Irrigation Fundamentals

Part 5, Sprinkler Irrigation, Big Guns

By Ed Butts, PE, CPI

Over the past several months we’ve discussed irrigation fundamentals. Now let’s expand our talk on sprinkler systems by focusing on larger sprinkler irrigation techniques and design—that is, the Big Gun technique.

Please note the Big Gun® irrigation sprinkler is a registered trademark of the Nelson Irrigation Corp. However, several firms produce and market similarly sized sprinklers. Although this column will use and refer to the term Big Guns and outline many of the characteristics of the Nelson Big Guns, for purposes of this discussion, the term is intended to be generic and does not infer any endorsement of any kind.

Big Gun Concept

When choosing an irrigation system, a permanent solid set system or conventional small impact sprinkler setup may not be viable. To provide flexibility for leased ground and irregular field dimensions and optimize available capacity, Big Gun or soft/hard hose systems may be necessary because of their ease of use, versatility, and portability.

When examining the available options for the appropriate sprinkler irrigation system, a portable or mechanized or automated system may be warranted. In fact, many of the irrigation firms selling irrigation systems often advise customers one of the biggest advantages of this system is it can be the only viable selection when other systems are either not practical or simply will not work.

This may be due to irregularly shaped fields; fields that are difficult to access or apply water with other types of sprinklers; fields with trees, roads, streams, and other obstructions or bariers; to lessen labor costs from moving pipe and sprinklers; and fields that cannot be easily or practically fed or watered by pipes, trenches, or natural (flood) irrigation methods.

Big Guns can also be used in fields where animals graze part of the time, as the system’s mobility allows it to be moved even when animals are present.

A Big Gun irrigation system will often work where other systems cannot and can help increase the yield possible on isolated or leased farm property because irrigation can be done with a portable system on land that is almost any physical size, shape, or layout.

Portable Big Gun systems are used on many different types of field crops—particularly taller crops where movement in the maturing crops is usually difficult. Typical crops include hay, cane sugar, sugar beet seed, corn, and pasture grasses. Also, with appropriate riser heights these systems can also be used for some over-tree and vineyard irrigation.

Portable Big Gun systems set on lateral stands or tripods can be removed and relocated to other sites or to facilitate field work as needed. Solid set systems may include permanent buried PVC water lines or temporary aluminum main and lateral lines that can be pulled up to allow for cultivation or moved as needed for use in another field.

Solid set systems can use Big Guns mounted to permanent risers to allow automatic or manual operation using a separate solenoid or manual valve or valve in head concept.This method of irrigation can be set up based on the available water pressure, and the sprinklers are easy to relocate and maintain.

The proper nozzle sizing, pressure, and time of application enable farmers to have complete control over the amount of water their fields receive, providing an economical solution to get a higher crop yield.

They are used on most types of plants, regardless of growth height, and help conserve water as water can be applied exactly where and when it is needed.

Even when used together with other systems as a hybrid system to irrigate hard-to-reach areas, Big Gun sprinklers can operate efficiently in either a traveling, portable (hand-move), or stationary (solid set) role. They are also often placed at the end of center pivots as an end gun to cover extended land within the perimeter beyond the reach of the pivot’s sprinklers.

Due to the large nozzle size, Big Guns are also useful for many wastewater applications including municipal sludge, dairy slurry, and effluent disposal.

Figure 1. Large volume sprinklers.

Big Gun Disadvantages

Typically, the capital cost for a Big Gun system is competitive with smaller systems on a per acre coverage basis. But even with all these advantages, larger or Big Gun sprinkler systems are not without a few distinct disadvantages.

