Underground pipeline systems (also known as buried pipelines) are being increasingly used for conveying irrigation water on the farm. The main advantages of these systems are – saving of land, elimination of seepage losses, and relatively little maintenance need. Under most conditions a properly installed pipeline system will function well for several years. However, they need a higher initial cost as compared to open channels.
Classification of the Pipeline Systems:
Buried pipeline systems may be classified depending on the working pressure as:
(i) Low pressure systems (less than 10 m),
(ii) Medium pressure systems (10 to 20 m), and
(iii) High pressure systems (more than 20 m).
The low pressure systems are used for water conveyance while the medium pressure ones are used with drip systems and the high pressures ones with sprinkler systems. Low pressure pipeline systems can be classified on the method of pressure control into closed, semi-closed and open systems, and on the method of providing the hydraulic head into gravity, pumped or mixed systems. These are shown in Fig. 14.15 (a), (b), (c) and (d).
The open pipe systems are used in sloping areas and arrangements have to be made to dissipate excess pressure heads. In the semi-closed systems float valves are used for dissipating the excess pressure heads. The closed pipe system does not require dissipation of excess head and the entire pipeline is hydraulically connected. These are the systems which are widely adopted now in flat agricultural lands.
The head required to supply water to different points in the system may be provided by gravity alone or by pump or in some situations partly by gravity and partly by pump.
Materials for Pipelines:
Concrete, vitrified clay, rigid P.V.C and mild steel pipes are the materials used for underground pipelines. Rigid P.V.C pipes are also known as uPVC (un-plasticized polyvinylchloride) pipes. Among these, concrete and PVC pipes are most commonly used.
The chemical conditions of the soil are to be taken into consideration while selecting the pipes. If sulphates are present, concrete is affected by the same. In such cases special cement is used in making the pipes. Chlorides in the soil affect steel pipes. Clay and P.V.C. pipes are not affected by any of the chemicals.
Concrete pipes used for farm irrigation systems are mainly classified as reinforced (R.C.C.), and non-reinforced (N.R.C.C.) pipes. Reinforced pipes are used for operating heads higher than 6 m (0.6 kg/sq.cm). These are especially required when the system is used with sprinkler irrigation or for large and undulating areas where the operating heads are higher than 6 m.
The general sizes of the R.C.C. pipes available are 15 cm, 22.5 cm, 30 cm, 37.5 cm and 45 cm diameter. The lengths vary from 2 to 2.5 m. The usual concrete mixes adopted are 1 : 1.5 : 3, 1 : 2 : 2, 1 : 2 : 2.5, and 1:2:4 (cement : sand : gravel).
The recommended pipe pressures are given in Table 14.4:
Design of the Underground Pipeline System:
The design of the pipeline system consists of the following:
1. Selection of the type of the system.
2. Selection of the material of the pipeline.
3. Design of the pipe diameter.
4. Design of the ancillary structures for pipeline protection and water distribution.
The selection of the type of the system is done keeping in view the site conditions. A topographical map of the area, the location of the source and rate of flow available are required. Selection of the pipe material is made depending on availability, soil conditions and cost considerations.
Design Velocities:
Recommended maximum velocities in low pressure pipelines are in the range of 1.3 to 1.5 m/s. Higher velocities reduce the diameter of the pipe and hence the cost but result in higher frictional losses and higher cost of water hammer protection. Minimum flow velocities should be around 0.5 m/s in order to prevent sedimentation of fine sands.
Diameter of the Pipeline:
The diameter of the pipeline is determined taking into consideration the rate of flow and the frictional losses in the pipeline and the ancilliary structures.
The frictional losses in the pipeline are given by what is known as Darcy-Weisbach equation.
Consider a uniform horizontal pipe with cross-sectional area A through which water is flowing at velocity V. Let P1 and P2 the pressures at two points at distance L (Fig. 14.16). If f’ indicates the frictional resistance per unit area at unit velocity, the total frictional resistance over the length L is given by –
Eq. (14.16) is known as Darcy-Weisbach equation. In this equation, f is a dimensionsless coefficient known as friction factor, and y is unit weight of water.
The pipe friction factor (f) depends on the Reynolds number (Re) of the flow and the roughness of the inside of the pipe. Reynolds number, Re which is dimensionless is given by–
Under operational conditions, pipe flow is usually turbulent, and friction factors are provided in the form of tables in the handbooks of hydraulics.
