INTRODUCTION
Irrigation is the watering of land by artificial methods. Without irrigation, agriculture is limited by the availability and reliability of naturally occurring water from floods or rain. Drip irrigation is widely accepted as the most efficient irrigation technique as it allows high uniformity of water and nutrient applications.
A drip irrigation system comprises many components, each one of them playing an important part in the operation of the system. The aim of this chapter is to provide an overview of the drip irrigation system components, their functions and properties.
- Water source
- Pumping station
- Air valve
- Pressure gauge
- Check valve
- Shock absorber
- Manual valve
- Main filtration unit
- Main filtration automatic drainage valve
- Water meter
- Hydraulic valve
- Secondary filtration unit
- Dosing unit
- Fertilizer tank
- Irrigation controller
- Mainline
- Sub mainline
- Distribution line
- Kinetic valve (vacuum breaker)
- Dripperline
- Flushing valve
- Flushing manifold
- Fertilizer filter
Water source
There are basically two main types of water sources: groundwater and surface water:
Many existing and potential water supply sources for irrigation systems are derived from surface water,
which does not tend to have high levels of salts (with the exception of some coastal areas), and thus
systems are usually less prone to the formation of precipitates in drippers when using a surface water source.
Surface water, however, tends to introduce biological hazards. If wastewater is being considered as a
source, quality and clogging potential will vary depending upon the extent of treatment.
Groundwater is generally of higher quality than surface water. However, iron and manganese levels should
be measured, as high levels may lead to dripper clogging, and treatment may be required.
Pumps & pumping stations
Unless the water at the source is supplied at an adequate flow rate and pressure (by municipal or other
entity supply, a pre-existing pump upstream from the irrigation system or gravitational pressure*), a pump
will be needed to push water from the source through the pipes and drippers.
Most irrigation systems include pumps as an integral part of the drip irrigation system.
*Gravitational pressure (also known as hydrostatic pressure) is the pressure at a point in a fluid at rest
due to the weight of the fluid above it. If the water source is at a higher elevation than the drippers in the
field, the elevation difference between them will determine the gravitational pressure in the system
(e.g. the water level in a tank is 5 meters above the elevation of the pump's axis, the gravitational pressure
is 5 meters = 0.5 bar = 7.25 PSI).
Selecting a pump for an irrigation system requires an understanding of the water conditions and local
system requirements.
Poor pump selection can lead to high operating costs and shortened pump life; this in turn impacts on the
performance and reliability of the whole irrigation system.
When a pump site is selected it is necessary to consider a range of factors, including availability of power,
proximity to the development site and water quality issues.
Power source for the pump
The power source for the pump will depend on the availability and accessibility of the energy resource in
the local area.
In most instances, electricity is preferred because of reduced labor requirements and higher efficiency,
resulting in lower energy costs. Three-phase power is usually required to operate over 10 horsepower (hp)
irrigation pumps.
If electricity is not available, alternative power sources such as diesel, gasoline, or solar may be used. The most common alternatives are gasoline engines for small pumps and diesel engines for larger pumps.
Pump types
In most irrigation applications, centrifugal pumps are used.
A centrifugal pump is a rotodynamic pump that adds energy to the water using a rotating impeller. It may
be either horizontal-shaft or vertical-shaft (including submersed pumps).
Horizontal pumps are more frequently used to pump water from surface sources such as ponds.
Horizontal-shaft pump
Vertical-shaft pump
Vertical-shaft submerged pump
Pump capacity
When selecting a pump, four basic
factors must be considered:
• Pump discharge (flow rate) defines the quantity of water supplied by the pump during the 1-time unit
(units: m3/hour, liter/second or gallons/hour).
• Pressure (pressure head) defines the internal energy of a fluid due to the pressure
exerted on its container's walls (also known as static pressure head or static head)
(units: bar or psi. 1 bar = 14.5 psi).
• Net Positive Suction Head (NPSH) is the required head value (suction lift) at the inlet of a horizontal pump enabling it to pull water upwards while keeping the water from cavitating* (inherently limited to 0.8 barnets).
*Cavitation - The formation of vapor cavities ("bubbles" or "voids") in a liquid.
It usually occurs when a liquid is subjected to rapid changes of pressure that
cause the formation of cavities where the pressure is relatively low. When
subjected to higher pressure, the voids implode and can generate an intense
shockwave causing significant damage to the pump's impeller and chamber.
• Friction head - Head loss caused by the friction between the fluid and the inner walls of the shaft the enclosure of a vertical pump (or in the outlet pipe of a vertical submerged pump) which pulls the water upwards. Friction loss increases with run length and by the square of the fluid velocity.
It affects the required pressure and flow rate.
