Crude oil tank fires pose a difficult operational and budgetary decision, as it relates to fire protection and emergency response for tank storage facility owners and operators due to
the probability of escalation, and Boilover. Although the probability of a full surface tank fire event is very low (approximately 9×10-5 year), the catastrophic nature of a Boilover can result in the total loss of a facility, fatalities, business interruption, and environmental impact.
Although a number of recent successes have been noted by the emergency response community in the extinguishment of large-scale tank fires (270’ diameter gasoline tank), the industry has experienced many more incidents where tremendous losses in both product and personnel were the result of these types of tank fires. Examples include Buncefield, Jaipur, Singapore, Puerto Rico; and most recently Santos (Brazil).
When planning for a potential crude oil tank fire, several options that provide fire protection and emergency response guidance are available to a facility owner or operator. These options exist in the form of prescriptive codes and standards, or a risk-based approach to fire protection.
Prescriptive codes and standards will provide the facility owner or operator with information that is useful, but does not take into account important site-specific factors that are critical in the decision making process of how to protect a crude oil facility. These factors include, but are not limited to, onsite risk (i.e. occupied buildings, other processes, etc.) and offsite risks (i.e. adjacent facilities, community, etc.), site location, public relations, business continuity, economic loss, corporate risk tolerance, and environmental impact. More importantly, prescriptive codes do not account for critical known factors, which are vital to the protection strategy of a crude oil storage facility, such as thermal radiation levels emitted from a full surface tank fire (to adjacent tanks, buildings, process areas, etc.), local emergency response capabilities and limitations, and time to Boilover.
Due to the fact that prescriptive codes and standards fall short in providing a comprehensive protection strategy to crude oil storage facilities, the best approach is a risk-based or performance based criteria.
When commencing the process of determining the best strategy to protect a crude oil storage facility, three (3) basic options exist:
1 Passive Protection
No fire-fighting activities and the stored fuel is allowed to burn out without any intervention.
Cool surrounding exposures (i.e. tanks, process equipment, structures, etc.) to prevent escalation and allow the involved tank to burn out.
Attempt to extinguish the fire, via fixed fire protection systems or mobile application.
Based on the potential for escalation and Boilover, allowing a tank to burn out is not an acceptable choice, especially where personnel, equipment, and other property could be affected.
The phenomenon of Boilover is an area of fire science that is generally not well understood. The specific mechanisms which can lead to it have been hypothesized and in some small way categorized; however all indications are that it is not a well understood phenomena.
The most dangerous effects occur when burning crude oil is expelled from the tank due to the vaporization of a second phase with a higher density but a lower boiling point than the fuel (typically water). This second phase usually consists of water present in the tank due to condensation effects, drilling and transport, or even the natural composition of the crude oil. Due to its higher density, water settles at the bottom of the tank, where it cannot be removed (completely) in most industrial cases. During the fire, the liquid under the fuel surface is heated up to a temperature exceeding the boiling temperature of the water and this heated zone layer begins to sink as the lighter ends are burned off. If this heated zone reaches the water phase, the water starts to vaporize, and fuel is ejected from the tank by the rapid volumetric expansion during the phase change of water to steam.
The ejection of fuel from crude tanks during full surface fires tanks can be divided into three (3) categories as shown in the illustration below. ‘Slop over’ is a discontinuous frothing over of the fuel on one side of the tank, which might be caused by fire-fighting attempts. The ‘Froth Over’, a continuous frothing of burning material with low intensity, is often related to the so-called ‘Roll Over’ effect that may occur during the filling of tanks.
The most dangerous form of fire escalation is a ‘Boilover’. In such cases, violent fuel ejections, frothing over of the whole tank content, flame enlargements and the formation of fireballs are observed. The intense heat radiation and the spilled oil represent an enormous danger to surrounding plants and fire-fighting personnel. This Boilover process may expel the liquid a distance of up to 10 tank diameters away (this however has not been studied in depth) igniting other materials and endangering emergency responders. Thus, Boilover is a situation that should be avoided, or at least minimized at all costs, as it can quickly escalate the fire into a catastrophic event that will be unmanageable.
The mechanisms that lead to Boilover aren’t well understood, however it has been noted that a wide range of conditions can affect whether and when a Boilover will occur. The addition of even a small quantity of firefighting water will typically cause Boilover in a short time, pump-down may contribute to Boilover conditions, temperature, and other factors all can play a role. It has been shown that a heat wave will descend through the tank at approximately 1-3 feet per hour, and thus depending on the depth of the fuel the Boilover can occur in as short a time period as 4-5 hours for a full tank.
The extinguishment of a storage tank fire is a complex problem that requires significant resources (water, foam, delivery devices, etc.), thorough pre-planning, and well-trained fire department or emergency response personnel.
To fully understand the potential impacts of a full-surface tank fire, the first step is to execute a radiant heat analysis of the facility. The radiation analysis considers radiant heat from a full surface tank fire, and its impact to surrounding tanks, equipment, buildings, and structures. This analysis is typically executed in both a 2-D format using empirical data (does not account for environmental data such as wind direction and speed and obstructions), and a 3-D format utilizing a computer-based fire modeling tool such as Fire Dynamic Simulator (FDS), which accounts for environmental data as well as obstructions.
The radiant heat analysis determines the maximum offset distances (i.e. affected zones) of crude oil storage tanks’ critical radiant heat levels. These critical heat flux levels are 4.7 kW/m2 – the maximum radiant heat exposure to personnel in bunker gear, 12-19 kW/m2 – the maximum radiant heat exposure to adjacent tanks where cooling is required within 1-3 hours (depending on exact heat flux), and 19-35 kW/m2 – the maximum radiant heat exposure where immediate cooling is required.
The radiant heat model’s results are then used to determine firewater and foam concentrate demands that will be utilized for both extinguishment of the affected tank (foam), as well as cooling (exposure protection) of adjacent tanks (and other structures and equipment where applicable).
Once the worst-case firewater and foam demands are calculated, the facility will have several options as it relates to types of fire protection systems. Industry standard systems include:
- Fixed foam system using a foam concentrate bladder tank or atmospheric tank and foam delivery devices located at the top of the tank (i.e. foam pourers or foam chambers);
- Semi-fixed foam systems using mobile apparatus to inject foam to delivery devices located at the top of the tank;
- Mobile or manual over the top devices such as large capacity firewater monitors; or
- Hybrid suppression systems, which utilize a foam delivery device to apply foam to the walls of the tanks, as well as a foam stream to the middle of the tank (normally utilized in large diameter tanks – above 200′).
Often the fire protection systems design approach for Crude Storage Terminals is to apply the criteria set forth in NFPA 11 without fully understanding site layout (in terms of radiant heat), possibility for Boilover, and emergency response capabilities. Risk-based fire protection is a far superior approach that takes into account site-specific variables and hazard modeling.
Full-scale tank fires can require in excess of 100 people in order to provide the staffing necessary to fight the fire, and thus staging/training are critical issues. Fires in tanks can produce their own weather, where the thermal updraft of the plume can be significant, and the true performance characteristics of delivery devices are often unknown, especially at high flow rates or when using custom fabricated equipment. These technical issues add to the complexity of the problem and require that adequate engineering is performed prior to the installation of a system to ensure that it will function as intended and can be deployed in an appropriate amount of time.
For more information, go to www.orcusfireandrisk.com