By Craig Weeks
I find it interesting that the least utilized fire apparatus are some of the most expensive. In this particular case, I’m talking about industrial fire suppression apparatus. Typically these apparatus are designed to suppress petroleum-based fires and can come in various configurations including, but not limited to, pumper, tanker, aerial, and I have even seen an elevated master stream constructed using a multisection articulating boom from a concrete pump truck. But regardless of the exact configuration, and with little exception, they all have similarities. They move big water; flow in excess of 2,000 gpm through multiple discharges; have a large master stream appliance; transport bulk quantities of suppression concentrate, such as AFFF foam and possibly even a pressurized dry chemical extinguisher system; have complex proportioning systems; and are just plain big and expensive.
Yes, I know fire apparatus are just expensive and that’s the price to pay for doing business. After all, when the day comes, and it inevitably will, having it there will far outweigh the cost—and I agree. However, you still have to have the budget to pay for and maintain it. But, cost is just one issue associated with these behemoths. Other issues include staffing, training, high maintenance, and concentrate shelf life. Why is it that proportioning system gremlins always seem to show up or the concentrate has turned to ballistics gel at one incident in five years? The unfortunate answer is because that’s the nature of the product.
I’m not going to get too deep into fire suppression concentrates as there are smarter people that know more about them than me. So for the sake of argument let’s just call them Class A foam, used for non-metallic solid-fuel fires, and Class B foam, used for liquid fuel fires. Now, different types of concentrates mix at different rates, which are identified in percent. So, a one percent mix rate means one gallon of concentrate is used to treat 100 gallons of water.
Class A foams are surfactants that improve the water’s permeation and cooling efficiency. They mix relatively easily and are quite effective at a mix rate of less than ½ percent. They can be used up to one percent, producing a whipped cream consistency for pretreatment.
Class B foams, on the other hand, require a much higher mix rate of at least one percent and as high as six percent, creating a blanket of foam to cap vapors or smother a pool of flaming liquid.
There are several types of concentrate proportioning, or mixing, systems. The most common are:
- Direct Injection: An electric or power takeoff (PTO) driven pump producing higher pressure than the fire pump injects concentrate directly into the water stream. This is the most common type used on pumping apparatus, typically flowing 1 to 5 gpm of concentrate with the electric pump and 60 to 100 gpm or more with the PTO pump.
- Eductor: Creates a pressure differential, or vacuum, between the incoming high-pressure and outgoing low-pressure water flow. The low-pressure side of the educator draws in the concentrate, mixing it in the hose. Eductors can be vehicle-mounted, connected inline between hose sections, or integrated into the nozzle. Vehicle-mounted and inline eductors typically mix concentrate from ½ to 6 percent at 60- to 125-gpm water flows. Self-educting nozzles can flow 2,000 gpm or more while mixing foam inside the nozzle.
- Pressure Balance: This is a combination of direct injection and educator systems, typically only used in high-flow applications, is the most common in industrial fire apparatus, and uses a PTO-driven concentrate pump.
- Around the Pump: A pressurized water line is connected to an eductor picking up concentrate and returning it to the fire pump where the mixing takes place and is sent to all discharges.
Pros and Cons
The foam concentrates work well but are specific for their application. Although there are some on the market claiming to work for both A and B fires, in general one does not perform well in place of the other. Class B foam is very expensive compared with Class A foam, and the smothering blanket it produces creates an inherent problem. You don’t want to run out before completing suppression operations. If you do, everything that was done will be quickly undone by the remaining fire. So as a general rule, never start what you can’t finish during a Class B fire suppression operation, which is why refineries maintain tens of thousands of gallons of the stuff. Foam concentrates also have a shelf life, which is shortened when subjected to air, sunlight, and heat.
Each proportioning system also has its advantages and disadvantages. Direct injection, pressure balance, and around the pump systems can support the high flow rates required for a Class B fire operation while feeding multiple discharges at varying fire pump pressures. Eductors are extremely simple and reliable but require specific operating pressures to work properly and have flow limitations to a single line, at least when inline or vehicle-mounted. Self-educting nozzles can be bulky and typically have cumbersome 1½- to 2-inch concentrate hose feeding them from the concentrate source. Direct injection and pressure balance systems are very complex and high-maintenance but allow for adjustable concentrate mix rates to each point of injection. Around-the-pump systems, like eductors, are fairly simple and reliable, but because the concentrate is mixed in the fire pump, every line that’s flowing gets foam, and it takes quite a while to clear it all out when done. The eductor itself is quite bulky and time-consuming to set up.
CRAIG WEEKS is division chief/fleet manager for the Los Angeles County (CA) Fire Department and president of Specialty Fleet Consulting.