Fact Sheet

FS 51/99

Setting up a boom spray PDF (63kb)

Introduction

Agricultural pesticides play an important part in maximising production from field crops and pastures. Products used today, when compared with those used 10 or so years ago, are generally more effective at lower dose rates and require greater application accuracy.

The aim in applying any pesticide is to effectively distribute the appropriate amount on the target to obtain the desired result while minimising non-target area contamination.

To achieve this it is essential that boom sprayers are correctly set up, maintained and calibrated so that the full benefit of the applied pesticide is obtained. See fact sheet Boom sprayer cleaning, maintenance and calibration.

Many factors can influence the performance of a boom sprayer. These include:

• nozzle selection (fan angle, size, material),
• nozzle type and operating pressure,
• boom height,
• boom stability,
• alignment of nozzles on the boom,
• filtration,
• water or carrier application rates,
• speed of operation,
• marking systems.

Nozzle selection

Fan angle of nozzles

The fan or spray angle of a flat fan nozzle refers to the angle of the spray sheet as it leaves the nozzles. The fan angle of standard flat fan nozzles is measured at a pressure of 280 kPa. Lower pressures decrease the fan angle and can affect distribution across the boom.

Most agricultural pesticides are applied using flat spray nozzles with a fan angle of either 80 or 110°. For general use a nozzle with a fan angle of 110° is preferred. The main advantage of using a 110° nozzle is that double overlap of spray patterns can be obtained at lower boom heights. There also tend to be slightly more and smaller droplets at a given pressure, which can mean better coverage but it can also mean a greater potential for droplet drift.

Nozzle size

The nozzle size, spacing and speed of travel will determine the amount of spray mixture to be applied by the boom sprayer to a given area, provided the operating pressure remains constant. You need the following information to select a nozzle of the correct size.

• The total volume of spray mixture (water or other carrier and the pesticide) to be applied to each hectare (that is the total boom output).
• The operating speed of the boom sprayer.
• The nozzle spacing on the boom.

To determine the nozzle output required:

Individual nozzle output (L/min) =

boom output (L/ha) x speed (km/h) x nozzle spacing (cm)
                                     60,000
 
Example:
 
The required boom output is 45 L/ha, the ground speed is to be 12 km/h and the nozzle spacing is 50 cm.
 
The nozzle output needed = 45 x 12 x 50 = 0.450 L/min
                                                    60,000
 
The correct nozzle can then be selected by consulting charts available from nozzle manufacturers or resellers.
 
Using nozzle bodies with either diaphragm or ball-spring check valves is preferred. (Diaphragm and ball-spring check valves are also called 'non-drip' nozzle assemblies.) These avoid overdosing of patches when the unit is turned off. However, they may lower the pressure at the nozzle and this should be considered when selecting the operating pressure.

Materials used for nozzle construction

Nozzles are made from a range of materials, some of which will wear quicker than others. Worn nozzles give a narrower spray pattern, uneven distribution across the boom and a greater nozzle output in a given time. They should be replaced.

Relative durability of nozzles made from different materials:
 
Most durable 

• sintered alumina
• ceramic
• thermoplastic
• hardened stainless steel
• plastic/nylon
• stainless steel

Least durable

• brass

Nozzle wear rates are difficult to estimate because of the range of product formulations and water qualities used. Wettable powders cause greater nozzle wear than do emulsifiable concentrates. Water containing very fine sand or clay particles will also hasten wear. It is important to check the output of nozzles at the beginning of and at least once during the spraying season. Worn nozzles or those seen to be giving poor spray patterns should be replaced.

Nozzle type and operating pressure

Standard flat fan nozzle

Most flat fan nozzles are designed to operate at a pressure of 200 to 300 kPa at the boom. This is the preferred range. Pressures lower than 175 kPa tend to cause a deterioration in the spray pattern. At pressures greater than 350 kPa the higher number of smaller droplets produced could cause drift.

When using standard flat fan nozzles the boom output should not be changed by altering the pressure, except when only small variations in output are required. The nozzle output is proportional to the square root of the pressure at the nozzle. This means that to increase nozzle output by 25 per cent, the pressure would have to be increased by at least 50 per cent which in most cases would then be outside the manufacturer's suggested operating pressure for that nozzle.

Low pressure (LP) flat fan nozzle

LP nozzles are designed specifically to operate at pressures as low as 70 kPa. At 100 kPa LP nozzles will provide the same fan angle, flow rate and liquid distribution as will equivalent standard flat fan nozzles at 275 kPa.

LP nozzles produce droplets up to 50 per cent larger than do equivalent flat fan nozzles. This reduces the number of droplets liable to drift. However, as a result of the larger and fewer droplets produced, LP nozzles are more suited to applying pre-emergent soil incorporated and post-emergent translocated broadleaf herbicides than to applying selective grass herbicides.

Lower operating pressure and larger orifices reduce nozzle wear and the chance of blockages.

Extended range (XR) flat fan nozzle

XR nozzles are designed to provide a consistent fan angle and liquid distribution pattern over a range of operating pressures (100 to 400 kPa). At lower pressures XR nozzles produce larger droplets with less drift potential and at these pressures should be used only with pre-emergent soil-incorporated and post emergent translocated broadleaf herbicides.

At higher pressures the droplets produced by XR nozzles are smaller, and these nozzles can be used for general post-emergent herbicide applications.

A change in operating pressure will produce a change in nozzle output. For example if ground speed remains constant, an increase in operating pressure from 100 to 400 kPa will double the application rate per unit area.

It is therefore very important to calibrate the boom sprayer and match ground speed with operating pressure to ensure that the application rate remains constant.

Boom height

Boom height refers to the height of the nozzle above the target. The target height can vary (for example soil, broadleaf weed rosettes or grass-seed heads). Therefore, the boom height must be adjustable to ensure double overlap at the top of the target.

