Distance from hydrant and water pressure

[Davo]

Is there a formula for calculating the effect on hydrant water pressure and what comes out of the hose, due to distance of hydrant from a building.
Looking at a PFI where the proposed FH is 60m away and site elevated from surround slightly, 3 floors.

davo

[kurnal]

Not likely to be a problem Davo provided the fire hydrant conforms to BS750 in terms of pressure and flow.

The ADB recommends the hydrant should be up to 90m from the fire appliance parking position. The hydrant should deliver a high volume at low pressure, the fire appliance pump converts this to a lower volume at higher pressure for firefighting jets. This will easily overcome the loss of pressure due to head and due to friction losses  in the hose lines. Typically a hydrant may have capacity to deliver say 1500 LPM at 1-2 Bars, (look up BS750 for the performance criteria- I cant remember ) the pump converts this to higher pressures- typically say 6-7 Bars  for firefighting probably with a nozzle delivering 300-600 LPM.

The pump operator will add 0.1 Bar per metre of rise and 0.2 Bar per 25m length of hose to counter friction losses.  

(Thats using hose in a radial manner of course. Losses would be less if there was a ring main. )

Hope this helps

 

[SamFIRT]

The discharge in litres per minute from a nozzle is measured by taking two thirds of the diameter of the nozzle squared and multiplying it by the square route of the pressure of the water received at the nozzle.

L=2/3 d2 √p  

To obtain this pressure you have to calculate the frictional loss in each length of hose and take into account the reduction and or increase in pressure due to changes in height. There is also a loss in pressure due to the creation of flow and the turbulence created by entry of the water into the pump.

That is the reason the correct tactic is for a fire fighting pump to be positioned close to a hydrant with as short a length as possible between it and the hydrant. With the pressure feed going directly into the eye of the pump through a collecting head rather than into the tank filler. This way you obtain the maximum pressure from the hydrant rather than only having the head pressure feed from the in-built appliance tank. The standard hose length is 25m. But this can be shortened down to as little as 18m. However many FRS use what is euphemistically called a short length, which is non-standard and may be as short as three metres. It is also the reason that twin headed standpipes are used with double lengths of hose to feed the pump, as by twinning the lines feeding the pump you will ¼ the frictional loss.

This is why at the design stage Hydrants are asked to be positioned where they are and why road access has to be sufficient for the appliances.

The idea is to have 3 bars pressure at the nozzle, in order to minimise jet reaction, and hose stiffness. (Modern rotary spray nozzles are designed to discharge around 300 – 500 LPM varying according to size).