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Flushing pipes

LMacNevin Member Posts: 13
Has anyone seen a research paper or report about the required water velocity for flushing new pipes, before systems are completed? For example, steel, copper, or plastic piping in a building’s hydronic system, or buried pipes for a ground source geothermal system.

Looking for an answer in the form of a minimum velocity (and therefore gpm) to ensure the system is well flushed. This applies to pipe sizes 1 inch and larger, mostly.

Flushing with water is necessary to clean out insects, rocks, cigarette butts, lost nuts and bolts, pipe shavings, maybe beer cans, and other junk that gets into pipes during construction. Good flushing can prevent many mechanical problems.

The industry standard recommendation seems to be 2 fps, but this number was developed to flush air, not necessarily stuff. In the 100+ years of hydronic systems, I’m hoping that someone has done this type of study already, but can’t find it. Thanks


  • Jamie Hall
    Jamie Hall Member Posts: 23,295
    The wastewater engineering folks have done some research on what velocities are required to move solids in pipes, but most of their work has to do with larger pipes which are not filled. Not sure whether it's applicable or not. Most low density solids seem to move in such applications at 2 feet per second. Higher density solids -- such as the nuts, rocks, and the like which you mention, will require much higher velocities to move more or less reliably -- depending very much on the particle size. Sand, for instance, has a settling velocity of foot per second, so should move at 2 fps. Pebbles may require as much as 10 feet per second (ask five experts and you will get 10 different answers).

    There is a further consideration: water hammer. If you get enough velocity in there to move some of these things, and they encounter an obstruction -- even an elbow -- they may lodge abruptly and effectively stop the flow. At those velocities, you will then get a spectacular and damaging water hammer.

    Bottom line: blow out your pipes and fittings before you put them together, if you have any doubt about cleanliness.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 22,144
    edited March 2020
    Hi Lance, I met this company at the IGSHPA show in Tulsa a few years back, they had a booth near mine. They specialize in clearing plugged lines of most any time, GEO loop fields for example.
    They claim 5 fps is the minimum velocity required to start moving solids thru a pipe. They use ultrasonic flow meters clamped one the pipe and rev up the flow based on the pipe diameter to attain the flow rate. Might look over their website or contact them.

    Another idea I had for clearing fouled radiants lines is a concept called ice pigging. A UK company www.suez-na.com developed this technology for cleaning sewer lines. It replaces mechanical pigs currently used which are prone to getting stuck on fittings and valves in the line. An article in MSWMag.com August 2019 on ice pigging.https://www.mswmag.com/editorial/2019/08/clean-your-pipes-with-ice

    The Alaskan pipeline gets pigged every so many years and they struggle with cameras and pigs getting stuck. Possibly this ice slush would be a better solution.

    I have a call in to Suez to see if it is practical for radiant tube applications.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • GroundUp
    GroundUp Member Posts: 1,907
    To flush suspended particulates, it seems 2FPS is adequate. But for actual solids, material which has settled, I find 4FPS to be a minimum in small 1/2"-3/4" bore piping on up to 10 FPS for larger 3"-4" bore, as the solids allowed in are physically larger.