Pressure for structural firefighting

[Clevelandfire 3]

I know the minimum flow rates required for structural fire fighting but can't find any up to date references regarding pressure, or am I missing the point (ie so long as flow rate is achieved then the pressure is largely irrelevant)

[Dinnertime Dave]

Yes, fire service pump will increase the pressure.

[SeaBass]

The dynamic pressure is critical. Too little and the throw of the jet won't be sufficient to reach the fire, too much and the branch operators will be knocked off their feet, or worse, and the water jet will feather and break up before it reaches the fire. The pump operator also needs to make allowances for the loss in pressure at the branch that will be incurred for every meter of rise, when the branch is taken aloft (0.1 bar loss per metre) as well as losses due to friction arising from the flow of water in the hose line.  The old Fire Service Manuals go into this in quite some detail, I think that it was in book 7, Hydraulics and pump operation, from memory.   

[Clevelandfire 3]

someone said these days that crews need at least 5 bar at the branch to undertake gas cooling?

[SeaBass]

 Five bar sounds about right. The rule of thumb for pump operators used to be to aim to provide no more than 70 PSI at the branch, which is just under five bar, and then increase or decrease as was necessary.  70 PSI with a standard, uncontrolled branch pipe and nozzle, was fine for a two person branch team, who knew what to expect, and could brace themselves for a known level of jet reaction.

[Clevelandfire 3]

Thanks Seabass the silence was deafening until you replied - I thought I must have been cracking up

[Golden]

Depends on the nozzle they're using and what they are using it for - a jet washer operates at a very high pressure but a small nozzle means that it can be operated with one hand however you get a large nozzle operating at much lower pressure its the mass of water that makes the difference and it'll need a few people to hold it.  Basic hydraulics give you all of the answers. Minimum flow rates are given as this is the critical factor - an incident I attended we had two firefighters seriously injured on the 7th floor from heat exhaustion as we simply couldn't get enough water through a 4'' rising main to cool the fire even though the pressures were high. A bit like the modern high pressure lances which are great when the fire size is small and confined but quite useless when trying to fight a large fully ventilated fire.

The function of a branch is to change the pressure to velocity and hence get a decent jet or spray hence dialling down the orifice size may give you a better jet if you're unable to get the pressures required at the branch but you'll lose flow and hence the fire may overcome the firefighting. Pressures required at the branch vary depending on the type of branch but most these days are multi-functional and can have a variety of settings with many working effectively down to 2-3 bars. Its often a matter of whose needs are greatest at the time!