Water Pressure Zone HGL and Service Pressure

All of it comes from the hydraulic grade line: pressure is the HGL less the ground elevation, times 0.433 psi per foot. THE HGL IS THE THING TO THINK IN. It is the elevation water would rise to in an open tube, so it makes every pressure in a system a subtraction rather than a calculation. Under static conditions it is the tank overflow elevation; under flow it falls by the friction between the tank and the point in question, which is why the same service reads one pressure at two in the morning and another at peak hour, and why a complaint about low pressure is a question about when. ONE ZONE IS ONE ELEVATION BAND, and the band's width is not a design choice. It is fixed by the pressure range the code allows -- typically 40 psi at the top of the band and 80 at the bottom -- which at 0.433 psi per foot is a little over 90 feet of relief per zone. A service area spanning more relief than that needs more zones, and the count is arithmetic rather than preference. That is the whole reason a hilly system has many zones, many tanks, many pressure reducing stations and many boundary valves, and a flat one has few. Services outside the band are not failures of the zone; they are services that need a pressure reducing valve below it or a booster above it, and identifying them from the elevation is cheaper than identifying them from complaints. AND THE STATIC CASE IS NOT THE ONE THAT GOVERNS. Codes generally require a residual pressure -- commonly 20 psi -- to be maintained at every service during fire flow, and that condition is what actually sizes mains, storage and pump stations. A zone comfortable at rest can fall below the residual at the far corner during a fire, and the static number gives no warning of it. This computes pressures and bands from an entered HGL. It does not model the distribution network or compute the friction that sets the HGL under any flow (which needs a hydraulic model), determine fire flow requirements or available fire flow, size storage, mains or pumping, locate zone boundaries or pressure reducing stations, address surge, thermal expansion, or the backflow protection a PRV arrangement needs, or evaluate what any jurisdiction requires. AWWA M32, the applicable plumbing and fire codes, and the utility's hydraulic model govern.

service pressure = (HGL - ground elevation) x 0.433 psi/ft; the servable elevation band runs from HGL - max/0.433 to HGL - min/0.433, and the zones a service area needs is its relief divided by that band.

Distribution pressure zones as AWWA M32 and the plumbing codes frame them. The minimum and maximum service pressures are ENTERED because they are jurisdictional; 40 and 80 psi are common.

One exact hydrostatic conversion and its rearrangements.

Estimate. AHJ and licensed professional govern.

Field names used by the API: hgl_ft, service_elevation_ft, min_pressure_psi, max_pressure_psi, service_relief_ft, fire_flow_friction_ft, static_psi, lowest_servable_ft, highest_servable_ft, elevation_band_ft, zones_required, fire_flow_psi

• Pressure limits are entered they are jurisdictional; 40 and 80 psi are commonthe plumbing code in force
• Static HGL under flow it falls by friction, which needs a hydraulic modelAWWA M32
• Fire flow governs the residual requirement sizes mains and storage, not the static casethe fire code and the utility's model