Here is a compilation of essays on ‘Tube-Wells’ for class 5, 6, 7, 8, 9, and 10. Find paragraphs, long and short essays on ‘Tube-Wells’ especially written for school students.
Essay on Tube-Wells
Essay Contents:
- Essay on the Definition of Tube-Wells
- Essay on the Types of Tube-Wells
- Essay on the Construction of Tube-Wells
- Essay on the Typical Section of a Tube-Well
- Essay on the Yield of Tube-Well
- Essay on Water Lifting Arrangement from a Tube-Well
- Essay on the Failures of Tube-Wells
- Essay on the Development of Tube-Wells or Tube-Well Education
- Essay on the Maintenance of Tube-Wells
Essay # 1. Definition of Tube-Wells:
Tube well is a small bore hole drilled into the subsoil formation. A well made by driving a tube into the earth to a stratum that bears water is known as tube well.The cross sectional area of this type of well is small does not have storage and unless mechanical power is used for lifting water, rate of water withdrawal will be low. The velocity of water withdrawal should be more to maintain high discharge.
Obviously the limit of critical velocity is exceeded. Under such conditions the soil particles are dislodged. To prevent the clogging of the bore hole a metal pipe with perforations is driven in the hole. It prevents the soil particles from entering the hole. To increase the efficiency of the tube well, in the space between perforated metal tube and the soil some straining device is always provided.
The perforations in the metal pipe are necessary only for the depth of water bearing subsoil formations. Remaining portion of the pipe is generally kept plain. The tube well may be taken down to 50 metres in the water bearing stratum.
Average discharge of a tube well is about 0.04 m3/sec. The factors which affect the location of tube well are more or less the same as those discussed for open wells. Tube well irrigation is very suitable where the subsoil formation is suitable for storing water.
In Haryana, Uttar Pradesh, Punjab and Bihar tube well irrigation is extensively in practice. The obvious reason is that most of the areas are made up of alluvium, which hold water to a great extent and there are massive ground-water reservoirs.
Essay # 2. Types of Tube-Wells:
Depending upon the mode of water contribution to the bore hole from the subsoil formation following main types of tube wells may be recognized:
(i) Strainer type tube-well.
(ii) Cavity type tube-well.
(iii) Slotted type tube-well.
(i) Strainer Type Tube-Well:
It is a bored hole in which a metal pipe with suitable perforations is inserted. The best and most commonly adopted practice is to provide a pipe with fairly big perforations and surrounding that is a wire-net or a strainer with smaller openings. The wire-net with finer openings excludes objectionable soil particles from entering the tube well.
The total area of opening in the metal tube and in the strainer is kept the same. This is because if the area of opening is same the velocity of inflow will be the same. Moreover some annular space is left between the tube and the strainer. If the space is not left the strainer might rest directly over the tube and consequently the open area of perforation will be reduced. Fig. 9.1 shows a section of a strainer type of tube well.
Normally the mesh size of the wire-net or the strainer is kept equal to D60 to D70 of the surrounding soil. This type of well derives water from one aquifer of unlimited extent or from a confined aquifer or from number of aquifers. The perforated pipe extends only for the aquifer portions of the soil formation while for other portions the pipe is kept plain or blind. At the bottom the tube well is plugged. The plug is kept a little above the bottom. The reason for it is otherwise the plug may fail due to the weight of the well.
Abyssinian Tube-Well:
It is the most common type of tube well used for household purposes. It is also a strainer type of the tube well. The length of the strainer used here is about 2 metres.
(ii) Cavity Type Tube-Well:
In this type, water contribution to the bored hole takes place through the bottom layer only (Fig. 9.2).
It derives water from the previous layer underlying the hard impervious layer. The tube well is taken down till it penetrates the impervious layer and reaches the water bearing layer.
In the initial stages when the water is pumped out fine sand comes in the tube well with the water. As a result a hollow or a cavity is formed at the bottom. The bottom of the cavity for some thickness is thus made free of finer particles. Thus after the cavity formation only clear water enters the tube well. The coarse sand layer reduces the critical velocity of the water entering the well though the rate of pumping is more.
