Design Norms
The demand on our scarce water resources has necessitated the efficient use of all water in South Africa.
The irrigation sector, which is presently responsible for 50 % of all water consumption, will need to become involved within the agricultural sector and other water users in finding a balanced approach to the continually increasing demands on water.
SABI, whose objective is the promotion of the science, practice and technique of efficient irrigation, can make a significant contribution towards ensuring that irrigation designs enable farmers to utilise their equipment and water efficiently.
SABI has compiled the irrigation system design norms as a standard, so as to assist designers with the hydraulic calculations during the design process.
During the compilation of these norms it was decided that the choice and requirements of equipment not influencing the hydraulic calculations be excluded from the norms, but are still subject to manufactures specifications.
Only industry rules for specific equipment that influence the hydraulic calculations of a system, will form part of the norms, e.g. the maximum allowable dirty water pressure drop over a filter is 40 kPa.
General
In South Africa there is a great need for farmers, crop, soil and fertiliser specialists to have information on crop water requirements, nutritional requirements and the scheduling thereof in terms of recommendations for the designer to optimally design an irrigation system for specific circumstances.
A multi- disciplinary approach is required when evaluating water quality for irrigation purposes, so as to identify any anticipated problems with drip systems, the determination of and management of available water sources e.g. boreholes, peak and annual crop water requirements, analysis of soil water holding capacity and infiltration rate.
The designer must highlight any problems e.g. the blockage of drip systems with irrigation water, and make recommendations to solve the envisaged problems.
The following norms are proposed:
1.1 Pipe friction in main and sub-main pipelines
The filling up of pipelines and examples of mainline design must be according to industry standards, which must be covered in manuals specific for designers. The designer must take into account the possible affect of water quality on pipes as well as the deterioration of pipes with age during the design process.
The following values for allowable pipe friction in mainlines are proposed as norms:
The following applies for pipelines with a diameter of 200mm or smaller:
- Rising pipeline: Maximum 1.5%(m/100m) friction
- Gravity pipeline: Maximum allowable flow velocity of 3.0m/s
If the above figures are exceeded, then the designer must show that the chosen pipe diameter's total cost (capital and annual running cost) have been optimised and is the best of the available options. For pipelines of larger diameter, the effect of water hammer is critical and must be investigated and optimised.
1.2 Application Efficiencies
These values mentioned are important when used to change nett irrigation requirement to system capacity (gross irrigation requirement).
The efficiency of a system is made up of two components, namely the losses that take place between the emitter outlet and before the water reaches the root zone as well as the distribution uniformity (DU) of the total system after operating for a number years (Burt, 1994). Although there are numerous figures in the literature, there is a lack of reliable figures for South African conditions.
In the interim the following figures are recommended as norms:
- Drip systems 90%
- Micro sprinkler systems 80%
- Permanent sprinkler systems 75%
- Moving systems 80%
- Movable quick coupling sprinkler systems 70%
- Traveling sprinklers and other moveable sprinkler systems 65%
- Flood irrigation (with piped supply system) 80%
- Flood irrigation (with earth channel supply system) 60%
1.3 Irrigation hours per week
These values are used to determine the required pump -/ stream flow size.
The norms recommended by DWAF (1985) are accepted:
- Micro and permanent sprinkler systems - 144 hours
- Centre pivots systems - 144 hours
- Moveable sprinkler and other movable systems - 110 hours
- Flood irrigation systems - 60 hours
1.4 Minimum pump capacity (safety factor for wear and tear)
These values are added to the calculated system capacity and are used to indicate the duty point (pressure and flow) when selecting a pump.
The present norms are accepted:
- Discharge rate 10%
- Pressure head 5%
Where an irrigation pump is also used for the mixing and application of fertilisers, then an additional 20% pump capacity must be provided for.
1.5 Permissible suction velocities
A foot valve's "open" area must be four times that of the open area of the suction hose, thus ensuring that the velocities through the foot valve do not exceed those of the suction hose by more than 25%.
The following is proposed:
- Suction hose (absolute maximum) 1,5 m/s
- Suction strainer 0,4 m/s
1.6 Maximum permissible velocity in filterbank manifold
- 0,5 m/s
Micro Irrigation
The manufactures' standards for equipment in the industry, for example the minimum back pressure / flow required for the backwash of filters must be adhered to. The choice of equipment, for example a pressure control valve at the inlet of a block, is not part of the norms.
The following norms are recommended:
2.1 Minimum gross application rate
The present norm for gross application rate of 3mm/h on the wetted area remains unchanged (Lategan, 1995).
The minimum recommended wetted area norm is scrapped due to management problems in the past, when irrigation controllers were not freely available.
2.2 Filters
Ring / mesh filter openings must be 1/5 that of the emitter orifice diameter. The appropriate micro emitter manufacturer's recommendations must be used for flow path openings of 1mm.
The following norms are recommended unchanged (ASAE EP405.1, 1997):
Maximum allowable pressure drop over ring / mesh filters:
- Recommended pressure drop over a clean ring filter - 10 kPa
- Recommended pressure drop over clean filter bank - 30 kPa
- Maximum allowable pressure drop over a filter bank before backwashing - 70kPa
2.3 Minimum emission uniformity (EU)
Design EU 90%
The minimum emission uniformity (EU) is used for calculating the available pressure band for the lateral and manifold diameters. The emission uniformity is used to calculate the pressure band, as the maximum design flow variation norm amongst others, does not make provision for the manufactures coefficient of variation (CV) of micro systems.
Each manufacturer of micro sprinklers is responsible to supply the required information (e.g. CV) to designers to determine the pressure band variation.
