Small Parshall Flumes, those from 1-inch to 3-inches in size, are well suited to a variety of applications, from stream gauging to industrial pretreatment monitoring. 

The flumes posses good flow ranges, are able to measure low heads / flow rates with accuracy, and are available in a variety of accessories and in number of configurations.

Sanitary Flows

When applying small Parshall Flumes to sanitary or mixed industrial flows, it is importatnt to make sure that the flume will not only handle the flow rates, but also be able to pass any sanitary solids.  

1 and 2-inch Parshall Flumes should not be used in applications where sanitary solids are present - even if only occasionally - as these flumes will eventually clog.  The 3-inch Parshall flume is the smallest that is recommended for use on sanitary flows.

Sidewall Height

One difficulty in using smaller Parshall Flumes is that flumes of the same size (throat width) may not be rated to handle the same flow rates.  The problem lies in the fact that, unlike the other 19 standard sizes of Parshall Flumes, the sidewalls heights of the 1-inch, 2-inch, and 3-inch Parshall Flumes are not fixed.

For flumes of the same throat width, there can be a variance of as much as 100% in the depth of the flume.  The range of "standard" sidewalls for these flumes are:

• 1-inch 
  • 6 to 9-inches [15.24 to 22.86 cm]
• 2-inch 
  • 6 to 10-inches [15.24 to 25.4 cm]
• 3-inch
  • 12 to 24-inches [30.48 to 60.96 cm]

Needless to say with such a wide range of sidewall heights, there can be considerable differences in the maximum rated flow rates for flumes of ostensibly the same size.  it is important, therefore, to not only look at the flow charts of the flumes when selecting a device, but also insuring that the device specified has sidewalls of sufficient height to actually handle the flow.

Geometric Similarity and 4-Inch Parshall Flumes

The sizes of small Parshall Flumes follow a regular procession - with each flume increasing the throat width of the flume before it by 1-inch [2.54 cm].  This does not mean, though, that the flumes are scale models of each other - they are not.  The flumes follow a general design, but each flume has its own inlet / outlet widths and angles of convergence / divergence.  The flumes are empirical devices and it should not be assumed that one can merely scale up / down one flume to obtain the dimensions or flow rates of another.

Dr. Parshall's intial research (1915-1922) into an improved Venturi flume concentrated on flumes from 6 to 120-inches in size.  By 1932, flumes from 144 to 600-inches in size had been added.  It was only in 1945 that one of the smaller (the 3-inch) Parshall flumes had been developed.  A further 15 years passed before Robinson rounded out the smaller sizes with the 1-inch and 2-inch flumes.  Since 1960, there have been no further extensions to the range of Parshall Flumes sizes that have been published.  In particular, the much requested 4-inch Parshall Flume has not been developed.  

MASTER DIMENSION DRAWING FOR SMALL PARSHALL FLUMES

 

 

 

Sources:  Parshall, R., The Improved Venturi Flume, Colorado Agricultural College, Colorado Experiment Station, 1922, Parshall, R., Parshall Flumes of Large Size, Colorado Agricultural College, Colorado Experiment Station, May, 1932, Parshall, R., Improving the Distribution of Water to Farmers by Use of the Parshall Measuring Flume, Colorado A&M College, Colorado Agricultural Experiment Station, May, 1945, Robinson, A., Parshall Measuring Flumes of Small Sizes, Technical Bulletin 61, Colorado Agricultural Experiment Station, 1960