Chemical systems and processes that require the handling of flammable gases and vapors should have preventative and mitigative measures in place to ensure the safety of employees and facilities. These measures can be properly implemented by first obtaining the needed test data from various flammability experiments, such as flash point tests, and following up by installing the recommended preventive/mitigative safeguards which may include LFL/UFL detectors, oxygen monitors, rupture discs, flame arrestors, and explosion relief panels.
Before a process that has flammable material is put into service, operators need to ensure they are operating well below the autoignition temperature of the flammable gas or vapor. The autoignition temperature is a flammable property defined as the lowest temperature of an environment at which a gas or vapor will spontaneously ignite without a distinct/localized ignition source (like a flame or spark). Knowing and understanding the autoignition temperature is critical if gases are being handled or processed under elevated temperature and/or pressure conditions. Next, the complete flammable range of the flammable material should be evaluated so that operators know the material’s flammable region and understand how to operate the process outside of the flammable region to prevent unwanted explosions.
The flammable region is defined by a material’s flammability limits and limiting oxygen concentration (LOC) and is best presented as a flammability or fire triangle on a ternary graph, containing the fuel on one axis, the oxidant on another axis, and the inert gas on the third axis (see Figure 1). The flammability limits for a fuel-oxidizer mixture consist of a lower flammable limit (LFL) and an upper flammable limit (UFL). These boundaries are also commonly referred to as the lower explosive limit (LEL) and upper explosive limit (UEL). Flame propagation will not be supported if fuel concentrations are either below the LFL or above the UFL in an oxidizing atmosphere. However, concentrations of mixtures between these limits are flammable, and a fire or explosion could occur. The flammable region is widest when there is no inert gas. The “nose” of the triangle corresponds to the LOC, and the methane gas mixture is not flammable if the oxygen concentration is less than about 12%. Also shown in Figure 1 for illustration are the stoichiometric line and the “air line” where the UFL and LFL for methane in air are readily identified (about 17% and 5%, respectively).
The LFL and UFL of a fuel and oxidizer are also dependent on numerous factors including temperature, pressure, vessel size/geometry, and the nature of the oxidizer/inert present in the system. Therefore, it is important to determine these limits experimentally (e.g., Figure 2) through testing as closely as possible to actual “real-world” process conditions. Determination of the LFL and UFL provides an understanding of the possible safety hazards present in operating at certain conditions and suggests how to keep a process outside of the flammable region.
Sometimes it is not cost effective to operate large scale processes under an entirely inert environment; therefore, knowledge of the flammability limits can give some flexibility on process operations. Once the LFL and UFL are determined, process operators can install calibrated LFL/UFL detectors into the system to continuously monitor conditions and ensure that the process is operating outside of the flammability limits. LFL/UFL detectors are generally designed with safety margins with the intention that alarms are triggered if the gas or vapor concentration is approaching the flammable limits, so that operators have sufficient time to cease operations or purge the vessel. If the process/system is continuously monitored and controlled with safety interlocks, NFPA 69 Standard on Explosion Prevention Systems recommends maintaining the combustible gas or vapor concentration at or below 60% of the LFL. If the process/system is not continuously monitored and/or controlled with safety interlocks, NFPA 69 recommends maintaining the combustible gas or vapor concentration at or below 25% of the LFL. Chapter 8 of NFPA 69 (2019) provides additional insight on proper safety considerations when attempting to operate outside of the flammability limits.
If a process might require a mixture to be within its flammable limits during normal operations, the process operator can potentially turn to another preventative measure that focuses on deflagration prevention by oxidant concentration reduction. In this instance, it would be appropriate to perform limiting oxygen concentration testing on the flammable gas or vapor. The limiting oxygen concentration (LOC) is the minimum amount of oxidant (typically oxygen) needed to support flame propagation. The LOC can be used to help determine proper inerting and purging procedures to keep a process material outside of the flammable region. Like the LFL/UFL, the LOC depends on factors such as temperature, pressure, and the nature of the inert/oxidizer present in the system. Therefore, it is critical to determine this parameter through testing as closely as possible to actual process conditions.
Once the LOC is determined, process operators can install calibrated oxidant (usually oxygen) detectors into the system to continuously monitor concentrations and ensure that the process is operating well below the limiting oxygen concentration. If the process/system is continuously monitored and controlled with safety interlocks, NFPA 69 recommends maintaining the oxygen concentration 2% below the determined LOC (if the LOC is greater than or equal to 5% O2) or no more than 60% of the LOC (if the LOC is less than 5% O2). If the process/system is not continuously monitored and/or controlled with safety interlocks, NFPA 69 recommends maintaining the oxygen concentration 4.5% below the determined LOC (if the LOC is greater than or equal to 7.5% O2) or no more than 40% of the LOC (if the LOC is less than 7.5% O2). Chapter 7 of NFPA 69 (2019) provides additional insight on proper safety consideration and use of purge gas systems when attempting to prevent deflagrations by oxidant concentration reduction.
Under certain circumstances, it can be necessary to operate a process inside of the flammable region, thereby, presenting the risk of a fire and/or explosion hazard. In this situation mitigative safeguards such as explosion protection equipment and controls are needed to operate the process safely. Maximum experimental safe gap and explosion severity tests are useful in determining the extent of the protection required in the process to mitigate potentially catastrophic damage to the system. The maximum experimental safe gap (MESG) is the maximum gap that prevents an ignition of a gas/vapor mixture from an inner chamber from moving into a second outer chamber containing the same gas/vapor mixture. In other words, MESG testing measures how easily a flame will pass through a narrow gap. The MESG is used for the design and/or selection of electrical equipment and flame arrestors for processes which include flammable vapors and gases.
Explosion Severity testing determines the maximum explosion overpressure (Pₘₐₓ) generated during an ignition event of the optimum concentration of the flammable mixture as well as the deflagration index, which is the maximum rate of pressure rise normalized to the vessel volume. These parameters are used to pressure-rate a vessel for containment purposes or to design an explosion relief system and/or properly size a vent. Chapter 7 of NFPA 68 Standard on Explosion Protection by Deflagration Venting provides additional insight on proper safety considerations when attempting to mitigate deflagrations by venting.
Deflagration venting is one of the most common forms of explosion protection. It enables a portion of the containment vessel, reactor, tank or even building structure to fail in a controlled manner before the rest of the structure is affected. In the case of a reactor tank, the explosion protection may be a rupture disc, but for a building it may be a flat “blowout” panel. The equation for gas deflagration venting provided by NFPA 68 is given below:
Many factors are involved in determining the correct deflagration vent size. Often these factors are equipment-specific, like the Pᵣₑₔ, Pₛₜₐₜ and Aₛ. Other factors are related to the process flow and gas dynamics, including the ρu, γb, Gᵤ, Cd, and λ. The remaining values, namely the Pₘₐₓ and Sᵤ, should be determined experimentally.
