The pump type and size required are determined by the pesticide used, the pressure required and delivery flow rate. A pump must have adequate ability to operate a hydraulic agitation system, and also provide the critical volume to the nozzles. At least 25 per cent more volume is required to allow for agitation and loss of capacity due to pump wear, above what the nozzles require.
Pumps should be pesticide corrosion resistant. The materials used in housings and seals should be chemically resistant
Types of pumps on agricultural sprayers:
1.Centrifugal
2.Roller or rotary pumps with rolling vanes
3.Piston
3.Diaphragm
CENTRIFUGAL PUMPS AND CONTROLS
Centrifugal pumps are good for low-pressure high-volume sprayers. They are simply constructed, and great for wettable powders and abrasive materials. They are durable pumps. The high volume(above 500l/m) capacity nature of these pumps makes it necessary to have hydraulic agitators to agitate spray solutions even in large containers.
Up to 6 bar pressures are developed by centrifugal pumps, but flow volumes drop instantly above 2.8 bar. This “steep performance curve” is an advantage as it allows controlling pump output without a relief valve. Centrifugal pump performance is very susceptible to speed and pressure variations. These variations may produce uneven pump output under the same operating conditions.
Centrifugal and roller pump performance. Speeds of about 3,000 to 4,500 revolutions per minute (RPM) are recommended for centrifugal pumps.When driven with the tractor PTO, a speed step-up mechanism is necessary.Increasing speed with a belt and pulley assembly is easier and cheaper. Planetary gear system can also be used. Gears are contained and hoisted directly on the PTO shaft. Direct connected hydraulic motor and flow control operating off the tractor hydraulic system could also be used to drive the centrifugal pump.
This frees the PTO for other missions, and a hydraulic motor may sustain a more consistent pump speed and output with minor variations in engine speed. Direct-coupling of gasoline engine with pump maintains a constant pressure and pump output free from vehicle engine. The supply tank should be above the centrifugal pump to aid in and maintaining a prime.
Centrifugal pumps don’t require pressure relief valve. A strainer located in the discharge line protects nozzles from clogging and avoids constraining I pump input. Pump discharge line and the agitation line use 2 different control valves . This allows control of agitation flow, independent of nozzle discharge. Pressure relief valve with a separate bypass line is not necessary in controlling the pressure as this can be done through a throttling valve. A different throttle valve is used to regulate agitation discharge and spray pressure. Electrically regulated throttling valves are common for remote pressure control and are installed in an optional bypass line.
Spray system with a centrifugal pump.
A shut-off valve allows the sprayer boom to be shut off while the pump and agitation system continues to operate.With electric solenoid valves chemical-carrying hoses that run through the cab of the vehicle are not necessary. Electric valve is controlled by a switch box mounted in the vehicle cab. operator safety is assured if a hose should break.
To adjust for spraying with a centrifugal pump, open the boom shut-off valve, start the sprayer and open the throttling control valve until the pressure comes up to 0.7 bar over the desired spraying pressure. Control valve for the agitator is then adjusted until good agitation is observed in the tank. If the pressure drop slightly as a result of the agitation, readjust the main control valve to bring the pressure up to 0.7 bar above spraying pressure. Hence open the bypass valve to bring the boom pressure down to the desired spray pressure. The valve can be opened or closed as desired to offset for system pressure changes so a constant boom pressure can be maintained.