Firefighting water run off

[JT]

Is there a way of quantifying reasonably accurately, the amount of water run off for different types of premises?
I imagine there's too many variables but would be useful to design a tank size. 

[kurnal]

A starting point might be to make an  estimate for water requirements for firefighting and therefore assume all will run off. There is national guidance (the tobin report)  on the provision of water supplies for firefighting but this is likely to be onerous- so I would use fireground calculations- how many main jets and monitors would be needed to surround a fire plus any sprinkler discharge. 1000l/min per main jet for one hour plus the capacity of the sprinkler tanks may be reasonably mid range. Not sure how this would be achieved with a tank however.

[Mike Buckley]

Very difficult as in theory the aim is to have no runoff at all as all the water will be turned into steam to acheive maximum cooling and fire extinction. This depends on the skill of the firefighters and the size of the fire.

Then of course for us oldtimers there was the question of Colin's favorite Brigade and the use of 'street enders'.

[kurnal]

Just another thought to add- for industrial sites I have worked on where firefighting water run off has been considered, this has been achieved by running surface water drainage into balancing lagoons adjacent to the site  from which the outfall can be controlled if required. .

[JT]

Thanks both.

Kurnal, 1,000lpm? I wouldn't like to be on the end of that branch. Isn't it 450lpm?
I'll have a good look at that report, thanks.

Mike, I was a cfbt instructor and no you can't rely on the skills of a FF to produce no excessive water. Quite often the most experienced guys still put a jet above their heads!
That was internal, but what I'm considering is external firefighting (for large premises) where the Environment agency like to limit the amount of run off into drains/streams etc.

[JT]

Just another thought to add- for industrial sites I have worked on where firefighting water run off has been considered, this has been achieved by running surface water drainage into balancing lagoons adjacent to the site  from which the outfall can be controlled if required. .

Yes, industrial is what I'm looking at.
I've had a look at this document for general requirements, but hopefully in the report you mention there's a more accurate way of determining it. 
https://www.sepa.org.uk/media/100544/ppg-18-managing-fire-water-and-major-spillages.pdf

[kurnal]

Hi Cardiff

Thats a useful link - if you get a sight of reference document 3 I would appreciate a heads up.

Yes the 1000l/min seems a lot and probably is so taking into account modern diffuser nozzles but for an established fire- which is when run off becomes an issue - it represents the discharge from a 25mm nozzle at optimum pressure. And when you consider aerial monitors run 30mm  up to 40mm nozzles........

In my day every appliance carried two 25mm noble branches and two 12.5 AWG jet spray hand controlled branches. As equipment officer in 1980 I changed the noble branches to 25/18mm hand controlled branches and replaced the AWGs with Akron turbojets. (Sorry swinging the lead as usual)

I think (if my memory serves me right - and it may not) the figure of 1000 L/min  originates from the 1980s manual of firemanship- hydraulics and water supplies -  and was in the context of calculations for 70mm hoselines. A nagging thought in my mind is telling me that this may have been downgraded in a later edition to 300 L/Min for 45mm hose lines and 600 for 70mm hoses - i will check and report back tomorrow.
Please keep us posted- this is a significant issue often overlooked by many designers and then the fire service catches a cold.

[kurnal]

heres another useful link

http://www.hse.gov.uk/pubns/priced/eh70.pdf

[JT]

Document sent to your email address.
Any problem let me know.

[Mike Buckley]

What I was trying to say is that you cannot rely on the idea that if you pump 1000 l/m onto a fire you are going to get 1000 l/m runoff. Hence the calcuations can be rather difficult.

[kurnal]

Mike thats true but for the purposes of environmental protection the Environment Agency would prefer an overestimate than an underestimate leading to an uncontrolled pollution event. The fire service have been fined for allowing run off into rivers.