Safety of centre pivots
Contents
1. S Stimie C M Uys W J Van der Merwe F P J and Viljoen P D 1996 Irrigation Design Manual ARC Institute for Agricultural Engineering RSA Peters U W 1990 Designing with travelling irrigators Directorate Irrigation Engineering Silverton RSA Reinders F B 1998 Spilpuntbesproeiing George Farmer s Association Farmer s day ARC Institute for Agricultural Engineering Silverton RSA Valley Irrigation Manual 1988 Valley Irrigation Equipment Pty Ltd RSA Van der Ryst C 1990 Identifisering kwantifisering en beperking van waterverliese onder spilpuntbesproeiingstelsels WRC Report No 153 1 91 University of Orange Free State RSA2. 16 Irrigation User s Manual Table 10 5 Maintenance schedule of pivots Action After each revolution After each 4 revolution Seasonal Electrical Switch on pivot and listen to each motor and starter If any abnormal sound is heard remove and have it serviced If end tower s light is out replace electric bulb and remove dust insects and water where necessary Check tower panel and main control cabinet Clean panels remove dust insects e g wasps etc Inspect condition of wiring of pivot Inspect electrical motor cable condition earth conductor and connections gt Structure Ensure that all bolts and nuts are tightened Tighten if necessary Ensure that earth conductors are clean Grease pivot Grease pin that holds swing mechanism of towable pivots to prevent rusting lt lt Check system for leakages Repair if necessary Replace gearbox oil Drain and replace lubricants in motors Grease moving parts and roller bearings gt Check U couplings grease if necessary Check wheel bolts and adjust as prescribed Check wheel pressure and adjust as prescribed Check flange fittings for leakages secure and replace if necessary Inspect framework for sturdiness tighten bolts if necessary P lt P lt PS Check if all the safety switches work Check if all the drainage valves work Clean sand trap if nece
3. electrician Contacts of limit switch corroded Replace switch Incorrect wiring of end tower and booster Contact electrician pump order Defective micro switch Contact electrician End tower wiring faulty Contact electrician Pivot s alignment faulty Contact electrician Moving irrigation systems 10 15 P Broken wireinspan able Removeandneplce Booster pump motor turns in wrong Contact electrician direction Booster pump pressure switch set at high Adjust pressure Booster pump wired incorrectly Contact electrician 8 Maintenance All routine services as discussed in the owner s manual must be followed precisely Since a pivot consists of moving parts and movement usually takes place as a result of electrical energy the equipment can be potentially dangerous Keep the following safety precautions in mind o Never try to switch on the system before it is thoroughly earthed o Never use circuit breakers greater than the recommended specifications o Never remove any panel or open a tower panel before you have switched off the power yourself Do not trust anyone else with this task o Keep the panel cabinet locked as far as possible This will prevent unnecessary tampering with the pivot o Prevent mist spray from the pivot on the skin and inhaling when chemicals are applied through the system o Never adjust the end nozzle while it is in operation o Never try to climb on the pivot with a ladder while it is in operati
4. light irrigation on a tilled field This ensures that the wheel tracks are not too deep The onus is on the farmer to ensure that wheel tracks are never deeper than 150 mm Deep wheel tracks can cause mechanical damage if the machine should move out of line e On soils where problems are experienced with deep tracks circular tilling can be considered Tilling can then be done between rows during the season to fill the tracks e With corner systems it must be kept in mind that the extended tower section is much heavier than the other towers It is also inclined to make deeper tracks Give attention to tracks with the first revolution The extended tower section s wheels cross the trench in which the alignment cables are buried in a few places Heavy irrigation in the first revolution can possibly cause the pivot to get bogged down General pointers e Do not leave implements standing in the field they may be in the way of the pivot and cause damage e Do not irrigate when it is too cold the pivot can freeze Park the pivot in the service road during tilling High equipment such as tractors or harvesters can damage the pivot when they try to move below it e Use a strong enough tractor to tow the pivot Avoid at all times that the pivot is tugged forward Also look out for wheel tracks in the towing path Water application of a pivot is changed by changing the running speed of the pivot point and can be determined as follows 10 1