Minor Losses:
In general for pipe system, head losses due to bends and valves comprise only 5 to 10% of total pipe friction losses and are frequently referred to as minor losses.
Head losses are usually expressed in terms of velocity head –
Design of the Pump Stand:
The purpose of the pump stand is to provide the necessary hydraulic head for the flow of water in the pipeline. Depending on the topography of the land where the system is to be laid, the pipeline might have a downward slope or upward slope (Fig. 14.15). The height of water in the pump stand is calculated as follows –
Losses in the ancillaries consist of the head losses at pipe entrances, bends, gate valves and risers. The losses are approximated as equal to –
Where, K1, K2, …Kn are coefficient for each item where hydraulic head loss occurs. The coefficients are 0.5 for pipe flush with wall, 0.1 for bell entrance, 1.0 for bends etc. The pressure required to operate a gated pipe is added in terms of head of water.
A small free board about 0.25 to 0.5 m is added to get the height of the pump stand. The hydrostatic pressure occurring at the lowest point in the delivery system should be calculated and it should be within the safe limits for the type of pipes used in the system.
The diameter of the pump stand is kept larger than the diameter of the pipeline in order to dissipate the velocity of water and release entrapped air before the water enters the pipeline. A minimum diameter of 90 cm is generally kept to make the mortar joint from the inside of the stand.
For uniform grades, knowing the length of the pipeline to be laid and the differences in elevations at the starting and end points, the upward or downward slope (S0) can be determined.
The gradient of the hydraulic head (S0) is calculated from the height of water in the pump stand and the length of run of the pipeline. Provision has to be made to take care of losses at the bends, gate valves and risers. This can be calculated using standard tables.
The calculated frictional loss should be less than the values of Sf calculated using Eqs. 14.21 or 14.22. If not the head of water in the pump stand should be increased or the diameter of the pipeline increased, whichever is practical and economical for a given set of conditions.
Example:
A concrete pipeline is due to carry water across a distance of 450 m from a pump which discharges 100 lps. The land has a down field slope of 0.1 per cent along the line the pipes are laid. What size of pipe to be used. It is desired that the maximum pressure in the pipe line should not exceed 5 m of water.
Solution:
Considering 30 cm dia concrete pipe, Sf= .01, less than the previous value. Hence 30 cm dia pipe is satisfactory.
Surge and Water Hammer Protection:
Surge and water hammer problems in low pressure pipelines are caused by the entrapped air in the pipeline. Surge is any transient pressure fluctuation and could cause shock waves resulting in damage to the pipeline.
Water hammer in pipeline systems could occur as a result of:
(i) Sudden stoppage of pump,
(ii) Sudden release of air, or
(iii) Sudden valve closure.
Pipelines are protected from surge and water hammer problems by providing air vents. In large closed loop systems surge towers may be required. Further maximum possible pressures due to sudden stoppage of the pump can be calculated and these should be within the permissible pressures for the pipelines and joints.
Air Vents:
Air vents are vertical pipe structures to release air entrapped in the pipeline and to prevent development of vacuum. Entrapped air must be removed to permit an even flow and to avoid the danger of water hammer.
Air vents are installed near the pump stand, at all high points in the line, at sharp turns, at points where there is a downward deflection of more than 10° and towards the end of the pipeline.
The first air vent is located near the pump stand at a point where the design velocity exceeds 30 cm per second. The height of the vent at a point is calculated taking into consideration the frictional losses from the pump stand to the air vent. Free board of the order of 0.25 m is provided.
Ancillary Structures and Devices:
Apart from the main components of the pipeline system, other structures and devices are used to control the water and protect the pipeline damage.
Some of these are as follows:
1. Inlet Structure:
The pump stand is an inlet structure used when water is pumped from the source. If the water enters the pipeline from an open channel, a gravity inlet as shown in Fig. 14.17 is used. Provisions for removal of sand by means of sand traps and debris by means of screens are made as per the needs of the situation.
2. Gate Stands:
Those are used to control the flow of water into branches of the pipeline system. Slide gates as shown in Fig. 14.18 or sluice valves are used for controlling the flow. Gate valves are also useful to increase the of the gate stands.
