A special application for priming systems involves maintaining flow in a siphon. This is done either to maintain a water level on a downstream sump (siphon control) or for maintaining a siphon to a pumping process (siphon assist).
Siphons are used to transfer water from a higher elevation source to a collection sump at a lower elevation where it can then be pumped for use within a process. Transfer requires raising the water level first (over a berm) before being able to transfer it to the lower collection sump. A priming system has two functions, filling the siphon and for maintaining the siphon flow. Since water contains dissolved air, a prime must be maintained on the siphon at all times so that the siphon is not lost due to out-gassing of the air in solution. The amount of air (and other non-condensables) that is in solution depends on the source (lake, river, ocean, etc.) as well as the temperature of the water. For a typical siphon only a portion of the dissolved air actually comes out of solution. The percentage of air that will out-gas increases with a rise in temperature and with a reduction in pressure. Therefore, as the differential between the source water and the high point in the siphon increases, the absolute pressure at the high point accordingly decreases with an accompanying increase in the amount of air that is released from the water. Additionally, for very long siphons that have exposed (surface) piping, temperature increases in the water will increase the amount of air that out-gases from the water. Without proper management of the air within a siphon such as incorporating a vacuum priming system, the siphon’s prime will eventually be lost.
Bernoulli’s equation for steady state flow provides the basics for proper siphon design.
Pump assist priming systems are another class of primers that are used to reduce the discharge head required by a pump. Pump applications that don’t need to prime the suction side may however utilize a primer on the discharge side. When the pump discharge piping has a siphon leg, a priming system can provide substantial savings in both operating cost and initial pump costs by removing the differential height of the siphon section from the pump’s discharge head requirement.
Give us a call, LYNN can help with your Siphon Priming needs.