Why aren’t all sewer pipes round? 

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 shaped sewer

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. 

Ovoid or egg shaped sewer

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. 

horseshoe shaped

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. 

rectangular sewer

Parabolic – wider at the bottom than the top, parabolic sewers may be preferred where there is limited vertical clearance such as a bridge crossing.  

Parabolic sewer

  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. 

U-Shaped sewer

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. 

Basket Handle sewer

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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