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A properly constructed and protected well can be an excellent source for domestic water needs. The terms borehole, dug well and tube well describe the manner in which water is reached. A borehole well is drilled with a cable or rotary drill. It will have a small diameter and can be 200 metres or more in depth. A dug well is a hole dug with a diameter large enough to allow a man to work, usually to a maximum depth of about 30 metres. A tube well is a perforated pipe with a pointed end which is either hammered or jetted into the ground.
When a well is less than 7m deep it is called a shallow well, and when more than 7m deep, a deep well. An earth well is unlined, a masonry well is lined with concrete blocks or stone, and a sinking well casing is constructed and sunk in stages from the ground level as the well is being excavated.
Location of Well Site
Water may often be found in one of the following locations:
Types of Well Casings
There are several methods of constructing well casings, the one chosen depending on purpose, soil structure, water source and local skills.
Oil barrel well are used to form this inexpensive well casing. The barrels are perforated to allow the entry of water. The life of this casing will be shorter than with other materials and the residue in the barrels may pollute the water so that it is unfit for domestic use.
Ferrocement well is a type of earth wall well that is excavated to a straight and smooth surface which is then plastered with a layer of mortar, reinforced with chicken wire, and finally plastered a second time.
A well head is built above ground level to limit the risk of children and animals falling into the well. To prevent contamination of the well a concrete apron, sloping away from the well head, is constructed on the surrounding ground.
Figure 14.8 Ferrocement well (Courtesy of Erik Nissen Petersen).
Sinking wells are so named because the casing is sunk into place. The method works well in sandy soils. Figures 14.9 and 14.10 show casings that can be sunk into place.
Concrete ring well is a method that requires either a steel casing ring mould for casting the concrete rings on site or for precast rings to be purchased and transported from factory to construction site. Both alternatives are expensive for a single well, but feasible when a number of wells are being constructed in a local area. The rings, measuring 0.9m in diameter and 0.5m in height are stacked upon each other in an excavated well hole, or they can be used for sinking wells or a combination of both procedures.
Figure 14.9 Concrete ring well (sinking well).
Concrete block well is a less expensive alternative to concrete rings is the use of concrete blocks shaped in a wooden form. These blocks are stacked on a concrete foundation ring which can be cast in a wooden form, or more cheaply, in a ditch in the soil at the construction site.
With either type of casing, by digging soil out from under the bottom of the casing, the whole structure will be allowed to settle. When the top of the well casing has reached the surface of the surrounding soil another section is added to the top. Thereafter digging is repeated until another section can be added on to the well at ground level, and so on until a satisfactory depth has been reached. The blocks must be tied together with vertical reinforcing rods to ensure that the casing sinks as a single unit.
Figure 14.10a Concrete block well (sinking well).
Figure 14.10b Section of foundation and reinforcement (Courtesy of Erik Nissen-Petersen).
Lifts for Wells
The simplest means of lifting water from dug wells such as a rope and calabash or a bucket and windlass have been used for centuries and unfortunately they continue to be used today in many parts of the world. The objection to their use is that too often they are a source of pollution both because the top of the well is open and because the water vessel is frequently set down on a badly polluted surface. An improved variation of these methods has a bucket with a hose attached to the bottom and to an outlet at the wellhead as shown in Figure 14.11. When the bucket is lifted water is discharged from the outlet while the top of the well remains covered.
Figure 14.1 1 Bucket lift in closed well (Courtesy of Erik Nissen-Petersen).
A pump is the most convenient and sanitary means of lifting water from a well or any other low level water supply. Pumps may be hand or power operated, designed to lift only or to lift and discharge against pressure and to lift from either shallow or deep wells.
As mentioned earlier, shallow wells are those in which the low water level is 7m or less below the pump. In deep wells the water level may drop well below the 7m mark. The maximum suction lift for shallow-well pumps of any type is reduced about 1m per 1000m of site elevation.
