Various types of pipes are used in irrigation systems. The type of pipe used will depend on cost, working pressure, type of system, friction characteristics, durability, fittings used and internal pipe diameter.

Types of pipe available include:

• Black steel

• Galvanised steel

• Fibre-resin

• Special composite

• Spun concrete

• Asbestos-cement

• mPVC and uPVC (microcellular polyvinyl chloride and unplasticised polyvinyl chloride) • HDPE (high-density polyethylene)

• LDPE and LLDPE (low-density polyethylene and linear low-density polyethylene) • Polycop and copper tubing

• Lay-flat piping (various materials)

• Special application pipes using:

◦ rubber

◦ silicon

◦ polypropylene

◦ other

Portable laterals may be made up of:

• Aluminium

• Light-weight galvanised steel

• HDPE

• or other pipes

Drainage pipes could consist of:

• Slotted PVC

• No-fines concrete

• Ribbed PVC

• Pitch-fibre

• Composite drains

• and many more

The system designer will select pipes that are cost-effective and suitable for use in the system. The farmer, who must then operate and maintain the system, should know the type and class of pipes used in his system and how to repair and maintain the pipeline.

The inner surface of any pipe is always slightly rough. Each particle of water that passes over this surface will be deflected away in such a way that it obstructs the flow of other particles closer to the middle of the pipe. If these particles of water move faster (more water has to pass through the pipe) they will cause a larger frictional resistance.

Consider a series of simple garden sprayers attached to a hose system fitted with shut-off valves at each clip-in (Gardena type) connection.

If two sprayers work, a certain spray pattern can be observed. As soon as a third sprayer is added to the system, the area covered by each sprayer is reduced visibility. The same effect can be seen if you increase the length of a common supply pipe. As the length of the pipeline increases, so does the total friction loss in the pipe. Friction occurs in all pipes that convey water.

Fittings

Each type of pipe used has its own set of fittings. Fittings for a steel pipe can not be used on a uPVC pipeline. Glued plastic sleeve fittings used on uPVC can, however, not easily be confused with threaded fittings used on steel pipes. On some types of pipe, the type of fitting that should be used is not always this clear. Asbestos-cement pipes are not really suitable for use in irrigation systems. At some time in the past, these pipes were cheaper than PVC or uPVC pipes and they were often used in sprinkler systems where large diameter discharge nozzles were fitted to the sprinklers.

Many of these older systems have been upgraded and converted. In all cases where asbestos-cement main line pipes are used, filters must be installed at the offtakes to irrigation blocks. The linings of asbestos-cement pipes deteriorate with age. PVC pipes that are exposed to the sun for long periods will become brittle and break easily. PVC pipes cannot be used above ground.

The protection of portable pipes against accidental damage is important. It is possible to remove dents from portable pipes, but the process is difficult and expensive. Each time light-weight steel pipes are repaired, galvanising is lost. Repaired pipes will rust and perish quicker. There is a limit to the number of times that dents in a pipe can be repaired. Welding and braising of holes will only extend the life of a pipe for a limited time. When not in use, portable pipes should be stored out of the way of machinery, and on racks, where they cannot be damaged. Similar precautions must be taken with dragline-mounted sprinklers and any other portable irrigation equipment.

For each type of equipment available, the manufacturer or supplier will recommend a range of valves and other control mechanisms. It is important to remember that fittings are often designed specifically for use with one type of equipment and it is not advisable to change or mix fitting types. Use the advice and guidelines given by manufacturers and suppliers.

This series is published with acknowledgement to the ARC Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.