Water System Design Manual
Any pipeline designed to provide fire flow must be at least 6 inches in diameter (WAC 246-290-
230(3)). Minimum fire flow requirements are in the Water System Coordination Act (WAC 246-
293-640). Counties and local fire protection authorities often have more stringent fire flow
standards than these minimum requirements.
Engineers must consider at least two demand scenarios when using a hydraulic analysis to size
mains (WAC 246-290-230(5) and (6)).
, the water system must be able to deliver the peak hourly demand at the required
pressure of 30 psi at every existing and proposed service connection.
, if the water system provides fire flow, the distribution pipelines must be able to
deliver the maximum day demand (MDD) rate, in addition to the fire flow, at the required
pressure of 20 psi throughout the distribution system.
There is more detail on this analysis in sections 8.1.5 and 8.2.3l.
8.1.3 Peak Hourly Demand
Distribution pipelines must be able to deliver enough water to meet peak hourly demand (PHD)
at 30 psi at every existing and proposed service (WAC 246-290-230(5)). PHD is the maximum
rate of water use expected to occur in a defined service area over a continuous 60-minute period,
excluding fire flow. Unless there are accurate water demand records identifying PHD, the
designer should use the equations in Chapter 5 to determine PHD. If there is more than one
pressure zone, the engineer must calculate PHD separately for each zone.
Prior to 1999, DOH design guidelines called PHD "maximum instantaneous demand."
8.1.4 Fire "Suppression" Flow
In most cases, the local fire protection authority determines fire flow rate and duration
requirements for water systems. If the local government does not establish minimum fire flow
standards, the engineer must use the standards in the Water System Coordination Act (WAC
246-293-640). These fire flow standards apply to new or expanding water systems:
Within the boundaries of a designated Critical Water Supply Service Area.
With more than 1,000 services (WAC 246-293-602 and WAC 246-290-221(5)).
Typically, the fire protection authority is the town or city fire chief, or county fire marshal in
unincorporated areas. Some incorporated areas may contract for fire protection services with a
district or the county.
In addition to protecting public safety, a water system's ability to provide fire flow is important
because it is one of the main criteria used to establish the insurance rating for that fire district or