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Design criteria are based upon general the peak crop-water requirements for different crops in different areas. The designer makes the final recommendations. The first step in the design process is to determine the block size.  A drawing of block layouts is a prerequisite for all quotations and the information included by way of example in the standard system specification, tabled below, is also a requirement:

If a sprinkler is blocked or partially blocked, poor distribution and lost of yield may result.

A typical layout of Floppy sprinklers can be seen in the Modular Design layouts. Sprinklers are typically spaced in a 12 x 15 meters odd triangular pattern or a 12 x 12 meter square pattern. In other words, sprinklers are spaced 15 meters on the laterals, while the laterals are spaced 12 meters apart. This spacing only applies to sprinkler heights of more than three meters with a clearance of two meters from crop canopy to sprinkler.

Note: It is important not to place sprinklers more than 4 meters from the boundary of a field. This will eliminate the formation of dry areas on the edges of the field. In areas where strong prevailing winds occur, sprinklers should be moved to the boundary of the windward side of the field.

The hydraulic design is splitted into:

• Laterals
• Sub-Mainlines
• Mainlines
• Pumps

The hydraulic design and the structure’s design work closely together.

The recommended maximum number of sprinklers on a 25mm pipe (26.5mm – internal diameter) is 5. This number keeps friction loss over the lateral to 5 meters or 5 Bar (50kPa). The recommended maximum spacing between sprinklers is 15 meters, resulting in a maximum span of 75 meters. The weight of the water and the pipe at 0.73kg per meter; gives an evenly distributed load of 55kg.

If the number of sprinklers is increased to 6, the friction will increase to 8 meters. A 32mm pipe is required if the friction loss is to be kept to an acceptable 5 meters. The load on the structure is very nearly doubled using 32mm pipe, therefore a 25mm pipe and a maximum span of 75 meters is recommended.

Graph 7.1 - indicates the friction loss in lateral pipes with differing numbers of sprinklers on a line.

Sub-mainlines are designed as per standard poly plot calculations. The following graphs are a quick reference; provided the Sub-mainline is supplying laterals with 5 sprinklers. The first graph applies to a sub-mainline which supplies laterals to one side only and the second graph to either side of a sub-mainline.

Standard PVC Pipe diameters:

Properly designed irrigation systems will have mainlines with friction lower than 1% or 1 meter of friction per 100 meters. In small systems with short mainlines friction can be higher.

The following table indicates the flow rates through PVC class 6 poly pipes, with frictions of 1, 2, 3 and 4 meters per 100 meters, the approximate flow velocities are indicated in different colours. High flow velocities must be used with caution as water hammer action might damage the line if valves are opened and/or closed too quickly.

Flow Rate m³/h

When specifying pumps it is important to include flow rate, pressure and power required. Remember to include the safety factors as per South African Irrigation Institute norms: flow rate + 5% and pressure + 10%. The pump's required pressure is the sum of all the component pressures, including the safety factor.

As an example:

The pump's required flow rate is the number of sprinklers per shift multiplied by the flow rate per sprinkler, including the safety factor.

As an example:

The power required by the pump using the data from the examples shown above.

*As a rule of thumb: take 0.777m³ per sprinkler.

Formula for calculation of Pump Power Requirements:

As a rule of thumb: 1kW will deliver between 4 and 5m³ of water per hour at about 5 Bar or 500kPa.

The Cable Structure Design are devided into:

• Poles:  Length & diameter
• Cables
• Anchors & Anchor cables
• Distance between anchor poles
• Tensioning of polyethylene lateral pipes

Before planting poles in their appropriate holes in the ground, screw the M12#90 couch screws and washers into pre-drilled 8mm holes, 100mm from the top of the poles. Make sure that the screws in the anchor poles face towards the inside of the field, and those in the support poles are at 90° to the direction of the cable before starting to backfill.

Backfilling of holes should be done in layers of not more than 150mm between compactions as soil does not compact very well if the layers are much more than this. Soil that is too wet does not compact well either, it should have a moisture content of not more than 15%.

The deflection is a function of the weight on the cable, the span of the cable and the tension of the cable. A maximum deflection of 0.9% per 100 meters is recommended. This will result in a 0.7 meter deflection on a 75 meter span. The most important factor influencing deflection is that the tension should remain in the cable after the cable has been secured to the poles. A small movement of 50mm can result in reducing the tension in the cable by several hundred kilograms.

