Full surface storage tank firefighting strategies

Full Surface Storage Tank Firefighting Strategies

Fire in oil storage tanks are luckily uncommon events, However, when they do happen, they require significant assets both in labor and hardware so as to smothered effectively.

 

Full surface storage tank firefighting strategies

Several Tank fires case studies referred to gather information related to the extinguishment of full surface tank fires and strategies suggested here as per experiences.

Extract of Full surface storage tank firefighting cases

Although large-scale tank fires are very rare, they present a huge challenge to firefighters, oil companies and the environment. There are only two alternatives for combating such a fire, either to let it burn out and thereby self-extinguish or, alternatively, to actively extinguish the fire, using firefighting foams by mobile appliances and equipment.

Extinguishing of a tank fire can only be obtained by using firefighting foams. DCP is not effective as there is chance of fire re-ignition.

Chances of successful fire control and extinguishment are low due to lack of resources maintained at facilities, especially for larger tanks because even tanks exceeding 30 m diameter have caused problems in many cases.

The main reasons for unsuccessful firefighting attempts have been lack of suitable equipment and thereby too low application rate, lack of foam concentrate, problems with logistics, and severe weather conditions.

Fires in cone roof tanks with internal floaters are potentially a real challenge from the firefighting point of view.

Tanks without internal floaters might also cause similar problems if part of the cone roof remains and obstructs the fuel surface.

The requirement for huge scale appliances, excellent coordination, top class fire fighting foam and a well-planned activity turns out to be basic.

With few exceptions where sub-surface injection was used in parallel, although over-the-top application using mobile equipment seems to be the dominating methodology. Subsurface and semi-subsurface injection shall not be used for protection of open-top or covered floating roof tanks because of the possibility of improper distribution of foam at the fuel surface. Also it shall not be used for protection of Class IA hydrocarbon liquids or for the protection of alcohols, esters, ketones, aldehydes, anhydrides, or other products requiring the use of alcohol resistant foams

With the use of large-scale equipment and high quality foam concentrates, Knock-down normally is achieved within 10-30 minutes, although the time to complete extinguishment is more difficult to estimate. Most recommendations specify a foam stock equivalent for not less than one 1 hour of operation but this might be too optimistic in many situations. If the foam supply is interrupted, the fire will quickly develop again and all that has been achieved is lost.

Most of the largest tanks fire reported are in floating roof tanks. Cone roof tank fires don’t seem to occur unless preceded by an explosion.

Product involved in most of the tank fires were Crude & Gasoline.

A common experience from these fires case studies is that the actual flow rates of water for extinguishment and cooling, quantities of foam concentrate, duration of operation, etc far exceed recommendations given in NFPA 11 and similar standards.

 

Full surface storage tank firefighting strategies

Recommended Full surface storage tank firefighting strategies

Fires not substantially controlled within the first 20 minutes may exceed the capability of the crew and the on board system. Therefore, early attack on fire plays vital role in firefighting.

Foam can fail to seal against the tank shell as a result of prolonged free burning prior to agent discharge therefore cooling of the tank shell is recommended. Also adjacent tank should be cooled by water spray to protect form radiation. (Care should be taken that water streams are not directed into the tank where they could disrupt the established foam blanket.)

Do not start an attack before an uninterrupted supply of water & foam can be guaranteed.

Follow the minimum application rate recommended in NFPA-11 at initial stage. In case of Prolonged burning ( > 20 minutes), application rate to be increased more than recommended in NFPA by considering foam loss due to thermal updraft. Also the foam application rate recommended by NFPA may have to be increased for very large diameter tanks.

Combination of Fixed/ Semi-fixed foam system & Mobile equipment for over the Top foam application shall be adopted for quick knock down. (Since all the discharge outlets are supplied from a common (ring) foam solution main, some vapor seal devices may not rupture due to pressure variations encountered as the system is activated)

The use of large-scale Monitors makes possible to achieve sufficiently high application rate in order to compensate for foam losses due wind and thermal updraft. Depending on the fixed-roof tank outage and fire intensity, the updraft due to chimney effect may prevent sufficient foam from reaching the burning liquid surface to form a blanket. Foam should be applied continuously and evenly. Preferably, it should be directed against the inner tank shell so that it flows gently onto the burning liquid surface without undue submergence.

All the foam streams shall be aimed towards one single landing zone on the fuel surface, resulting in a very high local application rate making the foam spread more rapidly & efficiently (When foam first reaches the burning liquid surface, there may be a momentary increase in intensity caused by the mechanical action of steam formation when the first foam contacts the heat of the fire.)

For fighting full surface tank fire, high application rated equipment with shall be used.

