Sprayer Workout: Will your pump put out? By Bill Casady

In a year when sprayers have hit the front page along with Asian Rust, sprayers have sold like wildfire. Most sprayers come equipped with standard accessories and pumps that will handle the typical pressure and flow rate requirements of most applications. However, those specifications are not always readily apparent and there are several rules of thumb that need to be considered when selecting a pump to provide the desired output. There is some concern that sprayers equipped for low volume herbicide applications may not cut it when we attempt to supply the higher carrier rates (e.g. 20 GPA) suggested for fungicides.

The most popular agricultural sprayer pumps are roller, piston, and centrifugal pumps. Centrifugal pumps are very common on farm sprayers. The centrifugal pump is a non-positive displacement pump that causes a spray solution to enter through the center of a rotating impeller. As the impeller spins, the centrifugal force causes the spray solution to be forced to the outer limits of the pump housing. This force causes a large output at low pressures or as little as zero output at high pressures. Hence, the output of a centrifugal pump is a function of the working pressure in the system.

Let's determine the required specifications for a medium size sprayer. I have chosen, for the sake of example, a spray width of 60 feet, an application speed of 8 mph, and an application rate of 20 gallons per acre (GPA). Let's also choose a pressure of 40 PSI, which is the typical design operating pressure for many nozzles. Without providing the calculations in this article, the pump must deliver approximately 20 gallons per minute (GPM) to the boom at 40 PSI.

The required flow rate changes by ratios of the three specifications: width, speed, rate. For example doubling speed doubles the required output. A marginal increase in speed from 8 to 10 MPH requires increasing flow rate by the ratio 10/8. In this case, the flow rate would increase by the following calculation: (20 GPM x 10/8) = 25 GPM.

But that's not where the story ends. There are other flow needs that need to be tacked on to the flow rate to the boom. For example, agitation requirements for tanks using hydraulic agitation from the pump should be, as a rule of thumb, approximately 5 to 10 % of the tank capacity. Hence, a 100-gallon tank should be agitated with 5 to 10 GPM and a 400-gallon tank should be agitated with approximately 4 times those amounts or from 20 to 40 GPM. Self-cleaning line strainers also require some additional solution flow rate of approximately 6 to 8 GPM for each strainer.

Each component of flow is added together to determine the actual flow rate. In this example, let's assume that we needed 20 GPM of product to the boom, that the agitation requirements are 20 GPM, and that we have a single line strainer at 8 GPM. The total flow is 48 GPM. Finally, we consider a 20% increase due to pump wear and as a factor of safety to choose a pump that is sufficient. The calculation is (48 x 1.2) or about 58 GPM. Because we selected the operating pressure to be 40 PSI, we need to select a pump that will provide 58 GPM at the selected operating pressure of 40 PSI. To do this accurately, you will need to find pump curves for the selected line of pumps. These specifications should be readily available from your pump dealer.

It is worth mentioning that the additional cost for an oversized pump may be well worth the money. The additional cost is usually proportionally smaller than the additional capacity, and the extra $100 for a bigger pump is like insurance that you won't come up short handed on capacity. Be prepared for the challenges of Asian Rust. Check pump capacity to see if you might be ready to meet the challenge, and I will be more than happy to help with the calculations as you determine if your pump will 'put out' under the potentially new scenarios of Asian Rust.

Bill Casady 573 882 4370