STUDY HALL: Cone Roof Tank Fire

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CONE ROOF TANK FIRE

Recently Williams Fire & Hazard Control was dispatched to a tank fire involving a 122-foot cone roof “blend” tank holding a mixture of diesel fuel and #6 oil used in asphalt production that ignited during product movements into the tank.

“Chauncey Naylor described the event, “These tanks are vented in different ways — this one being with what we call a Chinese hat. The so-called "hat" is a round hooded vent built into the center of the tank roof. If the venting cannot keep up with the volume of flammable vapor inside the tank, ignition of the contents can result in a vapor air explosion of the contained vapor space trapped within. Anticipating this risk, tank builders allow for a “weak seam” along most of the roof weld connection along the wall. If a vapor air explosion occurs this weak seam allows the roof to release the pressure, while the strong weld portion of the seam will keep the roof from flying upward. Usually the roof will partially peel back and fall back down onto the tank. This often becomes a hindrance to TYPE III foam attacks that ideally would access and cover the entire surface of the burning fuel without obstruction. We have seen this occur often in these types of tank related incidents and have always been able to modify our application to overcome that obstacle."

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"When we received the emergency call around midnight we were asked to bring pumping capabilities, hose, ThunderStorm® foam, and a large-volume end-of-line device for TYPE III applications. Our Ambassador 1x6 "stopped down" to 3,000 gpm suited perfectly for this response", Naylor said.

Prior to Williams' arrival the Sabine Port Neches Chiefs Authority was onsite appling water to surrounding exposures to manage heat loads.

"We were onscene at the Port Neches facility and began setup at 1:30 am. A firewater resource initially identified by facility personnel required a 20 foot lift, which would have been prohibitive to a successful attack. Running another 600 feet out of the facility gate, a dockside water resource offered a lift of 9 feet — just under the suggested 10-foot maximum lift for the DependaPower 4,000 pump we had in tow. Using four 5” lines we ended up laying 8,000 feet of hose and got the Ambassador positioned for an attack that began at 5:30 am", Naylor continues.

Within 15 minutes of making our foam application we had control of the fire with full flame collapse. Extinguishment was called at 6:02. Follow-on vapor suppression foam applications continued until 6:30 am to prevent reflash. At 6:45 we applied 15 minutes of new foam to refresh foam blanket. Following a one-hour watch the tank was "dead cold" and we ascended the ladder for photo confirmation.

We demobilized at 8:15.

Two Events in One

It is important to recognize the flammable characteristics inside a floating roof tank. Essentially two potential circumstances lie in wait ...

Under normal conditions, the vapor space within the tank is too rich to burn and will remain dormant with proper vapor exchange via the tank venting system.

However, it has been our experience that given the right conditions this rich vapor will lean out due to any given set of circumstances such as failure of tank venting systems, low-pressure exterior atmospheric conditions, or an abnormal breach in the tank structure.

Even in extreme situations, if ignition does occur at the vents the interior space of the tank MAY remain too rich to burn and a surface fire within the tank will be avoided.

IFR Animation HOWEVER, several incidents Williams Fire & Hazard Control has reponded to domestically and internationally have been due to an adequate open-air exchange leaning out the interior vapor space whereby a fire source at the vent flashes back insde the tank resulting in a devastating combustion of the residual gases.

Our experience is that an internal foam application resulting in a rich foam blanket will reduce vapor yield within the tank, and will inhibit any potential surface fire resulting from secondary ignition sources as outlined above.

MOST NOTABLE: Our Dual-Agent chamber - a COMPLIANCE+ product - follows the foam application with an abundant dry-chemical surge that overwhelms any residual vapor inside the tank, extinguishes the fire at the vents, and prohibits a potential explosve flashback event.

CONE ROOF TANK FIRE

Two Events in One

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