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June 2017

3-12

City of Morgan Hill

Water System Master Plan

3.6.3

Peak Hour Demand

The peak hour demand (PHD) is another high demand condition that is used in the evaluation and

design of water distribution systems. The peak hour demand is the highest demand that occurs

within a one hour period during a year. The peak hour demand is considered to be the largest

single measure of the maximum demand placed on the distribution system. The PHD is often

compared to the MDD plus fire flow to determine the largest demand imposed on the system for

the purpose of evaluating distribution mains.

An industry standard peak hour to maximum day ratio of 1.5 was applied to the maximum day

demand to yield the peak hour demand ratio of 3.0.This is a decrease in peak use trends from the

previous master plan peaking factor of 3.2. The peak hour demand can then be calculated using

the average day demand and the following equation:

Peak Hour Demand =

3.0

x Average Day Demand

3.7 FIRE FLOWS

Fire flows are typically based on land use, with the potential for increased fire flow based on the

building type. The following are the criteria for fire flows:

Category 1.

Fire flows for residential areas was calculated at 1,500 gpm for two hours.

Category 2.

Fire flows for commercial and institutional areas was calculated at 2,500 gpm

for two hours.

Category 3.

Fire flows for industrial areas was calculated at 3,500 gpm for three hours.

3.8 TRANSMISSION AND DISTRIBUTION MAIN CRITERIA

Transmission and distribution mains are usually designed to convey the maximum expected flow

condition. In municipal water systems, this condition is usually the greater of either the peak hour

demand or the maximum day demand plus fire flow. The hydrodynamics of pipe flow create two

additional parameters that are taken into consideration when evaluating or sizing water mains:

head loss and velocity.

Head loss is a loss of energy within pipes that is caused by the frictional effects of the inside

surface of the pipe and friction within the moving fluid itself. Head loss creates a loss in pressure

which is undesirable in water distribution systems. Head loss, by itself, is not an important factor

as long as the pressure criterion has not been violated. However, high head loss may be an

indicator that the pipe is nearing the limit of its carrying capacity and may not have sufficient

capacity to perform under stringent conditions.

Since high flow velocities can cause damage to pipes and lead to high head loss, it is desirable to

keep the velocity below a predetermined limit. The criterion for maximum pipeline velocity used in

this master plan is 15 feet per second. This criterion also ensures that the head loss is kept below

an acceptable limit, as the head loss in a pipe is a function of the flow velocity.