K.G. Kress and W.J. Dick
A pipeline syphon can be a simple and reliable alternative for water conveyance over earthfill dams, or other topographic features, as compared to otherwise more complex pumping systems or buried pipelines. In its simplest form, a pipeline syphon consists of an airtight pipeline constructed over the top of a dam or other high topographic feature, connecting a source pond to a receiving pond or channel. It typically includes a submerged intake, conveyance pipeline, priming device, flow rate control device, and outlet energy dissipater. Additional features can include flow measurement, chemical injection, and fluid mixing devices.
A pipeline syphon can be quickly constructed from readily available pipe materials such as steel, concrete, polyethylene, or PVC. Steel pipe of sufficient strength to resist potential vacuum pressures is most commonly used. High-density polyethelene pipe is often used where flotation is required or where site conditions require a more flexible pipe. Connections can be welded, fused, or flanged, depending on numerous factors including pipeline type, technical requirements, and cost.
The syphon intake can be designed to float from pontoons on the pond surface or can be placed on the pond bottom. Larger, high-capacity syphons require intakes with bell-mouth inlets, anti-vortex plates, and trash racks to minimize energy loss, prevent the entry of air, and manage pond debris. Proper intake design is a key element for ensuring continuous, uninterrupted operation of higher capacity pipeline syphons. More complex intake designs can include adjustment features which allow the intake inlet to be raised or lowered in response to changing pond levels or fluid transfer requirements.
The syphon action is initiated by removing air from the pipeline (priming) using water pumps, vacuum trucks, or simple vacuum devices which can be purchased “off-the-shelf.” Once primed, the syphon flow rate can be controlled or stopped using conventional gate, valve, orifice plate, or other devices. Repriming is not required even if the flow is stopped, provided the syphon was not drained.
A limiting factor in considering a pipeline syphon as a design option is the maximum lift (i.e., vertical distance from source pond water level to the summit of the syphon pipeline) which is approximately nine metres or less, depending on pipeline syphon features and site conditions. As an example, a syphon that has a long pipeline length upstream of the syphon summit, high conveyance velocity, or conveys heavier fluids, like fine tailings, would have significantly less maximum lift capacity.
Compared to more conventional pump systems and low level outlet conduits through dams, a pipeline syphon is often easier to construct, retrofit, modify, or move. It also provides flexibility to address conditions which include poor structure foundations, changing pond and/or dam levels, and temporary or emergency use. A pipeline syphon can provide a low-risk and low-maintenance fluid transfer system. Capital and operating costs can be significantly less than for a pump system or low level outlet installation. A pipeline syphon is particularly well suited to many of the unique conditions often encountered at mine sites. Existing systems have been designed to convey clear water or fine tailings with up to 30% solids, and with velocities sometimes significantly exceeding 5 m/s.
This paper identifies project challenges that are best resolved using a pipeline syphon. Basic principles for pipeline syphon design, including case histories of their successful application, are provided.