The simplest hand pump, often referred to as a pitcher pump, is satisfactory for use on wells or cisterns in which the water never needs to be lifted more than about 6m. A cross section of a pitcher pump is shown in Figure 14. 12. If these pumps are maintained in good condition, they are easily primed and will hold their prime from one use to the next. However, if the valves leak, the pump will need to be primed each time it is used. This is not only a nuisance but can be a source of pollution from the priming water.
Water from deep wells is lifted with a similar plunger type pump in which the cylinder, including the plunger and valves, is supported on the discharge pipe deep enough in the well to be submerged in water at all times. The pump handle is connected to the plunger by means of a long rod. While this type of pump is self-priming due to the cylinder being submerged in the water, it must nevertheless be maintained in good condition to work effectively. Figure 14.13 illustrates a deep-well pump. Both of these pumps allow the well top to be completely covered for maximum protection against pollution.
Occasionally it is necessary to use a hand pump to force water above the level of the pump. Models are available that are designed with a packing around the lift rod and a pipe connection at the point of discharge enabling them to force water to a tank higher than the pump. An even more sophisticated model is equipped with a small "differential" cylinder that causes the pump to discharge on both the "up" and "down" strokes.
Power Driven Pumps
There are a number of pumps on the market from which to select for a particular application. They all have characteristics which influence their suitability for a specific water supply as well as the volume and pressure required.
Centrifugal pumps are simple (only one moving part), durable, and relatively inexpensive for a given capacity. However, they are suitable only for low lifts of 3 to 4m and are prone to losing their prime unless the suction pipe is equipped with a good foot valve (check valve). Neither will they discharge against a very high head (pressure).
There are several designs of centrifugal pumps that further influence one's choice. The impeller may be an open type with a relatively large clearance between it and the casing or it may be a closed type with very close clearances. The open type will tolerate sand or silt in the water much better than the closed-impeller type. (Figure 14.14 and 14.15).
A centrifugal pump may have an integral electric motor or petrol-powered engine which the manufacturer will have sized correctly, or it may have a belt drive. In the latter case, great care must be taken to drive the pump at a suitable speed and with a motor or engine of adequate power.
Figure 14.12 Shallow-well handpump.
As with the propeller fans described in Chapter 7, centrifugal pumps have volume, pressure and power requirement characteristics that vary with speed as follows:
This means that if a pump was designed to run at 2,000rpm and be operated by a 1,000W motor, and the motor pulley is exchanged for one that is 11/2 times the original diameter, the pump will then turn at 3,000rpm. The corresponding changes in volume, maximum pressure and power required will be:
Figure 14.13 Deep-well handpump.
Figure 14.14 Open impeller.
Figure14.15 Closed impeller.
Consequently, the motor will be badly overloaded and may be damaged.
Jet pumps are centrifugal pumps for a shallow that may have a jet (ejector) built into the pump housing. This will improve both the lifting and discharge efficiency. These pumps are suitable for lifts of up to about 6m.
A deep-well jet pump will have the ejector installed below the low-water level in the well. Two pipes of different dimensions connect it to the pump which may be located at the top of the well or even some distance to one side. The smaller of the two pipes carries water to the ejector, while the larger one delivers water to the pump housing where most is discharged but some is returned to the ejector. These deep-well jet pumps are suitable for wells in which the water level drops to 30m. The correct ejector for maximum efficiency is chosen on the basis of the lowest expected water level in the well. (Figure 14.16).
Deep-well turbine pumps are multi-stage centrifugal type and may be driven either by a long vertical shaft from a drive head at the top of the well or by a submersible motor below the pump in the well. The shaft-driven units are large expensive pumps designed to supply large volumes of water for irrigation or community use.
The submersible pump, on the other hand, is available in a range of sizes and is an efficient, trouble-free design for medium-sized installations. Obviously it is a major operation to remove the pump from the well if something goes wrong. It should be noted that the motor is installed below the pump so that if the water level is reduced to the pump level, the motor will still be submerged in water which is essential for cooling.
