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June 2017
3-8
City of Morgan Hill
Water System Master Plan
commercial and industrial areas. Fire flow provisions for each pressure zone were calculated
based on the governing (highest) land use type within a reservoir service area as follows:
Residential: 1,500 gpm for 2 hours = 0.18 MG
Emergency: 2,500 gpm for 2 hours = 0.30 MG
Industrial: 3,500 gpm for 3 hours = 0.63 MG
Total Storage Requirement
The total storage is the summation of operational (equalization), fire, and emergency storage
requirements as follows:
Qs = 25% MDD (equalization) + fire flow (varies) + 25% MDD (emergency)
where:
Qs is the Total Required Storage, in gallons
MDD is the Maximum Day Demand, in gallons
3.4 PRESSURE CRITERIA
Acceptable service pressures within distribution systems vary depending on city criteria and
pressure zone topography. It is essential that the water pressure in a consumer’s residence or
place of business be maintained within an acceptable range. Low pressures below 30 psi can
cause undesirable flow reductions when multiple faucets or water using appliances are used at
once.
Excessively high pressures can cause faucets to leak and valve seats to wear out prematurely.
Additionally, high service pressures can cause unnecessarily high flow rates, which can result in
wasted water and high utility bills. The criteria for pressures in the domestic water system include
the following:
Maximum pressure, usually experienced during low demands and winter months
Minimum pressure, usually experienced during peak hour demands and summer months
Minimum pressure during fire flows and during the maximum day demand
The American Water Works Association Manual on Computer Modeling and Water Distribution
System (AWWA M-32) indicates that maximum pressures are usually in the range of 90-110
pounds per square inch (psi). In some communities, the maximum pressure may be limited to 80
psi to mitigate the impact on internal plumbing. In this case, the distribution system is usually
sized for the higher pressures, and individual pressure-reducing valves are installed on service
lines where the pressure may be exceeded.