The safety aspects inherent in industrial facilities that operate with volatile, flammable and explosive compounds – as is the case with the handling and processing of natural gas – need to be approached carefully. Natural gas explosions are very serious events, often fatal. Planning for the evacuation of a natural gas explosion requires deep consideration of everything from the feasibility assessments and conceptualization of operations, continuing through the different phases of design, construction and startup, enduring throughout the operational life of the facilities and even until its closing stages.
The successful application of any engineering code assumes that all design requirements will be complied with and that the system is safe as long as the protection systems operate when required. But, reality has shown that both human resources and protection systems are susceptible to failures.
The correct use of codes and design criteria allow checks of each of the possible ways that give rise to operational problems or threatening situations, and are very useful in the development of engineering. This philosophy is the basis for the development of appropriate safety manuals and procedures to guarantee the integrity of the personnel present and close to the place where the event is generated.
The facilities that operate with natural gas consider in their design all the physicochemical properties for an adequate sizing of all their protection systems and risk control plans. One of the main characteristics of natural gas is that it is odorless, colorless and tasteless. It requires an ignition point for combustion and its relative density makes it lighter than air, so leaks or emissions dissipate in the upper layers facilitating the formation of explosive mixtures in closed or ventilated spaces. Natural gas is the most volatile part of fossil fuels and is often associated with the crude.
Planning for natural gas explosions
Among the risk control plans used in the natural gas industry is the emergency and evacuation plan, which comes from technical information present in quantitative risk studies, in the definition of classified areas and other risk assessments where all the potential failure modes that lead to dangerous events are quantified.
The numerical estimates of the consequences associated with the potential risks come from the qualitative risk assessment and the probability of occurrence of each major accident risk.
The possible consequences are often combined to produce numerical levels of risk. The detailed evaluation of the individual risk, the potential loss of life and the social risk during the operation of the facilities that handle natural gas are evaluated by the Risk Acceptability Criteria and are presented in the corresponding evaluations.
With these type of studies, risk contours are obtained, which allows locating of the risk. The Location Specific Individual Risk (LSIR) is the risk in a particular place for a hypothetical individual who remains for 24 hours a day, 365 days a year. For onshore installations, the LSIR is generally presented in the form of iso risk contour maps that illustrate the extent of the risk associated with the frequency of deaths that a person located at a certain point of the installation would have.
The iso risk curves are obtained with the risk assessment carried out in each of the areas and equipment present in the facilities. In the evaluations, the locations and consequences generated according to a certain unwanted event are obtained. The fire and explosion models that generally occur in a gas natural installation are the jet fire and the flash fire. The jet fire is burning jet of gas whose shape is dominated by the momentum of the release. A flash fire occurs when a cloud of gas burns without generating any significant overpressure.
The following figure shows the risk in a natural gas processing plant.
The results of the risk studies are the basis for the preparation of the emergency and evacuation plan. All safety procedures are focused on reducing or preventing damage to people generated by the type of accidents. The accidents are classified as severe, major or catastrophic.
The Individual Risk per Annum (IRPA) is another important factor to consider during the preparation of the emergency and evacuation plan. This risk measure takes into account the amount of time a person is exposed to possible risks of serious accidents (death). Thus, the individual risk includes both the proportion of time on the site and the proportion of time in specific locations of the facility. Both the Location Specific of the Individual Risk and the Individual Risk per Year are important technical engineering elements considered during the development of the emergency and evacuation plan.
In the case of an explosion in a natural gas plant, the emergency and evacuation plan must contemplate well-defined strategies. When a contingency originates, the greatest probability of occurrence is associated with the operation, so the personnel involved in it must recognize it, measure it and give a quick response.
The rapidity of the response reduces their severity the damage or circumscribes the impact on the environment. In case of an unwanted event, the personnel present must have clear procedures to respond to emergencies and evacuation plans.
A correct emergency and evacuation plan considers the following design aspects, many of which relate to good preparedness:
- Consequence reduction through emergency operational procedures
- Formation of workers groups to assist in emergency response and evacuation plans
- Ensuring emergency response and evacuation procedures are known
- Ensuring roles, responsibilities and necessary resources are defined
- Actions for the stabilization of the emergency defined, considering the protection of the physical integrity of the affected persons, third parties, support and control groups as well as the conservation of company property
- Audible alarm and communication systems appropriate
- Availability of transport vehicles at emergency sites and their adjacent areas
For the correct application of evacuation strategies, it must be kept in mind that it is definitely the practice that makes the plan, since only its repetition will create the expected response pattern.
Below a graphic representation of a typical evacuation diagram based on the results of corresponding risk studies for an installation that handles natural gas.
It is an accepted principle that “nobody does what he does not know how to do”. Indeed, practice is what will allow us to identify existing problems and make the necessary modifications, and in the energy sector, there are few places where this is more important than in the context of evacuating a natural gas explosion.