause a deviation, resulting in a sudden drop in signal strength. When welding pipeline flanges, a laser level or precision machining reference must be used for alignment. For large pipe diameters or long-distance installations, it is recommended to use specialized mounting brackets to ensure coaxiality.

2. Gas mixing uniformity and stratification risk

The choice of installation location directly determines whether the measurement results are representative. The instrument must be installed in a straight pipe section where the gas is fully mixed. If installed near bends or valves, gas stratification can cause the laser beam to pass through different concentrations of gas flow layers, resulting in serious errors. The data sheet emphasizes that "measuring stratification in the gas path can lead to erroneous measurements." If there are eddies or dead zones in the pipeline, it should be considered to install a static mixer or re open the hole at a more upstream position.

3. Correct configuration of the blowing system

In the vast majority of applications, process side blowing is necessary, but the instrument casing itself does not require blowing. The function of blowing is to protect the optical window from dust pollution and high-temperature corrosion.

Purging medium: Dry, oil-free instrument air (compliant with ISO 8573.1 Class 2-3) or nitrogen must be used.

Purging interface: Using 1/4 inch Swagelok fittings, compatible with 8mm outer diameter pipelines.

Common faults: Insufficient blowing gas flow or oil and water content in the blowing gas can cause scaling and blurring of the window, resulting in a decrease in transmittance. When the transmittance is below 3%, the instrument will not be able to read properly. During maintenance, the flow meter should be regularly checked and purged to ensure stable air source pressure.

Electrical interface configuration and signal integrity

The junction box of LS4000 provides rich I/O interfaces, and reasonable configuration is the key to stable system operation.

Analog output (AO): up to three 4-20mA outputs (corresponding to measured components and transmittance), maximum working resistance of 500 Ω, and non isolated. When connecting DCS or PLC, it is necessary to ensure that the receiving end is not grounded or use a signal isolator to eliminate ground loop interference. If the transmission distance is long, shielded twisted pair cables should be used and the shielding layer should be grounded at one end.

Analog Input (AI): Two 4-20mA inputs for temperature and pressure compensation, with a maximum working resistance of 100 Ω. It is also non isolated. Debugging points: After connecting the signal, it is necessary to check whether the 4-20mA input value is consistent with the actual output of the transmitter through the service port (Ethernet) or dedicated debugging software. If there is a deviation in the signal, it will cause the compensation curve to shift as a whole, resulting in systematic measurement errors.

Digital Output (DO): Two digital outputs (1A @ 30VDC/AC, NO), used for fault alarm and gas concentration over limit alarm. It is recommended to connect the fault alarm point to the sound and light alarm system in the central control room, so as to promptly notify the operator in case of severe decrease in transmittance or laser failure.

Calibration maintenance and common troubleshooting ideas

The maintenance cycle of LS4000 mainly depends on the application conditions and dust load. But calibration is a necessary means of verifying instrument performance.

1. Calibration strategy: single point calibration and offline pooling

It is recommended to use test gas and an external offline calibration cell for single point calibration of the data table. The calibration cycle is usually once a year. If the composition of the process gas is complex, it is recommended to use a Validation cell for online validation. The verification pool is permanently installed between the process pipeline and the transmitting/receiving unit, and filled with a known concentration of standard gas inside. By comparing and verifying the readings of the pool with the actual measurement values, it is possible to quickly determine whether the measurement is drifting without disassembling the instrument.

2. One of the common faults: continuous high or low measurement values

If the instrument does not report a hardware malfunction, but the measured value deviates from the process assay value for a long time, first check whether the optical path is contaminated. Suggest viewing the transmittance signal through the service port. If the transmittance is lower than the recorded value during initial installation (usually above 80%), it indicates that the window may be scaled. The solution is to close the process isolation valve, disassemble or wipe the window. Secondly, check whether the temperature and pressure compensation signal is accurate. If the temperature transmitter displays 200 ° C but is actually 150 ° C, the compensation algorithm will erroneously increase the calculated gas concentration value.