5. mm Similar calculations can be done for any other required speed setting 10 14 Irrigation User s Manual 7 Troubleshooting tables Table 10 4 Troubleshooting table for centre pivots Pivot won t move Pivots move out of line Pivot won t move back automatically Pivot does not stop automatically Movement sensor timer switches pivots off Pivot keeps on going running End tower nozzle does not close Safety circuit open switchboard and misaligned tower Tyre pressure too low Pump tyres according to manufacturer s specifications Variation in slope of field too great Change tilling methods in field Wheels make too deep tracks in field Repair wheel tracks and change drainage direction of water Alignment limit switch inoperative Check operation Replace if necessary Reverse switch out of order Contact electrician Control switch in off or manual control Change switch to automatic position position Stop limit and stop selector switch is Contact electrician incorrectly wired Control panel badly wired Contact electrician Movement sensor out of order Contact electrician Timer switch not set Set time switch as prescribed in operating instructions First tower s switch out of order Contact electrician Loose wires between first tower and Contact electrician movement sensor time switch First tower activating switch out of order Contact electrician Short circuit between wires Contact
6. same principle as the cable method except that the impulse from the centre pivot to the pump is given by radio signal A radio transmitter is mounted at the centre pivot control panel and a receiver at the switchboard As soon as the centre pivot switches off a radio signal is sent to the pump cutting the power supply Moving irrigation systems 10 5 e Siren If the centre pivot is operating close to a home or if it is always under supervision a siren may be mounted on it As soon as the centre pivot switches off the siren warns someone who then physically switches the pump off The siren may use 220 V or 24 V e Electrical safety This method may be used if the centre pivot and pump operate from the same transformer The method is as follows A monitor is clamped around one of the live cores of the 4 core supply cable and connected to a relay in the pump starter Whenever the centre pivot does not draw any current for a specific period the pump automatically switches off Switching off of centre pivot If a short power failure occurs the pump may switch off while the centre pivot will continue to move If this happens after hours the centre pivot must return dry the following day which could cause falling behind in the cycle The general remedy is to attach a low pressure switch to the centre pivot As soon as the pump fails the centre pivot pressure will drop and the system will shut down Low temperature safety Temperature sw
7. 2 Irrigation User s Manual The revolution time at 100 speed adjustment can be calculated as follows 27 0 1047 r t Ee 10 1 60 v v where t rotation time at 100 speed setting hour r distance of pivot to last drive wheel m v travel speed of last wheel at 100 speed setting mm m The gross application per revolution can be determined as follows Qt 10 2 10A q where BT gross application at 100 speed setting mm Q pivot flow rate m h t rotation time at 100 speed setting hour A Si area ha A schedule can now be drawn up for the amount of gross application at different percentage speed settings of the pivot t t 10 3 vy where ty rotation time at a specific speed setting hour Vy specific speed setting fraction This equation can only be used as a first approximation for determining the water application Because of factors beyond the producer s control such as soil conditions wheel slip and pivot pressure variations can occur A better schedule can however only be drawn up after installation with physical readings in the field For the calculation of rotation times and application rates the applicable information e g motor speed for different types of drives and tyre sizes can be obtained from the manufacturer Table 10 3 Tyres and motors available for pivots Type of Motor Speed m min for different wheel size
8. CONTENTS Moving irrigation systems O 10 1 SN MM a A a e dada 2 1 Centre pivots 2 1 1 General ATA Satety OF COME eee 10 4 Zl 3 O 2 2 Travelling irrigators 2 3 Rotating boom systems 2 4 Linear systems RO RR 10 8 4 Evaluation after installation 10 8 Osc Mana penent traes SO eae es od a o o e A eee A Oe EO ee ee 5 1 On and off switch 5 2 Water application 3 3 Trouble shooting LADIES rairai a e a T ia a Med 10 14 E a Ce ANE aos 10 16 6 1 Maintenance Schedules iis iiss rd cedida 10 17 6 2 Flushing of pivots points oo csnnnnnnnnnnninnninnniiiiinnisinsinnvinnniinniinnsinnvinnnnans 10 19 6 3 Solution for corrosion problems e ao atest aoe is Oe ce Nh ee ND he DE ns a et rR nate 10 20 All rights reserved Copyright 2004 ARC Institute for Agricultural Engineering ARC ILD ISBN 1 919849 24 6 Moving irrigation systems 10 1 1 Introduction The number of moving irrigation systems in use in South Africa has increased since 1970 Moving irrigation systems are as the name indicates machines with sprinklers or sprayers that move across a field distributing water and are opposed to systems where pipes and sprinklers are moved by hand With moving systems management aspects are simpler scheduling can be done more accurately and labour problems are largely reduced or eradicated Growers are mainly persuaded to change from conventional irrigation systems to moving irrigation systems because of labour and management L