The fan angle of the nozzle and the nozzle spacing on the boom will determine the optimum boom height.

Table 1: Theoretical optimum height for nozzles above the target (with a nozzle spacing of 50 cm).

Nozzle fan angle	Nozzle height above target to obtain	Nozzle height above target to obtain
(degrees)	double	one and half overlap
80	60	45
110	35	26

If the distance between nozzles is less than 50 cm, the boom height could be lower. However, there is less effect on spray distribution across the boom if the nozzle height is greater than the theoretical optimum height rather than below the optimum height. In practice this means that with a 50 cm nozzle spacing, and using nozzles with a fan angle of 110°, a nozzle height of 50 cm above the top of the target should give acceptable spray distribution. The effects on distribution of any fluctuations in boom height during the spraying operation will be reduced.

Boom stability

One of the main limitations on the operating speed of conventional boom sprayers is boom stability. Both vertical and horizontal movement of the boom will affect the distribution pattern.

Several methods have been adopted by boom sprayer manufacturers to improve boom stability. These include outrigger wheels or skids, support wires and frames, self-levelling booms (these must be able to be pinned for spraying across slopes), hydraulic damping of the boom, low-pressure wide-profile tyres, wider wheel tracks and various configurations of tandem axles. No one method is perfect but various combinations of methods can improve stability.

Before selecting a boom sprayer test the unit under the field conditions in which it will be operating.

Alignment of nozzles on the boom

To prevent the spray patterns from adjacent nozzles clashing and causing uneven distribution, offset the nozzles 10 to 15° to the spray boom.

Snap-on self-aligning nozzle fittings are available. These have a self-locking mechanism that is pre-set to give the required alignment to the boom, and they are preferred. Snap-on nozzles have the added advantage of being quickly and easily cleaned or changed.

Filtration

Good filtration is essential to prevent blocked nozzles and uneven distribution patterns. If lower rates of spray mixture are applied, lower output nozzles with smaller orifice sizes should be used. These require an efficient filtration system to prevent blockages. Four stages of filtration are preferred.

• Filling the tank - 80 to 100 mesh filter - to remove foreign material and lumps of wettable powders.
• In-line filter - suction line to pump - 80 to 100 mesh filter - this should be a large-capacity filter and cleaned frequently to avoid pump cavitation.
• In-line filter - outlet side of pump - 100 to 120 mesh - this should also be a large-capacity filter.
• Nozzle filter - usually 80 to 100 - this filter depends on the nozzle size and the type of product being using (wettable powder or emulsifiable concentrate) and should be as recommended by the nozzle manufacturer.

Water or carrier rates applied

Many agricultural chemicals do not have a recommended water volume on the label. If a suggested volume is on the label then this should be the water volume used.

If water volumes lower than those recommended on the product label are used, then possible compensation claims against the manufacturer are unlikely to succeed.

Products with no suggested volume on the label generally have obtained registration from trial work using water volumes of 70 to 150 L/ha.

However, farmer experience and limited research have shown that lower water volumes are possible provided the boom sprayer has very good filtration, good agitation to ensure thorough mixing of product and carrier, is fitted with suitable nozzles and is accurately calibrated. If lower water volumes are to be used, reduce volumes gradually to avoid problems with application techniques.

Ground speed

The ground speed at which spraying is performed and the nozzle size and spacing have the greatest influence on water volumes applied to a hectare. The speed used should be one with which the operator feels comfortable. The upper limit, depending on spray boom stability, is probably 15 to 20 km/h with current application equipment. If ground speed is increased to reduce water volumes, consider the type and size of nozzle used to ensure that the number of droplets hitting the target is sufficient to obtain the desired result.

Check the ground speed and boom calibration over a measured distance, preferably on a surface the same as or similar to that on which the actual spraying will be done. Wheel slip and non-standard tyres on the towing vehicle can affect ground speed at a given engine speed.

Marking systems

Some form of marking system is essential to apply pesticides accurately with a boom sprayer. Several marking systems are used:

• disc or mechanical markers,
• foam, paint or dye markers,
• seeding equipment marks, sight markers or 'tramlines'.

No system is completely foolproof but the use of any marking system is better than not using one at all.

Selection of a system depends on user experience and preference.

Disc or mechanical marking systems usually are reliable and have low capital and operating costs. However, they can be affected by soil type (poor marking in hard soils) and are often heavy and awkward to handle and transport.

Foam and paint or dye markers are easy to operate once set up, are not affected by soil type and do not cause crop damage. However, they tend to be expensive, do not easily show the deposit on all surfaces and have in the past often been unreliable.

These marking systems are reasonably successful on narrow booms, but a system of mirrors would improve their performance on wider booms (more than 15 m).

Seeding equipment marks can be used if the boom sprayer is set up to cover the same width as does the seeding equipment.

Sight markers (such as discs or posts placed at the end of each run) can be useful but are suitable only if the paddock is level or fairly small.

'Tramlines' are widely used in Europe. Appropriate drill rows are left unsown, leaving a track for the boom sprayer to follow. This method is accurate but does need careful planning and machinery matching.

Last update: July, 2007

Author: Graham Fromm, Regional Plant Protection Agronomist.

Agdex: 744

Disclaimer

Use of the information in this Fact Sheet is at your own risk. The Department of Primary Industries and Resources and its employees do not warrant or make any representation regarding the use, or results of the use, of the information contained herein in terms of its suitability, correctness, accuracy, reliability, currency or otherwise. The entire risk of the implementation of the information which has been provided to you is assumed by you.  All liability or responsibility to any person using the information is expressly disclaimed by the Department of Primary Industries and Resources and its employees.

Setting up a boom spray

ISSN 1323-0409