Low Pressure / Gravity Flow


Gravity Sewer Systems – A sanitary sewer system is designed to carry wastewater away from our homes and businesses to a wastewater treatment plant (WWTP), before being released into streams or other water bodies. The network of sewer mains is designed to rely on gravity to move the flow along; grades are selected so that the velocity of the flow results in a self-cleansing action to move the liquids and solids through the piping network.

Similarly, storm water systems are also designed to move water by gravity. In sanitary sewer systems, when the terrain is very flat and the required velocity by gravity in the piping system cannot be sustained, sanitary sewer force mains are used to convey the wastewater with the help of pumps. It is not uncommon to have many pumping stations in relatively flat areas to continue the flow of wastewater to a WWTP.

King County’s $1.75 billion Brightwater Tunnel Project, utilizing polyurethane lined and coated steel pipe and specials

Outfalls, Force Mains and Other Steel Pipe Solutions – Spirally welded steel pipe is used in various gravity applications by municipalities. Sewers discharging into large bodies of water are usually extended beyond the banks into fairly deep water where dispersion and diffusion will aid in mixing the discharge with the surrounding water. These outfall lines, referred to as sewer outfalls, are often constructed with spirally welded steel pipe; construction may involve placing the pipe from barges or even using divers to join them.

Steel pipe is often specified for sewer force main applications also. Although pressures within a sewer force main are relatively low, steel pipe used in these applications will always have a minimum working pressure of 150 psi, and a surge pressure of 225 psi, based on min. yield strength of 36 ksi, and a D/t ratio of 240.

Sewer force mains that are designed to always flow full are typically lined internally with cement mortar. If non-full periods of flow are anticipated, it is typical to use either polyurethane or epoxy.

Corrosion Resistance Advances – The advent of effective bonded flexible internal linings such as chemically resistant polyurethanes are leading to an increased specification of steel pipes for sewer force mains. Northwest Pipe Company’s polyurethane linings used in raw sewage applications have been evaluated for acceptability against the guidelines of the City of Los Angeles Green Book Standard specifications, the State of Washington’s Sanitary Agency’s Orange Book, as well as a number of other stringent guidelines for sanitary sewer applications nationwide. One of the largest sanitary sewer projects in recent years, King County’s $1.75 billion Brightwater Tunnel Project in the state of Washington specified and successfully used polyurethane lined and coated steel pipe for both influent and effluent pipelines that included treatment, conveyance and outfalls.