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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.