Hydraulic calculations

Calculation procedure:

Calculations of the Parshall flume consist of the design of the throat's width, of the level design above the bottom, of the design incoming and outflow construction (e.g. pipe/ rectangular shape). It is necessary to determine hydraulic conditions at the outflow and at the inflow channel, calculate range of flow, consider quality of waters. Then designer consider all results and then select the optimum throat width, its floor position, shape of chamber.

Calculation scheme:

• Determination of range of flow

  A range of flowrates may be determined in dependence on a type of sewer system. In case of a separate sewer system, it is determined from directive values of production of water and maximum flowrate that depends on a type of operation, technology, type of pumping device. So called peak coefficients of non-uniformity are considered as well. In combined sewer systems a minimum flowrate of sewage and maximum value of rainwater are calculated using several methods.

• Demands on wastewater quality parameters

  Concentration of suspended solids should not be too high, and the wastewater must behave as Newtonian liquid. The size of suspended solids should not be greater than the value given as 0,8 x w where w is the throat width. It is quite important to prevent the flume throat from sediment problems. Chemical content should not result in forming a foam on the water surface? then the use of Parshall flume is possible.

• Flow conditions in the approach channel

  Flow at the approach channel to the flume has to be in certain length subcritical. The simplest and save design is achieved when the value of the Froude number is within a range of flowrates below 0,7 (the Fr number is defined as v2/g*y in which y is the hydraulic depth). If the flow in the approach channel is supercritical, it must be changed into subcritical flow. The design of right width of the flume ś throat and design of right levelling of flume above channel ś bottom ensure that the hydraulic jump is created in sufficient distance in front of Parshall flume. The velocities turbulence and waves at the entrance to the flume are sufficiently eliminated. Moreover, a sufficiently high approach velocity must be achieved in order to prevent sedimentation problems in the channel. Measurement is negatively affected by the sediments only in the case if sediments reach entrance profile to the Parshall flume (due to construction of Parshall flume this case does not often occur although channel is partly clogged).

• Flow conditions in outflow channel

  The level of the backwater behind Parshall flume must be precisely calculated. The calculations must be carried out by the water-level calculation using a well-established hydraulic method, e.g. the step-by-step method for steady non-uniform flow. The ratio between depth of back waters and the depth of in front of Parshall flume must be lower than 0,6 for small Parshall flume or up to 0,75 for greater ones. The small coefficient indicates possibility of supercritical flow in outflow channel (Fr > 1), For these cases we can design short version of Parshall flume -so called Montana Parshall flume. Montana flumes consist only of incoming part and the throat, outflow part is cut from (with this design we make calculation free).

All calculation we provide free if you send us situation, longitudinal profile and range of flow and source of flow.