  1. To start with, the system is energy intensive, generally requiring 80 psi or more at the sprinkler nozzle to affect the needed coverage and distribution. Gallon for gallon of applied water, this can increase pumping costs by 40%-50%. On hard-hose traveler systems, energy costs per gallon applied can double, mostly due to increased frictional loss in the hose.
  2. The system is often hard to control from excessive wind drift and overspray when used next to legal or physical boundaries such as roads, streams, or adjacent properties.
  3. This type of system requires higher flow rates of water, typically beginning at 100 GPM and moving upward to more than 1000 GPM. This may present a challenge when used with lower producing sources and wells.
  4. The pressure and sprinkler trajectory required to gain the distance needed to effectively operate a Big Gun system usually results in larger droplet sizes, and thus more falling energy and resulting disruption to the soil upon impact. This can cause sealing or erosion of the upper soil horizon as well as subsequent runoff.
  5. Adjusting or working next to a high-volume, high-pressure sprinkler can be potentially dangerous, especially if the sprinkler is situated high above the ground on a cart or sprinkler riser.
  6. The system is often limited to use on reasonably flat ground, particularly portable Big Gun and traveler systems that may not be able to traverse or operate over sloped or uneven terrain.
  7. Lastly, the system generally produces a higher application rate than a comparable smaller sprinkler system and is generally used on ground with a moderate to high soil intake rate of more than 0.50-inch per hour. This can extremely limit its use on tighter soils, including many loam and clay types.

Big Gun Styles and Nozzle Types

In order to be effective and avoid ground and crop saturation, the nozzle must continually change its discharge point or rotate, so it will not send excess water onto the same area. This allows the sprinkler to optimize its high rate of flow and application without applying excessive water to any one spot.

Larger sprinklers are capable of rotating using a variety of methods, with the two most popular consisting of impact and spinning turbine drives. See Figure 1 for these two examples.

Basically, impact sprinklers use a curved drive spoon attached to a counter-weighted center pivot arm that engages the flow stream. The counterweight is positioned on the opposite end of the drive arm and provides the counter-force necessary to allow the spoon to alternately drop back into the flow stream at a reasonably predictable frequency. This results in a momentary directional shift of the flow stream, which causes a short rotational travel.

A modern type of impact part circle Big Gun reverses the drive spoon’s direction upon engagement of a preset collar, resulting in the sprinkler head reversing direction until encountering the other preset collar that returns the gun to normal travel.

Turbine drive Big Guns use a small impulse turbine into a gearbox to rotate the sprinkler. Rather than rotating in short pulses, as with an impact sprinkler, the turbine drive type results in a gradual but continuous rotation.

Most Big Guns are manufactured in two styles: full circle and part circle. As the term implies, a full circle sprinkler is designed to apply water throughout a full 360° circle, while a part circle sprinkler can be adjusted to apply water to an area from a 360° full circle to as tight as a 10° to 20° rotational arc.

The diversity and type of Big Gun sprinklers and nozzles depends on the specific manufacturer. However, the most popular style, the Nelson Big Gun, is available in four sizes and model numbers: 75, 100, 150, and 200.

The Model 75 and Model 100 are the smallest of the Nelson Big Guns and are primarily used for center pivot end guns, portable move, and small solid set systems. The flow rate of Model 75 and Model 100 ranges from 30 GPM to 300 GPM at pressures between 40 to 120 psi.

Model 150 and Model 200 Big Guns are frequently used for traveling systems, large solid set irrigation (automatic and manual), and wastewater application use. Model 150 Big Guns operate between a flow rate of 100-550 GPM at accompanying nozzle pressures of 50 to 120 psig. Model 200 Big Guns operate within a flow range of 250-1200 GPM at an accompanying nozzle pressure of 60 to 130 psig.

The angle of the nozzle relative to the horizontal plane, or its trajectory, varies with the size and application of the Big Gun, but is generally 18°, 21°, 24°, or 27° for most irrigation service. High angle sprinklers up to 43° are available for dust suppression.

Nozzles are also configured to match the specific model of sprinkler in both flow rate and discharge pressure. There are three basic types of nozzles used for Big Gun systems: taper bore (Figure 2a), ring nozzle set (Figure 2b), and taper ring nozzle set (Figure 2c—used for Models 75, 100, and 150 only).

Size for size, nozzles are selected to provide either optimum or maximum distance of throw from a taper bore or taper ring nozzle, or better stream breakup and diffusion with smaller droplet sizes when using a ring nozzle (see Figure 3).