9. abour management and the availability of willing workers especially in the cold seasons are important considerations A further advantage is that it is much easier to apply smaller amounts of water more frequently for example 15 mm to 30 mm every 2 to 5 days This is a high advantage with some crops in certain growth stages especially at germination on problem soils It is also easier to apply overhead fertigation to meet crop demands Moving systems are mainly applicable to agronomic and gardening crops and pastures They are not applied to orchards and fruit cultivation as permanent systems are better suited and not much more costly The key to economic justification of moving systems is to utilise them maximally At least two and if practically possible three crops per year should be irrigated with the same system For economic justification therefore the system should wear down and not rust apart This chapter consists of an introductory portion with the information mainly originating from the Irrigation Design Manual 1996 and the Valley Design Manual 1988 as well as practical information regarding the installation evaluation and operation of moving systems 2 System types Moving systems include all systems that move while applying water to the surface of a field 2 1 Centre pivots A centre pivot consists of steel frames and pipes which are supported at approximately 50 m intervals by an A frame on two wheels 2 1 1 General A
10. ake use of pressure from an external pumping station and e an automatic coupling system where the linear system is coupled to the hydrants and use is also made of an external pump The system is guided by censors which follow a cable stretched across the length of the field Linear system routing is further accomplished by switches or valves on each tower which keep each span length in line 10 8 Irrigation User s Manual 3 Installation It is very important that the limitations as prescribed by the manufacturers for the placing of different towers for towable as well as non towable models be attended to during design and erection To simplify installation installation must preferably be done along a hard surface such as a road 4 Evaluation after installation A basic evaluation must be done on the system after installation to ensure that the system complies with the design specifications Table 2 Proposed basic evaluation procedure after installation Subject Item Measurement Evaluation Action if measurements evaluation does not conform to the design specifications Operating pressure Determine the pivot pressure Contact designer for trouble and pressure at the beginnning shooting of towers with a pressure gauge and compare with the required pressured as specified on the peak design form Emitter delivery Measure the delivery in a If or a delivery variation of container for one minute on at more than 10 occurs as
11. and 80 of the diameter depending on wind conditions A hydrant is installed on the main line opposite each strip for connection to the sprinkler Actuation can take place by one of three methods e A drum on the trolley is hydraulically driven and rolls up a cable which is anchored at the other end of the field In this way the sprinkler is dragged across the field The water source is connected to the sprinkler with a flexible hose which is dragged behind the trolley A disadvantage of this model is that it has a costly supply pipe which can wear while an advantage is that a strip of up to 400 m can be irrigated per set e Another model consists of a drum around which the water supply pipe is rolled The drum is mounted on a trolley The big gun is connected to the supply pipe and mounted on a smaller trolley or sled The large trolley is set up in the middle of the field directly opposite the main line The big gun on the sled is then dragged to the edge of the field by tractor unrolling the supply pipe from the drum When the big gun begins to irrigate the drum is hydraulically driven rolling up the supply pipe and dragging the big gun in When the sled reaches the trolley and drum the trolley is rotated through 180 and the remaining half of the field is irrigated in the same way e The drum around which the water supply pipe is rolled and the big gun are mounted on the same trolley The supply pipe of ordinary high density polyethylene is c
12. ds Butterfly valve method An electrically operated butterfly valve is installed in the pipeline at the centre pivot inlet The valve solenoid is connected to the centre pivot s 220 V safety circuit Furthermore a pressure release valve is installed near the centre pivot inlet A pressure switch or no flow switch may be installed at the pump As soon as the centre pivot switches off the valve solenoid is activated slowly closing the valve Pressure build up in the pipe causes the pressure switch or no flow switch to switch the pump off The pressure release valve limits water hammer in the pipeline Before this method is used pipe classes pump shut off pressure and adjustment of release and pressure valves must be thoroughly taken into account Cable method This method makes use of a safety cable which directly connects the centre pivot control panel with the switchboard of an electric or diesel motor The centre pivot control panel has a standard terminal connection point for a safety cable The cable is connected in such a way that it has a supply voltage of 24 V AC while the centre pivot is in operation The pump switch must be connected to a 24 V AC relay normally open As soon as the centre pivot stops power to the cable will be cut and the relay will switch the pump off The following sizes of two core cable may be used Length Shorter than 1 000 m 1 5 mm 1 000 2 000 m 2 5 mm Radio signals This method works on the
