Waste (noun): an act or instance of using or expending something to no purpose.
In agriculture, environment and economy are intertwined. Producers strive to obtain the maximum return on their inputs. They study incremental returns and avoid applying more inputs than necessary, especially if conditions don’t warrant it. The financial incentive is powerful, and waste is a four-letter word. This applies to seed, fertilizer, and pesticide. Pesticide labels identify the rate needed to obtain the desired result, and there is no incentive to over-apply. In fact, it’s illegal.
But there are plenty of other places where applications incur waste. As with time efficiency, it’s a good idea to identify where this waste occurs, and the only tool needed is a sharp pencil.
When might we incur waste in the spray application process?
Mixing more than we need because we don’t trust the flow meter or the tank gauge entirely, or don’t know the exact field size.
Priming the boom before the first swath.
Overlapping due to curvy terrain and coarse sectional control.
Spray drift away from the intended swath.
How big are the losses?
Let’s say we have a clean sprayer and need to spray 160 acres before moving to a new crop and product. We plan to apply 10 gallons per acre and have a 1,200 gallon tank with a 120 foot boom. That means we need 1,600 gallons of spray mix in total.
Once we’re at the field we prime the boom. Each sprayer is different, but depending on operator experience, 30 to 50 gallons are usually needed to push product from the tank to the last nozzle. Only part of that is lost to the ground, as boom sections can be shut off as soon as product has reached every nozzle of that section. We’re assuming 0.2 gallons per foot of boom is lost.
Spraying itself is relatively straightforward. Swath and sectional control handle the overlaps, but in less ideal terrain, double application is known to account for 4 to 5% of the area to reach non-square parts of the field. This is even more likely when the outer section is 10’ or more. Early turn-on of the boom prior to leaving the headland, to allow boom to reach operating pressure, adds to this.
Air-activated shutoff for individual nozzles reduces section size at a reasonable cost.
With an average nozzle, we can expect about 2% of the product to airborne drift. Most airborne won’t return to the ground within the field borders, so it’s a complete loss.
Most of the spray that travels more than 5 m after leaving boom stays airborne and should be considered a total loss from the field.
As we finish, the pump will draw air before the tank is empty due to sloshing or foaming, and a 50 to 60 gallon remainder may not be unusual. This simulation has assumed 5% of tank volume remains.
We also need to purge spray from the boom at cleanout, consuming approximately 0.4 gallons per foot of boom. This occurs after the field is completely sprayed and is therefore considered waste.
So how does this add up? The following table shows the approximate losses associated with five setups.
Table 1: Spray mix losses during a sprayer operation. Setup 1 = baseline, Setup 2 = low application volume, Setup 3 = baseline with recirculating boom, tank level monitor, and low-drift nozzles, Setup 4 = large area between cleaning, Setup 5 = large area with recirculating boom, tank level monitor, and low-drift nozzles.
In the first scenario, we spray just 160 acres at 10 gallons per acre. Priming the boom with 0.4 gallons per foot (allowing for all associated feed lines) consumes 48 gallons, but only wastes half of that, or 1.5% of the total volume needed for the field.
Four percent overlap consumes another 64 gallons.
If we have 5% of the tank volume left over, that’s 60 gallons. That amount is so small it doesn’t even register on the sight gauge but nonetheless it represents another 4% of the total sprayed amount.
Upon cleaning the boom, we need to push the spray mix out of all the plumbing after the pump, as it has nowhere else to go. At an assumed 0.4 gallons per foot, that’s another 48 gallons or 3%.
If we add to that a conservative 2% drift loss, it sums to a surprising 14% of the total spray volume. For those that use lower water volumes (the second scenario), the volumetric losses are slightly less, but their proportion is higher, now accounting for 23% (!) of the total spray mix.
In the third scenario, let’s assume we use a recirculating boom that returns the initial prime volume to the tank, eliminating any waste. We’ll also upgrade to individual nozzle sectional control, reducing overlap to 1%. And, since we want to know exactly what’s left in the tank, let’s invest in an AccuVolume system to precisely monitor tank volume. This allows us to make small rate adjustments up or down to be sure as much of the mixed product goes onto the sprayed swath as possible.
Recirculating booms allows the spray mix to pass through entire length of boom without being sprayed, saving waste during priming and allowing waste-free boom rinses.
When the sump begins to empty, we can introduce some water from the clean water tank to push the last of the mix to the boom (a continuous rinse system makes this easy).
An AccuVolume sensor shows the exact volume left in the tank at any slope position and with 1 gallon resolution, allowing greater accuracy when filling and emptying.
We’ll assume our sump waste is now reduced to 12 gallons. We still need to dispose of the content of the boom somehow, so the recirculating boom offers no saving there. But let’s also add better low-drift nozzles to reduce drift by 50% (now 1% total volume). Total loss is now just 6%.
Low-drift nozzles such as this AirMix (Agrotop) SoftDrop reduce airborne drift by 50 to 90%.
The last two rows in the table repeat the first and third scenarios for a larger sprayed area (1000 acres) before a tank cleaning is needed. This doesn’t change the magnitude of the volumetric loss, but reduces its proportion. Percent loss is down by a factor of two from the 160 acre interval, to 3 to 7%.