The difference between the two lies in the relative efficiency of the nozzles. A taper bore nozzle resembles a smooth-bore fire hose nozzle as the flow stream is tapered and gradual and without any sudden obstructions to the end of the nozzle. Taper bore nozzles exhibit an efficiency of 95% or greater while ring nozzles are typically between 65%-85% efficient depending on the size.

As an example, with common parameters of 100 psi discharge pressure and 27° trajectory, a 1.75-inch taper bore nozzle will pass 900 GPM with a throw distance (diameter) of 550 feet, while the virtually same size of ring nozzle (1.74- inch) is only capable of 660 GPM with a diameter of 480 feet, a difference of 27% less in flow rate and a 13% reduction in covered diameter.

A taper ring nozzle combines the changeability of a traditional ring nozzle while providing some of the higher efficiency of a taper bore nozzle, and as with a ring nozzle set, is available in several sizes to facilitate field changeout.

The use of lower trajectory angles generally results in better ability in fighting the wind, but reduced throw distances. The degree of throw reduction depends upon the nozzle flow rate, but in general the throw distance is reduced approximately 3% with each 3° drop in the trajectory angle.

Figure 2. Big Gun nozzle styles.

Big Gun System Design

When not specifically used on a mechanized or traveler system, a Big Gun system is generally set up to operate in either a stationary or portable configuration. Systems can be designed to be automatic by placing a solenoid valve on the main or lateral or under the sprinkler or by using a nozzle or integral sprinkler shutoff valve.

A master controller is used to send signals to each sprinkler in sequence to irrigate, with the master valve function generally used to operate the pump.

Figure 3. Pattern difference between Big Gun nozzles.

As with smaller irrigation systems, spacing between Big Guns on laterals and mainline should be conducted in accordance with the prevailing wind direction and velocity. Tighter and staggered spacings of up to 50% of the nozzle diameter between heads on laterals and 65% of the Big Gun nozzle diameter between laterals is often indicated. Refer to Figure 4 in this column and to Tables 2a, 2b, and 2c in Part 4a (July WWJ) to provide guidance on sprinkler spacings for various wind conditions.

The use of Big Guns on tight or sloped ground of 5% or more must be carefully examined as the application rate is generally higher than conventional impact sprinklers, usually between 0.40 to 1.00 inches per hour. Excessive water applied on slopes can result in runoff.

A solid set Big Gun system is comprised of a grid of Big Gun positions fed by a network of individual pipes. Although systems can be designed with one Big Gun per position, it is common for a system to use one or a few Big Guns moved from position to position.

Figure 4. Typical Big Gun spacing.

Usually only one Big Gun operates on a lateral pipe at one time in order to reduce the required pipe diameter. Portable systems may use just a few lateral pipes which are constantly moved. Solid set systems may have a full field of pipe laid out and then moved after harvest or when the system is required for another field. Permanent systems are similar to solid set systems except the piping is usually buried PVC pipe.

While both solid set and permanent set systems are commonly used and are both acceptable, they each have unique features. The 75 Series and 100 Series Big Guns are the most commonly used models for these types of systems. The 75 Series and 100 Series Big Guns offer the best balance of required pipe diameters, application rate, operating pressure, and sprinkler spacing.

The 150 Series and 200 Series Big Guns have larger nozzles which allow for a wider grid pattern, but the required flow per position increases to a greater degree than the increased spacing.

For the best system performance, use conservative sprinkler spacings which should generally be from 50% to 65% of the sprinkler diameter, with the tighter spacings recommended in high wind conditions and for those crops requiring the best uniformity.

When using a rectangular spacing, place the closer spacings perpendicular to the prevailing winds to improve coverage. Stagger the sprinkler for better distribution and use the lower trajectory angle models or the variable trajectory models of Big Gun in windy conditions.

While a 27° trajectory will give maximum radius in no-wind conditions, the 24° trajectory or 21° trajectory will typically perform better in the wind. Note that higher trajectory angles give more desirable droplet patterns. When using lower trajectory angles, use slightly higher operating pressures to enhance the droplet pattern.

