Siphoning line

Siphoning lines are used to safely draw liquids from storage tanks from the top. In addition, they allow the safe siphoning of liquid and the reliable venting of the suction line at the end of operation or in the event of a malfunction.

Siphoning lines are used in groundwater pumping stations to transport water from individual wells into a collecting well from which it is then pumped into a reservoir by self-priming pumps. The difference in levels (e) represents a gradient which generates the flow velocity (v) required to overcome the head loss (Σζ∙v²∙(2∙g)^-1) i.e. the resistance coefficient (ζ) and the acceleration due to gravity (g).

See Fig. 1 Siphoning line

The siphon flow rate (Q) is calculated using the following equation:

v      Flow velocity in m/s

Σ ζ   Sum of resistance coefficients of pipe sections, elbows, etc. (see Pressure loss)
d      Inside diameter of piping in m
e      Height difference between the two water levels in m
Q     Siphon flow rate in m³/h

The siphoning line can only operate if the absolute pressure (p_S) at the apex of the system exceeds the vapour pressure (p_D) as determined by the water temperature. This condition is satisfied if

or, if the apex lies close to the collecting well.

e_s         Height difference between apex and inlet side water level in m
Σζ_e,s    Sum of resistance coefficients from the inlet to the apex of the system
S         Safety margin of 1 to 2 m (depending on quality of
           pipeline construction)
p_D        Vapour pressure in bar
p_b        Atmospheric pressure in bar

Dissolved gases (see Gas separation) are continuously released out of solution at the apex of the pipe and have to be removed.

Often submersible borehole pumps which pump the water directly into the reservoir are used instead of siphoning lines. See Fig. 1 Domestic water supply system