Those of you who work with water distribution systems generally need to determine the flow that can be delivered to a fire at some minimum residual pressure, which in the US is 20 psi in most jurisdictions. In most cases, this is determined by conducting a fire hydrant flow test which measures the pressure at a residual hydrant while flowing a nearby test hydrant. The flow at 20 psi (called Q20 or distribution system capacity) is then calculated using this equation. (1) Where Q20 = flow at 20 psi residual, Qt = flow during test, Ps = static pressure, Pt = pressure during test Pressures are measured at residual hydrant, not the flowed hydrant. But where did this equation come from and what assumptions are contained in it? The equation comes from the Insurance Services Office (ISO) and it was most likely developed by its predecessor the National Board of Fire Underwriters. I can’t find the original paper, so I went back and re-derived it myself to understand what it’s made of. Write out the energy equation for three cases: Static (2a) Test flow (2b) Residual condition (2c) Where H = hydraulic grade for zone, z = elevation of residual gage, a = conversion factor

In order to do any kind of hydraulic modeling, whether it is for water distribution, sewage or stormwater, it is necessary to determine the head loss over a range of flow rates. Much of the early days of hydraulic engineering were consumed with engineers trying to come up with the right equation to do these calculations.

Water utilities everywhere struggle to reduce real water loss using a variety of tools from acoustic leak detection, to water audits, to pressure anomaly detection, to ground penetrating radar, to pressure management. Each of these addresses a portion of the problem and there is no single perfect solution.