Reciprocating pumps are available for both shallow wells and deep-wells. They are capable of delivering water at quite high pressures. The shallow-well type is usually reasonable in cost, but the deep-well type tends to be expensive and it must be installed over the top of the well.
Diaphragm pumps have a piston and cylinder thatare replaced with a diaphragm. As there are no sliding parts to wear, these pumps are suitable for pumping muddy water or high moisture slurries such as the waste from a biogas generator. See Figure 14.17. These pumps may be either hand or power operated.
Figure 14.16 Deep-well jet.
Figure 14.17 Diaphragm pump.
Hydraulic rams require no electricity or human power to operate, relying instead on the energy from flowing water. A minimum flow of 10 litres per minute with a head of at least one metre is required. As water flows through the ram, the waste valve alternately opens and closes. Each time it closes water is forced up the delivery pipe by the inertia developed in the flowing water which is abruptly stopped when the waste valve closes. Small quantities of water are thus lifted well above the original source. A ram can be useful for pumping domestic or livestock water to a storage.
Commercial rams are available in a number of sizes that can pass supply-flow rates from 10 to 400 litres per minute and can discharge to maximum heights of 100 to 150m. Although a ram will operate at as little as 1 metre of head, larger heads will increase discharge rates considerably, e.g. increasing supply head from 1 to 10m can increase delivery by up to 20 times. It is necessary to know the flow rate of the water supply and the head which is possible before purchasing a ram. The first cost is substantial, but maintenance is low, life is long and operating cost is nil, so if the natural conditions are available, a hydraulic ram can be a very good investment.
Choosing a Pump
Five main factors must be considered when selecting a pump:
The terms head and pressure are used interchangeably. The unit of measure of pressure is the pascal (Pa) while the unit of measure of head is the metre (m). One metre of water column = 9.8 kPa. Head is frequently used in discussing pump installations because there will be vertical distances from water level to pump and pump to point of discharge. Pipe friction tables are often given in terms of loss of head per unit of pipe length.
The daily water requirement influences pump size in that it is desirable for the pump to operate not more than 25% of the time.
The maximum rate of flow is determined by totaling suitable flow rates from all of the discharge openings that may be operating at one time. If the source of water is a dug well, pond or stream, undoubtedly the desired flow rate can be used in choosing a pump. However, if the source is a borehole or driven well with very low storage capacity, there is no alternative but to choose a pump that does not have a capacity in excess of the flow rate of the well.
The vertical distance between low water level and the location of the pump is the primary factor in the type of pump chosen, although the total head is also significant. Total head is made up of: a lifting head from well to pump, b vertical discharge head from pump to point of use, c working head or pressure at the point of use, and d friction losses due to flow through pipe and fittings.
Pump Storage Tanks
Regardless of the type of pump chosen, it must either discharge into a tank or have an open pipe discharge into an irrigation channel. Operating any of the centrifugal pumps against a closed line results in overheating and damaged shaft seals. Operating a reciprocating pump against a closed line will result in a stalled motor or the physical breaking of some part in the pump.
Figure 14.18 Hydraulic ram.
These systems consist of an enclosed tank combined with an automatic pressure switch which turns the pump motor on when tank pressure drops to a preset level. As the tank is approximately half full of air, several litres of water can be pumped into the tank before the air is compressed and the stock cut-off pressure is reached. The amount of water pumped into the tank can then be used as required before the pump needs to operate again. There are several advantages to the hydropneumatic system:
Table 14.5 Pump Applications
|Type of Pump||Vertical distance Pump to Water||Quantity Water Required||Operating Pressures||Applications|
|Centrifugal||up to 4m||large||low||stock or irrigation|
|Shallow-well jet||up to 6m||med||med||domestic or stock|
|Deep-well jet||6 - 30m||med||med||domestic or stock|
|Shaft-driven deep-well turbine||4 - 40m||large||low to high||irrigation|
|Submersible deep-well turbine||6 - 40m||med||med||domestic, stock, irrigation|
|Shallow well||up to 7m||low to med||med||domestic, stock|
|Deep well||6 -40m||med||med to high||domestic, stock|
|Diaphragm||up to 5m||med||low||slurries|
As air is soluble in water, a small continuous supply of air is required to prevent the tank from becoming waterlogged. Each type of pressure pump discussed will have an air volume control suitable to its mode of operation to provide the necessary supply of air. Alternatively, tanks may be equipped with rubber air bags or foam plastic floats for permanent air retention.
