LEL monitoring of solvent vapors in coil coating

The customer:

The European coil coating company supplies colour coated alumium, mostly for the building market and the transportation sector.

The process:

Coil coating is a process where a coil of steel or alumium sheet is unwound and coated with a paint containing solvents. Then the strip enters an oven which evaporates the solvents and cures the coating. Ventilation air is used in the oven to remove the evaporated solvents and keep the solvent concentration well below the explosive limit.

A gas detection system system continuously monitors the solvent vapor concentration and automatically initiates emergency measures if the concentration rises above preset thresholds given in % of the lower explosive limit (LEL).

The problem:

Due to the high oven temperatures and the complex solvent mixtures coil coating is a very demanding application for LEL monitoring systems. The drying oven of the color coating line at the customer had already been equipped with LEL monitoring systems which, however, frequently failed and therefore caused too many shutdowns of the coating line while driving maintenance costs to unsustainable levels.

The solution:

The existing LEL analyzers based on the flame ionization detector principle have been replaced by new PrevEx FTA analyzers of type SNR675-T2 which use the flame temperature analyzer principle. This analyzer model has been specially designed for the monitoring of coil coating operations: it operates at a higher operating temperature of 250°C and is less subject to problems caused by condensation.

It was critical that the time needed for the replacement of the LEL monitoring system was kept as short as possible to minimize downtime of the line. The new FTA analyzers were mounted at the same location as the previous ones, a part of the auxiliary gas supply system could be re-used. A single touch screen operator panel, the FTA Monitor 670 CT, replaced the various control units of the FIDs.

The result is a trouble-free LEL monitoring system without any shutdowns due to analyzer failures. Regular annual service by SCIMA personnel ensures that it stays that way.

As an additional bonus the customer realized that the FTA analyzers show lower concentration readings than the previously used devices because FTA analyzers in general have a much better accuracy when measuring complex solvent mixtures. This presents an opportunity to increase production speed or save energy by using less ventilation air, thus offering significant cost savings.