A Golf Course Irrigation System with a Single Submersible Pump
Controlled by a Variable Frequency Drive
Copyright © 2004 Francis J. Martino
As sprinklers are opened and closed by a separate, independent controller, changes in pressure will be created within the system which will then be regulated by the drive to maintain a constant system pressure. The source of water is from a well with a submersible pump.
In the event that the sprinkler openings and closings require the pump to be driven either below 30 Hz or above 105 lbs of system pressure in order to maintain constant pressure, then an automatic, unattended start and stop feature will be utilized.
If continuous, uninterrupted water flow is desired, it is the responsibility of the user to program the independent sprinkler control so that the number of valves opened or closed at any time will allow the system to remain within the specified and adjustable design limits of 60 and 105 lbs of pressure with continuous pump operation to be above 30 Hz and below 55 Hz.
A wall mounted set point potentiometer is provided to assist the user in adjusting the drive to regulate for continuous flow. The potentiometer allows the constant pressure point to be adjustable between 60 and 105 lbs.
The drive is enabled by a two-position selector switch for ON - OFF control.
Automatic, Unattended Start and Stop Feature
The automatic, unattended start and stop feature will provide for:
* Start upon low system pressure.
* Low speed shutdown at 30 Hz with restart upon pressure dropping to 60 lbs.
* High pressure shutdown at 105 lbs with restart upon pressure dropping to 60 lbs.
Either of the above two conditions may be caused by a high number of sprinklers or all sprinklers being closed simultaneously. In such a case the system pressure will then rise and the drive will attempt to maintain constant pressure by lowering the speed of the pump. With all sprinklers closed, the drive will attempt to lower the pump speed to below 30 HZ.
To prevent overheating of the pump per manufacturer's minimum low speed specification of 30 Hz and to prevent cavitation of the pump that may be caused by a lack of water flow, the drive will be shut off upon the drive output frequency lowering to 30 Hz.
To prevent high pressure within the piping system, the drive will also be shut off in the event that the system pressure rises to 105 lbs before the drive lowers the output frequency to 30 Hz.
The pump will then remain inoperative until the system pressure drops below 60 lbs. At that time the pump will be automatically restarted. If no sprinklers are opened and if the system is closed so that no water leaks out then the pump will remain inoperative.
Upon sprinklers opening, the drop in water pressure will cause the drive to restart and resume regulation for constant pressure. However, if the drive restarts due to low volume water leakage (without sprinkler openings), the drive will shut down a second time due to a high pressure condition that will be caused by the restart. If a rapid stop and restart cycle is allowed then the high number of starts may be detrimental to the pump motor.
Thus, as sprinklers are automatically opened and closed by the sprinkler control system, the drive will stop and restart as necessary for irrigation to continue throughout the night.
Broken Pipe Protection
In the event of a broken water pipe, the system pressure will drop and the drive will then attempt to maintain constant pressure by raising the pump speed. With sufficient water flow and a sufficient drop in pressure, the drive output will exceed 55 Hz for a continuous period of time. In order to prevent either flooding of the course or pump cavitation that may be caused if the well is unable to refill to meet the flow requirement, the drive will be shut off. A manual reset will be required via the Stop/Reset key on the drive keypad.
Note that a condition of a high number of sprinklers being opened simultaneously may be interpreted as a broken pipe. Adjustment to the broken pipe protection would be required.
In the event that the pressure gauge is located several hundred feet away from the pump, a fast acceleration rate will causes a surge at the pressure gauge. The pressure at the gauge will then fluctuate and cause a cycling on and off in a one or two second time span. The system may not stabilize and the subsequent repeated cycling may be detrimental to the pump motor. Therefore the acceleration time must be slow.
One (1) Any Variable Frequency Drive with High and Low PID outputs may be used. A timing function must be included or added separately which will allow the drive to interpret a broken pipe condition by virtue of the drive operating at a high frequency for an extended period of time. The PID outputs will be active only when the high or low condition is in effect and the outputs will return to the normal state when the high or low condition is relieved.
One (1) Signal Conditioner mounted in a separate enclosure. The conditioner will accept 0 to 10 VDC, 4 to 20 mA, or a potentiometer input and give two (2) Form C DPDT contacts output. 120 VAC power input is required.
One (1) Separate operator station with ON/OFF Selector Switch and Set Point Potentiometer.
One (1) Pressure Gauge for 0 to 160 lbs which provides a resistive feedback for pressure regulation or a pressure transducer that will give a 4 to 20 mA or 0 to 10 VDC output.