Safeguarding Your Process Against Gas and Vapor Explosion

Many industries use flammable chemicals (liquids and/or gases) and chemical mixtures as a
part of their processes. These flammable chemicals could present a substantial hazard in the
form of fire or explosion during the handling, processing, and storage of these chemicals or if
containment is lost through a leak, spill, or rupture of equipment. Depending on the magnitude
of the fire and/or explosion, losses to a facility can be substantial. The losses could be in the
form of injuries and fatalities, and substantial damage to equipment, facilities, and the
environment. In order to reduce the risk of a fire or explosion event at a facility handling
flammable chemicals and/or chemical mixtures, the site staff should be familiar with the
following;

• The flammability properties of the chemicals
• Factors affecting the flammability properties of chemicals
• Measures to reduce the risk of a fire and explosion hazard

Flammability Properties of Chemicals - Gases and Vapors

In order to assess the risk of fire or explosion at a facility – and to determine the necessary safety precautions needed to minimize the hazards of the process – the flammability characteristics of the liquids, vapors, and gases must be determined. Safety-critical flammability properties include: 

• Flash Point (FP) and/or Lower Temperature Limit (LTL)
• Lower and Upper Flammable Limits (LFL/UFL)
• Auto-ignition Temperature (AIT)
• Limiting Oxygen Concentration (LOC)
• Minimum Ignition Energy (MIE)
• Maximum Explosion Pressure (Pmax), Rate of Pressure Rise (dP/dt)max, and Deflagration Index (Kg)

Testing in laboratory equipment is required to obtain values for the above properties.

Factors Affecting the Flammability Properties of Chemicals

The flammability properties of gases and vapors such as FP, LFL, UFL, AIT, LOC, MIE, Pmax,
and Kg may be affected by the process conditions such as;

• Temperature
• Pressure
• Oxidant Concentration
• Size of Equipment
• Material of Construction

For example:

- The flash point of a liquid is lower at sub-atmospheric pressures (vacuum).

One of our clients had developed a new alcohol-based fuel-blend that could be used as
an aviation fuel. They were interested in determining the flash point of the fuel at
various altitudes above ground level. FP or LTL experiments were conducted for
various altitudes ranging from 0.0 to 40,000 feet (pressure equivalent of 14.7 to 2.72
psia) using a modified ASTM E1232¹ test method. From the experiments, it was
determined that the FP of the fuel decreased with increasing altitude. The test result for
the flash point of the new alcohol fuel-blend at 40,000 feet was found to be 21 C lower
than its atmospheric-pressure flash point.

- The auto-ignition temperature of a gas or vapor is generally lower at elevated pressures.

The reaction between high-pressure air and compressor lubricant in high-temperature
inter-stage and discharge regions of a compressor can be a problem (i.e., overpressure
due to auto-ignition of lubricant vapors). To help one of our clients in their incident
investigation, AIT experiments were conducted at atmospheric and 100-psig pressures.
Atmospheric pressure testing was conducted according to the ASTM E659₂ method.
The elevated pressure testing was conducted using a 2.25-liter pressure vessel, and
the test set-up was in general accordance with ASTM E659 test methodology. From the
experiments, it was determined that the AIT at 100 psig pressure was 126 C lower than
the AIT at atmospheric pressure.

Very limited data on the flammability properties of chemicals or chemical mixtures at low and
elevated conditions of temperature and pressure are currently available. In such cases, the
flammability properties of chemicals must be experimentally determined at the representative
process conditions.

Measures to Reduce the Risk of Fire and Explosion Hazards

The risk of fire or explosion can be minimized when one of the following measures is ensured:

• The formation of a ignitable mixture is prevented: