Troubleshooting "N/A" results in H2S Modeling for pressure pipes

Product(s): SewerGEMS, CivilStorm
Version(s): CONNECT Edition,
Area: Modeling


When modeling your system for septicity or H2S concentrations (using the Explicit SWMM Solver); the H2S (Out) results show “N/A” for pressure pipes in some cases.


This happens due to the H2S Loss Factor term in the pressure pipe properties being set to “0”. If the value is set to a considerable value (E.g. 0.9) the results for H2S (Out) would be computed.

In general, the H2S Loss Factor term is not used when computing H2S concentrations for pressurized pipes (such as force mains) and inverted siphons as they are assumed to be flowing full. Since the pipes are flowing full (hence under pressure) the H2S gas cannot leave the liquid phase or there is not enough headroom for the gas to escape. Due to this consideration the H2S Loss Factor is not used when computing H2S concentrations for pressurized pipes flowing full.

However, if the pressure pipes are flowing part full; the H2S Loss Factor is used and if its value is kept as “0”; then the H2S (Out) values downstream would show up as “N/A”. An example of this can be seen in the below profile;

This can happen at high points in a force main where the pressure pipe downstream would be partially full.

When modeling force mains through an undulating topography which would have several high points in the system, your system could contain air-valves at those high points. In the Explicit SWMM Solver used for H2S modeling air-valves are treated as bolted manholes. However, it is advisable to model force mains using the GVF-Convex Solver which is better equipped to handle pumped flows.

See Also

Modeling septicity / hydrogen sulfide (H2S)

Modeling Force Mains with Air Valves in SewerCAD (GVF Convex Solver)