One of the most difficult cleaning operations is large-diameter sewer cleaning. Also known as interceptors or collections systems, large-diameter sewers (greater than 36 inches in diameter) require a different cleaning process than standard small-diameter sewers. Large-diameter sewer cleaning requires increased water flows due to increased pipe flow, debris levels, pipe lengths and manhole depths.

An experienced sewer equipment operator should know the secrets to successful large-diameter sewer cleaning.

Factors to consider

Let’s look at what you are facing in cleaning a large-diameter sewer pipe:

• Condition of the conduit (any cracking of the pipe requires cleaning with lower psi and narrower nozzle angles)
• Diameter of pipe
• Slope/grade (flat pipe flows more slowly than pipe on a steeper grade)
• Type of pipe (clay, concrete, other?)

If the pipe is estimated to be a 1/4 full, 1/2 full or 3/4 full, just how much is in there? Let’s review the following table:

Naturally, as pipe size increases, the debris capacity also increases.

Pipe science
It’s important to understand the science behind what goes on inside the pipe being cleaned. Unlike smaller pipes that require more pressure and less flow, larger pipes require more flow and less pressure. 

Nozzle thrust
The formula for thrust is: 0.0526 x gpm x √psi x Cos angle for each orifice. We can simplify that in the following table:

The throw out of an orifice is 200 to 300 times the nozzle diameter — which means that the bigger the hole, the further the water goes at pressure.

To prevent the nozzle from lying on the bottom of the pipe when performing scaling or root removal, it’s important to use a sled to keep the nozzle centered in the pipe.

When pressurizing water, there are few variables: mass, science and fluid engineering.

Debris
Unlike small-diameter sewers that typically contain organic material, larger pipes have continuous flow and often contain a variety of debris, including broken bricks and concrete, gravel, grease, grit, rocks, sand, sanitary trash and sludge. 

In small-diameter sewer cleaning, production is typically measured in feet cleaned per day; with large-diameter sewer cleaning, productivity is typically measured in tons cleaned per day. 

It’s important to understand how flow and pressure affect large-diameter sewer cleaning. The more water added to the nozzle the more debris is moved through the pipe. Higher flow pumps found on combination trucks are often selected for this operation.

Pipe flow
With large-diameter sewer cleaning, flow cannot be turned off without bypass pumping. An effective extraction method is required, using attachments and pumps, to remove the debris. Pump size is a key factor. 

Water
Large lines are typically cleaned with higher flow machines, which require more frequent trips to refill the water tank, causing delay and downtime. 

Water recycling system
Contractors, municipalities and utility companies cleaning large-diameter pipe can all realize great benefits using a combination sewer cleaner equipped with a water recycling system. These customers are all facing many of the same challenges: reducing the impact on the environment, conserving resources, advancing productivity of the line-cleaning process, and cleaning more sewer line with the resources they have. A water recycling system provides a substantial step in the right direction. 

Vactor’s water recycling system is available as an option on the Vactor 2100i combination sewer cleaner. The system reuses water already in the sewer to clean sewer lines, offering the potential to eliminate the need for clean water and saving thousands of gallons of clean water during every shift. Vactor’s five-stage filtration process is simple to operate, easy to maintain and extremely efficient.

The Vactor system delivers uninterrupted sewer line cleaning, enabling operators to clean more lines in less time, without stopping to refill the water tanks with clean water.

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Gary Toothe, CET, is a full-time training manager for FS Solutions with more than 35 years of experience in the industrial vacuum and water blast industries. As the experts on industrial vacuum and water blast equipment, FS Solutions makes OSHA training and compliance more convenient than ever through interactive courses.