Large capacity Monitors, Large diameter hoses & Foam compound stored in bulk container shall reduce the knock down time & extinguish the fire completely.

In case of roof sunken & full surface fire, Current practice would be to apply foam to the top of a partially sunken roof through the eyebrow vents at the side of the tank.

High quality FP or AFFF is the most effective Firefighting foam for extinguishment of Tank fire. Fixed/ Semi-fixed firefighting system is reliable for Rim seal fires only.

Communication might be a problem; a mutual aid channel would have been of extreme assistance.

There is only one “best position” for an attack of a tank fire, be sure that this is remained clear.

The use of helicopter for an aerial view is invaluable. Helicopters are, however, noisy and the rotor wash may disturb the foam blanket.

The logistics of supplying enough foam concentrate to the foam vehicles must be given consideration.

Product of affected tank can be transferred to minimize the loss.

Fire in Water miscible Flammableproducts can be extinguished by dilution. Dilution shall be done by supplying water through product line from bottom of the tank. Also, alcohol-resistant foams can be effectively applied through foam monitor or foam hose streams to spill fires of these liquids when the liquid depth does not exceed 1 inch.

Tanks up to 92 M in diameter can be extinguished provided the application rate is sufficiently increased above the recommended value given in NFPA-11.

After prolonged burning, Foam application rate required to be increased in case of Flammable liquids with a wide range of boiling points because it might develop a heat layer. More than 8.1 Lpm/m2

The cone roof and the internal floater might form pockets, which are very difficult to reach by monitor application. If the tank is full, there is also a problem with overflowing product causing fires in the bund area resulting in very complex fire situations and risk for escalation. Storage tanks without internal floaters might also cause similar problems if part of the cone roof remains and obstructs the fuel surface. High-back-pressure foam makers shall be installed on a product line to enable sub-surface injection to achieve extinguishment. Where product lines are used for foam, product valves shall be arranged to ensure foam enters only the tank to be protected.

On fixed-roof tanks of over 60-m diameter, subsurface injection with two discharge points can be used to reduce foam travel distances for tanks containing hydrocarbons. Experience & tests have shown that foam can travel effectively across at least 30 m of burning liquid surface only.

For fighting bunds fire, Low and medium expansion foam are most effective while the high expansion foam is easily disturbed by wind and destroyed by heat. However, high expansion foam is also effective - used in proper quantity it quickly flooded bunds and reduce fire to tanks and pipes only. Dry chemical powder is ineffective in very windy weather conditions. Generally, portable monitors or foam hose streams or both have been adequate in fighting spill fires in diked areas. In order to obtain maximum flexibility due to the uncertainty of location and the extent of a possible spill in tank farms, portable or trailer-mounted monitors are more practical than fixed foam systems in covering the area involved. The procedure for fighting diked area spill fires are to extinguish and secure one area and then move on to extinguish the next section within the dike.

This technique should be continued until the complete dike area has been extinguished.

Interrupted foam application & delay in extinguishment may cause Boilover.

The faster extinguishment can be obtained only by higher foam application rate & shorter pre-burn time.

Very quick extinguishment of full surface fires requires high-volume, long-range foam nozzles to enable fire fighters to stay out of the dyke area.

After the fire has been substantially extinguished by the foam, some fire may remain over the point of injection. With flash points below 100°F (37.8°C) (Class IB and Class IC liquids), the fire over the turbulent area will continue until it is adequately covered by foam. With gasoline or equivalent liquids, when fire remains only over the area of injection, intermittent injection should be used so that foam will retrogress over the area during the time foam injection is stopped. Depending on local circumstances, it may be possible to extinguish any residual flickers over the turbulent area with portable equipment rather than continue the relatively high rate of application to the whole tank.

If the tank contains a burning liquid capable of forming a heat wave, a slop-over may occur from either topside or subsurface injection of foam, especially if the tank has been burning for 10 minutes or longer. Slop-over can be constrained by irregular foam infusion or decrease in foam maker pressure until slop-over stops. Once slop over has subsided, and in the case of liquids that do not form a heat wave, the pump rate should be continuous.

As per NFPA – 11, 3-2.2.2 Foam Application Rates. The specified minimum delivery rate for primary protection is based on the assumption that all the foam reaches the area being protected. In deciding real arrangement foam stream prerequisites, thought will be given to potential foam misfortunes from wind and different components.

Conclusion

Full surface tank fire can be extinguished successfully by proper planning, setting strategies, ensuring the availability of resources and logistics at incident site before initiating firefighting operation. Uninterrupted firefighting with high-volume long-range monitors, 1% AFFF mobile supply facility, large diameter water supply hoses, relay system, etc is the key for success.