The operation of a pressure tank is in accord with the universal gas law which states that:
P1V1/T1 = P2V2/T2 where:
P = absolute pressure, Pa
V = volume, l
T = absolute temperature, ° K
Although it is the water charge and discharge that is of interest, it is the pressure and volume of air that must be considered. The operation of the tank is essentially an isothermal process (constant temperature) although as fresh water is pumped into the tank the temperature is likely to change a little. The tank should be approximately half full at the cut-in pressure for the best operation. Water system problems are shown later.
A second system for storing pumped water is a gravity tank with the pump operation controlled either manually or by a float switch. The tank must be elevated above the highest point of water use, frequently on the roof of the building where the water is used. The tank is usually appreciably larger than a pressure tank. This is an advantage in that, in case of a power failure or pump breakdown, there will be a larger reserve of water available for use. However, the need to support a large tank on the roof requires strong structural support that will add to the cost of the installation. Finally, water pressure is seldom very high and may be barely adequate near the level of the tank.
If the rate of water flow in a pipe system remains constant, the equation of continuity of flow applies; that is:
Q = A x V where:
Q = flow (m³/s)
A = cross-section area (m²)
V = velocity (m/s)
If the area of the pipe is cut in half, the velocity of flow will be doubled and so on. The velocity is not uniform across a cross section of the pipe because of the friction affect of the pipe walls, but average velocity is used for calculations.
Friction loss in pipes occurs when water flows through a pipe. The amount of loss is principally related to pipe size, velocity of flow and the roughness of the interior pipe surface and to a lesser extent temperature. The friction is proportional to the square of the velocity, so the resistance, which is small at low velocities, builds up quickly as the velocity increases.
Roughness in pipes can change with age. Galvanized steel pipes may form rust or scale with age, thus increasing the roughness and friction and reducing the rate of flow. A smooth pipe such as plastic has less friction effect than a rough surface such as concrete.
The length is directly proportional to the friction head in pipes. Figure 14.20 gives the loss of head for both smooth and rough pipes of several sizes and for a range of flow rates.
Other Losses that can occur is when water flow in a pipe is interrupted such as by going through fittings, or from one pipe size to another, there will be a friction loss. This results from turbulence in the flow, which uses up energy, and so more energy must be used to produce a higher pressure at the start of the pipe lines. As friction loss is proportional to the square of the velocity of flow, it can be ignored at low velocities such as in drainage pipes. However, it can be significant in high-pressure irrigation lines or water-supply systems, especially if there are a large number of fittings. Adding 10% to the friction loss of the pipes to allow for all the miscellaneous fitting losses, is a common procedure.
Water System Problem
It is necessary to design the water system for domestic and stock watering for a family of five who keep 3 Zebu cows, and 10 goats. The water will be pumped from a dug well that is 3m below and 5m away from where the pump will be located. The pump will need to discharge into the storage tank at a minimum of 300kPa of pressure. The discharge from the tank between cut-out and cut-in pressure should be approximately '/ 12 of daily water consumption so that the pump will operate no more than 12 times per day. Water will be discharged from the tank a distance of 50m to a single tap and the head loss at a flow of 11/ s should not exceed 10% of the average pressure. The pump dealer has advised that his pumps are approximately 75% efficient in terms of power demand and the electric motors are 85% efficient.
Figure 14.19 Hydropneumatic water sistem.
Determine the following:
1 From Table 14.1, a single water tap supply indicates 1020 l/day per person. Choose 20 litres.
From Table 14.3 local cattle require 20 l/ day and goats 3 l/day.
