I don’t think I have done a process safety course or a HAZMAT course where I do not have a lengthy sidebar discussion to explain why NH3 is a flammable gas under OSHA definitions, but not under DOT definitions. Yes, NH3 has a LEL of 15-16% and a UEL of 25-28% so it has a flammable range, but it is this same data that exempts us from classifying it as a flammable gas while it is being transported. Both OSHA and DOT have their own way of classifying flammables and this is where so many get lost. Let’s break these down and explain:
First – the data on NH3 flammability:
LEL = 15 – 16%
UEL = 25 – 28%
Source: IIAR Ammonia Safety Data Book, Chapter 4
OSHA
Flammable gas means a gas having a flammable range with air at 20°C (68°F) and a standard pressure of 101.3 kPa (14.7 psi).
A flammable gas shall be classified in one of the two categories for this class in accordance with:
Category 1 – Gases, which at 20°C (68°F) and a standard pressure of 101.3 kPa (14.7 psi):
(a) are ignitable when in a mixture of 13% or less by volume in air; or
(b) have a flammable range with air of at least 12 percentage points regardless of the lower flammable limit.
Category 2 – Gases, other than those of Category 1, which, at 20°C (68°F) and a standard pressure of 101.3 kPa (14.7 psi), have a flammable range while mixed in air.
So as we can see, NH3 is NOT a Category 1 Flammable Gas because it does not have a LEL of 13% or less nor does it have a flammable range of at least 12 percentage points.
However, OSHA puts NH3 in Category 2 Flammable Gas as it does have a flammable range. This helps explain why “machinery rooms” without “adequate ventilation” must be managed as a Class I, Division 2, Group D Hazardous Location.
NOTE: See NFPA 70, Article 500.5(A) – Refrigerant machinery rooms that contain ammonia refrigeration systems and are equipped with adequate mechanical ventilation that operates continuously or is initiated by a detection system at a concentration not exceeding 150 ppm shall be permitted to be classified as “unclassified” locations.
DOT
A flammable gas means any material which is a gas at 20 °C (68 °F) or less and 101.3 kPa (14.7 psia) of pressure (a material which has a boiling point of 20 °C (68 °F) or less at 101.3 kPa (14.7 psia)) which –
(1) Is ignitable at 101.3 kPa (14.7 psia) when in a mixture of 13% or less by volume with air; or
(2) Has a flammable range at 101.3 kPa (14.7 psia) with air of at least 12% regardless of the lower limit.
So as we can see, NH3 is NOT a flammable gas per the DOT definition as it does not have a LEL of 13% or less nor does it have a flammable range of at least 12 percentage points.