Why aren’t all sewer pipes round?
When people think of a sewer pipe, they will picture a cylindrical pipe with a round cross-section that has a constant diameter. Cylinders have many properties that lend themselves to being perfectly suited to transport fluid, in this case, wastewater.
Oval or Egg-shaped sewer
Photo courtesy : https://www.flickr.com/photos/gee01/
Why are round pipes the most common design?
The overwhelming majority of sewer pipes are round, and this is for good reason.
Strength – round pipes have no corners or weak joins.
Construction – manufacturing a round pipe is generally an easier process than other shapes. Die extrusion, spin-casting, and filament winding are all common processes used to manufacture pipes. A circular shape can be easily controlled when looking to achieve a consistent thickness and strength during production.
Economical – a cylindrical shape is able to provide the maximum volume for the material required and result in the most economical shape to fabricate.
When are other shapes used?
There are a range of non-circular shapes used in the construction of sewers, these include?
Ovoid (or egg-shaped) – one of the most common non-circular pipe is the ovoid sewer. These are popular in some combined sewer networks. With a narrower width at the bottom, this pipe shape is particularly useful to combine self-cleansing velocity during low flows (i.e., reduced blockages/settled deposits) and the capacity to transport larger volumes with a widening width in the upper half of the pipe.
Horseshoe – with a wider profile this design may suit a situation where a sewer line needs to be installed at a shallow depth or where the ground conditions do not allow for the installation of a more regular shape. The flatter invert also allows for easier conditions to manually enter and walk through a pipe.
Rectangular – rectangular, or box sewers, are sometimes used in combined systems where space is at a premium along buildings or other infrastructure. This shape is very common for stormwater culverts.
Parabolic – wider at the bottom than the top, parabolic sewers may be preferred where there is limited vertical clearance such as a bridge crossing.
U-Shaped – Looking like an inverted horseshoe, with a larger cross-sectional area than a circular sewer the U-shaped design allows for a greater total flow capacity and can help prevent backups and overflows during periods of heavy rainfall. During lower flows a higher velocity is achieved to aid self-cleansing.
Basket handle – like the ovoid design, this shape increases the velocity under low flow conditions to prevent settling of deposits. This shape was used in many parts of the original Parisian sewer; the horizontal flat sections provided a space for walkways or cleaning equipment.
About the Author
Mark Lee is the Business Development Manager (Aus/Nz) at VAPAR and a former Senior Asset Engineer who has spent more than a decade managing the asset lifecycle of infrastructure. He has extensive experience managing pipeline networks, including design, construction, condition assessment and decommissioning.
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