← All tools

Water Supply Analysis

Sprinkler System vs. Water Supply Analysis

Enter the hydrant flow test results and the sprinkler system demand point. The tool plots the supply curve, the sprinkler demand, and the outside hose stream allowance on N1.85 paper, then reports the available pressure at the total demand flow and the safety margin in psi.

Project & Site Information

Hydrant Flow Test Static and residual at the pressure hydrant; total flow measured at the flow hydrant. For pitot & outlet-coefficient calculations, see the Hydrant Flow Test tool.

Sprinkler System Demand

The hose stream allowance is added to the system flow at the same demand pressure — a horizontal segment on the curve. The safety margin is then measured at the total demand flow.

Results

Supply: PS / PR @ QF
— / —psi @ — gpm
Demand point (system + hose)
—gpm @ — psi
Available pressure at total demand
—psi
From supply curve at — gpm
Safety margin (sprinkler only)
—psi
Safety margin (w/ hose)
—psi

Supply vs. Demand Curve N1.85 paper — supply plots as a straight line.

Water supply curve Sprinkler demand curve Hose stream segment Safety margin (dashed)

Method

Supply curve (N1.85 paper)

The water supply is modeled as the Hazen-Williams friction relationship anchored at the static pressure (Q = 0) and the flow-test point (QF, PR):

P(Q) = PS − (PS − PR) · (Q / QF)1.85

On N1.85 paper, where the horizontal axis is stretched by Q1.85, this relationship plots as a straight line from the static point to the residual point.

Demand point and hose stream

The sprinkler system demand is plotted as a point at (Qsys, Pdemand). The outside hose stream allowance is added horizontally at the same pressure, producing a total demand point at:

Qtotal = Qsys + Qhose   at   Pdemand

Safety margins

Two safety margins are reported, both measured as the vertical distance between the supply curve and the sprinkler demand pressure:

Safetysys = P(Qsys) − Pdemand   (sprinkler only) Safetytotal = P(Qtotal) − Pdemand   (sprinkler + hose)

The sprinkler-only margin shows whether the supply can deliver the system demand by itself, independent of the outside hose stream allowance. This is useful when the hose stream can be supplied from another source, or when the AHJ accepts a project where only the sprinkler demand is met from the same supply.

A positive value indicates the supply has pressure to spare at that flow; a negative value means the supply cannot meet the demand at the required flow.

Engineering aid only. The supply curve is a model fit to a single flow test and assumes the municipal system behaves per the Hazen-Williams friction law. Real water supplies vary with the day, season, and operating state of municipal pumps and storage. The engineer of record is responsible for confirming the test conditions, accounting for elevation between the test location and the system riser, and applying any appropriate de-rating factors before using these results for design.