Why is a Bigger Sewer Pipe Used Before a Smaller One?

Introduction #

Sometimes, when designing sewer systems, you might see a bigger sewer pipe followed by a smaller one. This can trigger warnings is design software like MiTS or cause confusion. But there are valid reasons for this setup, and it’s not necessarily a problem. In this article, we’ll explain why this happens in simple terms and how to troubleshoot it.

Possible Cause #

Gradients and Flow Speed #

A common reason for using a larger pipe at the start is to make sure the water flows properly, especially if the slope or gradient of the pipe changes.

• Shallow Slope: If the first part of the sewer line has a shallow slope (not much downward angle), a larger pipe is needed to keep the water moving at the right speed. Without enough speed, debris can settle in the pipe, causing blockages.
• Steep Slope: After the water flows through the larger pipe and the slope gets steeper, a smaller pipe can handle the flow, since the water will be moving faster.

So, the bigger pipe at the start helps maintain a good flow speed until the gradient changes and the flow speed increases as in the example shown below.

Pipe flow = Velocity(m/s) x Area, A(m2) x 1000L/m3  Velocity = 1/n x R2/3 x S1/2 R = A/P  A = πr2 P = 2πr
r = 0.125; R = 0.0625; A = 0.049; n = 0.012
Gradient, S = 1/200	Gradient, S = 1/50
Pipe flow = [1/0.012 x 0.06252/3 x 0.0051/2] x 0.049 x 1000 = 45.4730 L/s	Pipe flow = [1/0.012 x 0.06252/3 x 0.021/2] x 0.049 x 1000 = 90.9459 L/s

The calculation above shows that a shallow slope produces less flow speed compared to a steeper slope. Now, let’s see the size of a sewer needed to maintain a flow speed of 50 L/s with different slopes

Pipe flow, Q = 1/n x (r/2)⅔ x S1/2 x πr2 x 1000 $$r=\left(\frac{Qn}{500\pi S^{\frac{1}{2}}2^{\frac{1}{3}}}\right)^{\frac{3}{8}}$$
Q = 50; n = 0.012
Gradient, S = 1/200	Gradient, S = 1/50
$$r=\left(\frac{50\times 0.012}{500\pi \times 0.005^{\frac{1}{2}}\times 2^{\frac{1}{3}}}\right)^{\frac{3}{8}}$$ $$r=0.1294\:m$$	$$r=\left(\frac{50\times 0.012}{500\pi \times 0.02^{\frac{1}{2}}\times 2^{\frac{1}{3}}}\right)^{\frac{3}{8}}$$ $$r=0.0998\:m$$

From the calculation above, a shallow slope requires a bigger sewer to cater for the same flow speed as a steeper sewer.

Different Pipe Materials #

Another reason for having different pipe sizes is the type of material used for the pipes.

• Material Differences: Different materials, like Ductile Iron (DI) or Vitrified Clay (VC), affect how water flows inside the pipe. Some materials cause more friction, which slows down the water, so a bigger pipe might be needed at the start. A smoother pipe, like DI, might only need a smaller size because it causes less friction.

Pipe flow = Velocity(m/s) x Area(m2) x 1000L/m3  Velocity = 1/n x R2/3 x S1/2
R = 0.0625; A = 0.049; S = 1/50
Vitrified Clay, n = 0.017	Ductile Iron, n = 0.012
Pipe flow = [1/0.017 x 0.06252/3 x 0.021/2] x 0.049 x 1000 = 64.1971 L/s	Pipe flow = [1/0.012 x 0.06252/3 x 0.021/2] x 0.049 x 1000 = 90.9459 L/s

The calculation above shows that a smoother sewer produces higher flow than a rougher sewer of the same size. The, how much of a sewer size is needed to have a flow speed of L/s with different sewer materials being used?

Pipe flow,Q = 1/n x (r/2)⅔ x S1/2 x πr2 x 1000 $$r=\left(\frac{Qn}{500\pi S^{\frac{1}{2}}2^{\frac{1}{3}}}\right)^{\frac{3}{8}}$$
Q = 50; S = 1/50
Vitrified Clay, n = 0.017	Ductile Iron, n = 0.012
$$r=\left(\frac{50\times 0.017}{500\pi \times 0.02^{\frac{1}{2}}\times 2^{\frac{1}{3}}}\right)^{\frac{3}{8}}$$ $$r=0.1137\:m$$	$$r=\left(\frac{50\times 0.012}{500\pi \times 0.02^{\frac{1}{2}}\times 2^{\frac{1}{3}}}\right)^{\frac{3}{8}}$$ $$r=0.0998\:m$$

We can see the rougher sewer, Vitrified Clay requires a bigger sewer diameter to maintain the same flow speed as a smoother sewer, Ductile Iron.

Conclusion #

In conclusion, using a larger sewer pipe at the start of a system is a proper choice to ensure proper flow, especially when the slope is shallow. As the slope steepens, a smaller sewer can be used since the water flows faster. The type of sewer material also affects flow, with smoother materials like Ductile Iron requiring smaller sewers than rougher materials like Vitrified Clay for the same flow speed. These design decisions help maintain efficient and reliable sewer systems.