13. e mainline is filled with water Change the main switch to the on position Change the STOP RUN switch to the run position Set the percentage switch Press and select the AHEAD REVERSE switch Ahead means clockwise and reverse means anti clockwise To stop the system change the STOP RUN switch to the stop position change the main switch to the off position and switch off the pump Alignment of centre pivots is very important since problems with alignment can lead to excessively high tension in the pivot structure that can shorten the life of the motor and gearbox lifespan and can cause structural damage The manufacturer s prescriptions must be followed with caution 5 2 Water application The development of a computer supported control system for pivot and the use of pressure regulators with nozzles has simplified the effective control of irrigation water of pivots 10 10 Irrigation User s Manual Figure 10 6 Typical control panels of pivots It still remains extremely important to operate the system correctly to make effective water application possible Scheduling aids such as soil water sensors and scheduling programmes are discussed in Chapter 12 Irrigation scheduling The following is recommended to limit water losses Do not apply too little water per cycle It is normally advisable to apply not less than 5 mm per application The reason therefore is that with a smaller application a s
14. eed The maximum permissible speed variation between highest and lowest speed is 10 It is also important to allow the sprinkler at the beginning and end of the strip a specific recommended time to stand and irrigate to obtain an acceptable distribution of water 2 3 Rotating boom systems The design of rotating booms merely entails reading off applications per hour from manufacturer s tables A suitable travelling time or standing time can be determined from the crop requirements and the soil water holding capacity It is important to note that a triangular spacing is used with a static boom as a rectangular spacing causes weak distribution with a dry area between the four positions It is advisable to plot the positions and wetted circles on the site plans With rotating boom systems a strip spacing of 85 of the wetted diameter produces the best distribution efficiency namely 91 2 4 Linear systems Linear systems are very similar to centre pivots as regards construction except that they do not rotate about a central point Driving is usually electrical or hydraulic with a diesel motor mounted on the machine or also electrical or hydraulic where an external pump with draglines is used Water is supplied by e achannel in the middle or at the side of the field parallel to the movement direction from which the linear system s motor pumps the water e hydrants which are connected to the linear system with a flexible drag line and m
15. hallow depth of wetting is obtained and soil water application is less effective as a result of interception by the leaf cover and evaporation from the soil A low application can also lead to a too shallow root development In regions with constant prevailing winds the distortion can be compensated by making speed adjustment in multiples of half days This will result in the same spot in the field not being irrigated at the same time of the day Irrigation must be ceased if the wind speed is greater than 20 km h Van der Ryst 1990 Any nozzle that applies water with small drops at high rate must be avoided Small drops will be blown away by the wind and lead to evaporation By alternating irrigation during the day and the night losses will be limited to less than 10 at a time Nozzles must be checked for blockage regularly A sieve must be installed at the pivot centre to remove material from the system that can cause blockage The distribution pattern must be inspected once per season to identify and replace worn nozzles The water must be applied as close as possible to the crop to limit losses Moving irrigation systems 10 11 During periods of water restrictions the following can be considered for different types of set ups Permanent e Divide the field into segments and spread out planting dates for the same crop so that all the crops do not reach their peak requirements at the same time This can also reduce the risk if unfore
16. he nozzles and the additional weight of the collected sand can damage the structure The following flushing procedure can be followed Ensure that the main switch is in the off position Remove the sand filter drainage valves at each tower and the lid at the end of the overhang Switch on the pump Flush the system thoroughly Switch off the pump and replace the sand filter drainage valves and lid at the end of the overhang The flushing procedure will normally be carried out after installation of the system after repairs are done to the pump and the pivot point structure at the beginning of the irrigation season and when it seems necessary as a result of sand collection in the system 8 3 Solutions for corrosion problems Different indexes are used for identifying whether irrigation water is corrosive or will form a deposit A single index can create the wrong impression and it is therefor recommended that as many indexes as possible is used for estimating the water quality Tables 10 6 10 8 can be used to evaluate the effect of the water on irrigation equipment Table 10 6 Langelier index DWAF 1996 Langelier index Effect on irrigation equipment lt 0 2 Increasing problems with corrosion 0 2 to 0 2 Limited problems with corrosion or deposit forming gt 0 2 Increasing problems with deposit forming 10 18 Irrigation User s Manual Table 10 7 Ryznar index DWAF 1996 Ryznar inde
