A: Limitation of an explosive atmosphere

An application process has to be evaluated, with regard to the protective measures to be provided, in terms of the likelihood of an explosive atmosphere occurring.

An explosive atmosphere exists if the ratio of

• solvent vapour
• dust or a
• hybrid mixture

to the available oxygen content is between the respective upper and lower explosion limits.

In liquid coating application processes, the occurrence of dusts and hybrid mixtures can be largely ruled out, meaning that the description remains limited to the generation of an explosive atmosphere by vapours from flammable liquids.

Ο  Explosive atmospheres from liquids

When pure liquid mixtures are involved, vapour concentrations above the lower explosion limit, which lies between 0.88 and 2.76% by volume depending on the solvent used, can only occur within the paint preparation and drying processes at ambient pressure if the operating temperature reaches the temperature of the lowest of the individual components’ lower explosion points (LEP).

The lower explosion point of a liquid is determined by the temperature at which the concentration of the saturated vapour/air mixture forms the lower explosion limit. In the case of unknown coatings and solvents, 40 g/m³ can be assumed to be the lower explosion limit (LEL).

Example: Vapour pressure curve of a solvent mixture Formation of an explosive atmosphere possible from temperatures from the lower explosion point onwards (below the flash point)

If the lower explosion point is not known, the flash point of the liquid mixture may be used as an aid to evaluation.

The formation of an explosive atmosphere by combustible liquids can be ruled out for the processes described if the operating temperature is guaranteed to be 15°C below the lowest flash point of the individual components of the liquid mixture.

TOP≤ LEP

or

TOP≤ TFL - 15°C

where

T_OP    is the operating temperature  

T_FL     the flash point and

LEP    the lower explosion point

Ο   Explosive atmospheres from paint spray

Irrespective of their actual flash point, the occurrence of an explosive atmosphere must always be expected in finely dispersed liquid coating materials, as occur in the overspray during application, if these materials contain sufficient quantities of organic solvents.

Without further measures, therefore, a non-explosion-hazard area can only be achieved in spray application systems by using water-based materials with a high water content.

The PTB-W-57 calculation method can be used to evaluate the combustibility of a water-based material:

PTB-W-57 calculation method
The generation of an explosive atmosphere can be restricted or its spatial expansion can be limited by means of the measures (A-measures) described in the chapters that make up chapter A.

It may only be assumed in the most exceptional of cases that the occurrence of an explosive atmosphere can be completely ruled out with absolute certainty as a result of the measures chosen. On the other hand, the probability of such an occurrence can generally be effectively reduced. For example, the effectiveness of the A-measure most commonly used in application facilties of diluting the concentration below the lower explosion limit (LEL) depends on the probability of failure of the technical ventilation system and its measurement control systems.

Consequently, in the case of a residual probability that an explosive atmosphere can occur, the safe avoidance of effective ignition sources must be implemented in accordance with section I (I-measures), or design measures in accordance with section C (C-measures) must be implemented to limit the effects.

Ο  Classification into zones

The explosion-hazard areas are classified into zones as a function of the probability of an explosive atmosphere occurring. The implementation of the protective measures outlined in section A (A-measures) of this explosion protection concept can have an impact on the probability of an explosive atmosphere occurring, and thus on the classification into zones, or they reduce the spatial expansion of such a zone.

Ο  The following applies to gases and vapours:

Zone 0      place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of                  gas, vapour or mist is present continuously or for long periods or frequently.

Zone 0, example 0.1:                                                       Click to enlarge Zone 0, example: Product where TOP > TFL - 15°C e.g. 55 Line automotive refinishing paint where TFL = 32°C at room temperature of TOP = 25°C , where: TOP = operating temperature and TFL= flash point Interior of vessel = Zone 0