(iii) Slotted Type Tube-Well:
It can be used as an alternative when the first two types could not be constructed. There should be, of course, an aquifer present at the bottom. In the bored hole a 15 cm diameter education pipe is lowered till it reaches the bottom. The bottom of the education pipe is slotted as shown in Fig. 9.3.
The size of slots may be 25 mm × 3 mm with 12 mm spacing. As the slots are quite wide, to avoid sand entry in the pipe a filter of shingle is provided at the bottom surrounding the slotted portion of the education pipe.
Finally, before withdrawing the 36 cm diameter pipe casing shingle is poured in the annular space between the education pipe and the casing pipe. The development of well is done gradually with compressed air.
Interference of Wells:
The wells should be located in such a way that their circles of influence under maximum draw-down conditions do not cross the neighbouring property line. If the water is withdrawn from beneath some property, it is likely to damage the structure above.
Two wells of any area should be spaced in such a way that their cones of depression or the circles of influence do not cross each other. Sufficient spacing should be kept between the wells to avoid mutual interference.
Mutual interference reduces the dependable ground water storage of each well as shown in Fig. 9.8. In turn it reduces the discharge of wells.
Essay # 3. Construction of Tube-Wells:
The construction of a tube-well consists of various steps.
They are:
(1) Boring or drilling
(2) Preparation of a boring log
(3) Lowering of a tube well pipe and a strainer
(4) Installation of a pump to energize the tube-well, and
(5) Development of the tube well.
Drilling Methods for Tube-Wells:
The operation consists of sinking a hole in the crest of an earth. The diameter of hole varies according to requirement. Though in principle drilling is simple it requires skill and care in actual practice. Percussion boring method, Wash boring or water-jet boring method; and Rotary boring method are most commonly adopted methods of boring.
Selection of Drilling Method:
A choice of particular drilling method depends upon:
(a) Purpose of the well;
(b) Quantity of discharge required;
(c) Depth of water table;
(d) Type of sub-surface formation; and
(e) Type of equipment available.
Shallow wells of small diameter may be bored in unconsolidated formation by an auger operated manually.
Small capacity wells in unconsolidated formations may be drilled by water jet boring method.
All deep tube wells are constructed by drilling. Rotary methods are generally better for geological investigations whereas cable tool or percussion method is superior for study of water quality.
Preparation of Boring Log:
As boring progresses detailed account of the sub-surface formations met with is kept. From this data the position of underground water bearing layers can be ascertained by preparing a boring log. The material taken out of the plunger is inspected carefully. A continuous record of the type of material taken out at various depths is prepared. From the analysis of data position of the aquifer can be determined accurately.
Lowering of a Tube-Well Pipe and a Strainer:
When the tube well reaches predetermined depth the platform is removed and an assembly of a blind pipe with strainer at the determined levels of aquifers is lowered in the hole. After lowering it upto the required depth the strainer pipe is clamped to prevent it from falling down. Then shrouding is started. Shrouding is coarse material filled in the annular space between the strainer pipe and the casing pipe. It acts as a filter and during pumping prevents the average soil particles from entering the bored hole.
In the beginning about 60 cm length of shrouding is done. Then the pipe casing is slowly removed by 30 cm. Then again 30 cm of shrouding is done and again the casing pipe is lifted by about 30 cm. Thus the shrouding and pipe withdrawal is done slowly, successively and in small lifts of about 30 cm till the whole casing pipe is withdrawn.
The amount of material required for shrouding per 30 cm length can be accurately calculated beforehand. It depends on the thickness of the gravel pack. Normally thickness of shrouding varies between 7.5 cm to 25 cm. The thickness of gravel pack should be such that it would not allow even finest particles to move. In sandy and unconsolidated aquifers shrouding is always required because the shrouding works as a filter.
Types of Strainers:
To allow entry of water but to exclude average soil particles from entering the bored holes strainers are used.
