City of Morgan Hill Water System Master Plan 2017

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.

June 2017

3-12

City of Morgan Hill Water System Master Plan