The recommended cable size and the maximum tension are:

Fig. 8.2 Cable size

• Calculated at a 100% safety margin.

By using the formula and tables below it is possible to calculate the deflection and determine the minimum ground clearance.

25mm LATERAL DEFLECTION:   6 MM CABLE TENSIONED TO 1300 KG

Securing the cable:

After tensioning the cable, it must be secured to limit backward movement.  The following securing method is recommended:

Vine line grips / Guy grips

Anchors and anchor cables are fundamental to the success of a Floppy Sprinkler Overhead Cable system and it is crucial to ensure than once installed they do not move. 

A very small movement of the anchor or a slight slackening of the anchor cable can cause several hundred kilograms of tension in the support cable to be lost; there will be a large increase in the deflection of the cable and a resultant decrease in the clearance beneath the system. The different types of anchors rely on solid undisturbed soil, a meter or more below the earth’s surface to counteract the upward pull caused by the tension in the support cable.

The main types of anchors used in Floppy Overhead Cable system are:

Concrete blocks either pre-cast or cast in-situ, with an eye made of 12mm mild steel rod cast into them and having dimensions of approximately 500mm x 500mm x 500mm. These should be placed 1 meter deep and care must be taken to accurately align the eye with the row of poles the support cable stretches over.

Timber logs between 1.5 and 2 meters long with a diameter of at least 150mm may be used. Anchor rods made from 12mm mild steel with an eye on one end and a loop of 170mm on the other end, both welded closed are used to attach them to the vine line grips at the ends of the anchor cables. They are buried 1 meter deep in a trench as per drawings and photographs. Alignment as above is also very important.

The maximum distance is calculated as follows:

The maximum span between support poles is 75m (see reasons given under laterals). Water from sub-main pipes is delivered via a feeder pipe and then split in two directions into the laterals. Therefore a sub-main is needed every 150m and this becomes the maximum module. Four modules give a total of 600m which in turn is the limit that the load is properly propagated to the two anchor poles at either end of the structure.

If a field is wider than 600m then it is advisable to divide it into two or more equal portions and to install extra anchors and poles where necessary.

To ensure a neat Floppy Sprinkler Overhead Cable installation, pipe tensioning springs should be used at the end of each lateral line to compensate for the expansion and contraction in the polyethylene pipe due to temperature fluctuations. Polyethylene pipe expands and contracts about 2mm per meter for every 10°C change in temperature.

The easiest way to install a tensioning spring is to thread a flat steel hose clamp (Jubilee clamp) onto one end of the spring's hook or end loop, as one would attach a key onto a key ring. Then insert the end of the pipe through the hose clamp and pull the clamp and pipe up to the desired position.

Fasten the other end of the spring to the pole at the same height as the pipe, which is hanging in the pipe hangers. Pull the lateral pipe towards the pole so that excessive slack is taken up and then tighten the clamp around the lateral pipe.

To aid with the installation of pipe tensioning springs, consult the graph below. This is for a spring of 2.6mm material thickness, 25mm outside diameter and 400mm length

Graph 8.1

The first step in the design process is to determine the block size. To assist with this, three standard designs are supplied in the manual (Appendix A) with a 50mm valve at a flow rate of 30 cubic meters per hour (Page 1a, 1b), 65mm valve at a flow rate of 50 cubic meters per hour (Page 2a, 2b) and an 80mm valve at a flow rate of 76 cubic meters per hour (Page 3a, 3b).

The layouts are per the drawings in Appendix A .

Page ...a:
Reflects the different layout options for a specific flow rate.

Page ...b:
Shows the detailed layout drawings for the different types of blocks.
Note that sprinklers are never further than 4 meters from the boundary of the field.

Page 4:
Shows the detailed drawings of the critical parts of the system.

The equipment needed for the installation of a Floppy Sprinkler Overhead Cable System can vary according to the method used and the size of the installation.  This holds true for all types of construction.  However, we will focus on an average size installation of approximately 5ha of a field measuring 450 x 110m.

