04-14-26 Agenda Packet - Laserfiche WebLink

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City of Arden Hills, MN – 25X.137753 LIFT STATION CONTROL SYSTEM January 2026 PAGE 26 90 04-9 ©2026 Design Tree Engineering, Inc All Rights Reserved b. The level control mentioned below shall be as sensed by the level transducer. c. When the wet well level is below the PUMP START elevation, the pump shall be off. d. When the wet well level rises above the PUMP START elevation for a preset time, the pump shall start. The VFD shall then modulate to the adjustable PUMP MINIMUM SPEED setpoint. e. If the wet well level stays at the PUMP START elevation or starts to drop, the pump shall stay at the minimum pumping speed. If the level rises, the pump’s speed shall be increased until the maximum speed setpoint is reached. f. If the wet well level starts to drop, the pump shall decelerate until it eventually reaches the PUMP MINIMUM SPEED. If the level drops below the PUMP STOP elevation for an adjustable time, the pump shall be stopped. 4. A restart delay relay shall prevent the pump from starting for an adjustable time delay of 0 to 3 minutes after it has stopped. The RESTART DELAY indicator shall be displayed on the operator interface screen during the restart delay period. a. The restart delay timer setpoints shall be operator adjustable via the operator interface. F. Process engineer will provide programmer with initial VFD ramp times and operating speeds. G. When a VFD is required to operate while under PLC control, PLC shall produce a digital required signal to the VFD and a 4-20mA analog signal generated by the PLC for use of VFD speed control. The analog signal shall be proportional to the desired speed. H. If when in AUTO and the process level drops below the “Low Water Alarm” level setpoint, the system shall shut down the pump and signal a LOW LEVEL ALARM after an operator entered time delay. I. If when in AUTO and the process level activates the “High Water Alarm” level setpoint, the system shall signal a HIGH WATER LEVEL ALARM after an operator entered time delay. J. Two float backup control mode (While in control mode selector switch is AUTO position): 1. Backup control shall be provided by 2 floats. Control relays and timers is to be provided for the backup control and shall be independent of the transducer and PLC controller. 2. High Level Float: If the wetwell level rises to an elevation which activates the High-Level Float for an adjustable amount of time, the control system shall go into a Backup Control Mode. The High-Level Alarm float shall initiate a High Wetwell Alarm which shall be signaled over the SCADA system, and start the pump. 3. Low Level Float: If the wetwell level drops below the Low-Level Float the control system shall discontinue the required signal to the operating pump and the control system and shall go into the Backup Control Mode if not already in this mode. 4. When backup control mode is active, an alarm shall be indicated on the SCADA system screens. Operator may transfer control back to PLC control by utilizing a simulated pushbutton on the OIT screen. 5. The backup start signal shall be connected to a preset speed input of each VFD. Speed setpoint shall be coordinated with the process engineer at time of startup. 6. When in float backup mode, the control system shall not signal a high- or low-level alarm when the floats are activated. The low- and high-level alarms shall be signaled when the system is under PLC and transducer control. a. The exception to this, is if the high wetwell float remains activated for a preset amount of time after pump is called to operate while backup mode is activated, the control system shall signal a backup mode high level alarm.