17-tube Well Construction, Comparison Of Tube Well Irrigatio.ppt 6z4y5m

Tube well Construction Tube well construction involves three distinct operations, including: (a) Drilling Cable Tool Percussion Method Hydraulic Rotary Straight Rotary Reverse Rotary (b) Installation of tube well components Screen/Strainer, Blind Pipe, and Sand Trap/Bail Plug Housing pipe Gravel Pack/Well Shrouding

(c) Development of tube well Over pumping and exerting sudden suction jerk Mechanical surging Blowing compressed air in the hole Back washing

a) Drilling

merits and demerits of each method depend upon the formation in which drilling is to be done.

several types of drilling rigs to drill in different formations.

Cable Tool Percussion Method

In this method the drilling in the well hole is carried out by hammering action of the drilling bit, which is attached to the lower end of a string cable tools. The drilling bit attached to a string of cable tool is alternately lifted and dropped in the, hole to break the formation. Rig is truck-mounted and is equipped with ive parts/accessories.

Straight Rotary

This method is used for drilling large diameter holes in soft and unconsolidated material. Equipment used includes derik, a hoist, a power operated revolving table that rotates the bit, a pump for forming fluid through the drill bit and a power unit.

Reverse Rotary

reverse rotary system is opposite to the straight rotary system, i.e. instead of pumping of water in the hole, it is sucked from the hole.

same equipment as used in the straight rotary system.

(b) Installation of tube well components (i)

Installation of Screen, Blind Pipe, and Bail Plug On the basis of the well logs of the strata prepared during drilling, bail pipe, well screens and blind pipes of appropriate lengths are welded and are hung inside the hole from the top. To keep the component of the tube well exactly in the centre, centralizers are provided. Centralizers are iron strips welded to the blind pipe to keep the parts exactly in the centre of the bored hole.

(ii)

Housing Pipe On top of the bail pipe, blind pipe and well screen the housing pipe of the required diameter and length is fixed.

(iii)

Well Shrouding / Gravel Pack

Next step is to provide gravel pack around the well screen. A suitable grading of the filter is designed keeping in view the size of sand particles and the size of screen slots. The filter material is poured in small quantity at one time, in the space around screen and the casing pipe. In case of tube well for irrigation purposes, gravel pack can be provided up to the ground surface. After providing the gravel pack the casing pipe is pulled up.

c) Development of Tube Well i. Over pumping and exerting sudden suction jerks In this method pumping of water is done at higher variable rates. By over pumping fine material surrounding, the welI is agitated and carried out of the well.

ii. Mechanical surging In this method a surge plunger is moved up and down briskly in the well pipe and the screen. In this way, fines which have entered the well are agitated, and then they are pumped out. iii. Blowing Compressed Air In this method compressed air is continuously injected, which develops air lift pump system in the well and carries sand particles with water and are pumped out. After injection of the compressed air is completed, mechanical surging is also done to fully develop the well.

iv. Back Washing In this method water is forced in the well in the reverse direction by means of compressed air pressure. All the sand and clay particles, which have got stuck around the strainer, are agitated and are removed.

Design of a Tube well for Irrigation It includes: Design of Pump Housing casing i. Diameter ii. Length Design of Screen i. Diameter of Screen ii. Length of Screen Types of Pump i. Pump ii. Motor Spacing of Tube wells

a. Design of Pump Housing casing

i.Diameter Diameter of the pump housing casing should be such that vertical shaft of turbine pump and suction pipe of a centrifugal pump can be easily accommodated in it. Also, in case of small capacity tube well it can serve as the suction pipe of centrifugal pump. The minimum inside diameter of the housing should, therefore be at least 2 inches larger than the outer diameter of the proposed bowl assembly or suction pipe.

ii. Length

For turbine pump, the length depends on ultimate Pumping Water Level (PWL) and pump setting requirements So, it is necessary to estimate drawdown in the well.

The fluctuation of static water level (SWL) during the year as well as during prolonged periods of time may also be studied to determine the ultimate static water level. The minimum length of housing pipe can be computed by adding ultimate SWL to the expected draw down. Let: SWL (Ultimate) A, ft. Draw down= B, ft Then the minimum length of housing pipe = (A+B) ft. To allow for errors in the estimate of draw down and also for increase in draw down with age of time due to well deterioration, at least 10 ft. extra length is provided to the housing pipe. Therefore, the final length of housing pipe = (A + B) + 10. A good thumb rule to determine the length of pump housing casing is the lowest expected PWL plus 10ft. or so.

Design of Screen

i.Diameter of Screen The screen diameter may be equal to or less than the diameter of pump housing casing in case of vertical turbine pumps. But in case of centrifugal pump, it is generally equal to the housing pipe.

ii. Length of Screen Let Q = discharge capacity of tube well in (cfs) q = screen transmitting capacity per ft. length. D = diameter of the screen (ft.) P = percentage open area per ft. length of screen V = entrance velocity (ft./sec) = 0.075 ~ 0.1 ft./sec Then Length of screen, L = Q / q Surface area per ft. length = π D

Open area per ft. length = P x π D q = V x Open area cfs q =VxPxπD cfs q=πVPD cfs L = Q / (V x P x π D)

Types of Pump i.Pump

If the water table is not lowered beyond 15.0 ft., a centrifugal pump is quite suitable.

Because horizontal centrifugal pump works on suction, so it should be placed in a pit close to the sub-surface water level. In case of vertical turbine pumps there is no limitation of suction because they are normally installed in such a manner that in the conditions of lowest expected pumping levels, the first stage impeller is submerged in water. Mounting level of a turbine pump is generally 3.0 ft. above the natural surface level. Since, horizontal centrifugal pump is much cheaper as compared to vertical turbine pump, so it is always recommended to use centrifugal pumps for smaller capacity tube wells. ii. Motor

It has been found that an electric driven tube well is much cheaper than the diesel driven tube well so it is recommended that wherever electric power is available, tube wells shall be operated electrically.

Comparison of Tube well Irrigation with Canal Irrigation ADVANTAGES OF TUBE WELL IRRIGATION OVER CANAL IRRIGATION

1. The well is under the direct control of the owner. Hence wells may be sunk and equipped as required. 2. Isolated area can be irrigated by a well. Wells may be sited to command any desired land. 3. The supply from a well can be maintained fairly constant. Water can be turned off at any moment, taking advantage of rainfall. 4. Since a well is generally centrally located, the loss in transit is very much reduced. The duty of water in well irrigation is generally higher. 5. Volumetric assessment is possible. 6. Well irrigation is helpful in lowering the sub-soil water level and in draining off irrigated land which might become water logged. In canal irrigation, the chances of water logging is much more than in the well irrigation. 7. Unless drought continues. for several years, well irrigation does not fail in drought seasons, while a canal may fail. 8. With the help of well irrigation, more than one crop in a year can be grown. 9. The well water, which is warmer in cold weather, and cooler in hot weather, is more agreeable to crops. 10. The cost of construction of a well is low and irrigation in a locality can be introduced in stages.

DISADVANTAGES OF TUBE WELL IRRIGATION OVER CANAL IRRIGATION (1) Since water has to be lifted from the well, the working expenses are very high in comparison to the canal irrigation. (2) Due to mechanical defects in the pump or due to interruption in electric supply, water may not be made available to the crop in right time.

(3) The well water is clear and free from silt. Clear water does not have manuring value which silt in canal water provides. (4) The tube well strainer is subject to progressive deterioration to mechanical and chemical action. Thus, replacements are necessary after frequent interval of time. (5) The maintenance of mechanical and electrical machinery are also great care and more funds.