Big Guns require higher operating pressures than small sprinklers to produce good uniformity and desirable droplet characteristics. Higher pressures and larger diameter pipes are required, increasing the overall cost. Lower operating pressure does not necessarily reduce operating cost but results in reduced radius of throw and lower uniformity which require closer sprinkler spacing, greater labor, and longer run times to apply the net water requirements.

When it is desirable to use the larger models of Big Gun, oftentimes support stands, carts, or permanently set steel risers are used with buried systems strong enough to withstand the thrust forces. These support stands or carts can be moved or towed using farm ATVs or tractors. As with smaller sprinkler systems, the application rate for a Big Gun is determined from the following equation:

Application rate (in/hr) =

Nozzle GPM × 96.3Sprinkler spacing (ft) × Lateral spacing (ft)

Design example

A sample design of a Big Gun system is based on the following parameters:

Acreage:
25 (756 feet × 1440 feet)

Crop: Corn (peak CU = 0.275 inch/day)

Assumed application efficiency = 75% (0.75) for solid set Big Guns

Big Gun System Design

When not specifically used on a mechanized or traveler system, a Big Gun system is generally set up to operate in either a stationary or portable configuration. Systems can be designed to be automatic by placing a solenoid valve on the main or lateral or under the sprinkler or by using a nozzle or integral sprinkler shutoff valve.

A master controller is used to send signals to each sprinkler in sequence to irrigate, with the master valve function generally used to operate the pump.

As with smaller irrigation systems, spacing between Big Guns on laterals and mainline should be conducted in accordance with the prevailing wind direction and velocity. Tighter and staggered spacings of up to 50% of the nozzle diameter between heads on laterals and 65% of the Big Gun nozzle diameter between laterals is often indicated. Refer to Figure 4 in this column and to Tables 2a, 2b, and 2c in Part 4a (July WWJ) to provide guidance on sprinkler spacings for various wind conditions.

The use of Big Guns on tight or sloped ground of 5% or more must be carefully examined as the application rate is generally higher than conventional impact sprinklers, usually between 0.40 to 1.00 inches per hour. Excessive water applied on slopes can result in runoff.

A solid set Big Gun system is comprised of a grid of Big Gun positions fed by a network of individual pipes. Although systems can be designed with one Big Gun per position, it is common for a system to use one or a few Big Guns moved from position to position.

Usually only one Big Gun operates on a lateral pipe at one time in order to reduce the required pipe diameter. Portable systems may use just a few lateral pipes which are constantly moved. Solid set systems may have a full field of pipe laid out and then moved after harvest or when the system is required for another field. Permanent systems are similar to solid set systems except the piping is usually buried PVC pipe.

While both solid set and permanent set systems are commonly used and are both acceptable, they each have unique features. The 75 Series and 100 Series Big Guns are the most commonly used models for these types of systems. The 75 Series and 100 Series Big Guns offer the best balance of required pipe diameters, application rate, operating pressure, and sprinkler spacing.

The 150 Series and 200 Series Big Guns have larger nozzles which allow for a wider grid pattern, but the required flow per position increases to a greater degree than the increased spacing.

For the best system performance, use conservative sprinkler spacings which should generally be from 50% to 65% of the sprinkler diameter, with the tighter spacings recommended in high wind conditions and for those crops requiring the best uniformity.

When using a rectangular spacing, place the closer spacings perpendicular to the prevailing winds to improve coverage. Stagger the sprinkler for better distribution and use the lower trajectory angle models or the variable trajectory models of Big Gun in windy conditions.

While a 27° trajectory will give maximum radius in no-wind conditions, the 24° trajectory or 21° trajectory will typically perform better in the wind. Note that higher trajectory angles give more desirable droplet patterns. When using lower trajectory angles, use slightly higher operating pressures to enhance the droplet pattern.

Big Guns require higher operating pressures than small sprinklers to produce good uniformity and desirable droplet characteristics. Higher pressures and larger diameter pipes are required, increasing the overall cost. Lower operating pressure does not necessarily reduce operating cost but results in reduced radius of throw and lower uniformity which require closer sprinkler spacing, greater labor, and longer run times to apply the net water requirements.

