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P U MP S E L E C T I O N G U I DE L I N E S TECHNICAL REFERENCE PUMPS Every operation using spray nozzles requires a method to provide fluid flow. Fluid flow can be provided by gravity, air pressure or mechanical pumps. It is important to understand that pumping systems provide flow, not pressure. Pressure is the result of restricting flow. The output of an unrestricted pump is 0 psi (bar). When a restriction is placed in the flow, line pressure will result. HOW PUMP TYPE AFFECTS NOZZLE SELECTION The main types of pumps are positive displacement and centrifugal. There are others, but the operational principles are the same as for positive displacement and centrifugal pumps. • High pressures usually require a positive displacement pump Centrifugal pumps (velocity pumps) These pumps typically consist of a large vane (impeller) which is turned by a shaft inside a cavity (casing). The geometry of the impeller and casing moves the fluid in a tangential motion. The fluid gets restricted to a smaller volume and is then discharged into the system piping. These types of pumps typically operate at low pressure and high volume. They may also consist of several stages to increase the number of pressures available. These pumps have the unique feature of being able to run while the outlet is blocked. Since the pumps are velocity based, the impeller will spin in the casing fluid without “dead heading” the system itself. It will produce heat and may cavitate the fluid, but it will not build pressure like positive displacement pumps. However, a system bypass and pressure safety valve is still installed in the system to protect components. • Variable Frequency Drive (VFD) pumps may be an option. These pumps allow variable control of speed and flow rates • Consider the fluid. Specific gravity will affect pump flow rates just as it affects nozzle flow rates • Pump efficiencies, heat, available power, maintenance and plant conditions should also be considered PERFORMANCE PRESSURE (FT OR M) A fixed volume of fluid is delivered for every stroke of a piston, or plunger or rotation of a shaft. Examples include piston pumps, plunger pumps, peristaltic pumps and gear pumps. Positive displacement pumps provide high pressure, and regardless of the system characteristics, will deliver a fixed flow every rotation. These pumps must have an unrestricted bypass valve and a pressure relief valve. • High flows usually require a centrifugal style pump CENTRIFUGAL POSITIVE FLOW (GPM OR LPM) FLOW RATES AND FLUIDS 100 90 PERCENT Positive displacement pumps The flow rates and pressures required by the system will determine the pump choice. There are many styles, sizes and types of pumps available but these general guidelines should prove helpful. 80 CENTRIFUGAL 70 POSITIVE 60 50 40 VISCOSITY 80 EFFICIENCIES PERCENT 70 CENTRIFUGAL 60 POSITIVE 50 40 PRESSURE (PSI OR BAR) spray.com | 1.630.665.5000 A7