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 static pressure is less than 40 p.s.i., the NFPA standard allows available flows to be based on residual pressures which are half the static pressure. Thus a hydrant which has a 30 p.s.i. static pressure can be calculated for flows generated when pulled down to a 15 p.s.i. residual rather than the standard benchmark of 20 p.s.i., thereby improving its accredited performance.

FORMULAS

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.) STATIC PRESSURE Pressure reading before water flows. RESIDUAL PRESSURE Pressure reading while water is flowing (from an outlet other than the flow outlet.) PITOT PRESSURE Reading taken by a pitot gauge inserted into the center of the flowing outlet, at a distance away from the lip of the outlet of about half the nozzle's diameter. COEFFICIENT Since hydrant nozzles typically don't produce perfect discharge columns, this is a correction factor which is often used to compensate for errant pitot readings. Hydrant manufacturers should be able to provide coefficients for their products. For hydrants where the coefficient is unknown, we use .95 or .9 depending upon how uniform the discharge stream looks when the hydrant is opened. If a flow tube or "stream straightner" is used on the hydrant, the coefficient would be 1. RESIDUAL BASELINE A pressure which you determine is the lowest that the hydrant would be pulled down during actual use. NFPA states that the basis for fire flow calculations will be 20 psi residual, however in low pressure areas they allow calculations based on one-half the static pressure. Ergo, flow from a hydrant that has only a 30 psi static pressure can be calculated on a basis of drawing it down to 15 psi rather than 20. OBSERVED FLOW This is a calculation in GPM of the actual flow from one outlet flowing fully opened. AVAILABLE FLOW This is the calculated maximum capacity of the hydrant if it is pumped down to the basis residual pressure (usually 20 psi). Q FORMULA The Q formula produces a value in GPM based on the nozzle diameter and pitot pressure (solving for "Q".) Where Q=observed flow, c=coefficient, d=outlet diameter, p=pitot pressure. HAZEN-WILLIAMS FORMULA This formula calculates available flow based on the readings taken before and during the single outlet flow test (solving for "QR".) Where QF=observed flow, hr is the drop in pressure from the static pressure to the desired residual baseline and hf is the drop in psi from static pressure to the actual residual pressure that was measured during the test. Please note that we are calculating to the .54 power (a fractional number.) We loaded these formulas into a computer program to make them practical to work, however a programmable scientific calculator can also provide efficient results. Note: 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.