5 people x 20 = 1001
3 cows x 20 = 601
10 goats x 3 = 301
Total daily needs 1901 at 1l/s maximum flow
2 The lift from well to pump is low (3m) and the water demand is low. Choose a shallow-well jet pump with a 1.21/ s capacity. The extra capacity will allow for some loss of capacity due to wear over the life of the pump.
3 Calculate the loss of head per metre of suction pipe.
3m x 8% = 0.24m/8m of pipe = 0.03m/m. From Figure 14.20, the intersection of 1.2l/s and 0.03m/m head loss is 38mm plastic pipe. Choose a 38mm P.V.C. suction line.
4 Tank size. 190/12 = 16ldischarge/cycle. Choose a pressure range of 200 to 300kPa; atmospheric pressure equals l00kPa.
P1V1/T1 = P2V2/T2 but assume T1 = T2
V2 = V1 + 16 as P1 drops to P2
400 x V1 = 300 x (V1+ 16)
100 x V1 = 4800
V1 = 48l
V2= V1 + 16 = 48+ 16 = 64l
V2 = should be about 1/2 of the tank size
Approximate tank size = 2 x 64 = 128l .
Figure 14.20 Friction losses in pipes.
5 Average pressure at tank is (200 + 300)/2 = 250kPa 1 m of head = 9.8kPa, therefore 250kPa = 25.5m of head 25.5 x 10% = 2.55. Which gives:
2.55/50m = 0.5m/m allowable loss at 11/s flow.
From Figure 14.20, 20mm PVC pipe is small but 25mm PVC pipe is satisfactory.
6 Power to lift water from the well and overcome all head at a flow rate of 11/s is as follows:
Total head = 3 + 0.24 + (300/9.8) + 2.55 = 36.4m of head
1l water = 1 kg mass and gravitational force
= l kg x 9.8m/s2
Force required = 1 kg x 9.8 m/s2 = 9.8N
Work done = 9.8N x 36.4m = 357Nm
Since this amount of work is done each second,
Power = work/sec = 357Nm/s or watts (W)
356/0.75 pump efficiency = 475W input required by pump
475/0.85 motor efficiency = 560W input to motor
560/220V = 2.5 amp running current, which gives:
2.5 x 2 = 5 amp starting current.
Summary of Requirements
Water System Design Features
1 Even if the home water system consists of only one tap near the house, a complementary drainage system is essential. A pit that is one metre square and a half metre deep and filled with stones or gravel should be constructed under the tap to carry off leakage and spillage without creating a muddy area.
2 Perhaps the second step in the development of a rural home water system is a solar water heater. This can be as simple as a black 208 litre oil drum installed on the roof that is refilled periodically from the tap or it can be connected permanently by a branch pipe from the watersupply line. A combination of a check valve in the supply line to the water heater and a pressure safety valve at the tank is advisable. The check valve will prevent warm water from draining back into the cold water line at a time of low pressure, but a safety valve is absolutely essential when the check valve is used to prevent excessive pressure build-up from hot water.
3 If an extensive home water system is planned, complete with toilet, shower and sinks, it is prudent to plan a good drainage system at the same time.
Soakaways are necessary for disposing of shower and sink water unless the water must be saved for irrigation or stock watering in which case a collection tank should be constructed. Waste from a toilet is best treated in a septic tank and the effluent allowed to soak away in a pit or drainage field. These systems will be discussed later.
4 Pipe materials for cold water may be either plastic (PVC or high density polythene) or galvanized steel. The steel is more expensive and difficult to work with, but it is not easily damaged. Galvanized pipe has a relatively short life when exposed to acid water, but lasts very well when the water is neutral or slightly alkaline.
5 Twenty to 25mm pipe should be used as a main supply line, but 13mm will be adequate for branches to sinks, shower and water closet. Each branch should have a shut off valve to facilitate repair work.
6 Tropical areas are normally an ideal place in which to make use of solar water heating. Two square metres of properly positioned collector area should heat 20 litres or more to 45-50°C on most sunny days. Solar heaters are discussed in greater detail in Chapter 7.
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