Experienced operators might cheat the system a bit by mixing the required pesticide with some extra water to make up for the plumbing waste. Doing so prevents extra pesticide from being consumed, but it doesn’t reduce the inherent inefficiency.
Lessons
This exercise suggests that waste from spraying is probably higher than we assumed. If we average the scenarios, there is 10 to 15% waste. At, say, $200,000 spent on pesticide for a single spraying season, that’s $20 – $30,000 worth of product and water hauled that ends up where it doesn’t belong. Beyond the time and money, there can also be environmental consequences depending on how that waste is treated.
Improve monitoring of tank content to allow lower remainders.
Consider individual nozzle shutoff to improve sectional control. These are part of Pulse Width Modulation (PWM) systems, but can also be achieved with less expensive valves.
Plan spray operations to minimize the amount of product changeovers.
Consider direct injection.
The return on investment for plumbing improvements can be high and result in considerable future savings over the life of the sprayer. It’s worth thinking about.
A properly-sized pump should produce more flow than is needed and work in conjunction with the atomizers to regulate that flow. Typical to high pressure pumps, a piston relief valve (aka regulator) should maintain the desired system pressure through the normal speed range of the sprayer, regardless of the number of booms (or boom-sections) that are on or off. This is achieved by balancing the sprayer pressure against the relief valve spring, which must move freely across a range of flows.
But what does it mean when the pressure gauge briefly spikes off-scale when boom are turned on or off? This is bad for the gauge and will eventually cause it to fail. Quite often, pressure spikes are an indication of one of two things:
A dirty or stuck valve
An inappropriate spring size
A pressure gauge spiking beyond its range.
Relief valve maintenance
Sometimes, pressure spikes indicate a need for valve cleaning and maintenance.
The regulator spring cavity may be packed with dirt, which limits valve travel. Clean the housing and spring, and then lubricate and adjust.
The regulator may be partially seized or sticky. If the regulator piston and cylinder bores are caked with spray they will ‘hold’ the valve until the pressure/spring balance overcomes the friction.
Sometimes valve, and/or the valve guide pin are seized. Disassemble them, clean all sliding surfaces, then lubricate and adjust.
Valve/seat wear may have created a leak. You may have already tightened the spring to compensate, but this loads the spring past the pressure balance point you want to spray at. This means that when the booms are shut off, the pressure increases until it reaches the ‘new’ spring balance point. Repair (or replace) the regulator, then lubricate and adjust. Be aware that any leak (external or internal) can contribute to this condition and tightening the spring isn’t the solution.
The spring may be damaged (e.g. bent, corroded, etc.). Replace the spring, lubricate and adjust.
Note: Be sure to read the operator’s manual before you do anything. You should understand your sprayer’s design before you perform any maintenance, adjustments or calibration.
Spring size
Sometimes, the relief valve may be mechanically sound, but the spring may not be sized to match a reduced operating pressure. Relief valve springs match the maximum pressure range of the pump. Sprayers operated at lower pressure may be unable to compress the spring. This is common when people switch from disc-core nozzles operated at higher pressure to molded nozzles operated at lower pressure.
This would manifest when one boom is shut off for single-boom operation; there may not be enough pressure to open the bypass. As a result, flow increases over the remaining boom.
Recognizing this problem, some operators have teed-in a second relief valve capable of finer adjustments at lower pressures. Make sure you know what you’re doing if you’re considering this option.
Technically, a spring can either be too weak, or too heavy:
The spring may be too weak for the pressure being used (i.e. any adjustment bottoms out). In order to obtain sufficient pressure the operator tightens the spring until it is virtually collapsed, essentially creating a fixed orifice. When the booms are closed the ‘fixed orifice’ doesn’t compensate and pressure rises to force the increased flow through that small orifice.
If the spring is too heavy for the pressure being used (any adjustment barely touches the spring when pump is turned off). In this case, the pressure being used will not deflect the spring, so the operator closes the regulator until the ‘fixed orifice’ creates sufficient restriction to flow to achieve the desired pressure. When the booms are closed the ‘fixed orifice’ doesn’t compensate and pressure rises to force the increased flow through, or until the spring begins to deflect.
In either situation the spring must be sized so it is in the centre-third of its flex range (i.e. rest state > fully collapsed) at the desired pressure. You can buy springs from the sprayer dealer or hardware supply. Try to maintain original length and diameter of the coil, while varying the diameter of the wire.
Engineering
In some cases, it is not a matter of valve maintenance, or spring size, but poor engineering. Consider the following:
The valve supply and return may be too small for the pump flow. Consult hose and fitting catalogs for flow capacities and lengths. Re-size the hoses and fittings appropriately, and then adjust the regulator.
There may be kinks or sharp bends in in the supply and return lines. Re-route the hoses and/or fittings to avoid kinks and sharp bends, and then adjust the regulator.
The relief valve may be too small for the pump flow. Consult a regulator catalog for flow capacities and replace the regulator with an appropriate size. Calibrate the regulator spring and adjust.
Relief valves have a ‘cracking’ pressure (that’s when the valve just starts to open). Well-designed regulators have small pressure changes from ‘cracking’ to full flow. That information is in their catalogs. Poorly designed regulators have large pressure changes between these two ratings and these regulators should be avoided.
The pump may be too big for system. This often happens when sprayers are upgraded and pumps are replaced. Consult the catalogs and reduce pump size or speed, or increase the sizes of the hoses, fittings and regulator.