Following are some types of the strainers:
Cook Strainer:
It consists of a solid drawn brass tube. The wedge shaped slots are cut on the surface of the tube. The slots are horizontal as shown in Fig. 9.15. The slots are cut from inside of the tube. Thus the openings are wide inside of the tube. The width of the slots varies from 1/7 mm to 2/5 mm depending upon the coarseness of sand.
Tej Strainer:
Only difference between the Tej and the Cook strainer is, in the former the brass sheet is first slotted and then bent to form a tube. The tube is then brazed. As there are joints this strainer is weaker. It is cheap and hence widely used.
Brownlie Strainer:
It is made of a convoluted steel tube. There are perforations in the tube. The wire mesh made of copper surrounds the steel tube.
The convolution may be done in various ways as shown in Fig. 9.16 (a & b). This type is designed in such a way that there is some space left between the perforated tube and the surrounding wire mesh.
Ashford Strainer:
It consists of a perforated tube with a wire mesh surrounding it as shown in Fig. 9.17. To keep space between the mesh and the tube, a wire is wound on the tube.
The wire mesh is soldered to this wire. The wire mesh can be further protected by a wire-net above it.
Legget Strainer:
In this type of strainer cutters are provided in the tube. By operating the cutters from the top the perforations in the tube can be cleaned.
Phoenix Strainer:
It is made of a mild steel tube. To avoid corrosion of the tube it is plated with cadmium. The slits are made from inside.
Layne and Bowler Strainer:
It consists of a perforated steel or wrought iron tube. A wedge shaped steel wire is wound over this tube at suitable pitch.
Essay # 4. Typical Section of a Tube-Well:
A strainer type tube well is a most commonly used tube well. A typical cross- section of a tube well is shown in Fig. 9.9. On the left hand side names of various components of a tube well are given. Also given are the technical terms used in tube well technology. On the right hand side log of the sub-soil formation is indicated.
The tube well structure is protected at the bottom by means of a plug. At the top the well is protected from surface seepage water by a concrete ring. A blind or plain pipe is provided from the ground level to little below the sources of pollution.
Only after ensuring availability of good quality water a strainer is provided to cover water bearing layers. Impermeable layers are covered with blind or plain pipes. All round the tube well a gravel packing or shrouding is provided.
Essay # 5. Yield of Tube-Well:
It is necessary to clearly understand that in case of unconfined aquifer the top flow boundary is a free surface and it is unknown. In case of confined aquifer all boundaries are known from beginning. Approximate analytical solution to this problem was given by Dupit and the equations which give yield of tube well are known after his name.
Theoretically discharge estimation for a strainer type tube well. (It is most common in occurrence) is done after making following simplifying assumptions:
(1) Water-table is horizontal and at rest.
(2) The well penetrates homogeneous isotropic aquifer.
(3) The bottom of the strainer reaches a horizontal impervious layer.
(4) The inclination of free surface is equal to hydraulic gradient and it does not change with depth.
(5) There is no flow in the aquifer other than that towards the well. That is the flow is radial. It is also assumed that the flow is in horizontal direction.
(6) Total discharge remains constant across the surface of a series of concentric cylinders.
(7) The flow through soil is assumed to be steady and laminer. Therefore, Dacey’s law is applicable.
Most of the assumptions are not justified and hence the formula derived gives approximate discharge. But as no mathematical basis could be given until now this formula is used for finding out the discharge. The results given are reasonably accurate.
The discharge through unconfined aquifer is given by the formula:
Unconfined aquifer is that water-bearing layer which is not interposed by any impervious layer. In this case the water-bearing layer is unlimited in extent as far as depth of the tube well is concerned. The pressure at the top surface of this aquifer is atmospheric. That is the top of the aquifer represents the water table.
R is radius of circle of influence in metres. It is also called radius of zero drawdown.
r is radius of tube well in metres.
H1 is height of water table above the bottom of the well in metres.
H2 is height of water level in the tube well in metres, after depression.
Q is total discharge of the tube well.
K is coefficient of permeability of the soil.
