| Product(s): | SewerCAD, SewerGEMS | ||
| Version(s): | 08.11.XX.XX | ||
| Environment: | N\A | ||
| Area: | Calculations |
Problem
When using an equation-based Extreme Flow method such as the Babbitt Equation, along with Count Based unit load, the resulting calculated flow does not seem to be following the equation and considering the Extreme flow factor.
Solution
There are a few key things to check when using count based unit loads with extreme flow factors.
1) Proper model setup - First, ensure that the model is properly configured to use the Extreme flow Method you have configured
a) Go to Components > Extreme Flow Setups and select the appropriate extreme flow method for each unit load type
b) Go to Analysis > Calculation Options, open the calculation option set for the scenario of interest and select the entry from the "Extreme flow setup" dropdown.
c) Select Steady State as the Time Analysis Type
2) Unit count vs population - Keep in mind that the base load is equal to the Loading Unit Count (entered in the manhole properties) times the Unit Load flow rate (entered in the Unit Sanitary Loads dialog). The population is tracked separately to determine the extreme flow factor that will be applied to the total
3) Tracking of population and base load - Keep in mind that at any given point in the network, the total upstream population is tracked; see result field "Population (System Sanitary)". The total *base* sanitary load is also tracked, for each type of contributing upstream unit load type. The total population is then used along with the total base load and the respective extreme flow method for each unit load type to determine the adjusted total "extreme flow" at that point in the network. This is reported as the "Flow (System Sanitary)" result field that you see.
4) Population Units - Be mindful of the unit used in the extreme flow equation's "Population Unit in Equation". Some Extreme Flow equations such as the Babbitt equation use "Capita X 10^3" for the unit, which essentially means that you need to divide the population "P" in the equation by 1000.
5) "Cutoff Value" - If you are using a Extreme Flow Equation type make sure to proper input a value to the "Cutoff Value" field. As explained here the Extreme Flow Cutoff should be > 0. The Cutoff Value is the maximum possible extreme flow factor (EFF) for peaking methods. This is used to prevent unrealistically high values for small populations or land areas.
Example
Loading Unit count - 400
Associated Unit load flow - 150 L/d
Population Equivalent - 5 capita
Extreme flow Method: Babbitt Equation
Population Unit in Equation: Capita X 10^3
Proof:
EFF = (5+(0*P^1)/((1*(P/1000))^0.2)
EFF = (5+(0*2000^1)/((2000/1000^0.2)
EFF = (5+0)/(2^0.2)
EFF = (5+0)/(1.1487)
EFF = 4.353
Q = 4.353 X (400 X 150) = 261,165 L/d
Note that the division by 1000 in bold above is due to the Population Unit in Equation of Capita X 10^3