The output pressure of a pump is dependent on pressure head and flow rate (a higher flow rate causes a
lower pressure and vice versa, all other variables being unchanged).
Make sure the pump is able to deliver an adequate flow rate and pressure for the application. Obtain a
performance curve for the pump and have modifications made if it is not adequate - the energy savings
alone will easily pay for any upgrades required, which will also improve system operation and crop
production, resulting in a shorter ROI.
Pump selection
The irrigation system design will specify the required pump duty (flow rate and pressure head).
The best pump choice is the pump in which the Best Operating Point (BOP) occurs at this flow rate and
pressure head and that can operate at the available suction head.
The pump's performance curve
Each pump must be supplied with its performance curve, as an integral part of the product and the
supplier/manufacturer must commit to the data presented in it.
It is very important to keep the pump data documentation available for the whole lifetime of the pump.
The performance curve of the pump (flow rate/pressure range) is indispensable for the design and the
construction of the entire irrigation system.
The pump outlet pressure is related to the discharge rate. A change in the flow rate will cause a change in the
working pressure. Changes in the flow rate and pressure may be critical when considering the relationship
between the flow rate, the working pressure and the pump's efficiency curve in the planning process.
The steeper the pump's operating curve, the more a change in flow rate will affect the working pressure.
Filtration
Filtration is critical in any drip irrigation system. Effective filtration is essential for proper irrigation system
operation and long-term performance, as it prevents the irrigation water from clogging the drippers.
Water quality
The concept "water quality" relates to the variety and concentration of the dissolved and suspended
components in the water.
Water requirements for drip irrigation
The quality of water for irrigation relates to the parameters required to maintain the crop's health and the integrity of the irrigation system. Every type of pressurized irrigation system requires attention to the
water quality to avoid clogging of the irrigation components in order to enable orderly long-term irrigation
according to the irrigation program.
Water quality will dictate filtration requirements, chemical injection requirements, and management of the
irrigation systems to prevent dripper clogging.
Causes of dripper clogging in systems may be chemical (precipitates or scale), physical (grit or particulates
such as sand and sediment) or biological (such as algae or bacteria).
The water’s chemical characteristics are influenced by the variety and concentration of the substances
dissolved in it. These dissolved substances include ions of dissolved salts such as chloride, sodium, and
nutrients (nitrogen, phosphorus, potassium, and others). Calcium and magnesium influence the hardness
of the water, iron, and manganese are liable to be found either dissolved or as a residue, along with other
dissolved organic compounds and even poisonous substances.
The biological characteristics of the water quality include a variety of living organisms such as microorganisms, including bacteria, viruses, single-celled entities, algae and zooplankton, which develop in open
water along with creatures developing within the water transport system itself.
The water quality is expressed by the physical conditions and the variety and concentration of its
constituents.
Types of filters
The types of filters used most often in drip irrigation systems are:
Media filters (gravel or sand) are necessary for any surface water
source and especially so for wastewater. They consist of a metal or
plastic enclosure incorporating small gravel stones or sand, which
traps the dirt. This filter includes a flushing system for washing the
gravel or sand and returning the dirt to the water source.
ATTENTION
It is highly recommended to install a screen filter downstream
the media filter in order to prevent infiltration of the filter medium
into the system in the event of a malfunction of the media filter.
Disk filters are used with surface water systems, wells or
municipal water sources. These filters are comprised of a series
of grooved plastic disks stacked together with a total equivalent
screen size ranging from 40 to 400 mesh.
These filters enable deep three-dimensional filtering (e.g. allow
entrapping of more particles as water passes through the pores
created by the grooves in the surfaces of the filtering disks stacked
together in the filter).
Having more surface area than screen filters, disk filters are better
suited for higher flow rates.
Screen filters are used mainly as secondary filters with surface
water systems or as primary filters with well or municipal water
sources. A screen filter is comprised of a cylinder with a net that
traps the dirt. This filter is intended for relatively clean water; its
use is less common with water from a reservoir or pumped water.
ATTENTION
In any type of filter, the dirt returned to the water source
should be discharged as far as possible from the suction
point. In a streaming source (e.g. a river) the discharge the point should be downstream from the suction point.
Hydrocyclone sand separators are used as a preliminary stage of
filtration in the presence of sand or other heavy particles (50 microns or
bigger) in the source water. It utilizes centrifugal force to separate the
particles from the water. The separated material drops down into a
tank or reservoir where it can be removed later.
It is not a true filter since there is no physical barrier to separate out
the particles, but it is often used before a filter to first remove the
bulk of the contaminant, where the filter does the final cleaning. This
type of design reduces the time required to flush and clean the main
filter. Each hydro cyclone model has its specific operation flow rate
range, it will not perform outside this range.
REFERENCE
OTHER TYPE OF IRRIGATION
