Unit 2

Design of Parshall Flume for flow measuring

 Design equations and design of Parshall Flume 

A Parshall flume is a type of hydro-metering structure used to measure the flow rate of water in open channels. 

It is designed with a constriction in the middle of the flume, which creates a drop in water level as the flow rate increases. 

This drop in water level is proportional to the flow rate and can be used to accurately measure flow. 

The design of a Parshall flume includes specifications such as the shape and size of the constriction, the slope and width of the flume, and the location and size of the measurement device (such as a stilling well or bubbler system). The flume should be installed in a straight section of the channel with a stable bed and consistent flow. 

Design equations and design of Parshall Flume

The design of a Parshall flume involves determining the proper dimensions of the constriction and the flume to ensure accurate flow measurement. 

The most common method for designing a Parshall flume is to use the equations developed by Parshall himself in the 1920s, which are based on the principle of continuity and the conservation of energy.

The main equation used for designing a Parshall flume is the flow equation, which relates the flow rate (Q) to the head loss (h) through the constriction, the cross-sectional area of the flume (A), and the velocity of the water (V):

Q = A(V^2/2g)^(2/5) x h^(3/5)

Where:
  • Q = flow rate (m^3/s)
  • A = cross-sectional area of the flume (m^2)
  • V = velocity of water (m/s)
  • g = acceleration due to gravity (9.81 m/s^2)
  • h = head loss through the constriction (m)

The design of the constriction section in Parshall flume is critical to accurate flow measurement. This is typically done by specifying the head loss (h) through the constriction, and the discharge (Q) that will occur at that head loss. This can be determined using the following equation:

h = (Q^5/A^2) x (2g/V^2)^3

Where:
  • A = cross-sectional area of the flume (m^2)
  • V = velocity of water (m/s)
  • g = acceleration due to gravity (9.81 m/s^2)
  • Q = flow rate (m^3/s)
Once the head loss (h) is known, the dimensions of the constriction can be calculated using the geometry of the flume. 

The shape and size of the constriction will depend on the specific design of the flume, but it is typically a V-shaped channel that narrows as it approaches the measurement point.

The flume must be installed in a straight section of the channel, with a stable bed and consistent flow. 

The slope and width of the flume must also be considered in the design process to ensure that the flow is not too shallow or too deep.

It is important to note that Parshall flumes are designed and calibrated for specific range of flows and it may not work accurately outside that range. 

Also, the accuracy of the measurement is affected by the condition of the flume, such as siltation, debris, or other obstacles that can disrupt the flow.