Water is to be lifted from wells before it is used for irrigation. Sometimes water may have to be lifted from canals, streams, and rain water filled in low lying areas to use it for irrigation. The methods of lifting water vary depending on the source, depth, quantity to be lifted and availability of power.
The following are the various methods of lifting water:
1. Rope-Pulley and Bucket Method:
This method consists of a pulley over which a rope is passed. A bucket is tied to the end of the rope hanging in the well and other end of the rope is held by a man standing at the surface. The bucket is lowered into the well, and pulled out with help of rope and pulley after being filled with water. This method is suitable only when well is not deep and quantity of water to be lifted is small.
2. Dhenkli or Lever Method:
This method consists of a lever-rod rocking over the top of a vertical upright post. A suitable counter weight is tied on one side of the lever and a bucket is suspended from the other end into the well. Length of the lever rod on the side of the bucket is longer than that on the side of counterweight. The bucket when filled is lifted a little to set the lever action.
The bucket full of water moves up automatically due to moment of the counter weight. When bucket reaches the ground it is used for irrigation and empty bucket is again sent into the well. But this time lever will have to be pulled against the moment of counter weight so that bucket may reach water and get filled with it.
3. Mote, Charas or Leather Bag Method:
This method has a pulley fitted on the well on two uprights. A large leather bag also called Mote or charas is tied at one end of the rope. The other end is used to lift filled leather bag of water. In this case lifting force is applied by a pair of bullocks. This method is used when water level in the well is quite deep and irrigation is to be provided to a little larger area. About 1.5 hectare land can be irrigated by this method. In this case bullocks move on straight inclined path.
4. Nar, Rahat or Persian Wheel:
This method was very common in Northern India. Still it can be noticed in U.P., Haryana and Punjab. Persian wheel consists of an arrangement of wheels and gears run by bullocks. An endless chain of several buckets is put on a big wheel. The lower end of chain of buckets remains under well water.
As the wheel is moved by bullocks through a system of gears, the buckets filled with water come to the top and discharge water into a trough from where water flows to the fields. In this case pair of bullocks moves on circular path. By this method 3-5 hectare of land can be irrigated.
5. Basket Method:
This method is used for lifting rain water from low lying areas to the nearby fields. By this method water can be lifted for 60 to 80 cm only. The basket can be worked by one man or two men. The basket has ropes. It is filled and discharged by swing motion.
6. Doons:
Like Basket method it is also used for lifting water for small heights of say 1 m. Doon is a wooden or metallic channel closed at one end and open at the other. It is supported on a wooden bully which acts as its fulcrum.
This method is very much prevalent in Bengal.
7. Archimedian Screw:
It consists of a cylinder in which a screw is fitted. The cylinder is kept inclined with its lower end immersed in water to be lifted. Upper end is kept abutting the field to be irrigated. When screw in the cylinder is rotated, it lifts water as water enclosed in screw cannot go back. In this case also lift of water seldom exceeds 1 m. Inclination of the cylinder is about 30° with the horizontal.
8. Pump:
There are several types of pumps which can be used for lifting water.
Pumps may be divided into:
(1) Constant displacement and
(2) Variable displacement pumps.
Constant displacement pumps deliver the same volume of water against any head within which they can operate.
Variable displacement pumps deliver water in volume varying inversely with the head. The use of constant displacement pumps is confined to places where discharge is low.
Variable displacement pumps which are mostly used for pumping water may be divided into the following classes:
i. Centrifugal pump
ii. Bore hole type pump
iii. Jet pump
iv. Air lift pump.
i. Centrifugal Pump:
This pump lifts water by creating the required pressure with the help of centrifugal action. The maximum suction head under which the pumps can practically work effectively is about 6 m to 8 m. Hence this pump can be used only at places, where the fluctuations in water table plus the depression head is limited to about 8 m. For larger values the bore hole type pump is to be used.
In this arrangement a sump well is made so as to place the pump at the required level. The pump is to be set slightly above the water table so as to avoid the submergence of the pump. The minimum water level should not be lower than the pump level by more than 6 to 8 m, otherwise pump will not work.
The sump well is generally of a large size and deep enough and has to be plugged at the bottom with concrete. This arrangement is quite costly and the depression head plus the fluctuations in water table is also limited. This arrangement has become out of date these days. The bore hole type pump is generally used in the modern times.
ii. Bore Hole Type Pump:
Such pumps are of very compact designs and can be lowered in the casing pipe itself. The top 20 to 30 m of the bore hole and casing pipe is generally kept larger than the remaining bore, so as to accommodate the pump in the casing itself. The pump is driven by a direct coupled electric motor of a vertical shaft type, and is placed at the top of the line shaft at ground level. Thus construction of pump well is completely eliminated. This is a better alternative to the centrifugal pump.
iii. Jet Pump:
It is a combination of a centrifugal pump and an ejector pump. The pump is a single stage at the top of the well and the ejector is located at the suction screen in the well. A part of the water discharged by the pump flows down through the ejector, where it helps to improve flow into the pump and up the discharge tube. This pump is suitable for lifts more than 8 m and has a capacity of upto 200 lit/min. These pumps are inefficient and not used much.
iv. Air Lift Pumps:
Compressed air is released into the discharge pipe lowered into the well. When air is admitted into the well, the water recedes from the level of static head to the bottom of the discharge pipe. The displaced column of liquid rises up the discharge pipe. Air bubbles mix with water and this reduces the specific gravity of the column of water sufficiently to lift it to the surface. It is rarely used for pumping purposes.
Horse Power of Pump Motor:
Example:
Design a pumping station to lift water from a well to the field.
Following is the data:
Quantity of water = 40000 m3
Length of suction pipe = 30 m
Length of rising main = 170 m
Coefficient of friction = 0.01
Pipe diameter = 60 cm
Pumps work for two shifts each shift being of 8 hours.
Combined efficiency of pump and motor = 70%.
Static head through which water is to be raised = 25 m.
Solution:
Quantity of water = 40000 m3
Total pumping hours = 16
Per hour required pumping capacity
= 40000 / 16 = 2500 m3
Discharge in cumecs
