How to Design a Spot-Spitter System
High water application uniformity is critical for your nursery’s profitability. Without good uniformity, some plants get more water and nutrients than others, resulting in variable plant size and health, and wasted water and fertilizer. Spot-Spitter system design is about laying out your irrigation system for the application uniformity you need to be successful.
Parts of this tutorial rely on the Design Tool provided by Primerus Products, LLC, the manufacturer of the Spot-Spitter, on their website. To find the Design Tool, go to the Primerus website at www.PrimerusProducts.com, and click the Design tab.
Sizing your Spot-Spitter Supply Lines for high uniformity:
One of the most direct ways to ensure good uniformity is to choose poly tubing supply laterals that are the right size for the number of Spitters you have in each row and their flow rates (colors). If you put too many Spitters on a small diameter supply line you will exceed the supply line’s capacity, and the Spitters at the end of the line won’t output as much water as those at the beginning. Use the Primerus Design Tool to estimate the uniformity of your system before you build it, and make changes if it’s too low. Aim for uniformity of 90-95% for consistent plant size and health.
Enter the parameters of one row within your system (Spitter color, supply line diameter, etc.) into the Design Tool. The calculator tell you your uniformity and will warn you it’s below 90%, or if the flow velocity within the supply line exceeds 8 feet per second (this can damage fittings). It also gives the overall flow rate required per row, which you can use to size your zones and submains.
If uniformity is too low you can improve it by: 1) using a lower flow Spitter in each container, 2) using a larger diameter supply line, 3) reducing the number of Spitters on your supply line or 4) reducing the length of your supply line. Try a few different combinations to solve the problem. One possible trick to avoid this compromise is to center-feed your supply line from a Tee fitting, effectively making it into two supply lines that are half the length and have half the number of Spitters.
Sizing your zones:
An irrigation zone is a block of plants that you irrigate simultaneously by turning on a valve. Since all plants in each zone are watered for the same amount of time, they should share the same water requirements. This usually means that all containers in a single zone hold the same type of plant at the same growth stage.
Given the above constraint, it’s generally easier to schedule irrigation when you have a smaller number of larger zones. However, each zone must be small enough that it doesn’t exceed what your water supply can deliver.
Size your zones (number of rows you run simultaneously) so that the combined flow rates of the supply lines in each zone does not exceed the capacity of your water supply. For example, if the calculator tells you that each supply line requires 10 gallons per minute (GPM), and your water supply capacity is 100 GPM, then each zone can supply up to 10 supply lines.
Sizing your submains:
Finally, your submains (large PVC pipes that feed your supply lines) must be large enough to supply all of the supply lines attached to them that could be turned on simultaneously. Choose a diameter that results in submain in flow velocity below 5 feet per second for good performance. You can calculate flow velocity as
V = .408 x Q/d2
In this equation, Q is the total flow rate through the submain (supply line flow rate times the number of supply lines) and d is the submain diameter. For example, if you simultaneously run 10 rows on one submain, and each row requires 10 GPM, Q is 100 GPM. If your submain is 2” PVC, d is 2 so V = .408 x 100/22 = 10 feet per second, which is too high. Upsizing to a 3” submain results in 4.5 fps which will work well. Remember that if your supply lines are center-fed, each submain takeoff requires the flow rate of two supply lines.
Following the above guidelines will give you a system that can handle the flow rates needed to supply your Spitters without pressure drops that cause non-uniformity. It's also important to be sure that your system pressures stay at the right range to provide 15-25 psi to each Spitter. The best way to do this is to install pressure regulators at the right locations. Supplying Spitters with pressures outside of this range can result in out-of-spec flow rates, stringy or misty spray patterns, or can even cause spaghetti tubing to blow off the Spitter heads.
Two approaches to installing pressure regulators are to 1) install a small, low cost pressure regulators at the supply point of each poly lateral or 2) install a large pressure regulator at the water supply, large enough to supply the larges zone within your system. For each approach the best choice is a 25 psi regulator rated at the appropriate flow rate depending on the size of zone or lateral it supplies.
The advantage of installing a pressure regulator at each lateral entrance (first option above) is that it results in every lateral being supplied at the right pressure, even if there are pressure losses within the susbmains. Depending on the flow rate needed, you can use a 3/4" Senninger Medium Flow (PMR) Regulator which operates at 2-20 GPM, a 1" Senninger PMR Regulator (also supplies 2-20 GPM) or a 1.25" Senninger High Flow (PR) Regulator which supplies 10-32 GPM.
The advantages of using a single pressure regulator are cost and simplicity. However, depending on the size and flow rate of your zones, a large regulator may be necessary. One choice is the Senninger Ultra High Flow 2" PRU Regulator which can supply up to 100 GPM. A traditional agricultural diaphragm valve in a pressure reducing configuration is also a good choice, although diapghragm valves can be more expensive and require higher maintenance than a simple fixed pressure regulator.
Pressure control at the emitter:
The ultimate pressure control system is to install a very small pressure regulator at the entrance to each Spot-Spitter, ensuring that all Spitters operate at exactly the same pressure and uniformity is nearly perfect. GrowIrrigation has a product for this: the Primerus PC Assembly. Individual-emitter pressure control adds a significant amount of cost to the system, so PC Assemblies should should only be used in demanding situations which require them. These situations might include nurseries on hillsides where large elevation changes make it otherwise impossible to get the right pressure to each Spitter. PC Assemblies are also useful on high value crops such as Cannabis where high frequency irrigation is used. I will be writing a blog post dedicated to the topic of PC Assemblies in the near future.
Where to get help:
The experts at GrowIrrigation are happy to help with the specifics of your nursery. For more help, call (442)279-3152 or email me JMcDonald@GrowIrrigation.com.