The discharge through a confined aquifer can be calculated from the formula:
Confined aquifer is that water bearing layer which lies between two impervious layers. The pressure at the top of the confined aquifer is not atmospheric but it is positive hydrostatic pressure.
m is thickness of a confined aquifer. It is assumed that m is constant i.e. confined aquifer is of uniform thickness. The well penetrates aquifer fully.
Essay # 6. Water Lifting Arrangement from a Tube-Well:
In tube wells water is normally available at great depths. Secondly diameter of a tube well is much smaller as compared to an open well. Obviously water can be lifted from tube wells only with the help of pumps. There are many types of pumps available.
For deep open wells and tube wells suction lift is normally more than 8 metres. Also lift further increases due to fluctuations in water table and depression head. However, it is many times possible to reduce the suction lift to permissible limit of 7.6 m by installing the pump in the pump sump below ground level. A single stage centrifugal pump with horizontal spindle is commonly used in such cases.
The water lifting arrangement is shown in Fig. 9.18.
This arrangement has following advantages:
(i) It is very easy to install.
(ii) It is relatively trouble free arrangement.
(iii) Cost of installation is moderate.
(iv) It is capable of attaining good efficiency.
It has the main disadvantage that the suction lift is limited. In tube wells when the lift increases appreciably this arrangement becomes useless. As a result increasingly other types of pumps which can operate under high heads are coming into use.
Choice of a particular type of pump depends on the following factors:
(a) Diameter of the tube well,
(b) Depth of the well and depth of water table below ground surface,
(c) Variations in water table,
(d) Expected drawdown in the well,
(e) Quality of well water,
(f) Duration of pumping per day,
(g) Pumping capacity,
(h) Requirement of power for working of pumps, and
(i) Capital and maintenance cost of the pump.
Apart from single stage and multistage centrifugal pumps, other common types of pumps used to lift water from the tube wells are:
(i) Vertical turbine pump;
(ii) Submersible motor pump;
(iii) Jet pump; and
(iv) Air lift pump.
(i) Vertical Turbine Pump:
This type of pump is used to pump water from deep tube wells because it is a multistage diffuser-type pump. A vertical turbine pump consists of a vertical spindle electric motor at ground level. From it water flow pipe and a drive shaft within the pipe reaches the lowest water level. The pump is attached to the lower end of the water flow pipe and remains submerged under water.
The turbine pumps are available in varying sizes ranging from 10 to 35 cm in diameter. The pump length varies from 20 to 60 cm. The number of stages depends upon the head to be developed. In high head pumps the number of stages may reach 20 stages or more. This type of pump is capable of discharging 2000 l/sec and can develop heads upto 500 m.
(ii) Submersible Motor Pump:
This is a multistage centrifugal type pump. The impellers and diffusers are arranged in series to discharge water against sufficient head. The entire assembly comprising the pump and electric motor is kept submerged under water in the well.
The delivery of water is under pressure and there is no suction condition. The pump is placed above the motor with water inlet in-between. The electric motor is connected to the control system on the ground by means of special submarine type electrical cables.
This pump can be installed in a tube well of as small diameter as 10 cm. The pump can be installed at any depth subject to horse power and pipe size. Unlike turbine pump it does not need long drive shaft. Also sump well or pump house etc. is also not required.
(iii) Jet Pump:
It is nothing but a single stage centrifugal pump aided by a jet or an ejector nozzle. The pump is located at surface with foot valve reaching lowest water level in the well. The ejector nozzle enters suction pipe just above foot valve. Ejector is fed by another pressure pipe which receives part of the pumped water. The water flows down under pressure and enters suction pipe through the nozzle and helps to improve the flow into the pump.
The pump can be installed in a tube well having 10 to 20 cm diameter. Jet pumps are used to obtain small discharges upto 3.3 litres/sec. They are suitable for lifts between 6 to 50 metres.
(iv) Air Lift Pumps:
Due to low efficiency this method is rarely used. It is useful for removing objectionable gas, highly corrosive or abrasive water which damages parts of the pump submerged in the well. It is also used for testing or development of wells.
