Note: Comparison of the H2OMAP SWMM Hazen Williams Force Main Solution to a Steady State HW Solution
In this example, we compare the force main head loss in four links in H20Map SWMM to the head loss in a steady state Hazen Williams solution for the same length pipe, diameter and flow (Figure 1). The H2OMap SWMM model has a large constant dry weather inflow at the wet wells which floods the wet well and causes a constant pump flow to the force main (Figure 2). The HW calculator is located here http://www.engineeringtoolbox.com/william-hazens-equation-d_645.html and a comparison for HW head loss in PSI for 5000 feet long, 3 inch diameter pipes with HW Coefficients of 130, 120, 110 and 100, respectively, is shown in Table 1. The SWMM 5 equation loss (PSI Diff) and the PSI loss from the HW calculator are very close for all four links.
Table 1. Steady State comparison between HW Calculator and H2OMAP SWMM/SWMM 5 Force Main calculations.
HW |
SWMM5 |
SWMM5 |
SWMM5 Loss |
Loss |
Coefficient |
Psi UP |
PSI Dn |
PSI Diff |
PSI HW Calculator |
130 |
84.563 |
44.88 |
39.683 |
39.82 |
120 |
88.772 |
43.765 |
45.007 |
45.16 |
110 |
91.798 |
41.426 |
50.372 |
50.54 |
100 |
95.354 |
38.727 |
56.627 |
56.82 |
Figure 1. H2OMAP SWMM Wet Well, Pump, Force Main and Gravity Main Network.
Figure 2. Constant Pump Flows