3. Overflow Stands:
Overflow stands are constructed when the pipeline is installed on steep slopes. Fig. 14.19 indicates the functioning of the overflow stands. These serve both as check and drop structure and also help in controlling the pressure in the lower reaches of the pipe. To prevent air entrainment by the falling water which may cause surges in the pipeline, air release vents are installed directly below the overflow stands.
4. Float Valve Stand:
This is an open stand structure used in semi-closed pipe systems. This houses a float valve which permits reduction in the pipe head or pressure while connecting, hydraulically, the upstream and downstream sections.
5. End Plug:
End plug is installed at the end of the pipeline to close the pipe and absorb the pressure developed in the pipeline. The end plug is usually a concrete block cast in-situ (Fig. 14.20).
6. Outlets:
These are needed to deliver the water from the pipeline
system to the fields. The outlets consist of a riser pipe joined to the main pipeline vertically. At the end of the riser pipe near the ground level a valve is fitted.
Opening and closing of the valve control the flow of water. The diameter of the riser pipe is kept the same as the pipeline system where the entire flow of the pipe line is to be released through the valve.
In larger size pipelines, the size of the riser pipe is smaller than the main pipeline. Table 14.5 presents the discharge capacities of different diameters of risers used in low pressure pipelines commonly used on farms.
Riser pipes are installed on the pipeline by making an opening on the upper surface of the pipe and permanently attaching with cement mortar grout. The top of the riser is placed lower than the ground surface in order to prevent erosion of soil when the water is flowing through the riser.
The outlet valve (also known as alfalfa valve) is attached to the upper end of riser. The valve consists of a screw shaft to which a handle and a cap plate are attached. The screw is turned to close the valve. When the valve is closed, the cap plate fits on the circular edge making the joint watertight. The screw shaft is usually made of brass to make it rust resistant. Opening of the valve will allow the water to flow into the adjoining area.
When the water is to be conveyed through a portable pipe system on the surface or when gated pipes are to be used (especially with furrow irrigation) devices known as hydrants are used. The hydrants are portable and are made of cast aluminium or G.I. sheets.
They can be moved from outlet to outlet and can be fixed over them and connected to the portable pipeline system. Hydrants have a double screw arrangement such that one screw permits forming a tight seal between the hydrant and the top of the riser pipe, and the other screw is used for opening the riser valve.
Hydrants may have single or double outlets. The gated pipes connected to the hydrants are portable aluminium pipes with sliding gates. These gates help in regulating the flow of water into the furrows or to the border strips.
Installation of the Pipeline:
After the design and layout plans are prepared, installation of the pipeline system is undertaken. The installation consists of excavating the trench to the proper depth, laying the pipeline to the required grade, joining the pipes, installing the outlets and back filling the trenches.
In order to prevent damage to concrete pipelines from loads from the ground surface, the pipes are laid with their upper surface at a depth of about 60 cm from the ground level. At no place the pipeline should have a depth less than 45 cm from the ground level.
After the excavation of the trench, the bottom part is well finished so that the pipes are laid on firm, compact soil. A Dumpy level is used to obtain the required grade for the bottom of the trench.
The pipes are lowered into the trench carefully. Pipes with bell ends are laid such that the bell ends face upstream. To join the pipes, jute or hemp rope dipped in cement paste is wrapped round the plane end of the pipe and inserted in the socket.
The rope is rammed tight and the remaining space in the socket is filled with 1 : 2 mix cement mortar and finished with a bead on the outside. To join the concrete pipe with collars the following procedure is used.
A collar is slipped over the adjacent pipe and the pipes are placed above 1 cm apart. A rope dipped in hot bitumen is placed in the recessed end of the pipe and the pipes are pressed together using a screw jack.
The collar is now slipped over the squeezed joint so that half the collar width covers each side of the joint. To maintain uniform clearance between the joint and the collar, wooden battens are placed between the pipe and the collar.
After cleaning the pipe and the collar parts with a wet brush, the gap is filled with a dry mixture of cement and sand mixed in the ratio of 1 : 1. The outlet edges are now sealed with a jute or hemp rope dipped in a 1 : 1 wet cement sand mixture.
After pressing in the rope, rest of the gap is sealed with thick cement plaster (1:2 ratios) and bevelled off at angle of 45°. Installation of the risers is done by providing a hole in the upper surface of the pipe and joining the riser portion with cement mortar.