The Case of the Ancient Lift Station
Synopsis:
Sometimes, “They don’t make ‘em like they used to” is really true. Here, three venerable motors are saved from replacement by inferior new motors.
Background:
An ancient lift station in a Montana city failed and damage by lightning was claimed. TI’s client obtained a lightning report which did not show any lightning close enough to have hit the lift station.
Assignment:
TI was requested to determine the cause and extent of damage, possibility of repair, and commercially reasonable cost or repairs or replacement equipment.
Analysis:
TI first compared the search center in the lightning report to the actual loss location and found it was significantly in error. Then, the loss site and location of each lightning stroke were plotted. While one lightning stroke was significantly closer, it was still beyond the claimed maximum error for the lightning report. However, it is known that lightning recorders do occasionally miss lightning strokes. TI is highly experienced in differentiating damage done by lightning from all other electrical failure modes and in the site inspection confirmed lightning as the cause. The lift station consisted of three 60 HP pumps with wound-rotor motors. Each motor had a bank of resistors for reduced rotor energizing voltage that provided for two reduced energization levels, thus two reduced motor speeds plus full speed. With each motor having three speeds, the group of three pumps provided for nine different sewage pumping rates. Motor speed selection was performed by relays controlled by 11 diaphragm switches set in a progression of higher pressures using flows of compressed air to bubble in the sewage well.
Relay 1 was set lowest and represented all pumps off. The next nine were progressively higher pumping rates as the sewage level rose 8’ in the well and relay 11 was an emergency alarm for high sewage level. The controls were custom designed and built for the application and thus not replaceable. A contractor proposed replacing all equipment in the lift station with new pumps, variable frequency drives (VFDs), and electronic controls. TI strongly recommended against the proposal as not in the city’s best interest. The ancient and massive Fairbanks-Morse pumps and motors were very conservatively designed, and designed for maintenance. The robust motors had proved exceptionally reliable over more than a half century, a record unheard of with today’s lightweight motors and electrically tender VFDs. TI worked with a second electrical contractor to design replacement controls employing programmable logic controllers (PLCs) and pressure transducers, retaining the highly reliable resistor banks and wound rotor motors. For reliability, the PLCs were redundant and could be changed without shutting down (hot-swap) and there were redundant pressure transducers for the sewage well that could measure depth to 0.3” compared to 9.6” for the diaphragm switches. The major argument for the VFD system was energy savings but with nine pumping speeds for the Fairbanks-Morse pumps, the $100k additional cost would never be recovered due to far higher downtime and maintenance costs with the VFD system.
