What is residual pressure? 2

[DPAJR]

NFPA 24 and the IFC both mention 20 psi residual pressure. What does this mean? Where is the pressure measured or calculated? Is it the pressure at the hydrant or the calculated pressure after pipe losses, velocity head etc? I'm lost on this one

Thanks
DPAJR

 

[trashcanman]

Better illustrated with an example. Take a fire hydrant and take the static pressure, say 80 psig. Open an adjacent hydrant. Take the static pressure again at the 1st hydrant. Say it now reads 50 psi. That is your residual pressure.

From NFPA 13: The proper method of conducting this test is to use two hydrants in the vicinity of the property. The static pressure should be measured on the hydrant in front of or nearest to the property and the water allowed to flow from the hydrant next nearest the property, preferably the one farthest from the source of supply if the main is fed only one way. The residual pressure will be that indicated at the hydrant where water is not flowing.

 

[DPAJR]

Thanks trashcanman. That clears it up.

DPAJR

 

[sharique]

Trashcanman is correct!

Fire Protection Hydrant System shall be designed based on the residual pressure at the extreme end of the hydrant network.

Assume a case: If the pumping system has been designed to maintain (say) 6 bar pressure with (say) 100 GPM flow when four consecutive hydrants in operation at the extreme end of the network, the residual pressure of the system in this case would be 6 bar, on which the overall design of the system shall be based.