There may be a hydraulic agitator jet on the regulator ‘tank’ line. An agitator jet applies considerable back pressure to a system, and when booms are closed the increased flow causes more than a linear increase in pressure.
Broadly, the sprayer system as a whole may be poorly engineered. Inspect and draw a flow path of the sprayer system. Examine where everything is going (or not going). Is it possible someone made changes that the manufacturer did not intend? Consult the manufacturer if you are uncertain. Sometimes, it will have to be re-engineered, which may require expert consultation.
Note: Your pressure gauge can tell you a lot more than your operating pressure – it can indicate a problem with your regulator, pump, lines or overall sprayer engineering. Don’t ignore it – address it.
Thanks to Murray Thiessen, Consulting Agricultural Mechanic, for his contribution to this article.
The pump is the heart of the sprayer and a key component for producing the flow of spray material and sprayer output. Because various spraying situations require different pressures and flow rates, using the correct sprayer pump is essential to achieving desired results. In addition to sprayer considerations, a pump must also be durable enough to withstand harsh chemicals that may cause excessive wear. Even though pumps with added chemical corrosion protection are more expensive, they are a popular choice because of their durability.
Roller, centrifugal, diaphragm, and piston pumps are commonly used to apply crop protection products. Centrifugal and roller pumps are typically used for low-pressure sprayers, and diaphragm and piston pumps are more popular when high-pressure sprayers are needed (i.e., vegetables, orchards, etc.). Less common pump types include squeeze, gear, and turbine.
Pumps are typically either ground driven or powered by main or auxiliary engines, power takeoff (PTO) shafts, or hydraulic pumps. The choice of pump depends on the material to be pumped and the capacity or volume needed. However, no particular type of pump is ideal for all purposes.
Sprayer pumps can be divided into two general categories: positive displacement and non-positive displacement. Positive displacement pumps (roller, diaphragm, and piston) maintain a flow output directly proportional to the pump speed. These pumps require a pressure-relief valve and a bypass line for proper performance. Non-positive displacement pumps do not have a proportional output flow to pump speed and do not require a relief valve and bypass line. The centrifugal pump is an example of a non-positive displacement pump style. A summary of common pump types and characteristics is found in the following Table (contributions from ACE Pumps Corporation, Hypro Pumps Inc., and CDS-John Blue Company).
Characteristic
Roller
Centrifugal
Diaphragm
Piston
Ground Driven Piston
Cost
Low
High
Medium
High
High
Displacement
Positive, self priming; Requires relief valve
Non-positive, needs priming; Relief valve not req’d
Positive, self-priming; Requires relief valve
Positive, self-priming; Requires relief valve
Positive, self-priming; Relief valve not reg’d. Runs off drive wheel and can be lifted on hydraulic-controlled applicators, or can be purchased with clutches to to disengage pump when flow is not desired.
Drive Mechanism
PTO, gas engines, electric motors
PTO, hydraulic drives, gas engines, electric motors
PTO, hydraulic drives, gas engines
PTO, gas engines, electric motors
Primarily ground-driven. Although less common, can be used with hydraulic drives, electric motors or gas engines.
Adaptability
Compact and versatile
Good for abrasive materials; Handles suspensions and slurries well.
Compact for amount of flow and pressure developed.
Wide range of spraying applications; Dependable
Wide range of spraying applications from clear liquids to suspensions. Very accurate regardless of ground speed or back pressure. Very dependable.
Durability
Parts to wear; replace
Very durable, not much wear
No corrosion of internal parts
Parts to wear; replace
Very durable. With basic care and maintenance, pumps can easily be in service 30 years or more.
Serviceability
Easy to work on, repair
Basic maintenance extends life
Low maintenance
Potential for high maintenance
Low maintenance
Pressure Range
up to 300 psi
up to 180 psi
up to 725 psi
up to 400 psi
up to 120 psi
Output Volume
2 to 74 gpm; high volumes for size; proportional to pump speed.
up to 190 gpm; High volumes for size and weight; Proportional to pump speed.
3.5 to 66 gpm; Proportional to pump speed.
up to 10 gpm; Proportional to pump speed, independent pressure.
0.5 gpm to 68.4 gpm.
Revolutions per minute
540, 1000
Requires speed-up mechanism. Very efficient at higher speeds; up to 6,000 rpm.
540
540
Ground-driven. Maximum 450 rpm.
Notes
Best choice by farmers.
If hydraulic-driven, no PTO required. Popular in commercial ag. applications. Running pump dry i s a problem.
Good for higher pressure requirements. Popular for horticultural applications. Pump can run dry.
Similar to an engine; Low capacity.
No gpa flow variation due to pressure or ground speed changes. No concern of electric failures on controllers or radar systems. Dependable accuracy.
Pump Efficiency
Regardless of the type of pump, the necessary flow rate must be provided at the desired pressure. Enough spray liquid should be pumped to supply the gallons per minute (gpm) required by the nozzles and the tank agitator, with a reserve capacity of 10 to 20 percent to allow for flow loss as the pump becomes worn. Unfortunately, pumps lose efficiency for a number of reasons, such as drive friction or leakage.