Drip Irrigation
The dripper spacing should be determined through multi disciplinary collaboration between experts in the agricultural field.
As mentioned earlier pressure regulated valves and anti- vacuum valves installed at block inlets do not form part of the norms. The use of specific filter equipment for specific drippers depends on the specific manufacturer's recommendations because research results for sand filters on drip irrigation are not always conclusive. This norm is thus scrapped until such time that relevant research results are available.
The following industry specifications for sand filters are recommended:
A minimum of 50% of the maximum filtration rate (50m3/h per m2 sand surface area) is required to backwash the filters. The maximum backwash rate must not exceed 1,2 times the filtration rate. A minimum of 6 m inlet pressure is required during backwashing. The backwash time of sand filters can be between 90 -180 seconds.
Remembering that as the flush process starts, the raw water is above the sand bed, and at first appears to be clean. Thereafter the dirty water, which was trapped in the sand bed, is then expelled. During the flushing process the water will gradually appear cleaner.
Thus it is so important to allow sufficient time during the backwash operation to ensure all impurities are removed from the filter.
Pressure compensated drippers are recommended to operate at a maximum of 75% of the allowable pressure of the dripper so as to protect the dripper diaphragm.
The following norms are recommended:
3.1 Filters
When using a sand filter, a 200 mm control mesh- or ring filter must be placed on the downstream side of the sand filter to catch the impurities in case of damage to the sand filter. The drip manufacturers recommendations must be followed when using a ring- / mesh filter.
The present norms should be adjusted as follows (Van Niekerk, 1983):
The maximum allowable flow rate through a clean sand filter:
- Flow rate 50 m3/h per m2 with a maximum pressure drop over the sand filter of 10 kPa
The maximum allowable pressure drop over a sand filter with ring-/ mesh filters:
- Total pressure drop over a clean filter bank (including sand - and ring filter) 40 kPa
- The maximum allowable pressure difference over the filter bank before backwashing should be 60 kPa
- When using a ring-/ mesh filter, then the maximum allowable pressure drop norm as described in section 2.2 must be complied with
3.2.1 Minimum emission uniformity (EU)
Design EU 90%
The method for calculating pressure band is discussed in detail in the Irrigation Design Manual (2003) edition.
The minimum emission uniformity (EU) is used for calculating the available pressure band for the lateral and manifold diameters.
The emission uniformity is used to calculate the pressure band as the maximum design flow variation norm amongst others does not make provision for the manufactures coefficient of variation (CV) of dripper systems.
Each manufacturer of drippers is responsible to supply the required information (e.g. CV) to designers to calculate the pressure band.
3.2.2 - 10% Flow variation:
If the dripper you design have a CV of 5% and better the use of a 10% design flow variation is still acceptable.
As long as you remember that 10% flow variation as norm does not necessarily have the most economic system as a result.
If the CV of the dripper you design for exceeds 5% it will be necessary to use the method referred to in the Irrigation Design Manual (2003) If you don't the flow variation will exceed the 10% norm.
3.3 Flow velocity of laterals
A minimum flow velocity of 0.4 m/s at the furthest lateral end point is required. (T-Tape, 1998)
Sprinkler Irrigation
During the design stage, especially with moveable sprinkler systems, it is important that the designer can interpret the available water holding capacity and infiltration rate of the soil.
The following norms are proposed:
4.1 Minimum gross application rate
- Moveable systems - 5mm/h
- Permanent systems - 4mm/h
4.2 Maximum pressure variation
- £ 20%(Jensen, 1983)
4.3 Christiansen uniformity co-efficient (CU)
The CU- value of a specific sprinkler is influenced by the proposed operating pressure and spacing, and will give an indication of the uniformity of water distribution in an irrigation block.
The sprinkler spacing and operating pressure are chosen from a manufacturers catalog, bearing in mind the norms applicable to the CU -value.
The following norms are applicable for wind still conditions (Keller, 1990):
- CU 85% for vegetable crops
- 75% CU 85% for deep rooted crops e.g. lucern
- CU 70% for tree crops
- When applying chemicals through the system, the CU should be 80%.
For windy conditions the following adjustments should be made:
- Wind speed 0 - 5 km/h, reduces the chosen spacing by 10%.
- Wind speed greater than 5 km/h; reduce the chosen spacing by an additional 2.5% for every additional 1.6km/h wind speed.
Centre Pivot
The selection of a sprinkler package is a multi - disciplinary process involving the interpretation of the infiltration capabilities of the soil and determination of irrigation requirements.
The choice of specific bandwidths, pressure regulators and electrical motor for specific situations depends on the manufacturers specifications.
A new index for the evaluation of emitter delivery rate on centre pivots is proposed (Van der Ryst, 1990):
The following norms are proposed:
5.1 Christiansen uniformity co- efficient (CU)
- Emitter-CU 95%
5.2 Friction through centre pivot
- 2.5%(m/100m) over centre pivot length.
5.3 Effective radius of end gun
- 75% of the wetted radius of the end gun.
Flood Irrigation
Although flood irrigation appears to be a relatively simple system, it requires various design information to ensure a well-designed scheme. The infiltration rate of the soil must be thoroughly investigated and the results thereof taken into account during the planning phase of the system.
A runoff control plan must be implemented to ensure that rainwater is kept away from the irrigation area. During the planning phase remember that during construction not more than 20cm of topsoil must be removed during the construction of beds.
The following norms are proposed:
6.1 Slope of beds
- Slope along the length of the field must be < 0.7% to prevent erosion unless an insitu test is done.
- The slope across the width must be = 0% for basin and border irrigation.
6.2 Allowable flow depth in beds
- 50mm flow depth 150mm