• A 100 m tape-measure.  Shorter tape-measures allow "creep", leading to inaccuracies.
• About 100 setting-out pegs:  150 mm long ~ 6mm round bars are ideal.
• Standard hammer.
• Hack-saw.
• Screw-driver, flat or star, depending on hose-clamps used.
• Bricklayer's or "mason's" line at least 200 m long but preferably longer.
• Picks, shovels, crowbars and compactors.
• Soil-auger if available (200 - 300 mm).
• Drill-brace and 8mm bit.
• 19 mm flat or ring spanner or preferably a ratchet with a 19 mm socket.
• Fencing pliers.
• Small side cutter.
• 10 mm Poly-pipe hole-punch.  (Obtainable from Floppy Sprinkler (Pty) Ltd.
• Cable grip with long grip area. (2 Ton)
• 1 400 kg tension indicator.  (Obtainable from Floppy Sprinkler (Pty) Ltd.
• 600 mm wire-rope cutter.
• Pole-cap with pulley.  (Obtainable from Floppy Sprinkler (Pty) Ltd.
• 1 500 kg wire-rope puller "Tirfor" or alternatively a 1 500 kg lever-hoist with a 4 - 6 m, 12 mm wire-rope sling.
• 3 or 4, M16 D-shackles.
• A 3.6 - 4.5 m ladder, depending on the height of the poles, and/or scaffolding and scaffold-planks, preferably mounted on a trailer or small truck (bakkie).
• Light weight pole about 4 m long with a wire hook attached to one end and a 25 m cord is used to measure the spacing of the Floppy Sprinkler on the lateral line.

The installation procedure is straightforward and can be done largely by unskilled labourers with the help and guidance of someone having a basic knowledge of construction and mechanics. Several hundred Floppy Sprinkler Overhead Systems have been installed around the world and these guidelines have been compiled with the knowledge gained from the advice and input from many different people. Floppy Sprinkler (Pty) Ltd. will welcome and appreciate any suggestions you might have to improve and simplify installations.

STEP 1

Measure and mark out the boundaries of the field/s and clear the surface of all unwanted vegetation, large rocks and other obstacles such as old fences or structures. Do not plough or rip the land at this stage, as it will only make it more difficult to work on. If the field/s have already been ploughed and/or ripped, the surface should preferably be prepared in such a manner that a vehicle can move easily across the surface.

STEP 2

Measure out the positions of the poles and anchors, making sure that they line up in all directions and then mark them with a large lime cross with a peg in the center. (The lime crosses make it easy to accurately position a pole once the hole has been dug).

Ensure that the positions of the anchors are accurately aligned with those of the anchor poles.

STEP 3, 4 AND 5 - SQUARING METHOD

STEP 3

While the setting out is being done, drill a 8mm hole 70mm deep, 100mm from the top of all the poles. Screw in M12 x 90mm couch-screws into the holes in the anchor poles and do the same with the support poles but include a penny-washer.

When installing poles it is most important that these couch-screws face in the correct direction. In the case of the anchor poles the screws face inward towards the field away from their respective anchors and those in the support poles face across the field at 90°, all in one direction.

STEP 4

Dig or drill the holes for the anchor and support poles, do this in rows so that the installation of poles can be initiated while the balance of the holes is being dug. Ensure that the holes are not too big, but are the correct depth. Too large a hole does not make for a solid immovable installation, as too much soil must be back-filled around the pole.  (See also poles)

STEP 5

Start installing anchors only after the respective anchor or pole has been planted and backfilled.  This is to ensure allignment of the anchor's eye with the row of poles and the anchor at the other end.

If concrete anchors are being used, make sure that the holes are dug in the correct position. The distance of the front edge of the hole from the anchor should be; the height of the pole above ground level plus 1 meter. This is to ensure that the anchor cable will be at an angle of 45° to the ground. If the hole is positioned any closer to the anchor pole the angle will be greater and the effectiveness of the anchor's holding capacity will be reduced. The holes should be about 500 x 500 x 500mm and 1m deep.

Concrete cast in-situ should be at least 18Mpa and 300-400mm thick. It should be allowed to set for at least 7 days before any strain is placed on it.

A 12mm rod, with a small eye bent and welded closed on one end and some short rods welded on the other end, to give it some purpose, must be cast into the concrete.