When estimating the pump horsepower needed for an application, efficiency (Eff) of 40 to 60 percent should be assumed. The horsepower (HP) required to drive the pump can be estimated by using the following formula:
HP = (gpm × psi) / (*1,714 × Eff) *Constant derived when converting gallons, minutes, pounds, and inches to horsepower.
Example: How much horsepower is required to run a pump if the maximum output is 50 gpm at 40 pounds per square inch (psi)? Assume a pump efficiency of 40 percent.
HP = (50 gpm × 40 psi) / (1,714 × 0.40 Eff) HP = 2.92
Because of inefficiencies of the drive units, electric motors should be approximately one third larger than the calculated horsepower. Gasoline engines should be one half to two thirds larger than the pump horsepower required. Ground-driven pumps that vary flow rates as ground speed changes are accurate and dependable; they are often used when applying high volumes of materials such as fertilizer.
Many pumps are PTO driven, but most modern spray pumps are hydraulic driven because of mounting versatility, ease of maintenance, and customization for individual sprayers. Charts are available to match pumps to various tractor hydraulic systems. You can access these charts by following the links to the following major pump manufacturers:
Proper pump size is an important consideration when selecting a sprayer pump. Requirements for nozzle capacity, hydraulic agitation, and overcoming the efficiency loss noted previously are essential points to consider. Nozzle capacity is determined by multiplying the number of nozzles on the boom times the output (gpm) of each nozzle for a specific application. Be sure to give consideration to the range of spray pressures that will be used for the given application. Agitation requirements typically account for another 5 percent of the sprayer tank capacity. Efficiency losses due to friction and pump wear may account for an additional 10 to 20 percent increase in the required flow rate. Spray pump manufacturers provide useful Web page worksheets to help determine pump sizes based on typical field application scenarios.
Manufacturers also make product guides available to help match sprayer pumps and hydraulic motors to the tractor’s hydraulic system (Table 2). A simple pump selection worksheet is provided at the end of this article.
No matter what type pump is used, it must be plumbed to route liquid from the pump to the spray boom with a minimum amount of restriction, a necessity for achieving the pump’s maximum rated capacity. The hoses should be the same size as the pump’s suction and discharge ports. Other recommendations include installing a pressure gauge and valve on the pressure side of the pump to measure the shut-off pressure and using a minimum number of elbows, fittings, and valves to reduce pressure losses.
Following these guidelines is necessary for delivering the highest pressures to the boom.
Pump Rotation
Pump rotation is critical for PTO and belt and- pulley driven pumps. The direction of rotation is always determined when facing the pump and drive shaft, and pumps are available in both clockwise and counter-clockwise rotation. Thus, when direct coupling shafts, the opposite rotation pump should always match the shaft. When mounting a pump with belts and pulleys, either pump rotation can be used to match the drive shaft rotation and the desired direction of the pump. Gasoline engine and electric motor shafts rotate in a counter-clockwise direction, and a tractor PTO shaft rotates in a clockwise direction.
Pump Types
Roller pumps are popular for small sprayers because of their low initial cost, compact size, ease of repair, and efficient operation at PTO speeds of 540 and 1000 revolutions per minute (rpm). Roller pumps are self-priming, positive displacement pumps, and a variety of models is available. Maximum outputs range from 2 to 75 gpm, and pressures range up to 300 psi.
Figure 1 – Roller Pump
Roller pumps are usually constructed with cast iron or corrosion resistant housings (non-symmetrical in shape), rotors, four to eight rollers (either nylon, Teflon, or rubber), and seals (Viton, rubber, or leather). The type of material selected depends on the chemical being pumped. A typical roller pump is shown in Figure 1.
Nylon or Teflon rollers are the most resistant to agricultural chemicals and are recommended for multipurpose sprayers. Rubber rollers are preferred when the pump is used only for water solutions and wettable powder slurries at pressures less than 100 psi. Because sand and scale are abrasive to the rollers, the solution being pumped must not contain these materials. Polypropylene rollers wear better than either nylon or rubber rollers when applying weak solutions or solutions with little or no lubricating qualities.
Some operators have experienced problems with excessive wear of the rollers, especially when using wettable powders. Other operators have achieved long pump life by allowing the pump to run continuously when spraying with wettable powders, and by properly maintaining and storing the pump, including keeping abrasive materials out of the sprayer. Specific seal, roller, and casting materials can be selected for compatibility with certain herbicides, insecticides, fungicides, and fertilizers Consideration should also be given to the adjuvants used in the spray solution.
Centrifugal pumps are the most popular type of low-pressure sprayer. They are durable, simply constructed, and can readily handle wettable powders and abrasive materials. Because of the high output of centrifugal pumps (70 to 190 gpm), the spray solution can be agitated sufficiently even in large tanks at pressures up to 180 psi. The initial cost of a centrifugal pump is somewhat higher than that of a roller pump, but its long life and low maintenance make it an economical choice. Pump housings of cast iron, stainless steel, and polypropylene are advantageous because they withstand strong chemicals. Stainless steel pumps are ideal for use with glyphosate or other acid applications. Polypropylene pumps are lightweight and provide excellent resistance to corrosive chemicals. Figure 2 shows a typical centrifugal pump.
Centrifugal Pump – Exploded View.
Because centrifugal pumps are not self-priming, they should be mounted below the supply tank to aid in priming. In addition, a small vent tube should be installed from the top of the pump housing to the supply tank. This positive vent line allows the pump to prime itself by “bleeding off” trapped air upon starting and when the pump is not operating.
