Pumps on Cruise Control
In 1988, the Bristol Township Wastewater Treatment Facility in Croydon, Pa., installed two screw pumps that were intended to move effluent from a secondary trickling filter to a chlorination chamber and then discharge.
Each of the two pumps, powered by a 75-hp AC motor, was started across the line with a direct-on-line (DOL) starter. One pump acted as the lead pump, with the second, or lag pump, only engaging during peak demand. Due to the operating stresses and conditions of the motors and pumps, the system incurred problems including corrosion and bearing failures, with an estimated annual maintenance cost of $25,000.
In an effort to avoid freezing during extremely cold winter spells, the lag pump often had to be operated even when flow rates did not require it, resulting in unnecessary energy consumption. The operating characteristics of using the DOL starters—starting immediately at full speed with current spikes of 600 to 800% of the motors’ full load amp rating and the mechanical stresses of these instantaneous starts—increased the maintenance costs and downtime of the system.
Finding a better way
In October 2003, Stan Rockovich, sales engineer for Godwin Pumps, began work with Unitech Engineers, Inc. on a pumping project for Bristol Township. When the screw pumps finally became inoperable, Unitech called Godwin Pumps for immediate technical assistance. Based on Unitech’s recommendation, Bristol Township rented a 6-in. CD150M Dri-Prime pump for a 14-month period to move flow and keep the beds active.
This allowed Bristol Township to test the reliability of a Godwin Pump throughout an entire range of temperatures and elements. In the meantime, a team including Rockovich, Godwin Pumps’ engineering department and the engineers at WEG Motors and Drives worked with Unitech engineers Kanti Patel and Partha Tallapragada to provide a permanent solution.
In January 2005, the township removed the two temporary screw pumps and replaced them with two permanently installed 12-in. DPC300 Dri-Prime pumps. Seventy-five horsepower 900-rpm WEG motors power each pump. To provide variable speed capabilities to the pumps, two WEG CFW-09 variable frequency drives (VFDs), built in custom outdoor NEMA3R panels, were installed to control the motors.
Where the original DOL starter was limited to running only at the motors’ base speed and caused a large mechanical shock at every startup, the VFDs allow for variable speed control and a soft and controllable acceleration and deceleration of the pumps. The variable speed ability of the VFDs provides a wide range of required flows from 800 to 4,000 gpm, depending on the conditions. The VFD parameters allow simple settings for minimum and maximum speeds and the acceleration and deceleration rates.
Most VFD applications require the AC motor to run at a specific speed as set by the keypad, speed potentiometer or analog input. The WEG CFW-09 VFD provides an alternate option that allows precise process control through a setpoint controller or PID mode of operation.
Just as the cruise control system in an automobile allows the driver to establish a set speed, the VFD allows Bristol Township to provide a setpoint to control some variable of the equipment that the motor is connected to, according to Rockovich. When on cruise control, a vehicle’s electronic system monitors the speedometer or process variable and will increase or decrease the motor speed to keep the automobile running at the set speed. As such, the proportional, integral and differential gains of the system, or PID, adjusts motor speed based on the effect of variables like terrain and weather conditions that would speed up or slow down the vehicle.
Similarly, by constantly comparing the desired setpoint and the value of the process variable using the VFD, Bristol Township is assured proper operation automatically and can save considerable energy and operating costs without having to manually monitor pump flow. At the Bristol Township Wastewater Treatment Facility, a level transducer that provides feedback on the process variable that measures the influent channel fluid level was installed. The beds need a constant flow of water to keep the bugs/bacteria from drying out and dying. Too much flow wastes energy and water; too little flow will affect the process operation. The PID function in the VFD compares the potentiometer setpoint with the process variable data supplied by the level transducer. The PID gain settings will control how quickly the motor speed will change to resolve any error between the setpoint and the process variable.
To maintain simplicity in the operation, the system control panel contains Start and Stop pushbuttons, a Hand-Off-Auto (HOA) selector switch, an Emergency Stop pushbutton, the Speed/Setpoint potentiometer, a PID On/Off selector switch, and Run and Fault Pilot Lights. The HOA selector switch allows Bristol Township to select the mode of starting and stopping the system to either pushbuttons or by remote contact closure.
The Bristol Township system’s control panel uses the same potentiometer to manually set motor speed in the PID Off manual mode and also to provide the setpoint in the PID On or automatic mode. This allows the motor to keep a very even flow with a maximum-programmed speed of 900 rpm.
According to Rockovich, to meet Bristol Township’s needs, the lead pump typically runs at approximately 800 rpm. Godwin Pumps has designed the system to engage the second pump only during high water conditions, based on the feedback or process variable signal received through the level transducer. The PID system only runs the motor at the speed required to keep the influent channel fluid level to match the adjustment of the setpoint potentiometer.
Benefits of variable frequency drive
The benefits of the VFD in this pumping application became apparent immediately at system startup. Using the VFD, the motor can slowly ramp up to the desired speed, eliminating the mechanical stresses of the across-the-line start. Once the water level reaches a predetermined level, the pump and motor are slowly ramped down to a smooth stop. Monitoring the level transducer, the pump is turned on automatically when required and runs at an appropriate speed set by the flow. This process alone conserves energy and eliminates the low cycle fatigue experienced in across-the-line starts. In addition, because the pumps and motors are ramped up slowly for starts and stops, there will be less wear and reduced maintenance costs over time.
“We’ve been pleased with this project,” said Steve McClain, director of the Sewerage Authority for Bristol Township. “Things are working fine, and we have a good flow now. It’s a good fit for us.”