This rod should lie in a groove cut into the front edge of the hole, angled up at 45° and aligned with the anchor pole. It is important that the groove is cut deep enough so that when strain is placed on the rod, it does not with time cut further into the soil and cause the anchor cable to become slack.

Pre-cast concrete blocks of similar dimensions and rods can also be used. This can speed up the anchor installation process as they can be cast in bathes, well ahead of the time that they are required. Both these types of anchors should be back-filled in layers as described under anchors.

Timber log anchors are installed in a very similar manner, except the dimensions of the holes are different but the position of the front edge and the groove for the rod remains the same. The rod must have a large loop of 170mm bent and welded on the one end so that it can be slid around the timber log, the other end remains the same as for the concrete types.

The holes should be about 300mm wide and 2m long with the same depth of 1m. A small undercut must be made at the bottom of the front edge just large enough to accommodate half the diameter of the log. The log will nest snuggly in this undercut and will be seated against solid undisturbed soil. This will eliminate any "creep". Back-fill as before.

STEP 6

The anchor cables or stay-wires play an important role in the structure, as they carry a tension of about 1.4 times as much as the main suspension cable when the system is loaded. In the interests of economy and to virtually eliminate the joining of any main suspension cables, exact lengths of the main suspension cable plus 1m, may be laid out between anchor poles and cut-off. At the end of a drum of cable there will usually be a length of cable not quite long enough to reach all the way between two anchor poles. Cut this remnant up in exact lengths to suit your anchor cable requirements. These lengths can be calculated by multiplying the height of the pole above the ground by 2.9 or 3. Remember anchor cables are double the distance from the top of the pole to the eye of the anchor rod protruding above the ground.

Take cable off a drum by suspending it on an axel, a strong steel pipe with a crowbar through it works well. Place this axel on two trestles or between arms of a tractor's three point hitch and then pull the cable off by rotating the drum. If a tractor is used it can be driven forward slowly and the end of the cable held in place. Have someone walk behind the tractor regulating the rotation of the drum to prevent an over-wind. If the drum is not rotated and the cable is merely uncoiled, the cable twits and then tends to untwist, at a later stage during the installation process, causing problems.

Attach one end of the cable to the eye of the anchor rod with a vine line grip, loop the cable around the top of the pole above the coach-screw and pull it down towards the eye.

The top of the pole must then be pushed towards the anchor, leaning it about 2-3° off vertical and the cable being kept as tough as possible must then be attached to the eye of the anchor rod by a second vine line grip. When the system is loaded the anchor will return to vertical. Now attach an anchor cable to the anchor pole at the other end of the row in the same manner.

STEP 7

While the ladder or the scaffold is still at the far side of the field, attach the main suspension cable to the top of the anchor pole just above the loop of the anchor cable with a vine line grip.

Now proceed across the field alongside the main suspension cable that is lying on the ground and attach it to the support poles by looping it over the coach-screws and penny washers. The cable will at this stage be some 3-4m or even more away from the anchor pole. Attach one end of the tension indicator with a D-shackle to the eye of the anchor rod and the other to the pin in the body of the wire rope puller, ("Tirfor") or to the hook on the body of the lever-hoist, if one is being used.

Place the pole cap on the top of the anchor pole, align it with the main cable and the anchor cables and secure it squarely with the four screws on the side.

Attach the wire rope grip to the hook on the cable of the wire rope puller, release the cable and pass it over the pulley on top of the pole cap. Pull the cable down towards the main suspension cable at the same time pull this cable upwards so that the two meet and pass one another. Attach the wire rope grip to the main suspension cable and then slowly start to tension the cable with the wire rope puller. A lever hoist is used in the same manner except a wire rope sling is needed between the hook at the end of the chain, to pass over the pulley and connect to the wire rope grip with a D-shackle.

Continue tensioning the cable until 1400kg is indicated on the tension indicator, then stop tensioning.

Pull the slack portion at the end of the main suspension cable towards the anchor pole and while keeping it taut, secure it with a vine line grip to the anchor pole across the anchor cable. (If this process is not done correctly, much of the tension can be lost when the pulling cable is slackened). Slowly release the tension by reversing the action of the wire rope puller or lever hoist.

When the tension in the main suspension cable is propagated to the anchor pole and the pulling cable is a little slack, release the wire rope grip and remove the pole cap. The main suspension cable should now be absolutely straight between poles and the anchor cables should be taught. This is the end of the structural portion of the system. The remainder of the work may be regarded as the plumbing portion.

