4. THE IRRIGATION SYSTEM 1. Introduction
5.2. Pipe materials
5.2.1. Iron, Steel and Copper
Cast iron was in extensive use in the early years of pressurized irrigation. Due to its vulnerability to corrosion, its life expectancy time-length was relatively short. Pitting and rust accumulation on the inner wall surface decreased the wall smoothness and increased friction head losses in the flowing liquid. Ductile cast iron had improved pipe performance but corrosion remained a severe disturbance. Carbon steel further improved pipe functioning. Later on, in order to avoid corrosion, the inner wall surface was coated by reinforced cement that kept wall smoothness and prevented corrosion. Steel pipes are expensive and are used mainly in water supply networks.
In underground installed pipes, the outer wall surface is coated with asphalt, to avoid damage to the outer surface by corrosive elements in the soil.
Stainless steel is too expensive to be used in irrigation and water supply networks.
Copper is used mainly in residential and industrial plumbing. In irrigation systems, copper small diameter tubes are utilized in hydraulic control devices of automated systems.
Galvanized tin pipes were used in the past in hand-move irrigation but were replaced by aluminum pipes.
5.2.2. Aluminum
The use of aluminum pipes for irrigation commenced after WWII. They replaced the galvanized tin pipes used in hand-move irrigation. Their low weight and sturdiness eased the operation of hand-move systems. High quality pipes were manufactured from corrosion resistant aluminum known as alclad aluminum. Aluminum pipes are manufactured in two technologies. In one of them, pipes are produced from aluminum flat sheets that are rounded and welded. The other technology is extrusion, in which seamless pipes are produced.
5.2.3. Asbestos-cement
Pipes made of asbestos-cement were in wide-scale use in water supply networks and as mains for irrigation. The use of these pipes is now prohibited because it was found that asbestos fibers are carcinogenic, endanger workers in the production line and may disintegrate by some chemicals inherent in the water and endanger the users.
5.2.4. Concrete
Concrete pipes are used mainly in drainage and sewage systems.
5.2.5. Plastic Materials
5.2.5.1. Overview
After the introduction of aluminum pipes that were used mainly in hand-move irrigation, began the production of pipes made from plastic materials for use as replacement of galvanized iron pipes as laterals and single emitter extensions as well as replacement to carbon-steel pipes in water supply networks and as distributing mains, sub-mains and manifolds in the irrigated parcels.
Plastics are solid materials comprised of one or more polymeric substances that can be shaped by molding or extrusion. Polymers, the basic ingredient of plastic materials, are a broad class of materials that include natural and synthetic substances. In professional terminology, polymers are frequently defined as resins.
For example, a PolyEthylene (PE) pipe compound consists of PE resin combined with colorants, stabilizers, anti-oxidants and other ingredients required to protect and enhance the quality of the material during the fabrication process and the operation in the field.
Plastic materials are divided into two basic groups: thermoplastics and thermosets, both of which are used for the production of plastic pipes.
Thermoplastics include PolyEthylene (PE), PolyPropylene (PP), PolyButylene and PolyVynil Cloride (PVC). These materials can be re-melted by heat. The solid state of thermoplastic materials is the result of physical forces that immobilize polymer chains and inhibit them from slipping past each other. When heat is applied, these forces weaken and allow the material to soften or melt. Upon cooling, the molecular chains stop slipping and are held firmly against each other in the solid state. Thermoplastics can be shaped during the molten phase of the resin and therefore can be extruded or molded into a variety of shapes, such as pipes, flanges, valves, sprinklers and micro-sprinklers components, drippers and other accessories.
Thermoset plastic materials are similar to thermoplastics prior to a chemical reaction (“curing”) by which the polymer chains are chemically bonded to each other by new
cross-links. That is usually performed during or right after shaping of the final product.
Cross-linking is the random bonding of molecules to each other to form a giant three-dimensional association. Thermoset resins form a permanent insoluble and infusible shape after applying heat or a curing agent. They cannot be re-melted after shaping and curing. This is the main difference between thermosets and thermoplastics. As heat is applied to a thermoset component, degradation occurs at a temperature lower than the melting point. Thermosetting resins can be combined with reinforcements to form strong composites. Fiberglass is the most popular reinforcement and fiberglass-reinforced pipes (FRP and GRP) are a common form of thermoset-type pipes.
