Pressures in Sewer Cleaning

By NASSCO member Barry Howell, General Manager, Visu-Sewer, Inc. | April 2010 Vol. 65 No. 4
Pay attention to pressure and flow vs. the diameter of the pipe to be cleaned.

Diameter and other characteristics of the pipe What is the pertinent information about the pipe being cleaned, including type of pipe, diameter, slope, and condition of the conduit? Eight inch (8”) pipe should be cleaned with less flow in GPM than fifteen inch (15”) pipe. Flat pipe will flow more slowly than pipe on a steeper grade, so minimizing GPM on flat grades is a good idea. Finally, clay pipe with severe cracking should be cleaned with lower PSI (and narrower angles on the nozzle), than concrete pipe in relatively good condition. Armed with this information, an operator is ready to begin the cleaning process.

The general rule of thumb in jetting and cleaning is this: In smaller pipe, pressure is more important than flow; in larger pipe, flow is more important than pressure. Here is how it works: Pressure in PSI is a measure of force applied to the pipe wall. Pressure produces a scouring effect and is important in “stirring up” debris for removal. Smaller jetters typically operate at high pressures, for example, most 1/2” system operate at up to four thousand (4,000) PSI. Larger jetters and combo trucks typically operate at half that pressure range, i.e. two thousand (2,000) PSI. GPM is a measure of flow induced into the pipe to move debris. Pipe of any diameter has a finite capacity to store or hold water (debris loads and defects only reduce that storage ability). Small, high pressure jetters typically don’t exceed twenty five (25) GPM and clean small pipes effectively. Large jetters and combo machines can operate at eighty (80) GPM or higher and clean large pipes effectively. In general, GPM and PSI required are diameter driven. Smaller pipes require more pressure and less flow. Larger pipes require more flow and less pressure.

Finally, a word on the dreaded “blown toilet” is necessary. If you ask ten people in the sewer industry why they blow toilets, eight will reply “too much pressure was used”. WRONG! The correct answer is too much flow was used in the pipe. For example: an eight-inch (8”) pipe 20 feet long can only hold 30 gallons of water. Adding debris loads and defects (sags, bellies, etc.), limiting a pipe’s ability to discharge water (flat slope), plus inducing huge volumes of water and air that are generated by a cleaning nozzle only increase the likelihood that a backup will occur and the dreaded “blown toilet” will occur. For more information, go to www.nassco.org and order the Jetter Code of Practice.