Lastly, install Cross Brace Vine Line Grips at all poles.  Cross bracing requires the same procedure, however must be pulled to 600 kg's.

STEP 8

Make sure that the polyethylene lateral piping conforms to specification.

Check the size, in South African terms, a 25mm pipe has an internal diameter of 26.7mm. ISO standards, used in almost all countries using the metric system defines a 25mm pipe as having an internal diameter of only 21.2mm and a 32mm pipe having an internal diameter of 27.4mm.

The area of a South African 25mm pipe is 560mm² and an ISO 25mm pipe is only 356mm²; this is some 37% less. Water flow will be drastically affected.

An ISO 32mm pipe is therefore much closer to a South African 25mm pipe. Make quite sure that a polyethylene lateral pipe has an internal diameter of more than 26.5mm.

Make sure that the pipe is made from Low Density Polyethylene (LDPE) and not High Density Polyethylene (HDPE). This is important as HDPE is not flexible enough to seal properly around a Floppy Sprinkler push-in fitting.

Check that the pipe is rated Class 4 and able to withstand 4 bar (400 kPa) pressure, that it is UV resistant (has an anti-oxidant added to it) and that it is made from virgin material.

Poly-pipe can usually be ordered in specific lengths if quantities are reasonably large, this can save time and money by eliminating joints and off-cuts.

The length of the pipe may be determined as follows:

1. The distance between two rows of poles, i.e. 75m plus.
2. The height the pipe is to be suspended above the ground, i.e. 4.7m plus.
3. The distance from the base of the pole to the saddle on the sub-main, i.e. 1.7m plus.
4. The height the pipe is to be suspended above the ground minus 1.5m, i.e. 3.2m
5. Then round this off to the next full meter.

STEP 9

Connect the Laterals to the Sub-mainline, complete the mainline, filter, and pump installation.

STEP 10

Open the ends of the laterals, and flush the system thoroughly. Open valve slowly to give air time to escape. This procedure is very important and will eliminate the blockages of sprinklers with debris that came into the system during the installation process. Close the laterals and reinstall the springs.

If the sprinkler does not rotate:

a. The sprinkler is blocked. Especially during new installations foreign material may find a way into the pipeline;

b. The pressure is too low;

c. There is a leak in the feeder lines;

d. There is a blocked sieve on the suction line;

e. There are air locks in the feeder line.

Knowing when crops need water and how much they need are the key elements to effective irrigation management. With basic knowledge of the soil-moisture crop relationship, an irrigator can easily schedule more scientifically and anticipate irrigation requirements. Effective irrigation practices require the development of a good irrigation schedule. Irrigation scheduling is simply knowing when and how much to irrigate. An effective irrigation schedule helps to maximize profit while minimizing water, energy and fertilizer use.

The following factors influence an efficient irrigation schedule:

• Soil type;
• Crop requirements;
• Climatic conditions;
• Irrigation system.

TYPICAL IRRIGATION SCHEDULING PROGRAM

It is advisable to install two tensiometers at different depths - one in the root zone and one below the root zone of the plants.

The tensiometer installed at the root zone (A) indicates when to irrigate. This tensiometer reading should be kept at a low soil-moisture tension i.e. between 10-20 kPa. The tensiometer installed below the root zone (B) indicates the irrigation time, or the duration of the irrigation cycle. This tensiometer reading should be kept between 20-50kPa to ensure optimum conditions which will result in higher oxygen content in the soil. Placement of tensiometers By applying this concept when irrigating sugar cane it has been found that while it is standard practice to irrigate 50mm once a week, this quantity can be reduced down to three applications of 12mm, or only 36mm per week.

This principle applied resulted in a 20%-40% increase in yield compared to hand movable systems. The increase in yield is due to less water applied more frequently, which eliminates stress conditions between irrigation cycles, creating a continuous optimum growth environment. Furthermore, there is a reduction of water and fertilizers moving past the root zone. Thus, savings on water consumption and fertilizers is another advantage of the system. It is therefore safe to conclude that effective scheduling will result in lower water and energy input costs and more efficient fertilizer use whilst increasing yield.

Interpreting tensiometer readings