A reader sent in some great questions regarding scouring velocity in pressurized systems.

“What is your understanding of scouring velocity?”

Scouring velocity is meant to define a pipe velocity (in feet per second) that will prevent buildup (or settling) from occurring in a pipe. The goal to keep a pipe clear; what velocity in feet per second do we need to maintain a clear pipe. 

The answer isn’t simple, because of variables such as size and type of pipe and the type of solids you are trying to clear from the pipe.

In 1975 the Environmental Protection Agency contracted Water Resource Engineers out of Walnut Creek California to perform the following two objectives: A) a review of feasible methods for reducing the first-flush of solids in overflows from storm and combined sewers, and B) the development of a mathematical model that could be used to evaluate the behavior and cost of first-flush mitigating alternatives.

Early in the EPA report the very first conclusion they discuss is relative to this article and I will quote it here for reasons that will become apparent:

“Sewer design based on velocity criterion only for self-cleansing does not consider all the relevant variables that affect deposition and scour, and hence such design can lead to situations in which deposition may still occur and “first-flush” pollution will not be prevented. Where velocity becomes the controlling factor, however, 2 feet per second appears to be adequate to initiate scour of deposited materials. This conclusion is based only on the analyses used here which assume noncohesive deposits are involved.”

This quote really says a lot of important things. If we are only considering velocity, we are not considering other important variables that affect our ability to keep the pipe clean. One of those important variables is what material we are trying to keep from depositing in the pipe. The last sentence of this quote is very important: the studies that determined 2 feet per second were specific to noncohesive material only. As the type of depositional material and type of pipe changes, so will adequate scouring velocity. 

“Is there any credible evidence that supports a 2 feet per second scouring velocity in pressurized pipe?”

The conclusion reached above in the 1977 EPA document states that “…2 feet per second appears to be adequate to initiate scour of deposited materials.” This has since been followed as a general guideline, and in most instances seems to have guided the industry well. I have not found instances of this general guidance being changed to any large degree in my cursory reviews. It seems where velocity needs the biggest change are in other industries where the material going through the pipe is such that it is harder to keep the pipes clean. In the onsite industry, if we are only clearing pipe of the typical sanitary items we work with daily, in most cases the 2 foot per second velocity seems to maintain relatively clear pipes. As the flow begins to contain other items, higher in fats, oils, grease and other chemicals (that should not be going through our systems) we see additional issues with pipes clogging more easily, but not as much in the pressure pipes.  

Is there such a thing as "better" scouring velocity?

Depending on size of pipe used and the gallon per minute requirements, I know that at least in Wisconsin, the scouring velocities that are approved in our pressure component manual range from 2 feet per second to a maximum of 10 feet per second in schedule 40 PVC pipe. I cannot think of instances where we are anywhere near 10 feet per second. 

Scouring velocities of 2.5 to 3.5 feet per second are not unheard of, and are achieved in many typical instances of general pressure design. The general rule of thumb is not to exceed 10 feet per second, but that seems more an issue of limits for check valves and other fittings that have limits rather than limits on the pipe itself. 

“Better scouring velocity” has many variables — type of solids one is trying to prevent accumulating in the pipe, size of pipe, etc. As stated previously in the conclusions of the report that determined 2 feet per second as a good scouring velocity, if velocity is the only variable used, other variables could be overlooked. 

If these questions were not answered to your satisfaction, please comment below.

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About the author
Todd Stair is vice president of Herr Construction, Inc., with 34 years’ experience designing, installing, repairing, replacing and evaluating septic and mound systems in southeast Wisconsin. He is the author of The Book on Septics and Mounds and a former president of the Wisconsin Onsite Water Recycling Association.