Syphon and its application

A syphon is a long bent pipe which is used to carry water from a reservoir at a higher elevation to another reservoir at a lower elevation when the two reservoirs are separated by a hill or higher level ground in between as shown in Figure below

Fig: Two Reservoirs are Connecting by Siphon

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Since the siphon is laid over the hill or the high level ground, for some length from the entrance section it will rise above the water surface in the upper (or supply) reservoir, and then for the remaining length it will drop to be connected to the lower reservoir. The rising portion of siphon is known as 'Inlet leg (or Inlet limb)', the highest point is known as 'Summit (S)' and the portion between the Summit and the lower reservoir is known as 'Outlet leg (or Out leg limb)'. The 'Inlet leg (or Inlet limb)' of a siphon is usually shorter than the 'Outlet leg (or Out leg limb)'

As the siphon is also a long pipe, the loss of head due to friction will be very large and hence the other minor losses may be neglected. Further the length of the siphon may be taken as the length of its horizontal projection. Hence, the HGL and the EGL (or TEL) for a siphon, as shown in fig may also be obtained in the same manner as in the case of ordinary long pipe. It will be seen from fig that the HGL cuts the siphon at points 'C' and 'D', so that some portion of the siphon is above the HGL. The vertical distance between the HGL and the pipe Centre line represents the pressure head at any section. If the HGL is above the Centre line of the pipe then the pressure is above atmospheric, land if the HGL is below the Centre line of the pipe, the pressure is negative (or below atmospheric). Thus for the portion of the siphon below 'C' and 'D', the pressure will be above atmospheric and at points 'C' and 'D', the pressure of the water flowing in the siphon is equal to atmospheric pressure. For the portion of the siphon between 'C and 'D', the pressure will be below atmospheric.

As the highest point of the siphon above the I-IGL is the Summit (S), the water pressure at this point is the least. Further as the vertical distance between the summit of the siphon and the I-IGL increases, the water pressure at this point reduces. Theoretically this pressure may be reduced to -10.3 m of water (if the atmospheric pressure is 10.3 m of water) or absolute vacuum, because this limit would correspond to a perfect vacuum and the flow would stop. However, in practice if the pressure is reducing to about 2.5 m of water absolute or 7.8 m of water vacuum, the dissolved air or other gases would come out of the solution and collect at the summit of the siphon in sufficient quantity to form an air - lock, which will obstruct the continuity of the flow, (or the flow will completely stop). A similar trouble may also be caused by the formation of the water vapor in the region of low pressure.

•  For the continuous supply of water through siphon, following condition should be met:

The siphon should be laid so that no section of the pipe will be more than 7.8 m above the HGL at the section. Moreover, in order to limit the reduction of the pressure at the Summit (S), the length of the Inlet leg (or Inlet limb) of the siphon is also required to be limited. This is because, if the Inlet Leg (or Inlet limb) is very long a considerable loss of head due to friction is caused resulting in further reduction of the pressure at the Summit (S).

• Starting of Siphon:

 

A syphon can be put in action either by exhausting air thus creating vacuum in it or by filling it with water. The air can be exhausted by a vacuum pump. The water in the siphon can be poured through an opening made at the summit which is only possible by closing he outlets at two ends.

While in operation, the air separates itself form the flowing water and has the tendency to be collected at the bend. An air vessel can be provided to get rid of this difficulty, otherwise there will be an interruption in the flow.

Atmospheric, Absolute, Gauge and Vacuum pressures

Fluid pressure may be measured with respect to any arbitrary datum. The most common datums are Absolute zero pressure and Local atmospheric pressure. When pressure is measured above absolute zero (or complete vacuum), it is called an 'Absolute pressure'. When it is measured either above or below atmospheric pressures as a datum, it is called 'Gauge pressure'. This is because practically all pressure gauges read zero when open to atmosphere and read only the difference between the pressure of the fluid to which they are connected and the atmospheric pressure.

All values of absolute pressure are positive, since in the case of fluids the lowest absolute pressure which can possibly exist corresponds to absolute zero or complete vacuum. However, Gage pressures are positive if they are above that of the atmosphere and negative if they are vacuum pressures. Figure illustrates the relation between atmospheric, absolute, gauge and vacuum pressures.

Fig: Relationship Between Atmospheric, Absolute, Gauge and Vacuum Pressures

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