The inlet of a centrifugal pump should never be restricted. A partially clogged suction strainer, collapsed suction line, or a suction line with insufficient capacity causes a loss of pressure control and possible damage to the pump. Centrifugal pumps can handle small pieces of foreign material without damage, so a suction strainer is not always required. If a suction strainer is used, however, it must be capable of handling the large capacities of the pump with a minimal drop in pressure across the strainer, and it must be cleaned frequently. Typical centrifugal pump plumbing would place the strainer on the pressure side of the pump.
Centrifugal pumps for low-pressure sprayers can generate pressures of up to 70 psi when the impellers are running between 3,000 and 4,500 rpm. The output volume drops off rapidly when the outlet pressure exceeds 30 to 40 psi. The decrease in volume is an advantage because the nozzle pressure is able to be controlled without a relief valve. See Figure 3 for a typical centrifugal pump performance curve. The pump performance curve describes the relationship between flow rate and pressure for the actual pump.
Figure 3 – Centrifugal Pump Performance Graph
The need to operate at high impeller speeds requires a type of step-up speed mechanism when operating centrifugal pumps from PTO shafts. The simplest and least expensive of these mechanisms is a belt and sheave assembly. Other step-up mechanisms have planetary gears that are completely enclosed and mounted directly on the PTO shaft.
Another method of driving a centrifugal pump is with a close coupled, high speed hydraulic motor. Using the tractor hydraulic system to drive the pump keeps the tractor PTO shaft free for other uses. It is essential to consult manufacturer pump selection guides to match the proper pump to your tractor. Pumps can also be driven by direct-coupled gasoline engines when other drive mechanisms cannot be used.
Airplane pumps may be wind-driven, directly powered from the aircraft engine, or powered by an electric or hydraulic motor. The pump may also power the tank agitation system. For fixed-wing aircraft, the most common type of pump is a wind-driven centrifugal pump mounted under the aircraft (Figure 4). The propeller slipstream drives a fan mounted on the front of the pump. Some fan-driven pumps have variable pitch blades that allow for changing pump speed, and thus output. The centrifugal pumps commonly used on aircraft produce high volumes (up to 200 gpm) at typically low pressure, usually ranging between 10 and 100 psi. These pumps usually require operating speeds from 1,000 to 5,000 rpm.
Figure 4 – Airplane Pump
Diaphragm pumps are popular when higher pressures are needed for applying foliar herbicides, insecticides, and fungicides. Models are available that provide maximum outputs ranging from 3.5 to 60 gpm and maximum pressures ranging from 200 to 700 psi. These pumps are extremely durable because all moving parts are sealed in an oil bath and spray solutions. Diaphragm pumps are self-priming and considered positive displacement pumps. Figure 5 shows a typical diaphragm pump. Smaller electric diaphragm pumps (Figure 6) are available for use by homeowners, ranchers, and hobbyists to apply pest control products. A good example is a spray system mounted on an ATV for spraying pastures and rights-of-way.
Figure 5 – Diaphragm Pump
Piston pumps are positive displacement pumps that are favored by many users for their priming ease, higher pressure capability, and constant volume spraying. Piston pumps are often used to apply crop protection products and fertilizers in combination with a ground drive so that flow rate stays proportional to ground speed and application rates remain constant. A pressure relief valve is required, though. Figure 7 is an example of a piston pump used to accurately meter liquid fertilizers.
Figure 7 – Piston Pump
Turbine pumps are also available for low‑pressure sprayers. A turbine pump consists of a rotating turbine within an enclosed housing. These pumps are similar to centrifugal pumps, except they provide higher flow capacity and pressures of up to 70 psi when mounted directly on a 1,000 rpm PTO shaft, eliminating the need for step‑up mechanisms. Because of the close tolerances between the turbine blades and the casing, turbine pumps are better adapted for clean fluids of low viscosity but may have difficulty with wettable powders and suspensions. Figure 8 shows a typical turbine pump.
Figure 8 – Turbine Pump
Gear pumps are positive displacement pumps capable of providing a smooth, low-volume, continuous flow of material. Gear pumps are typically two gears meshing together revolving in opposite directions within a casing. Abrasive materials such as wettable powders rapidly wear the gears and pump housing. Figure 9 shows a typical gear pump.
Figure 9 – Gear Pump
Squeeze pumps are low-pressure, positive displacement pumps with output proportional to speed. Pump flow is created when liquid is trapped by squeezing the hose between a roller and casing. Pump flow is determined by the size and number of hoses. This pump is ideally suited for metering small quantities of fertilizers or pesticides and would be practical for injection-type pumping systems. Figure 10 shows a typical squeeze pump.
Figure 10 – Squeeze Pump
Pump Maintenance
Proper pump maintenance is critical for maximum pump life. Regular cleaning is essential to removing all chemical residues and preventing wear to the pump from corrosive solutions. Do not allow spray solutions to remain in the sprayer for extended periods of time. Using lightweight antifreeze or a motor oil as the final spray solution after cleaning can preserve the pump during a period of non-use.