17. itches are freely available throughout the market place but should be used with great caution on centre pivots The problem lies therein that water does not always freeze at the same temperature Depending on atmospheric conditions freezing may occur at temperatures as high at as 9 C Depending on the setting on the switch it may cause the centre pivot to switch off unnecessarily or even too late Wind safety In windy areas centre pivots must be anchored when not in use 10 6 Irrigation User s Manual 2 1 3 Application Centre pivots can be used for different applications because of the variety of available sprinklers Water source Hydraulic Ultra low 40 kPa High 375 550 kPa Drive Medium Low Sprinkler pressure 280 350 kPa 140 275 kPa Sprinklers Small to Medium Sprayers large different nozzles different nozzles Wetting diameter Figure 10 4 Schematic classification of pivot points adapted Reinders 1998 2 2 Travelling irrigators Travelling irrigators consist of a big gun sprinkler mounted on a trolley The trolley moves slowly across the field while the sprinkler irrigates Parallel strips are irrigated in this way Water is supplied by an underground main line which is laid in the middle of the field The sprinkler moves perpendicular to the main line in specifically spaced strips Strip spacing depends on the wetted diameter of the big gun and varies between 60
18. least three emitters on a tower specified in the technical report contact the designer System lay out Verify the emitter package by Re install according to plan determining whether the correct nozzles and pressure regulators if applicable are installed in the correct positions Compare installed pipe diameters with those on the plan Equipment Compare the installed electric Contact designer for motors and motor speed with replacement of faulty equipment the specifications as per design report High speed motors must be half of the total motors 1 General installation Examine if any leakages occur Repair leaks in the system System capacity Determine system flow rate by If a flow rate deviation of more taking the reading from the flow than 10 from the average meter occurs as specified in the peak design form contact the designer Moving irrigation systems 10 9 5 Management It is important that moving systems are managed correctly to ensure the effective application of water Figure 10 5 Placing of rain meters for pivot point evaluation 5 1 Switching on and off The following procedure can be followed when irrigation is to be done with a pivot point Ensure that there are no vehicles or any other equipment near the pivot that can cause an obstruction Change the WET DRY switch to the WET position Close the mainline s valve partially Switch on the pump Open the valve slowly until th
19. ll centre pivots are constructed from the following basic components e Pivot structure This is used to anchor the centre pivot The whole structure rotates around it e Span This is the basic structural unit with which a centre pivot is constructed A unit consists of the main pipe in which the water flows the frame which supports the pipe and the driving mechanism Spans are distinguished from each other based on the following variables 10 2 Irrigation User s Manual Table 10 1 General centre pivot spans available Span length m Pipe diameter mm Mass kg 52 corner system 56 corner system e Length and pipe diameter Any combination of pipe sizes and spans on a specific centre pivot may be identified by a model number e Crop clearance height The manufacturer can supply details of the crop clearance height of the different span lengths for a specific type of centre pivot The crop clearance must be considered where a hill is present between two towers Depending on the height of the structure the length of a span and the slope of the hill the crop clearance of 4 m for the high profile cannot shrink reduce to 0 3 m for a low profile 56 m span on a 5 slope ken SLOPE b SLOPE a CLEARANCE HEIGHT Figure 10 1 Crop clearance height e Towability The following restrictions apply to towable centre pivots o Soil type and topography Guard against clay soils and cross slopes along the towing route o Ce
