Choosing Cleaning Solvents Safely - Flash Point and Ex Zones

What does the flash point tell you about a solvent?

The flash point is the lowest temperature at which a solvent gives off enough vapour to form a mixture that can be ignited by an external source above the liquid. If the flash point is below the ambient temperature, the liquid is already ignitable at the workplace. It is therefore the single most important figure for the fire risk of a cleaner.

Under the CLP regulation, liquids with a flash point below 23 °C and an initial boiling point above 35 °C are classed as highly flammable (H225), while flash points up to 60 °C fall under H226. Isopropanol (IPA) sits at around 12 °C and acetone at roughly -18 °C, whereas many modified alcohol or hydrocarbon cleaners only ignite above 60 °C.

Why do vapour pressure and explosion limits matter?

Vapour pressure governs how quickly a solvent evaporates and how much vapour reaches the air. A high vapour pressure means fast release and therefore quickly reaching an ignitable concentration. Acetone at around 246 hPa at 20 °C evaporates far faster than IPA at roughly 43 hPa.

A vapour-air mixture only becomes ignitable between the lower (LEL) and the upper explosion limit (UEL). For IPA the LEL is around 2 vol %, for acetone about 2.5 vol %. The goal in operation is to keep the concentration safely below the LEL through ventilation.

• High vapour pressure plus a low flash point gives the greatest ignition risk at room temperature.
• The LEL in vol % shows the vapour concentration at which ignition becomes possible.
• Effective mechanical ventilation keeps the concentration well below the LEL.
• Small containers and closed vessels reduce the amount of vapour released.
• The occupational exposure limit protects health, not only against fire.

How do you assign solvents to ATEX zones?

Wherever ignitable solvent vapours can occur, explosive atmospheres may form. Under the ATEX workplace directive 1999/92/EC these are divided into gas zones: Zone 0 (present continuously or long term), Zone 1 (occasionally) and Zone 2 (rarely and briefly). The zone classification determines which equipment and safeguards are allowed.

Equipment for these areas carries a category under the ATEX product directive 2014/34/EU: category 1G for Zone 0, 2G for Zone 1 and 3G for Zone 2. The Ex marking is followed by an explosion group (IIA to IIC) and a temperature class (T1 to T6) that must match the solvent's ignition temperature.

How do you choose the safe solvent for your operation?

Safe selection starts with the safety data sheet (section 9): flash point, vapour pressure, ignition temperature and explosion limits provide the basis. Where possible, a cleaner with a higher flash point should replace the highly flammable product (substitution principle).

• Check the safety data sheet: note flash point, ignition temperature, LEL/UEL and exposure limit.
• Consider substitution: modified alcohol or HC cleaner with a flash point above 60 °C instead of pure IPA.
• Ensure mechanical ventilation or extraction at the workstation.
• Eliminate ignition sources: ESD grounding, Ex-rated equipment, no open flame.
• Keep only small hand containers at the bench, stock in a type 90 safety cabinet.
• Use suitable PPE and bond and earth containers during transfer.

Frequently asked questions