Application story: LEL monitoring of waste gas before a thermal oxidizer on a chemical site
A chemical site unites various plants from the chemcial industry and from the life sciences. The site provides services and a common infrastructure which includes the incineration of waste gas in a central thermal oxidizer.
Various production processes generate waste gas which needs to be incinerated in a central regenerative oxidizer (RTO) before the air can be released into the ambient atmosphere.
The waste gas contains flammable gases and vapors and their concentration could at certain conditions be high enough that they might cause an explosion in the RTO. The European standard EN 12753 requires that the concentration of flammable gases and vapors in the waste must be below certain levels so the the danger of explosion can be averted. In the present application the maximum allowed concentration is 20% LEL.
Conforming with the safety standard therefore requires the continuous monitoring of the concentration of flammables in the waste gas in percent of the lower explosive limit (% LEL) and the activation of a bypass if the concentration surpasses a preset threshold.
The gas detection devices employed were based on the flame ionization detection (FID) principle. They have proved to require very high maintenance and still exhibited frequent failures. Due to their measurement principle, these FIDs could not measure the non-hydrocarbons and some other flammable substances in the waste gas. Addtionally, the installed FID analyzers did not match the most recent state of the art and did not conform to the requirements of the ATEX directive concerning the performance of gas detection devices.
The composition of the waste is very complex and variable: this caused a large error in the measurement in % LEL with the FIDs. The corrosive nature of the waste gas caused problems.
The customer chose PrevEx FTA analyzers, specifically the model SNR674-T3. These analyzers can measure also non-hydrocarbon flammable gases and due to their extremely close response factors they minimize the variation in sensitivities versus different substances. This results in significantly more accurate readings considering the highly complex and variable composition of the waste gas.
The analyzers have been installed in a 2-out-of-3 system to increase both functional safety as well as availability.
The experience shows that the PrevEx FTA analyzers cope very well with the difficult and corrosive sample gas: after a few years of operation there are almost no sign of corrosion inside the analyzers. Regular filter changes, cleaning of sample probes and calibrations by the customer help to eliminate analyzer failures.