20. ntre pivot length The maximum number of spans for a towable centre pivot must be obtained from the manufacturer o Roads A six metre wide road with a level surface must be made to tow the centre pivot along Take care where centre pivots have to be towed over contour mounds Moving irrigation systems 10 3 o Span combination Only certain span combinations may be towed Contact the manufacturer for relevant details Seeing that short spans are more stable than long ones they should always be placed closest to the centre o Electricity supply The centre pivot s electricity supply must be disconnected before the centre pivot is moved to another position Pivot flex A flexible pivot coupling is necessary where the anchor tower is on level ground and the first tower moves on a slope of more than 4 downhill or uphill regarding the anchor tower The manufacturer must be contacted to obtain the flexibility of the centre pivot coupling at the centre of the pivot Contact the manufacturer to obtain the flexibility of the coupling at the centre of the centre pivot The coupling can only accommodate a certain percentage slope a between the centre and the first driving unit The allowable slope differs for the different tower lengths and between different centre pivot models PIVOT FLEX SLOPE a Figure 14 2 Pivot flex Overhang The overhang is mounted at the end of the centre pivot to irrigate an additional area Slope When a centre pivo
21. on Overhang Since the overhang is suspended on cables the build up of sand and silt can cause structural problems especially if the pivot is towed It is therefor essential that the pivot be flushed regularly Start with checking how soon the drainage valves get blocked after the pump is switched off This gives an indication of how regularly the pivot must be flushed It is no use removing the drainage valves after the pump is switched off Remove the sand trapper and drainage valves at each tower and rinse out the dirt while the pump is in operation Sprinklers As soon as blockages are noticed the sprinklers must be removed and cleaned Never use a piece of wire to clean the nozzle from the outside It is advisable to check the regulator at the same time In many cases the regulator blocks up partially before the nozzle gets blocked If chemicals are applied sprinklers must be checked for wear at least once per season Uneven application of irrigation can be catastrophic for the crop Gearboxes The lubricants for pivot gearboxes as prescribed by the manufacturers must be used The oil must be drained from the gearboxes after each irrigation season 8 1 Maintenance schedules Seasonal maintenance must be done at the end of each growing season This will protect the pivot during the months when it is not in use and lead to the minimal problems at the start of the next season A maintenance schedule is proposed in the table below 10
22. onnected to the hydrant on the main pipe at one end of the field The trolley is then dragged to the opposite end of the field by tractor When the water supply is activated the trolley is hydraulically driven and moves all along the supply pipe to the hydrant When the hydrant at the end of the Moving irrigation systems 10 7 strip is reached the water supply is automatically cut and the machine is ready to be moved to the next strip The advantage of this system is that strips do not necessarily have to follow a straight line The supply pipe may be laid along the contour and the trolley which has a steering mechanism will follow it Travelling irrigators are especially suited to pastures and sugar cane but are also applied to other types of agronomic crops They still require more labour than centre pivots as they have to be moved once every 12 or 24 hours by tractor They are generally used on uneven surfaces not suited for centre pivots but where travelling irrigators may be used practically The moving direction must be such that the pressure difference between the upper and lower ends of a strip does not exceed 20 of the working pressure The limiting factors with travelling irrigators are the condition of roads prevailing winds and the high working pressure required A point of importance with traveling guns is among others the variation in speed with which they travel The amount to be irrigated will vary in the same relation as the sp
23. s drive speed 14 2 x24 13 6 x 24 14 9 x 24 16 9 x 24 11 2 x 38 rpm eee 3 25 1 55 1 65 1 72 1 83 2 12 Hoe 47 2 89 3 08 3 21 3 42 3 95 e D 28 1 76 1 87 1 88 2 07 2 40 dido 56 3 51 3 74 3 90 4 16 4 80 Tyre pressure is very important for the effective operation of the system and must be checked at least three times during the irrigation season Low tyre pressure will damage the tyres and Moving irrigation systems 10 13 propulsion mechanisms The manufacturer must be consulted about the correct tyre size for a specific pivot point and slope The following directives can be used Sandy soils 11 2 x 24 Heavier soils gt 6 clay 13 6 x 24 cash crops 14 9 x 24 perennial crops Example 10 1 Determine the revolution time and gross application at a 50 speed setting for the following case Pivot point length 308 3m Distance to furthest wheel 282 8m Pivot flow rate 101m h Motor speed according to manufacturer s brochure 1 64 m min Wheel slip allowed 3 Solution Drive speed 1 64 x 100 1 59 m min According to Equation 10 1 y 0 1047r Rotation time v _ 0 1047 x 282 8 h 1 59 18 6 hours Area mR 2x 308 3 m 29 85 ha According to equation 10 2 _ Gt 10A 101x 18 6 _ mm 10x 29 85 6 3 mm Gross application At a 50 speed setting According to Equation 10 3 18 6 t hours 37 2 hours GA 63 mm 0 5 12 6
