To study flow characteristics through Parshall flume
Objectives:
- To draw water surface profiles of flow through Parshall flume
- To determine coefficient of discharge of Parshall flume
Apparatus:
- S6 tilting flume
- Point gauge
- Parshall flume
Related theory:
Parshall flume:
The Parshall flume is an open channel flow metering device that was developed to measure the flow of sub critical waters and irrigation flows.
The Parshall flume is a fixed hydraulic structure. It is used to measure volumetric flow rate in industrial discharges, municipal sewer lines, and influent/effluent flows in wastewater treatment plants. The Parshall flume accelerates flow through a contraction of both the parallel sidewalls and a drop in the floor at the flume throat. Under free-flow conditions the depth of water at specified location upstream of the flume throat can be converted to a rate of flow.
Under laboratory conditions Parshall flumes can be expected to exhibit accuracies to within +/-2%, although field conditions make accuracies better than 5% doubtful.
Figure: Parshall flume
Difference between Venturi flume and Parshall flume:
Beginning in 1915, Dr. Ralph Parshall of the U.S. Soil Conservation Service altered the subcritical Venturi flume to include a drop in elevation through the throat of the flume. This created a transition from subcritical flow conditions to supercritical flow conditions through the throat of the flume.
Modifications to the Venturi flume that Parshall made include:
- Decreasing the angle of convergence of the inlet walls
- Lengthening the throat
- Decreasing the angle of divergence of the outlet wall
- Introducing a drop through the throat of the flume
Parshall Flume Venturi Flume
Function of different parts:
The Parshall Flume acts essentially as a constriction, a downward step, and then an expansion: the upstream section is uniformly converging and flat, the throat is a short parallel section that slopes downward, and the downstream section is uniformly diverging and slopes upward to an ending elevation that is less than the upstream starting elevation. The width of the throat determines the flume size; 22 standardized sizes have been developed, ranging from 1 in. to 50 ft. (0.005 ft3/s to 3,280 ft3/s).
Free and submerged flow conditions:
There are two conditions of flow that can occur in a Parshall Flume: free flow and submerged flow.
When free flow conditions exist, the user only needs to collect one head measurement (Ha, the primary point of measurement) to determine the discharge. For submerged flow a secondary head measurement (Hb) is required to determine the flume is submerged and the degree of submergence.
The primary point of measurement (Ha) is located in the inlet of the flume, two-thirds of the length of the converging section from the flume crest. The secondary point of measurement (Hb) is located in the throat of the flume.
A hydraulic jump occurs downstream of the flume for free flow conditions. As the flume becomes submerged, the hydraulic jump diminishes and ultimately disappears as the downstream conditions increasingly restrict the flow out of the flume.
The free-flow discharge can be summarized as
Where,
Q is flow rate
Ha is the head at the primary point of measurement
Construction
A wide variety of materials are used to make Parshall flumes, including
- Fiberglass (wastewater applications due to its corrosion resistance)
- Stainless steel (applications involving high temperatures / corrosive flow streams)
- Galvanized steel (water rights / irrigation)
- Concrete (large Parshall throat widths 144″ [3.66 m] and above)
- Aluminum (portable applications)
- Wood (temporary flow measurement)
- Plastic (PVC or polycarbonate / Lexan) (teaching/laboratory investigation)
Installation
Dr. Parshall’s initial focus was for the use of his namesake flume to measure flows in irrigation channels and other surface waters.
Over time, however, the Parshall flume has proven to be applicable to a wide variety of open channel flows including:
- Irrigation channels and ditches
- Furrows
- Surface waters (streams, rivers)
- Elevated, above grade piped flows
- Below grade piped flows (concrete vaults / manholes)
Drawbacks
- Parshall flumes require a drop in elevation through the flume. To accommodate the drop in an existing channel either the flume must be raised above the channel floor (raising the upstream water level) or the downstream channel must be modified.
- As with weirs, flumes can also have an effect on local fauna. Some species or certain life stages of the same species may be blocked by flumes due to relatively slow swim speeds or behavioral characteristics.
- In earthen channels, upstream bypass and downstream scour may occur. Armoring of the upstream and downstream channels is recommended.
- Parshall flumes below 3 inches in size should not be used on unscreened sanitary flows, due to the likelihood of clogging.
Procedure:
- Level the tilting flume at zero slope and insert the Parshall flume, ensuring that the larger inlet contraction is positioned upstream
- Take great care to seal the unit is the flume because leakage will tend to invalidate measurement of flow
- Two gauging wells are provided within the Parshall flume fabrication and are to be used with the point gauge to determine the water levels.
- Following the sealing of the unit, the water inlet valve is opened and a fixed amount of water is admitted to the flume.
- Note the depth of flows at u/s, converging, throat, diverging and d/s portion of flume to plot water surface profiles
- Change discharge and again measure depths
- Also note the value of water depth at 1st well, this will be used to measure discharge
Observations and calculations
Water surface profiles:
Calculations for coefficient of discharge:
| Sr. No. | Qact | Ha | Qth | Cd |
| m3/s | m | m3/s | ||
| 1 | 0.002828 | 0.09 | 0.004237 | 0.67 |
| 2 | 0.003999 | 0.106 | 0.00546 | 0.73 |
| 3 | 0.004898 | 0.116 | 0.006279 | 0.78 |
| 4 | 0.006323 | 0.128 | 0.007314 | 0.86 |
| 5 | 0.007481 | 0.132 | 0.007671 | 0.98 |
Average Cd= 0.8
Comments:
- The average Cd is 0.8 which is less than 1 and is correct.
- From the water surface profiles it can be observed that the Parshall flume converts sub critical flow to super critical flow.
- Parshall flume measures discharge more accurately than venturi flume because the water depth at converging section is found by well where the depth can be measured accurately without turbulent flow.
- The graph of Qth vs Qact is approximately straight line which shows the accuracy of discharge measurement from this flume.