When it is desirable to use the larger models of Big Gun, oftentimes support stands, carts, or permanently set steel risers are used with buried systems strong enough to withstand the thrust forces. These support stands or carts can be moved or towed using farm ATVs or tractors. As with smaller sprinkler systems, the application rate for a Big Gun is determined from the following equation:

Application rate (in/hr) =

Nozzle GPM × 96.3Sprinkler spacing (ft) × Lateral spacing (ft)

Design example

A sample design of a Big Gun system is based on the following parameters:

Acreage: 25 (756 feet × 1440 feet)

Crop: Corn (peak CU = 0.275 inch/day)

Assumed application efficiency = 75% (0.75) for solid set Big Guns

Soil: Light sandy loam, maximum continuous application rate: 0.75 inch/hr, water-holding capacity: 1.50 inches/foot

Projected wind: 0-3 mph

Available source: 125-foot-deep water well, flow rate limited to 190 GPM firm capacity at 105 feet PWL

An experienced irrigation system designer would look at this application and become immediately suspect as trying to use only 190 GPM to irrigate 25 acres of corn may not be feasible except on a full 24 hours per day schedule. The first procedure is to determine if it can even be done by using a modified version of the equation from Part 3 (May WWJ):

Q (in GPM) = 453 × Irrigable area (in acres) × Gross depth of water application (in inches)Irrigation rotation frequency (in days) × Total operating hours of irrigation per day

Assuming a maximum 12-day rotation and irrigation efficiency of 75% (0.75) for a solid set system:

453 × 25 acres × [0.275 inch/day (CU)/0.75 (Irr. Eff.)] × 12 days190 GPM

= 262.2 hours = 21.8 hours/day × 12 days

This means that in order to cover the field in 12 days, the system must operate almost continuously at around 22 hours per day. This is unreasonable for a manually operated irrigation system as it leaves virtually no time for equipment breakdown or moving the gun to different sets. At least three 6- to 7-hour sets will be required each day to cover the field in 12 days, leaving less than one hour per set to move the gun.

In this case, the owner needs to consider the added cost of automating the system with three rotating guns versus the labor cost and associated hassle to move a single gun three times a day before proceeding to a full design. Following an analysis and review of labor costs versus automating a Big Gun system using automatic controlled valve laterals originating from a single mainline in the center of the field, the owner has opted to select the automated system.

Figure 5. Crop coefficient for corn.

Next, consideration of the type of crop and consumptive use should be evaluated. In many cases, this factor will impact the needed water storage for irrigation systems with insufficient source capacity to meet peak demands. In some cases, irrigation systems cannot possibly provide enough water for the highest crop demand or peak consumptive use.

In these instances, the use of deficit irrigation by using the stored water in the soil may be required. This type of crop example will generally work with deficit irrigation as the crop has deep roots and is fairly stress resistant.

In this example, the crop is corn with a stated peak consumptive use (CU) of 0.275 inch per day. Although this is the value used for design, this will obviously vary with the stage of growth which will require adjustment of the hours of watering.

Figure 6. Example of Big Gun irrigation system layout.

This is referred to as the crop coefficient and is shown in Figure 5 for corn. As can be seen on the chart in the figure, the crop coefficient varies from 0.30 (30%) of peak CU during the earliest stages of growth to 1.00 (100%) during the later stages.

This value, when multiplied by the local evapotranspiration rate, provides a daily CU value for irrigation scheduling, and along with tensiometers or gypsum, blocks to ascertain soil moisture, and is often used to adjust the amount of water applied per set.

The next step would involve the physical layout of the system. Using a combined element of experience, wind factors, available gallons (flow rate), setbacks from field boundaries, and degree of overlap, a 0.93-inch ring nozzle needs to be selected. This nozzle will optimize the available well yield as it will pass 189 GPM at 80 psi with a diameter of 305 feet.