Pump Selection Worksheet
Acknowledgements
Excerpts for this article were adapted with permission from University of Illinois Circular 1192 developed by Loren Bode and Jack Butler (May 1981), Extension Agricultural Engineer and Professor of Agricultural Engineering, Univerity of Illinois at Urbana-Champaign. Contributions for this article were also received from ACE Pumps Corporation; Hypro Pumps Inc.,; and CDS-John Blue Company.
For more information on pump selection, check out this article.
This article was co-written with Murray Thiessen, Consulting Agricultural Mechanic.
Sprayer wheel assemblies should be cleaned and inspected as part of regular annual maintenance. Wheel bearing maintenance before long-term storage may prevent water from corroding the bearings. The exploded diagram details the parts found in a typical trailed air-assist sprayer wheel assembly.
Exploded diagram of typical airblast sprayer wheel assembly.
The following procedure was performed on a 2012 Durand-Wayland sprayer by Mr. Murray Thiessen, Consulting Agricultural Mechanic and renowned “Sprayer Whisperer”. The steps are applicable to most sprayer makes and models. The entire process should take approximately half-an-hour per wheel.
Step 1
Empty the sprayer and park it in a well-lit, level spot. Un-hitch the tractor and raise one side of the sprayer using a bottle or floor jack to clear the wheel. Secure the sprayer with a jack stand.
Raise with one jack, secure with another.
Step 2
Remove the lug nuts and take the wheel off the hub. Do not remove the wheel and hub together because it is heavy and you might bang the delicate seal on the spindle. Check the wheel rim for signs of corrosion or distortion (often caused by either loose or over-tightened lug nuts). Check the tread for wear or cuts and check the tire pressure.
Remove the lug nuts and take the wheel off the hub.
Step 3
Remove the hub cap and pull out the cotter pin. Then remove the nut and washer that hold the hub on the spindle. Put all the small parts in a plastic container with some de-greaser (e.g. Varsol) to clean the parts and keep them from getting lost.
Remove the nut and washer that hold the hub on the spindle.
Step 4
Knock out the seal and hub bearing and put them in the plastic container. Unless it is damaged, there should be no need to remove the bearing cup (or race) from the hub. The seal is designed to keep dirt out of the assembly, not to keep grease from escaping. Be sure to note which way it is facing. The seal is often ruined during disassembly; have a replacement on hand.
Knock out the seal and hub bearing.
Step 5
Clean the old grease out of the hub. This hub has too much and it has filled much of the air space (or cavity) within the hub. That air space is provided so grease is not forced out as the hub heats up, and so dirt is not pulled in as the hub cools. Note the colour of the grease – if it is black and stains your hands, it has burned because too much grease has caused overheating. Look for evidence of dirt or water in the bearing, which indicates seal failure.
Clean the old grease out of the hub.
Step 6
Wipe dirt from the spindle. Never pressure-wash wheels when they are on the spindles because the spray drives dirt and water past the seal and into the hub. Inspect the sealing surface of the spindle for damage or wear.
Wipe dirt from the spindle.
Step 7
Clean the seal thoroughly. Seals are easily damaged and may need replacement.
Clean the seal thoroughly.
Step 8
Clean the hub bearing. Compressed air is a good way to get all the old grease out, but do not spin the bearing with the air.
Clean the hub bearing.
Step 9
Look for scratching, pitting or blue metal (indicating heat). This scorch mark indicates the bearing was moving on the spindle, and the friction created heat. Agricultural wheel bearings do not fit tight to the spindles. If there is too much clearance, the bearing race will turn on the spindle where it is not supposed to.
Look for scratching, pitting or blue metal (indicating heat).
Step 10
Repack the bearings, reassemble the hub and re-grease the hub. Bearings should only be ~40% full. Too much grease creates heat and does not let the bearing roll properly. Too little increases friction. No matter which grease you choose to use, never combine greases; they may not be chemically compatible.
Re-pack and reassemble.
Step 11
Mount the hub tightly on the spindle. Replace the washer, cotter pin, nut and cap. There is no need to bend the arms of a cotter pin all the way back – it weakens the metal. Just bend one arm to 90° and cut off the excess. Use anti-seize on the wheel pilot to make the rim easier to remove next time.
Mount the hub tightly on the spindle.Some airblast sprayers (such as this Durand-Wayland) have wheel assemblies that can be rotated to four different positions in the chassis. This will raise or lower the sprayer to better align it with the tractor hitch and PTO shaft.
Step 12
Replace the wheel and rim. Do not grease the lug nuts or they might loosen. Over- or under-torqueing lug nuts can cause damage. Look in the manual for your correct torque and consider using a torque wrench. Tighten the nuts in a star-shaped pattern – not sequentially.
Funny how some issues never go away. For as long as I’ve been in the sprayer business, the question of ideal droplet size for pesticide application has remained a hot topic. At its root are the basic facts that small droplets provide better coverage, making better use of water, but large droplets drift less. So why are we still debating this? Because we need both of these properties to be efficient, effective, and environmentally responsible. Ultimately, the droplet size question is reduced into one of values, where everyone’s individual priorities play a role.
First, let’s talk about basic principles. To be effective, an active ingredient must make its way from the nozzle to the site of action in the target organism. On the way, it encounters several obstacles as summarized by Brian Young in 1986.
