12.2 Water Demand, Peaking Factors & Fire Flow

Demand Terminology

In California these factors are wider than in most states because of climate. A Central Valley system with heavy summer landscape irrigation can see an MDD three times its winter average, while a coastal system with year-round mild weather may see 1.4. Know your own system's factors from your own meter and production data - that is exactly what 22 CCR 64554 requires.

The 22 CCR 64554 Method

And the capacity tests:

• All systems: sources must be able to meet MDD at all times, evaluated system-wide and within each pressure zone.
• 1,000 or more service connections: must meet four hours of peak hourly demand from source capacity, storage, or emergency connections.
• Fewer than 1,000 service connections: storage capacity must equal or exceed MDD, unless additional sources or emergency connections are demonstrated.
• Community groundwater-only systems: at least two approved sources, able to meet demand with the largest source out of service.

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Estimating Demand

Unit demand approach. A residential development of 480 homes at an average of 2.8 persons per home and 110 gallons per capita per day:

ADD = 480 x 2.8 x 110 = 147,840 gpd = 102.7 gpm MDD at a factor of 2.0 = 295,680 gpd = 205 gpm PHD at a factor of 3.5 x ADD = 517,440 gpd = 359 gpm

Equivalent dwelling unit (EDU) approach. Commercial, industrial, and institutional customers are converted to EDUs by meter size or by measured use, then added to the residential count. This is the basis for most California connection fee schedules and for Proposition 218 rate structures.

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Fire Flow

Fire flow is expressed as a rate in gpm, for a duration in hours, at a residual pressure of at least 20 psi in the main while flowing. All three parts matter; a system that can deliver 1,500 gpm only by dropping to 12 psi has not met a 1,500 gpm fire flow requirement.

Required flows are set by the fire authority having jurisdiction using the California Fire Code and the referenced fire flow tables, adjusted for construction type, occupancy, area, exposure, and the presence of automatic sprinklers, which can reduce required fire flow substantially.

$\text{Fire Storage (gal)} = \text{Required Fire Flow (gpm)} \times \text{Duration (min)}$

Example: 2,000 gpm for 2 hours = 2,000 x 120 = 240,000 gallons of dedicated fire storage.

[!IMPORTANT] Fire flow comes out of storage, not out of the wells. A source that produces 900 gpm cannot supply a 2,500 gpm fire. The correct design statement is that the system must supply maximum day demand from its sources while simultaneously supplying fire flow from storage, and must recover the fire storage volume within a stated period afterward. Sizing questions that ask "is this source adequate for fire flow" are testing whether you know that fire flow is a storage question.

Hydrant Flow Testing

Flow testing measures what the system can actually deliver:

1. Select a residual hydrant (where pressure is measured) and one or more flow hydrants (which are opened).
2. Record static pressure at the residual hydrant with nothing flowing.
3. Open the flow hydrant(s) and measure discharge using a pitot gauge at the outlet, and record the residual pressure at the residual hydrant.
4. Compute flow at the outlet, then project the flow available at 20 psi.

$Q = 29.83 \times c \times d^{2} \times \sqrt{p}$

where Q is gpm, c is the outlet coefficient (typically 0.90 for a rounded outlet, 0.80 for a square-edged outlet projecting into the barrel), d is the outlet diameter in inches, and p is the pitot pressure in psi.

Example: a 2.5-inch outlet with c = 0.90 and a pitot reading of 16 psi: Q = 29.83 x 0.90 x (2.5)² x √16 = 29.83 x 0.90 x 6.25 x 4 = 671 gpm

The available flow at 20 psi is then projected from the static and residual readings using the standard hydrant flow test formula, giving the number the fire authority needs.

Safety and Courtesy During Flow Tests

• Notify the fire department and, for large tests, customers - flow tests cause discolored water and pressure drops.
• Open and close hydrants slowly. A fast closure is a water hammer generator.
• Control the discharge - erosion, flooding, traffic hazards, and dechlorination of the discharge where it reaches a storm drain or watercourse are all real obligations.
• Watch the residual pressure floor. If the test would pull the system below 20 psi, stop; you are creating an intrusion risk, not gathering data.