Essay # 7. Failures of Tube-Wells:
The life of ordinary tube-wells in Northern India is between 15 to 20 years.
The tube-wells fail due to the following reasons:
(i) Corrosion
(ii) Incrustation.
(i) Corrosion:
Due to the continuous effect of the acids, chlorides and sulphates are present in the ground water, the tube-well materials gets corroded. Due to corrosion the strainer screens get damaged and the sand particles start coming out along with the water.
Following methods can be commonly used in reducing the corrosion:
(a) Using thick pipes for the construction of tube-wells.
(b) Using galvanizing or other corrosion resisting coatings on the pipes.
(c) Reducing rate of pumping, this will reduce the coming out of sand particles.
(d) Increasing the diameter of the well, in other words using big capacity tube-well.
(e) Using stainless steel pipes. But as they are very costly, it is not possible to use them in poor or developing country like India.
(ii) Incrustation:
It is caused due to deposition of alkali salts on the inside walls of the tube-well. Calcium carbonate, magnesium sulphates, and silicates are the main materials present in the ground water. These materials get deposited on the walls of the tube well and reduce the diameter of the pipe as well as the effective area of the screens. This cause reduction in the yield of the tube well.
The incrustation can be reduced by the following methods:
(a) Using larger area of the screens in the beginning and keeping allowance for the future incrustation.
(b) By using acid resistant material screens, so that after sometime the incrustating deposits.
(c) By periodical maintenance of the tube-well and removing the incrusting deposits.
(d) By using bigger capacity tube-well or pumping at low rates.
Essay # 8. Development of Tube-Wells or Tube-Well Education:
Definition and Need:
The tube well is not completely ready for use just after construction. The next important step is to develop the drilled tube well. The tube well can function successfully only after proper development. It is the process by which the finer particles from around the screen are removed to increase the permeability of the formation through which water moves towards the well.
Development serves following functions:
(i) It clears the water-bearing formation clogged by the mud in the drilling operation.
(ii) It causes the gravel pack and surrounding formation to settle and to get compacted against the screen, thus it makes the tube well structure stable.
(iii) It breaks down the bridging of sand grains across the screen openings and in the surrounding gravel pack and aquifer formation and makes the well efficient.
(iv) It helps in obtaining sand-free water by stabilising the sand formation round the screen.
(v) It helps in reducing head losses near the screen.
(vi) It brings the well to its maximum capacity that is maximum yield is available at minimum drawdown.
(vii) It gives a measure of available water supply and helps in determining the required characteristics of a pump and power unit to be installed.
(viii) It increases useful life of the screen or strainer.
Methods of Development:
When the water bearing strata in which well is drilled is made up of sand and gravel or alluvium, development is accomplished by removing finer particles from the area surrounding the tube well.
The following are the methods commonly adopted for development of a tube well:
1. Development by pumping,
2. Development by compressed air,
3. Development by surging,
4. Development by back-washing,
5. Development by high velocity jetting, and
6. Development by using chemicals.
Essay # 9. Maintenance of Tube-Wells:
For the proper running of tube wells, they must be maintained. The main works during maintenance include cleaning or changing the screens and other worn out materials.
The mesh or screens of the tube-wells which arc placed against the water-bearing stratums if clogged due to deposition of fine clay or sand particles, will reduce the yield of tube-well. The cleaning of the screens is done by the method known as ‘Surging’.
In this method the water is forced inside the well and taken out alternatively at very high rate. Sometimes compressed air is also passed inside the tube-well before pumping water inside it. This method removes the clay particles from the screens.
In another method ‘Dry ice’ which is solid carbon- dioxide is dropped inside the tube-well and it is closed from the top by tightening cap. The dry ice vaporizes immediately and increases the pressure inside the tube- well sufficient to force away the soil particles from the screens.
The corrugated or rusted screens are cleaned with the help of sulphuric acid or hydrochloric acid.
The tube-well parts which are totally rusted or corrugated should be replaced with new parts.