The Marine Firefighting Institute

Newsletter # 3
Please do not reprint in any form without the permission of the author.

Straight Stream vs. Fog

At Shipboard Fires


By Tom Guldner
The Marine Firefighting Institute

Warning: All firefighters should be aware that the techniques and methods of applying water and ventilation to fires discussed in this article require extensive training by qualified flashover instructors and any attempt to follow this style of firefighting without such training may be ineffective and potentially dangerous...

For years, structural firefighters have relied on a straight stream for attack lines at structural fires. Fog streams have either been relegated to the position of providing ventilation after the fire has been knocked down, providing water curtains, or to direct hazardous gas plumes away from populated areas and possible sources of ignition. Also for years, fog has been used by the US Navy and merchant vessels around the world as a primary attack line to fight shipboard fires. Shipboard firefighting has many similarities with structural firefighting; however there are also some definite differences.

At a structural fire, ventilation is generally of initial concern, once water is available. At a ship fire the opposite may be true. Instead of opening up the fire area, the shipboard firefighter may actually be shutting down mechanical ventilation and "buttoning up" or sealing the fire area. If this were attempted at a structural fire, it would be expected that the fire would burn through the retaining perimeters, and extend to uninvolved areas. Ships however, are constructed of steel, and to prevent sinking they are sub-divided by watertight walls or Bulkheads. These Bulkheads also create excellent fire boundaries, which separate the ship into "Vertical Fire Zones".

A localized ship fire may be contained to an area and even at times permitted to burn itself out or at least give you time to set up a coordinated fire attack. This can often be done with little structural damage to the ship. If an interior fire attack into one of these compartmented areas is warranted, it must be remembered that the fire area is, in effect, a steel box that is not unlike an oven. If the fire has been burning for any length of time, all six sides (ceiling, floor, & four walls) may have now become red hot metal. Remembering our fire science we may recall that as water passes from a liquid to a gaseous state (as when it becomes steam) its volume expands. The rate of this expansion is an extremely relative figure but in the past it has been projected as anywhere from 1600 to 1800 times. For this article I will use the figure of 1650 times. Let's consider what will happen if you are entering that red hot fire compartment with a straight stream. Imagine the dimensions of the compartment are 20' by 20' by 8', which is 3200 cubic feet. Eight gallons of water (7.48 actually) occupies a space of slightly more than one cubic foot (1.06624). If those eight gallons were converted to steam it might fill an area of 1,759 cubic feet. That alone is more than half of this compartments capacity. Most attack hose lines will produce approximately 150 gallons of water per minute. That 150 gallons you are pumping in could convert to 35,267 cubic feet of steam. (We are working with inexact figures, and all of the water would not convert to steam, but even if this number is cut in half you have a problem). Remember, the compartment we are generating all this steam in can only hold 3,200 cubic feet. The rapidly expanding steam will forcibly attempt to find an area of lesser pressure. Due to the fact that everything has been closed up, that area is the hall just outside of the doorway to the compartment. The only impediment to the escape of this superheated steam is you and your attack team kneeling in that doorway. That may prove to be an uncomfortable position!

In tests performed by the US Navy (see footnote) it has been found that several short bursts of a fog stream through the partially opened door to the fire compartment will immediately reduce the heat at ceiling level. Unlike the straight stream however, this temperature reduction will not immediately climb back up. (Note: The fog was directed into the superheated air and not at any hot surfaces). The steam is able to absorb more heat quicker than the solid stream. This is because its smaller droplets present more surface area. The more surface area, the greater and more rapid the heat absorption. During an actual fire test aboard a decommissioned vessel, two to three short bursts of fog cooled the fire area enough so that the attack team was then able to switch to straight stream, move into the compartment, and apply water directly on the fire. Extinguishment time was two minutes and was effected with all attack team members in the compartment. When the exact same conditions were presented to an attack team employing only a straight stream, they were driven out of the compartment by the high heat and steam and forced to operate from the door. They were unable to extinguish the fire. It was also found, contrary to popular belief, that the fog stream does not disrupt the thermal layer in the fire area as much as the straight stream does. This thermal layer is what allows us to enter a fire area by crawling in with the lower temperatures at floor level below the high heat found at ceiling levels. The test showed that a straight stream would tend to drive these extreme temperatures from the ceiling level down onto your attack team. The short burst fog attack did not disrupt this thermal layer as much.

Another major benefit of the fog stream at ship fires is the reduced water accumulation. Stability at a ship fire is of utmost concern and may be handled in future article. Suffice it to say that any way to reduce the volume of water put aboard a ship during a fire operation will greatly enhance the safety of the operation.

At past ship fires, one of the main impediments to advancing hose lines in enclosed spaces has been the high heat and burns to the attack teams caused by the conversion of firefighting water to steam. I am not recommending that we throw away our straight stream nozzles. I've crawled down too many charged hallways to recommend that. In fact at many large-scale ship fires large quantities of cooling water are needed. But perhaps we should begin to re-think our long-standing preference and sole reliance on straight streams, even if only for Marine fires. At least consider the use of fog streams in conjunction with the straight stream for some operations and let's have some more tests and training.

Offensive Fog Water Attack Reduces Firefighting Time and Heat Strain During Shipboard Firefighting Hagan, RS; RD Bernhard, KA Jacobs, JP Farley, LR Ramirez, SJ Feith & JA Hodgdon, Navel Health Research Center NHRC Publication 96-22

Remember the Warning: All firefighters should be aware that the techniques and methods of applying water and ventilation to fires discussed in this article require extensive training by qualified flashover instructors and any attempt to follow this style of firefighting without such training may be ineffective and potentially dangerous...

 

Stay Safe.

 

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Previous Newsletters:

Newsletter # 1 "Marine Firefighting Training, Who needs it!"

Newsletter # 2 "Shipboard Basics"

An excelent web site with a wealth of information about research on water fog application is maintained by Paul Grimwood who has written several articles on this subject. His most recent article appeared in the October issue of Fire Engineering. Please BOOKMARK my site for easy return before clicking on the link to his site below. His site is called Fire Tactics and its web address is

http://www.firetactics.com/