24. seen breakages are experienced e Plant different crops so that the peak requirements are spread out e If unforeseen circumstances should occur supplementary irrigation can be applied to a portion of the field Seasonally movable In some regions with the correct choice of crops it is possible to put in two harvests per year As soon as the requirement for the first crop begins to decrease the pivot can be moved to a second circle to begin soil preparation Cycle movable Never try to handle too many circles with one pivot point It is difficult to irrigate 3 circles with one pivot Also consider different crops or the same crop with different planting dates on the various circles The following is recommended for solving problems regarding infiltration and the forming of wheel tracks e Some soils tend to compact towards the end of a season Keep check on the soil for weakening in infiltration rate and do not irrigate too much at a time e For soils with poor infiltration alternative tilling practices must be considered The forming of puddles between rows or stubble tilling or gypsum application can improve infiltration rate tremendously e Some soils tend to make deep tracks early in the season if it is tilled too shallow If heavy irrigation is applied water remains in the tracks and the tracks become deeper with each revolution Deeper tilling or lighter initial irrigation can avoid this e It is important to first apply a very
25. ssary lt ISS TPS TPS PS P lt od Sprinklers Check nozzles for wear replace if necessary Check is the pressure meter works correctly Check the condition of the sprinklers Check pivot pressure and pressure at beginning of towers Check for blockages in nozzles Flush the system mK gt lt P lt gt lt gt lt Equipment Check functioning of end nozzles and check nozzle for wear gt Moving irrigation systems 10 17 Inspect cut off action of end nozzle repair or f X replace if necessary Check stop in slot micro switch adjust if x x necessary Test the automatic reverse action movement of pivots by switching the hand lever forward and X back Fill wheel tracks deeper than 150 mm with timber or stones X With linear systems the following additional measures must be kept in mind when maintenance is done e Drive All electrical cables must be checked regularly and replaced if necessary Check bearings and belts and adjust if necessary e Alignment Check alignment according to manufacturer s prescriptions Where a system uses a supply line that must be towed the road must be as even and dry as possible to make the towing of pipe easier 8 2 Flushing of pivot points The purpose of the flushing of the system is to remove sand and dirt collected in the pipeline Sand can cause unnecessary wear of t
26. t is designed for a specific field the topography of the land must be thoroughly investigated in addition to the above factors Each centre pivot has certain slope limitations due to the structural and driving power design The adaptability of the system to all remaining slopes must be investigated VERTICAL SLOPE orizontal A 0 VERTICAL HORIZONTAL Figure 10 3 Slope limitations for centre pivots 10 4 Irrigation User s Manual The climbing ability of a centre pivot is limited by structural requirements and driving power Attention should be given to ascending as well as descending slopes as it may result in variations in speed The slope absorption at the towers and torque and swivel action at the couplings must be kept within the particular manufacturer s specifications A slope combination of 14 to 30 is possible Reinders 1998 2 1 2 Safety of centre pivots There are two safety mechanisms for centre pivot irrigation systems With the first type the pump switches off if the centre pivot should stop and with the second type the centre pivot switches off if the pump should stop Switching off of pump If the centre pivot should stop after hours it will discharge at that position until someone switches off the pump To prevent such overwetting conditions a signal must be sent to the pump to switch off the moment the centre pivot stops moving The signal may be transmitted by one of the following metho
27. x Effect on irrigation systems lt 6 5 Tendency to form deposit gt 6 5 Tendency to be corrosive Table 10 8 Corrosion index DWAF 1996 Corrosion index Effect on irrigation systems lt 0 1 Limited corrosion problems gt 0 1 Increasing corrosion problems There are two approaches that can be followed to solve the problem of corrosion Firstly the thickness of the galvanising on the pipes can be increased from a minimum of 55 micron as proposed by the SABS to a value of 100 micron and more This will lengthen the lifespan of the pipes Alternatively the pipes can be manufactured from a zinc alloy that contains magnesium aluminium and lead or the galvanising can be protected by three coats of epoxy paint De Beer 2002 Valley has developed a pipe with polyethylene lining for this purpose The owner of the pivot can prevent problems regarding water quality by following maintenance measures such as by ensuring that the drainage valves function correctly If the drainage valves are blocked the fertilizer concentrate will accumulate at the lowest points of the pivot point and aggravate the corrosion process References De Beer H 2002 Prevent corrosion of center pivots SA Irrigation December January RSA DWAF 1996 South African Water Quality Guidelines Volume 4 Agricultural Use Irrigation RSA Koegelenberg F H Burger J H Heyns P J Kleynhans E P J Lategan M T Mulder D J Smal H
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