© 2001 Capt. Willis Lamm, Water Supply Officer, Moraga-Orinda (CA) Fire District
|VARIATIONS FOR LOW PRESSURE ZONES|
As a general rule available flows are calculated based on what the hydrant can
provide when pulled down to a residual pressure of 20 p.s.i. Typically this flow
is greater than the observed flow. On weaker zones it is possible that the
calculated available flow will actually be less than the observed flow when
the residual falls below 20 p.s.i. with one outlet flowing.
In pressure zones where the
We use the "Q" formula and Hazen-Williams formula to determine single outlet hydrant
flow. The Q formula produces the actual volume of water in GPM that
we are getting from the outlet we are flowing. The Hazen-Williams formula takes
this information and calculates what the hydrant would flow if all outlets
were being used simultaneously without having to actually open all outlets
and max-flow the hydrant.
In order for these formulas to make sense, one needs to understand specific meaning of the following terms. (Most are pretty basic but a couple are generally misunderstood.)
We loaded these formulas into a computer program to make them practical to work, however a programmable scientific calculator can also provide efficient results.
In most instances the Hazen-Williams formula will calculate available flows that are greater than observed flows. However on extremely weak water mains hydrants may fall below the baseline residual when an outlet is opened up. If there is sufficient residual pressure to take an accurate reading, the formula will still calculate what the hydrant produces at 20 psi (or half the static pressure) which in these cases will be slightly less than the observed flow. According to NFPA, these hydrants are to be rated at their available flows at the appropriate residual pressure so the lower GPM reading is the one to be used for rating the hydrant. If the residual pressure is too low to take accurate readings discharging an "open butt" outlet, a small smooth bore tip can be attached to the outlet, the calculations based on the size of the tip, and the formula will still work.
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