Measuring open channel (non-full pipe) flows in round pipes can be difficult when measurement costs need to be low or where power is not available at the site. There are proprietary weir inserts available, but they can be inconveniently sizes or insensitive or unrated for the flow rates anticipated.
General Circular Weir Flow Equation
The source equation is drawing from Addison’s Hydraulic Measurements – A Manual for Engineers.
|
Q = Cd [10.12 (h/d)1.975 - 2.66 (h/d)3.78] d5/2 Where: Q = discharge (liters / sec) d = diameter of circular orifice (decimeters) h = height over the weir (decimeters) Cd = coefficient of discharge as given by: Cd = 0.555 + [1/(110 h/d)] + 0.041 (h/d) |
Example: Calculating Discharge Over a Circular Weir
Source: Flow in Circular Conduits
Pre-Generated Circular Weir Flow Tables
To aid users, Openchannelflow has developed flow (discharge) tables for three medium sized Circular Weirs. The weirs are 2-inches H and flow is calculated from 0.2-feet of head (the recommended minimum for thin-plate weirs) to the top of the circular weir.
While three flow tables have been developed, additional tables can be readily generated for different weir height / pipe diameter combinations.
- 14-inch circular weir for 18-inch pipe
- 20-inch circular weir for 24-inch pipe
- 26-inch circular weir for 30-inch pipe
Finally, remeber that in using these flow tables, the approaching flow must be subcritical. Supercritical flows cannot be accurately measured by thin-plate weirs (circular or otherwise).
