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sanitary flow through a Parshall flumeOne of the most common uses of flumes today is to measure sanitary flows; whether this is at an industrial discharge, mobile home park, resort, or wastewater treatment plant.  Unfortunately, flumes are often selected bsed solely on their ability to handle anticipated flows – without regard to the make-up of those flows!  When the sanitary component is overlooked in lower flow applications, problems with clogging can ensue.

Remember, in residential construction, to avoid clogging, the smallest toilet line size is 3-inches [7.62 cm]. When multiple toilets are added to the line, the line size increases to 4-inches [10.16 cm] and up.  It makes sense, then, that throat sizes below 3-inches [7.62 cm] shouldn’t be used for flows containing unscreened sanitary solids.  With throat widths smaller than 3-inches [7.62 cm] it is not a matter of if the flume will clog, it is a matter of when.  Time and again we have seen customers absolutely insist that a flume with a smaller opening will be fine for their application. Time and again those same customers come back asking what they can do to solve their clogging problem!

The matter of clogging can be of particular concern in fiberglass packaged flow metering manholes where the flume is molded into the structure.  Here it is extremely difficult to remove an undersized flume as many times portions of the flume stick out upstream/ downstream of the manhole.  Installing a new one may very well not be an option – we’ve seen customer that have had to replace both packaged metering manholes and precast vaults because of flume clogging issues.  Take the time and get it right the first time!

HS and H flumes pose unique problems when it comes to use on sanitary flows.  This is because of the V-shape of the discharge of the flume tapers considerably as it approaches the floor of the flume – almost, but not quite, to a point.  Remember that for a 0.4-foot HS flume, even at the top of the flume, the opening is only 1 27/32-inches [4.67 cm] wide while the 0.6-foot HS flume is only slightly larger than 2 3/4-inches [7.00 cm] wide.  It is only when you get to the 0.8-foot HS flume that the top is wide enough to pass sanitary solids at the top with a width of 3 43/64-inches [9.34 cm].  Even here, though, the discharge width is not 3-inches [7.62 cm] wide until 7 9/16-inches [19.22 cm] of depth have been reached – a level that is 79% of the flume’s total depth.

Prisons can pose an additional problem – with all that free time on their hands, prisoners love to flush all sorts of unusual stuff.  Here an even larger opening may be advisable if a sewage grinder is not upstream of the flume.

With this in mind, the following flumes are recommended that they not be used on unscreened sanitary flows:

  • Parshall
    • 1-inch [2.54 cm]
    • 2-inch [5.08 cm]
  • HS
    • 0.4-foot
    • 0.6-foot
  • Cutthroat
    • 18-inch [45.72 cm] x 1-inch [2.54 cm]
    • 18-inch [45.72 cm] x 2-inch [5.08 cm]
    • 36-inch [91.44 cm] x 2-inch [5.08 cm]
  • Montana (Short Parshall)
    • 1-inch [2.54 cm]
    • 2-inch [5.08 cm]

Trapezoidal flumes are generally immune to the problem of clogging.  There the V-shaped cross-section of the two flumes allows for solids to push through readily as flow backs up behind an obstruction.  Really the only time sanitary solids are an issue for Trapezoidal flumes is where overly deep end adapters are used with flumes experiencing low flows but high solids.  Here solids may deposit upstream of the flume in the inlet end adapter.  

RBC flumes are rarely, if ever, applied to measuring sanitary flows.  The flumes do not have end adapters for connecting to pipe and have relatively low maximum flow rates in their standard sizes so their use on sewage flow monitoring would be extremely limited – particularly when so many other types of flumes hand such flows well.

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