Shock relay units stop costly plant breakdowns

Following a series of equipment breakdowns and service interruptions at its Wetzel Road wastewater treatment plant, the Onondaga County Department of Drainage and Sanitation in Syracuse, N.Y., placed a high priority on reducing downtime and maintenance costs.

The department was seeking a way to eliminate jams from sludge or debris that damaged equipment and led to large repair costs in unmanned or partially manned treatment or pumping stations.

Jamming can occur when heavy sludge loads, excessive debris on the bottom of settling tanks or ice buildups put undue strain on motors and chains.

Such conditions frequently lead to broken chain and flight boards, requiring the tank to be shut down until repairs can be made. Not only does this put the system out of service for several days, but the repair or replacement costs put additional pressure on tight maintenance department budgets.

This type of problem is more common since plastic chain has come into widespread use in settling tanks. While the light weight and non-corrosive characteristics of plastic chain make it well-suited for many wastewater treatment plant applications, it is also not as strong as steel chain.

When a jam occurs, and motors strain to overcome the unexpected load, something has to give. Shear pins, commonly installed in drive motor hubs, are supposed to be the weakest points in a drive system, protecting expensive equipment from damage. Too often, however, the plastic chain breaks before a shear pin fails, and the resulting damage can be severe.

Like the Wetzel Road Plant, the treatment facilities in the Onondaga County wastewater treatment system either operate with daytime crews, were set up as unmanned pumping stations or serve only as small package plants.

In the north section, just one 24-hour-per-day plant is equipped with a central control room to monitor the operation of other nearby facilities.

And since not every operation in the unmanned stations can be fully monitored, it is possible for a breakdown to occur on Friday evening and go unnoticed until the following Monday morning.

To avoid such problems, the department installed a new type of protective device that monitors a motor’s output current. When the motor is drawing excessive electric current to overcome the load, the device recognizes a potential problem and cuts off power to the motor.

When a user-selected “load current” and “shock time” are exceeded as the motor labors against a heavy load, the shock relay protective device, manufactured by U.S. Tsubaki, Wheeling, Ill., immediately trips and shuts off the power and prevents damage to the treatment plant.

All movement stops, and damage to equipment, including the plastic chain, is prevented. At that point, removing obstructions is all that is necessary before resetting the units with the push of a button. Previously, the tank would be taken out of service, a cumbersome and costly process.

The relay unit’s continuously adjustable trip points provide the sensitivity and precision lacking in mechanical protective devices such as shear pins. Since four of the units were installed on drive motors in primary and secondary treatment tanks at the Wetzel Road Plant in August 1994, damage from broken chains or overloaded motors has been virtually eliminated.

The relay devices are installed in enclosed electrical panels, protected from the treatment plant environment. The units can be wired directly to a central control computer, so operations personnel will be informed immediately if a drive motor is stopped for any reason.

Since installing its first four units, the department has purchased two additional units for installation on screen rakes, which can jam up when heavy flow brings large objects into the plant.