Gravity-powered Systems

Overview

There are two forms of gravity-powered systems: gravity flow and siphon.

Gravity flow uses elevation differences to move water from a higher elevation, such as a storage tank, to a lower elevation, such as a watering tank. A pond or storage tank with a pipe or hose extending to a tank located at a lower elevation uses gravity to transfer water as long as all components are below the water source. Water can be moved to a higher elevation using a pump.

A siphon system allows water to flow to a higher elevation than the water surface with the final elevation below the water source. The tube or hose cannot contain any air voids. A simple siphon system may consist of two adjacent water tanks connected by a U-shaped hose over the upper edges of the tank. A pond that has a tube with one end in the pond and the tube goes up and over the dam and down into a tank located below the dam is a siphon system.

It is common for an air trap to accumulate at the high point of the siphon system, causing the flow to stop. The producer will need to be prepared to “restart” the siphon when this occurs.

Gravity-powered systems for livestock most often have a valve and float system to control the water level in the tank. Otherwise, water in gravity-powered systems will flow until source and tank have identical water levels.

Advantages — Gravity Flow

  • No power required to operate.
  • Relatively inexpensive.
  • Gravity flow is dependable.

Limitations — Gravity Flow

  • Relatively low pressure.
  • Requires planning to get system installed during construction.

Advantages — Siphon

  • No power required to operate.
  • Relatively inexpensive.
  • Serves well in emergency situations.

Limitations — Siphon

  • Periodic repriming will be required.
  • Lack of assurance the system is working.
  • Temporary system that requires labor each time to set it up, including temporary tanks.

Design Considerations

Most gravity-powered systems operate on low pressure. One foot of elevation can provide 0.43 pounds per square inch. The system requires 2.31 feet of head to equal 1 pound per square inch, (6 feet of head is only 2.59 pounds per square inch). Friction line losses can become significant with such low pressures.

For livestock watering using gravity-powered systems, a 6-foot head is normally the minimum recommendation to ensure a sufficient flow rate. That means the top of the source water pool (pond level, storage tank), is at least 6 feet or more above the livestock water tank location. For 50- to 100-head cow herds, a 2-inch PVC pipe with 6 feet of head should meet livestock needs.

Siphon systems have a limitation on how high the water can be lifted above the source water level and this is dictated by the barometric pressure (one atmosphere is approximately equal to 33 feet of sea water or 14.7 pounds per square inch). Siphon lifts over 30 feet are difficult to maintain. The water molecule pulls apart at siphons approaching 30 feet.

Gravity-flow systems are easy to start as they always have positive pressure. Valves regulating inline water flow are required.

While siphon systems may be difficult to start, after the siphon flow is established, the flow rate and pressures are similar to a gravity-flow system, except for the friction loss from the additional length of pipe necessary to establish the siphon.

A siphon is started by completely filling the tube/pipeline between the water source and the tank, ensuring all air is eliminated. This can be difficult. In smaller lines, up to 2 inches in diameter, the solution is to pump the pipeline full at a high speed from the bottom end, allowing the air and water to escape to the surface of the water source. This process works with larger diameter pipes if an adequate flow-rate pump is available. A flow rate fast enough to push the air and air bubbles out of the pipe is necessary for this method of air elimination.

When the pipe diameter/volume is too large to be easily pumped with the equipment at hand, an alternative method can be used to eliminate air. Install shutoff valves on both the intake and the discharge ends and a tee at the highest point in the siphon pipe. Close the end valves, open the tee valve, and fill the pipe with water. Close the tee valve, first open the intake valve, then the discharge valve and siphon should begin flowing.

Siphon issues have been addressed by using a battery-powered sump pump connected to the pipe intake and running it for a brief period each day to provide continuous air elimination. A timer turns the pump on and off. A solar panel recharging setup can supply power to the battery, or, for occasional need, a vehicle battery can be used instead with a cord to the pump.