The next step would be to select the spacings, using Table 2a in Part 4a (July WWJ), showing the maximum spacing of sprinklers for water-critical crops based on the diameter of coverage of the sprinkler being used. Since the projected wind shown in our design example is 0-3 mph, refer to that same column in the table.

Laterals: sprinkler diameter of 305 feet, spacing between heads = 305 feet × 0.50 = 152.5 feet. Use 150 feet.

Main: sprinkler diameter of 305 feet, spacing between laterals = 305 feet × 0.65 = 198 feet. Use tighter spacing of 180 feet.

Even though more overspray will result, using a closer spacing to boundaries will help negate the impact due to a loss of overlap from an adjacent lateral. These spacings will also provide uniform setback distances from field boundaries (100 feet and 120 feet) and between lateral lines to fit the dimensions of the field. This is the type of decision where experience plays a definite role.

Thus, with the spacings decided, the application rate and hours of operation per set can be determined:

Application rate (in/hr) =

190 GPM × 96.3150 feet × 180 feet

= 0.67 inch/hour < 0.75 inch/hour maximum allowed.

Alternating sets to opposite ends of the field will prevent overwatering to one site and allow the applied water an opportunity to soak into the soil. This will usually prevent runoff from overapplication.

The next design step is to determine the frequency (rotation): Irrigation frequency or rotation (days)

=

Total waterholding capacity in roots (inches)Peak consumptive use (inches/day)

=

1.50 inches/feet × 3 feet =16.36 days0.275 inch/day

As a safety factor for soil water loss through drainage, to ensure that some water is left in the soil to prevent plant stress, and to accommodate system downtime and possible equipment failure, a 20% safety factor is recommended and applied. However, this will vary with the soil type, waterholding capacity, and crop root depth:

Design irrigation rotation

16.36 days × 0.80 = 13.09 days. Round down to a 12-day rotation schedule.

To confirm

Required number of sets (from Figure 6): 36 sets/three sets per day = 12 days. OK

Required hours per set: 0.275 inch/day/0.75 × 12-day rotation = 4.40 inches gross water/0.67 inch/hr = 6.56 hours. Use 7-hour sets.

Note: Even though the maximum rotation is 16 days, using a 12-day assumed rotation for design will provide reserve water in the soil and prevent stressing the crop from depleted storage within the soil horizon, should the irrigation schedule temporarily fall behind. It will also evenly coincide with the number of gun sets.

However, in all cases the net total applied depth of water must not exceed the available water-holding capacity that remains after crop use, percolation losses, and evaporation. Otherwise root saturation and runoff is likely. For this example, the presumed net application of water (3.30 inches) equals approximately 2¼ inches in 2 feet or less of soil.

The application rate of a well-designed, overlapped Big Gun system will generally be in the range of 0.3 to 0.8 inch/ hour. Always verify the soil will accept the instantaneous and continuous application rate or that the Big Guns can be shut down or moved to their next position before runoff occurs. The final design is shown in Figure 6.

Main and Lateral Sizing

Mainline and lateral sizing generally include combined elements of friction loss and velocity. Dedicated mains and laterals for irrigation systems should operate at velocities of 5 feet per second (FPS) or less. Since only one gun will be operating at any one time, the friction loss must be determined for the longest distance from the source. In this case, 1500 feet.

Typically, the best balance of pipe size cost to head loss below 1500 feet in length occurs between velocities of 3.5 to 5 FPS. In this example, the maximum flow rate of 190 GPM falls just below a 4-inch pipe at 5 FPS. Therefore, a 4-inch Class 160 psi PVC pipe is chosen for both main and lateral lines.

Irrigation Design Head and Horsepower

The next step involves determining the head and horsepower for selection of the well pump. The head is found by adding the individual components of well lift, riser pipe friction, elevation rise, frictional loss in the mainline and laterals, and operating pressure at the nozzle.

For this example, the following head values are:

Well lift:                               105 feet PWL
Estimated riser pipe hf:    5 feet
4-inch PVC friction loss:   32 feet (based on 1500 feet of 4 inches)
Misc. hf and riser height:  13 feet
80 psi at nozzle:                  185 feet

TDH: 340 feet

Estimated well pump HP

=

190 GPM × 340 feet TDH3960 × 0.75 (estimate)

= 21.75 BHP. Use 25 HP.