Figure 1: The dose transfer process of pesticides (after Young, 1986)
After atomization and before impaction, the spray encounters two main losses, evaporation and drift. Both of these are more severe for smaller droplets. Smaller droplets have a greater ratio of surface area to volume for any given spray volume, and can evaporate to a much smaller size, even to dryness depending on the formulation, in seconds. For water-soluble formulations, one consequence is lower uptake. Oily formulations may maintain efficacy, but neither type can escape the second effect, spray drift.
Figure 2: Time to evaporate all water from droplets of various sizes, based on the “two-fluid” model developed by Wanner (1980). Based on 0.8% v/v non-volatile, non-soluble addition, 20 ºC, and 50% RH. This model suggests that final droplet diameter is 20% of initial diameter. Reproduced from Microclimate and Spray Dispersion by Bache and Johnstone (1992, Ellis Horwood Ltd).
Small droplets are more susceptible to displacement by wind currents due to their small mass. There is no magical size above which drift is no longer possible, but we’ve generally used diameters of 100, 150, or 200 µm as a theoretical cutoff. The proportion of the spray’s volume in droplets smaller than these diameters can be called “drift potential”, and this value is useful to measure the impact of nozzle type, pressure, or formulation on that phenomenon.
But it’s not quite that simple. Even a small droplet may resist drift if its exposure to wind is limited, perhaps through a protective shield shroud, or lower boom height. Or by increasing its speed through air assist. Higher energy droplets resist displacement.
These mitigating strategies aren’t lost on sprayer manufacturers who have used them for decades to build lower drift sprayers.
The next phase of the dose transfer process is interception. The droplet has to encounter its target, but the process is mostly coincidence. Simply put, the target has to be in the way of the droplet’s flight path for the two to meet. Denser canopies are therefore more effectively targeted. A larger number of droplets (smaller droplets or more carrier) also improve the odds. But it’s not that simple. Flight paths can change. That’s where small droplets are more inventive. Because they respond to small air currents, and because such small currents surround most objects, the smaller droplets can weave around objects, following the small eddies generated by air flows. As a result, we’re more likely to find smaller droplets further down in denser, more complex canopies where the eye can’t follow. They simply cascade through.
Larger droplets, on the other hand, resist displacement by air and travel in straighter lines. They tend to hit the objects they encounter. For that reason, larger droplets are intercepted by the first object they reach and only make their way deeper into a canopy if the path is clear. In other words, vertical, sparser objects allow larger droplets to pass by.
These properties are related to the droplet’s inertia, and are best described by a parameter known as “stop distance”. Assuming an initial velocity, stop distance is the distance required by a droplet to slow to its terminal velocity.
Figure 3: Stop distance as a function of droplet size. Assuming a 20 m/s initial velocity (similar to exit velocity of a hydraulic nozzle) and gravity assistance. Note that smaller droplets without the benefit of air assist lose their initial velocity within a few cm of the nozzle exit. Reproduced from Microclimate and Spray Dispersion by Bache and Johnstone (1992, Ellis Horwood Ltd).
These characteristics, combined with the aerodynamic properties of objects such as tiny insects, cotyledons, leaves, stems, etc. govern the collection efficiency of sprays. Small, slow moving droplets are thus best captured by small objects that don’t create strong enough deflections of airflow to steer the droplets past. Large objects that redirect air around them very effectively are better collectors of the larger or faster droplets whose kinetic energy can guide them through this turbulence. It’s also a matter of probability, as the smaller objects tend to have a lower likelihood of encountering the relatively scarce large droplets of any given spray.
But once again, that’s not the end of the story. Interception is followed by a critical stage, retention. Objects must be able to hold onto the droplets they intercept. Slow motion video has shown that droplets flatten out on contact with an object as the liquid converts impaction velocity into lateral spread. Once at full extension, the flattened droplets will collapse even beyond their original round shape, pushing them away from the surface and possibly causing rebound. A rebounding droplet may eventually land on target, but that would be a matter of fortune. It’s better if the leaf can offer enough adhesion, diminishing the power of the rebound oscillation, allowing droplet to stick the first time.
Figure 4: Droplet deformation during impact (C. Hao, et al. 2015. Superhydrophobic-like tunable droplet bouncing on slippery liquid interfaces. Nature Communications. August 2015).
Small droplets have less mass, and tend to be retained more easily. But more than size is at play here. The morphology and chemistry of the leaf surface is also important, with crystalline or more oily surfaces offering less adhesion for droplets. The physico-chemical properties of the spray mixture becomes important, as characteristics such as dynamic surface tension and visco-elasticity affect spray retention. These properties are optimized through the product formulation effort, and possibly via adjuvants added to the tank.
We sometimes classify targets as “easy to wet” or “difficult to wet” to summarize these properties. Most grassy plants (foxtails, cereals) are difficult to wet (there are exceptions, such as the sedges) and broadleaf plants vary from the easy to wet pigweeds to the difficult to wet lambsquarters and brassicas. Easy to wet species can retain larger droplets than difficult to wet species, and that’s one reason why finer sprays are preferred for grassy weed control (leaf orientation and size are another).
Figure 5: Droplet deformation, and surfactant molecule alignment, during impaction. The inability of surfactants to reach optimal alignment quickly, and for the target surface to absorb these forces, leads to rebound.
