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Tuesday
Oct152013

Foam - 106 Discharge Devices and Nozzles

The discharge device is one of “key” components or pieces if equipment that you must use for applying firefighting finished Foam to hazardous material spills. The solution created by the eduction of Foam concentrate is not effective to fire extinguishment, post fire security, or hazardous material vaporization in itself. This solution must be properly “aspirated” to form a properly “finished Foam blanket”.

Standard water fog nozzles provide expansion be means of “air entrainment”. They function by drawing in a small amount of air after leaving the interior of the nozzle as the foam solution is projected onto the spill surface. This type of expansion is typically in the range of 4 to 6:1 and far below what is necessary for a properly finished Foam blanket. Depending on the situation responded to, this small amount of expansion “may” be acceptable for AFFF Foam (used as a light water/class A medium) solution but does NOT provide vapor suppression or post fire security. “Protein” type Foam that specializes in vaporization control and post fire security requires high energy “aspiration” to produce this acceptable finished Foam. Aspirating nozzles must be used with these Foams, for this result to occur. Foam nozzles and or attachments increase the aspiration ratio (listed above) to form a homogenous finished Foam blanket by means of this mechanical agitation – thus the term “mechanical Foam”.

Compatibility between nozzle and proportioning system is critical. When using any system, the nozzle device must “match” the flow of the Eductor/system at the specified pressure. Selecting the correct systems will provide the proper level of expansion for your purposes. The NFPA further defines these levels to make the proper choices for most needs. “Low” expansion for firefighting with class “A” type Foam concentrates are from 1 to 20:1 expansion rate. “Medium” expansion for firefighting against flammable liquids and hazardous materials expansion range from 20 to 200:1 expansion rate. “High” expansion ranging from 200 to 1000:1 ratios are seen in fixed systems for the protection of high value targets, equipment, aircraft, explosives, etc,.

Expansion rates thus become a “scale” to inform you of how “much” volume of finished Foam your application will end up with on the target hazard. For example; one gallon of Foam solution expanded properly at a 20:1 ration will yield 20 gallons of finished Foam blanket on top of your target hazard. A quality finished Foam blanket, that has been properly expanded, is not only the “key” to post fire security, but “key” to successful vapor suppression. Tests concur that AFFF used with structural fog nozzles can, at best, produce results ranging from 3 to 6:1 expansion. This is ineffective application for hazardous materials vapor suppression as well as flammable liquid firefighting.

Finished Foam must be aspirated to provide the best fire security post extinguishment and for hazardous materials vapor suppression. This practice additionally demonstrates “best use” practices of your resources. One technique utilizing both methods has proven very useful in the field. This system involves the combination of the following two (2) tactics;

1] Initial attack with firefighting fog nozzle for quick knockdown.

2] Organized follow-up attack with aspirating equipment.

This gives your organization a fast knockdown attack coupled with post fire security and hazard vapor suppression. By following-up with aspirated Foam you will also make better use of your remaining resources. NEVER initiate a Foam attack unless you have ALL of the Foam concentrate that will be required ON-SITE. As we will look at next month, in “Application Techniques” any Foam that is used, prior to having the volume for extinguishment or total vapor suppression becomes lost from your resources via “burn-back”, and is wasted.

                                        Haz Mat Mike

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