Typically, this application would be performed by either a vertical turbine or submersible pump.

Big Gun Thrust

Big Guns will impose horizontal and vertical thrust during operation. Thrust is a combined factor of the separate components of flow rate and applied pressure. The vertical force from thrust is typically resisted by the buried pipe and soil. However, the moment arm applied to the gun due to the height of the riser can impose considerable thrust horizontally. This thrust is converted to a horizontal force by applying the cosine of the gun’s trajectory.

The thrust generated by a Big Gun can be substantial and provisions must be made to prevent tipping or overturning of the riser.

For example, the horizontal thrust of a 24° 150 Series Big Gun with a 0.9-inch nozzle operated at 70 psi is 79 pounds. The horizontal thrust of a 24° 200 Series Big Gun with a 1.5-inch nozzle operated at 90 psi is considerably higher at 285 pounds.

All solid set Big Gun systems should be sufficiently anchored with adequate resisting ballast to resist movement. This is generally performed using a concrete block set around the base of the riser. Determining the thrust and force on a vertical Big Gun riser is illustrated using the following equations:

Thrust (pounds) =

(2)(P) × Q(38) × P

Q = Flow rate in GPM

P = Nozzle pressure in psi

Horizontal force (pounds) = Cosine of trajectory angle × thrust

Moment arm (MA) (in feet) = √R2 + D2 where:

R = radius

D = ½ depth

Required weight of resisting concrete (pounds)

=

H2 + MA2MA

× Horizontal force (pounds)

For example

189 GPM at 80 psi with 100 Big Gun (trajectory = 24°), riser height (H) = 9 feet (to clear corn)

Thrust = (2)(80 psi) ×

189 GPM38 × 80 psi

= 88.97 pounds

Force = cosine 24° × T = 0.9135 × 88.97 = 81.27 pounds

Assuming a round block of 30-inch diameter (R = 1.25 feet) × 12-inch thick:

Moment arm =

(1.25)2 + (0.5)2

= 1.346 feet

Required weight of concrete block:

(9)2 + (1.346)2

/1.346′

= 7.82 × 81.27 = 635.66 pounds

Check weight of 30-inch × 12-inch concrete block: 3.14 × (1.25)2 × 1 ft × 145 lbs./ft3 = 711.4 pounds > 635.66 pounds required. OK

Alternatives to Fixed Risers

Big Gun risers can consist of buried fixed steel risers for a solid set system with the lower ell set into a concrete block to withstand thrust, or a portable, commercially manufactured, collapsible stand for Model 75 or 100 Big Guns.

These are often called tripods. Tripods are popular in use for irrigating many crops, including corn, beet seed, and pasture grasses where portability and manual moves of the sprinkler are desired or needed.

The portability and adaptability of a lightweight and collapsible tripod and Big Gun allows their use on existing aluminum mainline and laterals that were previously used for operating small impact sprinklers by plugging the sprinkler port and converting the laterals to supply lines.

In accordance with standard spacing criteria, the lines are commonly laid out in a rectangular or triangular pattern with the sprinkler set every 3-40 feet (120 feet-160 feet) lateral lengths and at 150-foot to 200-foot spacings on the mainline. This generally provides an application rate of 0.30 inch to 0.80 inch per hour.

Tripods can be designed with an integral valve to plug directly onto a hydrant on a buried mainline as a semisolid set system, a portable aluminum mainline, or with an ell that permits clamped attachment to the end of a lateral line for a portable system. Thrust restraint is either provided by clamping a three-legged tripod to the lateral or mainline along with the other two legs and pads used for lateral support or by using a four-legged tripod that supports the gun directly from the ground at 90° intervals. This style is most commonly used for hydrant water delivery feeding from mainlines.

__________________

This concludes this month’s edition of Engineering Your Business. Next month, we will continue this series on irrigation practices with an overview on soft and hard hose travelers.

Until then, work safe and smart.


Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.