| Hydrogen sulfide gas (H2S) can sometimes pose a problem in gravity sewers if the pipe has an air space allowing O2 to combine with H2S, creating sulfuric acid (H2SO4). Since H2S is extremely hazardous to personnel, the designer should provide adequate slope and maintain an adequate velocity in the piping system, which should help prevent the waste stream from becoming anaerobic and generating H2S.
Gravity sewers are designed utilizing the Manning formula for uniform flow. The Manning roughness coefficient, "n," is generally accepted as 0.011 for cement-mortar lining, and 0.01 for polyethylene and epoxy linings. Ductile iron pipe typically has a larger inside diameter than other piping materials, which allows for greater design flows and carrying capacities. In larger diameter sewers, for example, ductile's larger inside diameter could accommodate several million gallons per day additional flow.
Ductile iron's long service life and bottle-tight joints make it especially suitable for gravity flow applications.
Additional flow with ductile iron pipe: Based on full flow, 1% grade and approximately 8% more flow with ductile iron pipe. |
Well-suited for difficult installations due to its inherent strength, ductile iron is an excellent piping material for use in gravity sewer applications. Its superior ring strength, combined with its bottle-tight joints, makes it ideally suited for deep trenches and installations in areas of high water table.
Ductile iron pipe has long been the material of choice for water and waste water treatment plant piping, due to its wide selection of joints and fittings, and the fact that it can be fabricated for any application. |
