Many new and improved pesticides have become available and costs have risen sharply. Consequently, many users have had to replace or modify their sprayers or spraying methods to meet changing requirements or to improve efficiency. Those who use their sprayers for several different weed control problems; herbicides, insecticides, fertilizers, etc.; are faced with the need to change nozzles or nozzle replacement; to provide more agitation, increase ground speed; or apply chemicals at higher rates. Often these changes result in a need for a different or larger pump. Only when the nozzles have been selected and their total volume and pressure requirements measured (or computed), can the pump and other elements of the sprayer be chosen or properly adjusted. Another item that must be considered, before the pump and other elements can be selected, is the need for agitation of the spray material in the tank. This need arises from the fact that many herbicides are supplied in the form of finely ground wettable powders which disperse readily in water but settle out if not continuously agitated. A sprayer pump, therefore, must have sufficient capacity to operate the agitators as well as to supply the nozzle requirements. In fact it is recommended that the pump capacity be 20% greater than the sum of these requirements.

In order to prevent unnecessary pump wear, it is important to match the pump and the spray materials being used. The first step in pump selection, of course, is to determine the volume of fluid the pump must deliver and the pressure required to maintain the flow in the system. The types of pumps used in pesticide power spray equipment are as follows.

• Centrifugal – consists of a fan-shaped impeller rotating in a circular housing, pushing liquid towards a discharge opening Simple design; only wearing parts are the shaft seal and bearings (if so equipped). Usually used where a flow of liquid at relatively low pressure is desired. Not self-priming unless provided with a priming reservoir or foot valve: works best with the liquid source higher than the pump (flooded suction/gravity feed). As the discharge pressure (head) increases, flow and drive power requirements decrease. Maximum flow and motor loading occur at minimum head. Centrifugal pumps deliver higher volume (up to 200 gpm) at lower pressures (to 70 psi). Some models can be driven at speeds to 6000 rpm to develop higher capacities and pressures.
• Diaphragm – consists of a flexible diaphragm which moves up and down in a chamber, creating suction and pressure. As the diaphragm is moved up, it creates a vacuum, opening the suction valve and drawing fluid into the chamber. As the diaphragm is forced down, fluid is forced out through the discharge valve. Diaphragm pumps handle fluid mixtures with a much greater percentage of solids, such as mud, silt, and sludge.
• Flexible Impeller – a flexible, vaned member, usually rubber, rotating in an eccentric housing. The volume of the spaces
  between the vanes changes as the pump rotates, and pumping action is created. Usually used with pressures up to 30 psi.
• Gear – consists of two meshed spur gears in a housing. As gears rotate and mesh, fluid is forced out of the space between the teeth. Because of close running of gears, a gear pump will not handle abrasives. Suitable for pumping more viscous liquids at slower speeds and for pressures up to 100 psi.
• Immersion – a pump which has its inlet port immersed in liquid, but the motor and electrical components remain dry.
• Piston – fluid is drawn in and forced out by pistons moving within cylinders. Generally, choose a piston pump where you have limited volume (up to 25 gpm) at high pressure (up to 1200 psi).
• Roller or Vane – this pump type uses rollers or vanes in a rotor, rotating in an eccentric housing like a flexible impeller pump. Rolling vane, or roller, pumps offer an economical solution in many applications requiring moderate output (up to 47 gpm) at moderate pressures (up to 300 psi).
• Rotary Screw – a screw-shaped rotor, turning within a flexible stator, usually of rubber. Progressing cavities between screw and stator carry the li quid. For pressures up to 75 psi; also may handle slurries and/or abrasives at slower speeds.
• Submersible – a pump which operates only when totally submersed in the fluid which is being pumped, with waterproof
  electrical connections, using a motor which is cooled by the liquid.