What is siphon?

Short Answer:

A siphon is a device used to transfer liquid from a higher level to a lower level through a pipe or tube, passing over an intermediate point higher than the liquid level in the source reservoir. The flow occurs due to the difference in pressure and gravity once the tube is completely filled with the liquid.

Siphons are commonly used to empty tanks, reservoirs, and ponds, or to convey water from one container to another. The working of a siphon depends on atmospheric pressure and the gravitational head difference between the two liquid levels.

Detailed Explanation:

Siphon

A siphon is a simple hydraulic device that allows liquid to flow from one container at a higher elevation to another at a lower elevation through a bent or U-shaped tube. It operates without the use of any pump, relying only on gravity and atmospheric pressure to move the liquid.

Once the siphon pipe is filled with the liquid, the fluid flows continuously from the higher level to the lower level as long as the outlet end remains below the level of the liquid in the upper container. Siphons are widely used in engineering, agriculture, and household systems for transferring water or other liquids conveniently and efficiently.

Construction of a Siphon

A siphon typically consists of the following parts:

  1. Inlet Arm:
    The portion of the pipe that dips into the liquid at a higher level (the source).
  2. Crest or Summit:
    The highest point of the siphon tube, which lies above the liquid level in the upper reservoir.
  3. Outlet Arm:
    The portion of the pipe that discharges the liquid into a lower reservoir or open space.

The siphon tube must be completely filled with the liquid before it can operate. This is usually done by priming — filling the tube with liquid and closing both ends before placing it in position.

Working of a Siphon

The working principle of a siphon is based on atmospheric pressure and gravity.

  1. Initiation of Flow:
    The siphon tube is first filled completely with the liquid. When both ends of the tube are placed in their respective reservoirs — one end in the higher tank and the other in the lower tank — the liquid starts flowing from the higher level to the lower level.
  2. Pressure Difference:
    • The pressure at the top of the siphon (crest) is lower than atmospheric pressure due to the height of the liquid column.
    • The liquid in the upper container is pushed up into the siphon by atmospheric pressure acting on the surface of the liquid.
    • Once the liquid crosses the crest, gravity pulls it downward through the outlet arm, creating a continuous flow.
  3. Continuity of Flow:
    As long as the outlet end remains below the liquid surface in the upper tank and the siphon remains airtight, the flow will continue.

The flow stops when air enters the tube or when the levels of the two reservoirs become equal.

Conditions for Proper Siphon Operation

  1. The siphon tube must be completely filled with liquid before operation.
  2. The outlet end must be below the level of the liquid in the upper reservoir.
  3. The entire tube must be airtight to prevent air from breaking the liquid column.
  4. The height of the siphon summit above the liquid level should not exceed 10.3 meters for water, because atmospheric pressure can only support a water column of this height.

If the summit exceeds this limit, vapor cavities form, and the siphon action stops.

Theoretical Analysis of Siphon

Let,

  •  = height of siphon crest above the upper reservoir surface (m)
  •  = height of outlet below the upper reservoir surface (m)
  •  = length of siphon pipe (m)
  •  = friction factor of pipe
  •  = velocity of fluid in the pipe (m/s)
  •  = acceleration due to gravity (9.81 m/s²)

By applying Bernoulli’s equation between the water surface in the upper reservoir and the outlet of the siphon:

Since the velocity at the reservoir surface  is negligible and , the equation simplifies to:

The head available for flow is the difference in levels between the upper and lower reservoirs, which is used to overcome frictional losses and provide kinetic energy to the flow.

Thus,

The discharge (Q) through the siphon is:

where  is the cross-sectional area of the siphon pipe.

Advantages of Using a Siphon

  1. Simple and Cost-Effective:
    • Does not require mechanical pumps or power supply.
  2. Portable and Easy to Use:
    • Can be used temporarily or permanently for transferring liquids.
  3. Continuous Flow:
    • Once started, the flow continues automatically as long as the outlet is lower than the inlet.
  4. Useful in Remote Areas:
    • Can function without electricity, suitable for agricultural or field use.

Limitations of a Siphon

  1. Height Restriction:
    • The crest of the siphon cannot be more than 10.3 m above the liquid surface for water due to atmospheric pressure limits.
  2. Air Leakage:
    • Even a small air leak breaks the continuity of flow.
  3. Priming Required:
    • The siphon must be filled with liquid before starting.
  4. Limited to Liquids:
    • Cannot be used for gases or liquids with high vapor pressure.

Applications of a Siphon

  1. Emptying Tanks and Reservoirs:
    • Used to remove water from tanks, ponds, or swimming pools.
  2. Irrigation:
    • Commonly used to transfer water from canals to fields.
  3. Hydraulic Engineering:
    • Used to bypass water around dams or weirs.
  4. Automobile Fuel Systems:
    • Transfers fuel between compartments or tanks.
  5. Chemical and Process Industries:
    • Used for liquid transfer between vessels.
Conclusion

A siphon is a simple but effective device for transferring liquid from a higher level to a lower level without using mechanical pumps. It works on the combined action of atmospheric pressure and gravity. The flow continues as long as the outlet end is below the liquid level in the upper container and the siphon remains airtight. Although limited by height and the need for priming, siphons are widely used in engineering and everyday applications for their simplicity, efficiency, and reliability.