A drip irrigation system consists essentially of mainline, sub mains, lateral, drippers, filters and other small fittings and accessories like valves, pressure regulators, pressure gauge, fertilizer application components etc.
It is the heart of drip irrigation. A filter unit cleans the suspended impurities in the irrigation water so as to prevent blockage of holes and passage of drip nozzles. The type of filtration needed depends on water quality and emitter type. A two-stage filter unit is usually needed.
a) Gravel Filter (Sand Filter):
These filters are effective against inorganic suspended solids, biological substances and other organic materials. This type of filter is essential for open reservoir, when algae growth take place. The dirt is stopped and accumulated inside the media in the filter. Gravel filter consist of small basalt gravel or sand (usually 1-2 mm dia) placed in cylindrical tank, made of metal. Water enters form the top and flows through the gravel while leaving the dirt in the filter. The clean water is discharge at the bottom. The filter is cleaned by reversing the direction of flow. Pressure gauges are fitted at the inlet and cutlet of the filter. When the dire accumulates, the pressure difference between the inlet and outlet increase and when the pressure difference is more than 0.5 to 1.o kg/ cm2 ( 5-10 m) , then filters must be cleaned by opening the clover or back washing, Automatic self cleaning filter are also available.
The flow rate of the filters may be 10,15,20,25,30,40,50 cu/m/hr and the tank diameter may range from 10-50 cm depending on the capacity of the system.
b) Screen Filter:
These are installed with or without gravel filter, depending upon quality of water. The screens are usually cylindrical shape and are made of non-corrosive metal or plastic material. Screens filters are specified as below:
1. By the diameter of inlet and outlet (range from ¾ “to 4” inches)
2. By the recommended range of flow rate (ranges from 3, 5,7,10,15,20,30, 40, cu. m/hr).
3. By the size of holes in the screen (in mm, micron or in mesh i.e. the number of holes per square inch). As a approximation, 20, 40,80, 100,120,150 and 200 mesh ( 0.15, 0.1 and 0.08 mm) respectively. The most common mesh selected for drip irrigation is 100 to 200 meshes (0.15 to 0.08 mm dia).
4. By the total surface area of the filter ( in sq. m) or the active or net filter area, which is usually about 1/3 of the total filter surface are.
5. By the cleaning methods: manual or automatic. The head loss across the filter should not be more than 3 m and otherwise needs cleaning. The filters are cleaned by flushing the screen with a stream of water. After cleaning the screen is checked for tears and the gasket should be checked and replaced when necessary.
6. With relatively clean water, screen filters can be used alone.
c) Disc Filter:
The filtration elements are grooved plastic disc, which are piled together around a telescopic core, acceding to the desired degree of filtration. Both sides of the discs are grooved and the grooves cross each other when piled up and frightened together. The housing is made of plastic or metal and comes in many different sizes mainly 3 ¾ to 3. The water passes through the filter from the outside to the inside. There is no danger of filter tearing. The filtration is affected in two stages: the larger outer surface operates as a screen filters and collects the larger particles. The grooves inside the disc allow the adhesion of fine particles, mainly organic matter. The filter element can be cleaned easily. When opening the core, the discs are released and can easily be rinsed under running water. The pressure drop is slightly higher than screen filter but disc filters have better cleaning capacity than screen filter. The water flow should be on a tangent to the disc to allow them to spin freely.
2. Main Line:
The main line conveys the water from filtration system to the sub main. They are normally made of rigid PVC pipes in order to minimize corrosion and clogging. Usually they are placed below the ground i.e. 60 to 90 cm ( 2 to 3 ft) , so that they will not interfere with cultivation practices. Their diameter is based on the system flow capacity. The velocity of flow in mains should not be greater than 1.5 m/s and the frictional head loss should be less than 5ml /1000 m running length of pipeline.
The Submain conveys the water mainline to the laterals. They are also buried in ground below 2 to 2.5 ft and made of rigid PVC. The diameter of Submain is usually smaller than main line. There may be number of Submain from one mainline depending upon the plot size and crop type.
Laterals are small diameter flexible pipes or tubing made of low density polyethylene (LDP) or liner low density polyethylene (LLDPE) and of 12 mm, 16mm, and 20 mm size. Their colour is black to avoid the algae growth and effect of ultra- violet radiation. They can withstand the maximum pressure of 2.5 to 4 kg/cm2. They are connected to Submain at predetermined distance. The pressure variation between two extreme points of lateral should not be more than 15-20 % and discharge variation should not be more than 10%. On slopping ground, the laterals are placed along the contour with 1% extra length for sagging purpose.
5. Emitters or Drippers:
It is the main component of Drip irrigation system for discharging water from lateral to the soil. i.e. to the plants. There are various types and size of drippers, based on different operating principles. They are made of plastic, such as polythene or polypropylene. Their discharge range is between 1-15 ph. Each dripper has it’ s own characteristics, advantages and disadvantages which determines its use.
The drippers can be classified according to working principle, discharge, type, structure, working pressure, designation, durability, regulated and non regulated discharge.
The main principle when planning a dripper is to achieve the minimum discharge with maximum size of water passage. The large water passage is essential to minimize clogging and provide the minimum discharge for cheapest set-up. Therefore, an emitter is necessary, ( a hole in a pipe is not a dripper). Emitters may be on the lateral or inside to lateral, accordingly they are called on line or inline emitters.
6. Controls Valves (Ball Valves):
These are used to control the flow through particular pipes. Generally, they are installed on filtration system, mainline, and on all Submain. They are made up of gunmetal, PVC cast iron and their size ranges from ½” to more than 5”.
7. Flush Valve:
It is provided at the end of each sub main to flush out the water and dirt’s.
8. Air Release Cum Vacuum Breaker Valve:
It is provided at the highest point in the main line to release the entrapped air during the start of the system and to break the vacuum during shut off. It is also provided on Submain if Submain length is more.
9. Non Return Valve:
It is used to prevent the damage of pump from flow of water hammer in rising main line.
10. Pressure Gauge:
It is used to indicate the operating pressure of the drip system.
11. Gromate and Take-off:
These are used to connect the lateral to Submain. A hole is punched with hand drill of predetermined size in Submain. Gromate is fixed into the hole. Take off is pressed into the hole. Take off is pressed into the gromate with take of punch upto the step provided. Gromate acts as a seal. The sizes are different for 12 mm, 16mm, and 20 mm lateral.
12. End Caps (End Sets):
They are used to close the lateral ends, Submain ends or mainline ends. Sub mains and mains are preferably provided with flush valve. They are convenient for flushing the line.
13. Fertilizing System:
It is used to add the chemical irrigation water; however, fertigation is not free of hazards. Chemicals added to water may be toxic human begins and animals so, safeguard must be taken to prevent back flow of irrigation water into the water source, which might be used for drinks purpose. Only water-soluble fertilizers should be used to minimize the clogging hazard.