5.2.5.2. Polyethylene
Polyethylene (PE) is the most prevalent material in pipes and laterals in pressurized irrigation systems. There are four types of PE, classified by material density:
Type I – Low Density (LDPE), 910 – 925 g/l Type II – Medium Density (MDPE), 920 – 940 g/l Type III – High Density (HDPE), 941 – 959 g/l Type IIII – High Homo-polymer, 960 and above g/l
Two percent, by weight, of carbon black agent are added to increase pipes’
resistance to the detrimental impact of exposure to ultraviolet (UV) sun radiation.
Another classification relates to the working pressure that the pipe withstands (PN).
Common grades of PN used in irrigation are: 2.5, 4, 6, 10, 12.5 and 16 bars (atm).
Certain thin-walled laterals withstand lower PN: 0.5 – 2 bar. The pressure tolerance depends on pipe material density and wall thickness. Tolerance data published by the manufacturers relate to standard temperature of 20 C0. At higher temperatures, the tolerance decreases significantly, hence pipes are tested at twice the designated working pressure.
Plastic pipes are designated according to their external diameter, in mm. In the USA and some other countries, pipe diameter is marked in imperial inch units (“). 1” = 25.4 mm. Pipe wall thickness is designated in mm units (in the USA by mil units. Mil = 1/1000 of inch).
1 mil = 0.0254 mm.
Laterals are commonly made of LDPE (PE – 32 grade) while delivering and distributing pipes of diameters greater than 32 mm are mostly made of HDPE.
HDPE pipes are further classified according the grade of the material: PE-63, PE-80, PE-100. The higher the grade, the higher the pipe quality and pressure tolerance.
Table 5.1. PE (PolyEthylene) Pipes for Agriculture PE type ND (Nominal
Table 5.2. LDPE Pipes Internal (Inner) Diameter and Wall Thickness - mm
OD = External (Outer) Diameter. In plastic pipes, mostly equivalent to the ND.
ID = Internal (inner) Diameter
Table 5.3. HDPE Pipes Internal (Inner) Diameter and Wall Thickness - mm
Adapted form "Plastro" brochure
5.2.5.3. PVC Pipes
PVC (PolyVinyl Chloride) is a rigid polymer. Addition of plasticizers renders flexibility to tubes made of soft PVC. PVC pipes are sensitive to UV sun radiation. Soft and flexible, they are used mainly in gardening and landscape. In agriculture and water supply systems, rigid PVC pipes are mainly used for water delivery and distribution.
PVC pipes are installed only underground to avoid damage from UV radiation.
Currently, un-plasticized PVC (UPVC) pipes are manufactured with improved UV and pressure surges tolerance. PVC pipes appear in discrete 4 – 8 m long segments and have to be attached in the field. The working pressure of rigid PVC pipes is 6 – 24
Table 5.4. PVC Pipes for Agriculture When PVC pipes are installed in heavy or stony soil, it is recommended to pad the trench with sand to prevent damage to the pipe wall by swelling soil pressure and contact with sharp protrusions of stones.
Table 5.5. Internal Diameter and Wall Thickness of PVC Pipes
PN---> 60 m 80 m 100 m feeding pipes in mechanized irrigation systems. The hose is impregnated with anti-UV radiation protecting agents. When the water is shut-off, the hose lays flat on the ground and can be crossed-over by tractors and other farm machinery. The lay-flat hoses can be laid out on the soil surface or in a shallow trench. These hoses are available in diameters of 75 – 200 mm.
5.2.5.4. Fiberglass Pipes
In addition to UPVC and HDPE pipes, reinforced fiberglass pipes are used to deliver water under high pressure from the water source to the irrigated area, as a substitution for steel and asbestos-cement pipes.
GRP (Glass Reinforced Polyester) fiberglass pipes are manufactured in diameters of 300 – 3600 mm and PN grades of 40 – 250 m. They are particularly useful in delivery of reclaimed water.