Water System Design Manual
If there are pressure limits associated with ES
, engineers can use Equation 6-6 to determine
the amount of ES. The SB would then be the amount of water remaining after the OS, ES, FSS,
and DS are removed from TS.
If distribution pressure limits do not restrict ES and SB
, engineers can determine the physical
capacity associated with the combined quantities established for ES and SB as follows:
First, define the capacity-related storage (CRS) as the amount of storage that relates to ERU
determinations (ES + SB). Using that with Equation 6-5 and Equation 6-7, leads to the
( )( )
( )( )( )
After solving for N, engineers can use Equation 6-8 to determine ERUs when the total storage, or
"capacity related" storage (ES + SB), is known and not restricted by distribution system pressure.
Of course, engineers also haves to determine the water system parameters for Qs, MDD, F, C,
SBi, and td.
Equation 6-8: Determine ERUs when distribution pressure limits do not restrict ES and SB
( ) ( )( )
6.7.4 Step 4: Physical Capacity Based on Distribution Facilities
Water system distribution and transmission infrastructure must be adequate to accommodate
design peak flows (WAC 246-290-230). Distribution adequacy is determined on a pressure zone
Each design parameter below is pertinent to the distribution system capacity determination:
Minimum domestic pressure (psig) under normal peak day operating conditions and fire
flow, if applicable (WAC 246-290-230 and Section 8.15).
Maximum flow velocities (fps) within piping.
Maximum fire flow requirements (gpm) in combination with MDD (gpd/ERU).
Piping sizes (inches in diameter and lengths) and appropriate friction factors.