Chapter 13

Wells and Pumps for Greenhouses

Centrifugal Pump Control

Pumping water for irrigation can be a major expense for greenhouses.  Improving the efficiency of irrigation pumps has many benefits, including improving the profitability of the greenhouse operation. When a single pump is required to operate over a range of flow rates and pressures, standard procedure is to design the pump to meet the greatest output demand of both flow and pressure. For this reason, pumps are often oversized, and will be operating inefficiently over a range of duties. But actual operating conditions may vary considerably from the design conditions, as when a single pump is used to irrigate several greenhouses. The performance of pumps can be regulated in several ways. The most common approaches for centrifugal pumps are:

Stop-Start/Pressure Level Control Operation

his method involves a pressure header or tank with on/off control. If properly designed, the pump doe not cycle much, and the on/off control produces acceptable results. An example, is with smaller watering systems using a pressure tank to supply the needed pressure to the irrigation system. A pressure tank is placed between a pump and the point of use of a water system to allow the water to become pressurized in the tank. The pump forces water into the tank, compressing the air in the tank. As the air compresses, the air and water pressure in the tank increases. Tanks have a pressure switch that controls the range of pressure that occurs within the tank.

Throttling Valve Operation

The easiest, but likely the not the least expensive is to design the pump system to meet the maximum pressure and flow and add control valves, which can regulate (reduce) the pressure or flow, control start-up rate, or other specific demands.

Parallel Operation of Multiple Pumps

Water wells located in unconfined aquifers are known as water-table wells. The water level in these wells is the same as the water level in the surrounding aquifer. Water wells located in confined aquifers are known as artesian wells. Wells are designed to be open to the aquifer; that is, water is free to move into the well from the aquifer. When completely at rest, the water level in a well and the groundwater level in the aquifer outside the well are equivalent.

Operation Centrifugal Pumps in Series

Pumps are frequently operated in series to supply head greater than either pump can supply on its own. Booster pumps fall in to this category, where one pump is used to booster the pressure of the main pump. Two pumps are connected in series if the discharge of one pump is connected to the suction side of the second pump.

Pump Speed Adjustments

Another method is to control the power unit and take advantage of the affinity laws and pump curve to produce the desired result. If the power unit is an internal combustion engine the control is by adjusting the fuel flow to the engine, which in turn affects the rotational speed.

Variable-frequency Drive

Variation of the pump speed, or rpm, is the most effective way to regulate a pumps performance. The use of variable frequency drives (VFD) is becoming increasing popular as an effective tool to vary the speed of a pump and thereby the pump performance. Electric motors used for irrigation pumping plants normally operate at a constant number of revolutions per minute (rpm), preventing the irrigator from adjusting the pump performance to match the operating conditions.

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