CAFS Question

[vindicator]

CAFS Question.

During our intial CAFS trainging we were told that our engine pressure should be between 90 and 120 psi. How did they come up with this number?
What is the difference between 90 and 120 psi (besides being 30 psi)? Is 120 psi a more wetter foam?
When should a drier foam be used? (exterior?)
When should more wet foam be used? (interior?)

Thanks

[Sleuth]

From my limited experience:

Dry foam - clings to vertical surfaces better, used for exposure protection.

Wet foam - Penetrates better, for grass/hay fires, pine needles, wildfires, whenever you want the water to penetrate the surface. In some cases, we have turned the air off, and just used the water/foam mix to get deep into piles of duff.

[vindicator]

Thanks

Thanks Sleuth

Yeah we use foam water mixture prior to getting CAFS. I understand that drier is "clingy" and "wet" penetrates. But at what point do you go wetter or drier?
Any idea about the PSI?

[PrestoC1]

Vindicator,

In my research on CAFS, the rule of thumb that the experts have told me is equal amounts of solution versus equal cfm. 60 gpm of solution takes 60 cubic feet of air. The systems we have looked into say to run them at 120 psi, no rhyme or reason, just that that is the recommendation.

Wet or dry is controlled by the outlet valve, cut down on the water and the volume is replaced by air...drier. Open the valve, more water, less air, wetter foam. Application depends on what your doing, coating exposures (dry), deep seated fire (wet) etc.

I'm not an expert and don't have a system yet (specing a new pumper with CAFS now), but that is what I have found. Odin Foam (Darley) has a page on the internet where you can send questions to their CAFS engineer.

Be safe, Dan

[GreggGeske]

An attack pressure is usually desided upon by the department. I think L.A. County uses 90. My own department uses 100 PSI. At 100 PSI+ you will get less line kinks. The higher pressure will provide further reach, but also a higher reaction force. I usually run at 100 PSI for attack and run it back to 70-80 PSI for overhaul. When you are running a drier/fluid foam it helps for reach to run a higher PSI. You do not have the reaction force problem due to the small amount of water you are discharging. The higher pressure also spins the compressor faster helping to provide the large amounts of air required for drier/fluid foam. Usually the automatic balancing systems will balance the air and water pressure from 40-150 PSI +/- 5%.

[vindicator]

Thanks GreggGeske and PrestoC1

GreggGeske

On a interior residental attack, what does your department run your valve settings at? "Half-Gate",fully open?

Just curious Thanks

[GreggGeske]

The discharge valve is fully open, which will provide about three GPM to 1 CFM (1 3/4" hose @ 100 PSI, 1" smooth bore tip, yields about 115 GPM and 38 CFM. The ratio should be at least two to one for structural firefighting. The only time we gate the discharge back is for providing fluid/dry foam for exposure protection and grass fires.

[Resq14]

Changing to a smaller tip will also give you a drier foam.

The recommended PSI isn't entirely arbitrary. It is based on the ideal pressure for effective stream reach, and effectove mixing of air/foam/water.

[ilfireinstructr]

while I admit my knowledge of CAFS is llimited, it is my understanding that the type of foam produced is due to the foam ratio and not the amount of air induced. Any thoughts?

[Resq14]

By "type" of foam, I assume you mean wet foam and dry foam, and everything in between.

To vary between these, you adjust the air:water ratio. The foam percentage can be kept constant. Adding more foam than you would normally use based on the manufacturer's injection rates is pretty much a waste.

We run standard 0.3% class A foam (preset). We hit our preset throttle (100ish PSI). Our standard structural line is a 200' 1.75" flowing 70gpm of class A solution, and about 40cfm of air @ 100ish PSI with 0.3% of foam through a simple 15/16" smoothbore. Personally, I do not notice a change in foam quality at different PSI's... 70psi CAF looks like 120psi CAF to me. The casting distance can be effected with drastic increases or reductions in pressure.

Now each manufacturer has a different way of adjusting the air:water ratio. Typically, you simply cut back on the water you are flowing by gating the line down. With a constant pressure, a reduction in GPM's of class A solution will result in an increase in CFMs of air. The truck will compensate because it automatically balances air and water pressure. This will result in more air being introduced (proportionally). And at the nozzle, you will see a thicker/drier type of foam.

To truly take advantage of "dry foam" use a smoothbore with a large nozzle diameter. Often we remove all the tips and just use the flow from the nozzle itself. You can even just let it flow out the end of the hose (if you don't care about casting it a long distance). As you narrow the discharge diameter, you will have a wetter foam with a longer reach.

Keep in mind that CAF is a "finished product" before it reaches the nozzle. Agitation and friction in hoselines, fittings, and appliances all result in better foam at the end. It is for this reason that most manufacturers specify minimum lengths of hose for CAF lines... you need the friction of the hose to complete the mixing process.

[Sleuth]

I just completed the State hydraulics course, and the instructor is the CAFS instructor for Phoenix FD. They are going to all CAFS, with 27 new trucks (must be nice to have $$$$$).

I asked, and he said they run .3% foam for fire, .1% for overhaul ($65 a 5 gal. can - save the foam!).

He also recomends 100PSI on your CAFS lines, saying the friction loss when pumping CAFS is negligable. So, we are going to save wear & Tear on our trucks by throttling back to 100 PSI.

[Fire304]

One of the comments I remember from our CAFS school for the wet vs. dry question was that you still need water to put the fire out. Wetter foam can absorb more heat therefore put out more fire, hence we want to use wet foam for attack work (the big water theory). A specific point the instructor made was that if you are going to defensive ops don't even turn the CAFS on, big fire still needs big water, you'll just waste your 55gals or so of foam.

Dry foam put on a high heat load fire will quickly dry out and you will be left with just the dry components of the foam (looks like foam but crumbles to the touch) and leaves you with no heat absorbing protection.

So to the question of Dry or Wet, if you've got a lot of fire you need wet, if you're providing exposure protection, mopping up, or going after smaller brush fires dry if fine.

As for pressure just like with any smoothbore nozzle you can overdrive your stream and it will blow out on you. The exact pressure you run should be determined by seeing how your nozzle/foam combination flows with different pressures and pick the one with the best reach and stream qualities.