Finding water pressure at hydrant

[WaterRookie]

The question is....what is the water pressure at a fire hydrant located off a tank (5 million gallon ground storage, ground HGL is 505) that has a HGL of 555 (overflow) but only has 31 feet of water in the tank The HGL at the hydrant is 392. Please give the means of finding PSI off of this question.

 

[semo]

You keep using HGL (Hydraulic Grade Line); but, don't think you mean to use that term.

I assume you mean the HGL (water elevation) at the tank will be 536 (505 ground elev + 31 water depth). I also assume the hydrant discharge elevation is 392 not the HGL. So I will answer what I think you mean. In a closed system, the HGL at the hydrant would also be 536. Therefore the pressure at the hydrant will be the water height above the discharge point divided by 2.31 which is the conversion factor from feet to psi. This gives (536 - 392)/2.31 or 62.3 psi.

If the HGL at the hydrant is 392 you would need the discharge elevation of the hydrant before you could calculate the static pressure.

The HGL is, in a nut-shell, a snap-shot of the water level at a given instant based upon the system node elevations, line sizes, and flows through the lines. Different system conditions will provide a different HGL.

 

[cvg]

it sounds like the hydrant is some distance from the tank to be at a much lower elevation. Under static conditions, Semo is correct - the HGL is 536. However, static pressure is somewhat meaningless for a hydrant. Residual pressure with the hydrant fully open, during average daily demand period is more useful. You will need to calculate headloss in the transmission main from hydrant flow as well as normal flow to the customers. This may be best done using a hydraulic network model.