Spraying System Pumps -------- ------ ----- The pump must deliver adequate flow and pressure, and it should handle the desired chemicals without rapid corrosion and wear. Pumps generally fall into two categories - positive displacement pumps and non-positive displacement pumps. Positive displacement pumps include piston pumps, diaphragm pumps and roller pumps (at low operating pressures). Non-positive displacement pumps depend on external restrictions like fluid viscosity and downstream pressure. Several common sprayer pumps included in this category are centrifugal pumps and roller pumps (at high operating pressures). Four of the most common pumps used on turf and landscape sprayers are roller pumps, centrifugal pumps, piston pumps and diaphragm pumps. * Roller pumps produce moderate flows and pressures. The rollers are held in a slotted rotor revolving in an eccentric case. As the rollers pass the pump inlet, the cavities between and under the rollers enlarge and draw liquid. When nearing the outlet, the cavities contract due to the eccentric housing and force the liquid out of the pump. Roller pumps handle a variety of pesticides and have low initial and maintenance costs. These pumps operate very efficiently, but as pressure increases the volume (output) decreases. Pump capacity is a function of pump design and rotational speed. If you use wettable powders primarily, a roller pump probably isn't your best choice. Roller pumps are not well-suited to abrasive materials because abrasives rapidly wear the pump housing, the slots in the rotor and the rollers. Replacing the rollers is easy but may not restore the pump to satisfactory working condition. Nylon rollers work well with most chemicals, but rubber rollers are slightly better if you must use abrasive materials. * Centrifugal pumps create flow and pressure by centrifugal force from an impeller to the fluid. Liquid enters through the center of the impeller; as the impeller spins, centrifugal force throws the liquid into a spiral passage leading to the outlet. The only moving parts in centrifugal pumps are the shaft and impeller. The impeller must operate between 2,000 and 4,200 rpm to give rated performance. If pump speed decreases by 10%, volume drops by 10% and pressure decreases by 19%. PTO-powered centrifugal pumps require speed-up drives and high engine rpms that can waste fuel when spraying. A good alternative is to power the pump either by a hydraulic motor connected to the sprayers hydraulic system or by auxiliary gasoline engines. Centrifugal pumps last a long time (even with wettable powders) and produce a high flow volume that is ideal for hydraulic (jet) agitation in the sprayer tank. They develop pressures up to 100 psi, but pump capacity falls very rapidly above 30 to 40 psi. Typical maximum working pressure is 50 to 60 psi. * Piston pumps propel liquid by a piston moving in a cylinder. The intake stroke draws the liquid in through one valve, and the output stroke forces the liquid out through another valve. Piston pumps should have either an internal or external air chamber (surge tank) to dampen pulsations in the liquid flow associated with each pump stroke. Without the surge tank, the sprayer will pulse rather than spray a steady stream. Piston pumps develop high pressures that can increase the sprayer s versatility. However, the relative capacity of piston pumps is often low. Because the higher-volume piston pumps are expensive, mechanical agitation is normally used instead of hydraulic agitation. As a professional applicator, look for larger-capacity pumps driven by gasoline engines. The large pumps achieve higher flow rates with two to eight cylinders, and the multi-cylinder design produces a more even flow. * Diaphragm pumps have at least one chamber sealed at one end by a membrane or diaphragm. The other end has an inlet and outlet valve. The diaphragm connects to a piston. As the piston moves, suction draws the liquid through the inlet valve by moving the diaphragm, which enlarges the chamber. The return of the piston forces the diaphragm inward, shrinking the chamber and propelling the liquid out. A compression chamber smooths out line pulses and needs to be added if it isn t incorporated in the pump. Diaphragm pumps require minimal maintenance because there is less contact between the spray material and moving parts. You will need to periodically check the diaphragm for pinhole leaks that can cause problems and lower pressure. Look for a diaphragm that resists the chemical you use. Abrasive materials are less likely to damage this type of pump. Pump Capacity The pump should have sufficient capacity to supply a number of nozzles, provide jet agitation (if used) and offset pump wear. To allow for normal wear on a roller pump, your new pump s capacity should be 20 to 25% higher than originally necessary. BOOM SPRAYER PUMPS ----------------------------------------------------------------- Pump Types Roller Centrifugal Piston Diaphragm ----------------------------------------------------------------- Materials Non abrasive Most Most Most. Some handled materials chemicals may damage diaphragm Relative Low Medium High High purchase price Durability Pressure Long life Long life Long life decreases with wear Pressure 0 - 300 0 - 100 0 - 1,000 0 - 850 ranges(psi) Operating 300 - 1,000 2,000 - 4,200 600 - 1,800 200 - 1,200 speeds(rpm) Flow rates 1 - 35 0 - 120 5 - 60 1 - 60 (gpm) Advantages *Low Cost *Handles all *High pressures *Wear materials resistant *Easy to *High Volume *Wear *Resistant *Operates *Long Life Resistant to most at PTO *Handles all chemicals *Medium *Self-priming *Self- Volume Priming Disadvantages *Short *Low pressure *High cost *Needs Life if *Not Self- *Needs surge surge tank material priming tank *High cost abrasive *Requires speed up drive or high speed hydraulic motor (Submitted by: Vernon L. Hofman, Extension Agricultural Engineer)