LEL monitoring of industrial processes with PrevEx FTA analyzers

PrevEx FTA analyzers can be used in all applications where the concentration (in % LEL) of flammable gases and vapors in industrial processes must be monitored.

Many coating systems (e.g. coil coating, lacquering, laminating, gravure printing, flexo printing, offset, …) use solvents. After the coating of the substrate a dryer evaporates these solvents using hot air or infrared radiation. The solvent vapors generated in the dryer must be diluted with air to avoid an explosive mixture in the dryer which could be ignited by a source of ignition which is often present in dryers.

Standards such as the European EN 1539:2015 “Dryers and ovens, in which flammable substances are released – Safety requirements” prescribe safety measures to ensure that the atmosphere in the dryer will not explode.  These measures include a minimum ventilation rate (which needs to be monitored), the declaration of hazardous zones and the use of gas detection systems to measure the concentration of solvent vapors in % LEL:

  • Ventilation rates: ventilation rates must usually be set according the maximum solvent intake at the highest production speed to ensure that the dryer is safe in all operating conditions. This would mean that at lower production speeds or with coatings that contains fewer solvents the ventilation rate is higher that necessary: since the ventilation air must heated and usually fed into a thermal oxidizer this will use up more energy (and increase costs).
  • Declaration of hazardous zones can help to increase the safety of the dryer but is not sufficient.
  • A solvent vapor monitoring system measures the solvent vapor concentration in % LEL and generates alarms when the concentration surpasses the allowed maximum concentration. This concentration alarm will then trigger measures to put the dryer into a safe state (e.g. stop of the coating, increase of ventiliation rate, shutdown of the heating system, …).

Coating processes which do not inherently prevent the excess input of solvents into the dryer, require the use of a LEL monitoring system according to EN 1539:2015. A mere monitoring of ventilation rates which are chosen based on the normal solvent intake cannot detect this error nor prevent an explosion.

Using LEL monitoring systems allows to optimally adjust ventilation rates according to the actual solvent concentration in each drying zone. This will increase solvent concentrations to the allowed limits with a number of benefits:

  • energy savings by using less drying air. Depending on the drying temperature these saving can be quite significant.
  • increase of production speed. This is an alternative to saving energy where the speed of the web is increased.
  • saving money on the oxidizer: increasing solvent concentration might finally allow you to run the oxidizer in an autothermal mode and use less natural gas. The lower volume of waste gas from the dryer might offer the opportunity to connect another emission source to the oxidizer or allow you to use a smaller oxidizer.

The adjustment of the ventilation rate to actual solvent concentration can be done in a completely automatic way with continuous damper control, or instead by using a recipe based system with fixed damper settings for each product.

In many coating processes the complex and changing mixtures of solvents require an LEL monitoring system that can not only measure all of the solvents but in particular will be able to measure them all with (almost) the same sensitivity (response factor). If the response factors versus different solvents variey significantly then, in order to be safe, the readings for some of the solvents will be quite exaggerated and might lead to early, unnecessary alarms. This would require higher ventilation rates  and thus counteract the whole idea of the installation of the LEL monitoring system.

PrevEx FTA analyzers are the measurement system with the least variation of response factors for solvents and are therefore optimally suited to be used for the LEL monitoring of complex and variable –  sometimes even unknown – solvent mixtures.

Coating processes are often running 24/7 and a shutdown of the production line due to a failure of the LEL monitoring system can be expensive. A high availability even in demanding environments is therefore a must. The PrevEx FTA analyzer of the 670 series are rugged devives which have been specially developped for use in industrial environments and have proven their superior performance over decades. Heating all parts in the sample train (up to 250°C) and mounting the analyzer directly online at the sampling location allows to deal with difficult, high dew point vapors in the sample gas by effectively preventing condensation.

Industrial ovens or debindering kilns often cause flammable gases and vapors to be released at high temperatures, these can be solvents, milling oils or others substances. Typically these ovens are batch-operated, but some are continuous ovens. The standard EN 1539:2015 describes safety requirements also for these types of ovens and a continuous monitoring of the concentration of flammable gases and vapors will increase the safety of the process and often will help to minimize the amount of ventilation air required.

Because of the large range of operating temperatures of the various models of the PrevEx FTA analyzers the right analyzer configuration will match the requirements of the various applications and oven temperatures. For instance, using special ceramic sample probes the analyzers have successfully been used on debindering/sinter ovens for technial ceramics with temperatures up to 1800°C.

The waste gas of many industrial processes – especially in the chemical, pharmaceutical, printing and converting industry – that contains hydrocarbons must be treated before the air can be released into atmosphere. There are different systems for treating the waste gas but most prevalent are thermal and catalytic oxidizers which incinerate the hydrocarbons and thus destroy them before the clean air is released.

These oxidizers usually are a source of ignition and safety regulations (e.g. EN 12753) require that the concentration of flammable vapors and gases at the inlet of oxidizers must not exceed certain limits (usually 20% or 25% LEL). If it is possible that the waste gases might  ever exceed this limit then the concentration of the flammable gases must be measured in % LEL and certain emergency measures must be activated if the allowable limit is exceeded. Often the emergency measures include the venting of the waste gas into an adsorber or the atmosphere instead of sending it into the oxidizer.

The LEL monitoring system must be able to match the requirement of the measurement task in many respects, but two properties will have significant effects on the investement and operating costs for the oxidizer:

  1. Usually the waste contains complex mixtures of flammable gases and vapors. If the sensitivity of the LEL monitoring system varies significantly versus the various constituents of that mixture then the reading of the analyzer will often be highly exaggerated because it must be calibrated to the substance for which it is least sensitive. Exaggerated readings lead to early, uneccessary alarms and require a dilution of the waste gas and thus a higher total flow into the oxidizer and an oversized oxidizing system. The dilution will also decrease the calorific value of the gas fed into the oxidizer and may require the use of natural gas to support the required minimum operating temperature of the oxidizer: these energy costs can very high. It is therefore imperative to choose an LEL monitoring system with the least variation in sensitivity (response factor) versus the various constituents in the waste gas stream in order to obtain a concentation that is as accurate as possible in spite of the complex composition of the waste gas.
  2. The transport time of the waste gas from the location of the LEL monitoring device to the valve which redirects the waste gas away from the inlet of the incinerator must be longer than the response time of the LEL analyzer plus any processing time and the time needed for the valve operation to take full effect. The internal diameter and the length of the duct must be dimensioned accordingly. That also means that the longer the reponse time of the analyzer, the longer the duct. A short response time will therefore usually translate into savings.

FTA analyzers have the least response factor variation of all measuring principles employed for LEL monitoring in industrial processes and are therefore optimally suited for the measurement of complex and variable mixtures with maximum accuracy. Additionally, FTA analyzers will measure all flammable gases, including non-hydrocarbons (such as hydrogen, carbon monoxide, hydogen sulfide, ammonia…) as well as those hydrocarbons which  flame ionisation detectors (FID) cannot detect, such as formic acid or formaldehyde.

To comply with regulatory requirements it must be observed that the measurement function of any LEL monitoring device used must be approved in an ATEX type examination certificate  (currently the harmonized standards EN 60079-29-1 and EN 50271 must be applied) and therefore must be listed in an EU declaration of conformity according to the ATEX directive 2014/34/EU. PrevEx FTA analyzers of course conform to all these requirements.

Functional safety is usually also an important issue when evaluating the requirements for an LEL analyzer. PrevEx FTA analyzers are available for SIL1/SIL2 (HFT=0) and SIL3 (HFT=1) applications as well as for PL d (EN ISO 13849-1) requirements in a redundant installation.