A few words about surface tension. Although surfactants reduce surface tension and facilitate spreading, this may not be enough to improve spray retention. To be effective, surfactant molecules need to align themselves with the surface of the droplet so they can be a “bridge” at the interface where the droplet meets the target surface. This takes time. The oscillations that occur during impaction continuously create new surfaces, and if surfactant molecules don’t follow suit immediately, the droplet will behave as if no surfactant is present. Specialists measure “dynamic” surface tension, i.e., the surface tension at young surface ages – a few milliseconds – to better predict spray retention. Very young surface ages have surface tensions of plain water, even with a surfactant present. Only certain surfactants, or higher concentrations of surfactants, can actually improve spray retention.
When air-induced nozzles were introduced in the mid 1990s, one of their claims was the improved spray retention due to air inclusions (bubbles) in the individual droplets. These bubbles made the droplets lighter, and also reduced their internal integrity, promoting breakup on impaction. As a result, the coarser sprays they produced actually had some of the same efficacy performance as the finer sprays they replaced. And indeed, research showed that coarser, air-induced sprays did in fact maintain good performance. Interestingly, performance of non-air-induced coarse sprays used with pulse-width modulation also showed similar robustness of performance. Research comparing air-induced to conventional sprays of similar droplet size rarely showed differences, and when they occurred, they were small in magnitude and could be corrected through improved pattern overlap.
Figure 6: Air Bubbles in spray droplets (Source: EI Operator. Believed to originate with Silsoe Research Institute, UK)
One reason larger droplets still work well is due to the pre-orifice designs of modern low-drift nozzles. This design reduces the internal pressure of the nozzle itself, with the effect being a slower moving large droplet. This reduced velocity takes away some of the force at impaction, reducing rebound.
Figure 7: Droplet velocity of larger droplets is reduced by lower pressures from pre-orifice and air-induced design nozzles. Lower velocities reduce droplet rebound.
Another neat effect of coarser sprays is their ability to entrain air. All sprays move air (simply spray into a bucket to see this), and larger droplets do this better and for longer distances. The entrained air is a form of air assist for the smaller droplets, increasing their average velocity and thus reducing their drift potential while they move in the spray pattern.
The final stage of the dose transfer process is deposit formation and biological effect, and that’s where we once again see differences attributable to droplet size.
Once established on a target surface, the active ingredient usually needs to move to its site of action. In some cases, resting on the surface is sufficient, it depends on the specific product. But for the majority of herbicides, the active ingredient must move across the cuticle into the cytoplasm where it eventually migrates to the enzymes involved in photosynthesis or biosynthesis of fatty- or amino acids. The cuticle is waxy, with only a few water-loving pathways and the uptake process is basically driven by diffusion and concentration gradients. As such, it is more effective when the product is in solution and the longer the droplet can stay wet, the better. That’s one reason why spraying during hot, dry days may reduce performance. Again, it depends on the formulation and the mode of action. Too high a concentration can damage membranes, physiologically isolating the active ingredient and reducing its subsequent translocation. It’s always a balancing act.
If you’ve been keeping track of the score, it’s more or less a tie between large and small droplets. One deposits better and makes more efficient use of lower water volumes, while the other has lower losses from drift and evaporation, helps smaller droplets resist drift, and may improve uptake of some products.
And this draw is why the venerable hydraulic nozzle has been so successful for so many decades. Hydraulic atomization, by its nature, creates a wide diversity of droplet sizes, ranging from 5 to 2000 µm or greater. As Dr. Ralph Brown of the University of Guelph used to say, this nozzle provides a drop for all seasons. Some small ones for coverage and retention in hard to reach places, and some large ones for uptake and drift-reduction. The result is a robust delivery system that provides reliable results on many different targets under many conditions. In recognition of the heterogeneity of sprays, we don’t refer to specific droplet sizes, but rather their composite, grouped into international categories of Spray Quality such as Medium, Coarse, and Very Coarse.
Our challenge is to find the spray quality sweet spot, the ideal blend of these contradictory and yet complementary features of our agricultural sprays. And I believe that task is very achievable. Simply put, broadcast agricultural sprays in field crops work reliably when applied as Coarse and Very Coarse sprays in volumes between 7 and 12 US gpa. There is no need to spray any finer than Coarse for good efficacy, as coverage is already sufficient and any additional coverage has small marginal returns. There is, however, value in adding more water when canopies are denser or when leaf area index grows as the crop matures. To gain coverage, adding water is preferred to reducing droplet size because of the value of environmental protection. It so happens that Coarse to Very Coarse sprays provide or ecxeeed the drift protection required by most agricultural labels.
There is occasional reason for spraying even coarser than what I’ve suggested. It’s certainly required by law for dicamba products on Xtend traited soybeans and cotton, but even then, only in conjunction with higher water volumes to offset losses in droplet numbers. In practice, moving to Extremely Coarse or Ultra Coarse sprays may allow an application to proceed in higher than average wind without adding drift risk. The use of some additional water is a relatively small price to pay for that additional capability.
There will always be opportunities for efficacy improvement in specific cases for those willing to spend the extra time to optimize that situation. That’s one of the reasons I’m excited to see the widespread adoption of pulse width modulation (PWM) in the industry, allowing users to change spray pressure and therefore spray quality with no impact on application rate or travel speed. Or the introduction of nozzle switching from the cab, employing the optimal atomizer for a specific situation. Although it remains difficult to define the ideal spray, selecting a spray quality has never been so easy.