Zone 1 place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapour or mist is likely to occur in normal operation   occasionally.
Zone 1, example 1.1:  Application unit for processing liquid coatings that are combustible. Spray booths and spraying stands with concentrations of combustible solvents >25%, but <50% LEL to EN 12215 (irrespective of the flash point of the solvent!). Note: The example only applies to spray booths without operating staff! According to EN 12215, the solvent concentration in spray booths with operating staff has to be restricted to < 25% LEL (see example below)!
Area	Type of ventilation	Classification of areas into zones
Interior of stands and booths	Technical ventilation	Zone 1:in interior
Around stand openings (constant openings, but no doors)	Technical ventilation	Zone 2:1 m
Zone 1, example 1.2:  Dryers and ovens for drying liquid coatings Dryers working on the concentration dilution principle (type A to EN 1539) Dilution with concentrations of combustible solvents between >35% and less than <50% LEL and a corresponding operating temperature in area 5 as per the graph in EN 1539 (for details see chapter AV). Note: Please refer to the user guide for details on the classification of drying ovens in zones with conditions outside the zone definition.
Area	Type of ventilation	Classification of areas into zones
Entire vapour space	Technical ventilation	Zone 1

Zone 2  place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapour or mist is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
Zone 2, example 2.1: Application unit for processing liquid coatings that are combustible. Spray booths and stands with concentrations of combustible solvents <25% LEL to EN 12215 (irrespective of the flash point of the solvent!).  Note: In the case of spray stands and booths with operating staff, the technical ventilation system must guarantee dilution to <25% LEL as standard practice.
Area	Type of ventilation	Classification of areas into zones
Interior of stands and booths	Artificial ventilation	Zone 2:in interior
Around stand openings (constant openings, but no doors)	Artificial ventilation	Zone 2:1 m
Note: In the area of the spray jet, compliance with the requirements for Zone 1 is re- commended even in the case of dilution to < 25% LEL.This recommendation is binding within BASF Coatings AG.
Zone 2, example 2.2: Combined spray and drying booths to EN 13355; Application unit for processing liquid coatings that are combustible. Spray booths and stands with concentrations of combustible solvents <25% LEL (irrespective of the flash point of the solvent!). The evaporation quantities during the drying process also only reach concentrations of <25% LEL. Note: In the case of combined spray and drying booths, the technical ventilation system to EN 13355 must guarantee dilution to < 25% LEL as standard practice (with and without operating staff). The example shown thus applies to combined booths in principle!
Spray booth with drying arch
Area	Type of ventilation	Classification of areas into zones
Entire spray room	Technical ventilation	Zone 2 (Note: The control panel must be outside the booth)
Note: In the area of the spray jet, compliance with the requirements for Zone 1 is re- commended even in the case of dilution to < 25% LEL.This recommendation is binding within BASF Coatings GmbH.

Zone 2, example 2.3: Mixing rooms for coating materials to EN 12757-1; mixing of coating materials involving the addition of thinners and hardeners with flash points that are not at least 15°C above the processing temperature. Max. capacity of the individual containers: 5 l  (to BGR104, example 2.2.4.)
Area	Type of ventilation	Classification of areas into zones
0.5 m above mixing scales at a distance of r = 2 m around mixing location	Natural ventilation	Zone 2
Entire floor area of mixing room up to a height of 0.5 m	Natural ventilation	Zone 2
Rest of mixing room	Natural ventilation	No explosion hazard (provided it is not classified as a zone due to neighbouring processes)

Zone 2, example 2.4: Mixing rooms for coating materials to EN 12757-1; mixing of coating materials involving the addition of thinners and hardeners with flash points that are not at least 15°C above the processing temperature. Max. capacity of the individual containers: 5 l  (to BGR104, example 2.2.4.)
Area	Type of ventilation	Classification of areas into zones
In immediate vicinity of mixing scales	Technical ventilation	Zone 2
Entire floor area of mixing room up to a height of 0.2 m	Technical ventilation	Zone 2
Rest of mixing room	Technical ventilation	No explosion hazard (provided it is not classified as a zone due to neighbouring processes)

Zone 2, example 2.5:  Dryers and ovens for drying liquid coatings Dryers working on the concentration dilution principle (type A to EN 1539) Dilution with concentrations of combustible solvents of >25% but <35% LEL and a corresponding operating temperature below the limit temperature of the coating material in area 4 as per the graph in EN 1539 (for details see chapter AV). Note: Please refer to the user guide for details on the classification of drying ovens in zones with conditions outside the zone definition.
Area	Type of ventilation	Classification of areas into zones
Entire vapour space	Technical ventilation	Zone 2