Gravity Fed Low Pressure
Steel pipe and RCP are leading the pack as product of choice over competing products for for sewer mains.
Sanitary sewer and stormwater systems are two common systems that utilize low pressure and gravity flow pipelines. These networks rely on gravity to move wastewater while the dropping grade creates velocity and self-cleans the pipes. In flat terrain without grading, sanitary sewer force mains use pumps at pumping stations to convey the wastewater.
A primary benefit of selecting steel pipe over iron ductile, high-density polyethylene (HDPE), or polyvinyl chloride (PVC) pipe for Low Pressure and Gravity Flow Systems is its brute strength. Steel pipe and joints do not crack or burst under abnormal pressure loads. Pressures within a sewer force main are relatively low, but steel pipe used in these applications will always have a minimum working pressure of 150 psi, and a surge pressure of 225 psi.
Serious and proven health concerns are associated with PVC, to the point where some cities have banned or restricted its use. PVC pipe is also sensitive to ambient temperatures and become increasingly brittle in low temperatures and weaker in warm environments. HDPE and PVC pipe also require frequent performance checks. While plastic is quite strong, it does not have the strength of steel or the ability to resist damage from internal or external forces.
Ductile Iron pipe used in wastewater and stormwater systems faces its own set of challenges. Steel pipe has integral coating and lining systems to protect against corrosion. Ductile iron pipe’s only external corrosion protections is a polyethylene bag that wraps around the pipe. This encasement is attached with duct tape during installation and routinely folds and tears, exposing it to corrosive agents. Ductile iron pipe is not engineered to a site’s specific conditions or parameters. The material comes in set sizes, just as you would find at a home improvement center.
After evaluating performance, many municipalities are including steel pipe as a choice for sewer mains including the City of Los Angeles Green Book Standard Specifications and the State of Washington’s Sanitary Agency’s Orange Book. One of the largest sanitary sewer projects in recent years, King County’s $1.75 billion Brightwater Tunnel Project in Washington, specified and successfully used polyurethane lined and coated steel pipe for both influent and effluent pipelines that included treatment, conveyance, and outfalls.
