Appendix “” - Pumping Station and Rising Main Calculations
Sewage pumping station designed using 6DWF
Wet well design
The design of the wet well was calculated using the following equation:
c
c
T = q + Q−q (1)
Where, T= Cycle time (s)
q = Inflow (l/s)
Q = Pumping rate (l/s)
c = Volume between start and stop levels
From the foul sewage design tables the value if the inflow entering the wet well is found to be
q = 28.4l/s.
The value for the Cycle time of the system is calculated to be T = 240s, this was calculated
by assuming pump is limited to no more than 15 starts per hour.
The value of the pumping rate can be calculated using the following equation:
Q = 2q
From this equation the value of the pumping rate can therefore be calculated to be:
Q = 2 x 28.4 = 56.8 l/s
Therefore using equation (1) it was possible to calculate the value for the voliume between
the start and stop levels.
240 =
c
c
+
28.4 56.8 − 28.4
c = 3408l/s = 3.408m3/s
Knowing the volume between the start and stop levels it was then possible to calculate the
height between the start and stop levels. To do this a diameter for the wet well had to be
assumed, and was assumed to be 3m.
c
Therefore the depth between the start and stop levels = π x 1.52 = 0.48m
The Wet Well was therefore to have an internal diameter of 3m and the height between
the start and stop levels was to be 0.48m.
Retention time in the rising main
For the rising main to function correctly the velocity of the sewage flowing through the pipe
must lay between 0.75m/s and 1.8m/s.
From the “Tables for the Hydraulic design of pipes and sewers” the velocity in the pipe was
found to be 1.147m/s, the flow 0.284m3/s and the diameter of the pipe was found to be
0.225m.
From these the volume of the pipe was calculated to be 2.54m3.
It was then necessary to calculate the retention time using the following equation:
Retention Time = Volume of the pipe / Flow going through the pipe
= 8.96 seconds
This value was found to be acceptable as it was lower than 4 hours.
Head losses in the Rising main
Flow = 56.8l/s
Velocity = 1.147m/s
Diameter =0.225m
Length = 64m
Reynolds Number =
𝑣 𝑥 𝐷𝑥 𝜌
𝜇
= 2.26580
Relative Roughness = ks/D = 0.00067
From the Moody Diagrams the Coefficient of Friction (λ) = 0.022
With these values the head losses due to friction could be calculated,
𝐿
𝑣2
Head loss (ℎ𝑓 ) = 𝜆 𝑥 𝐷 𝑥 2𝑔 = 0.42𝑚
All the values calculated in this section could then be inputted into the software known as
‘FLYPS 3.1’and the pump required for this sewage system could be obtained. The pump that
was selected from the software was as follows:
Pump Name – 1C 3102 181 ht 53-252-00-5205
See Pump drawing for further details.