Electronic safeguards eliminate equipment damage

Jam-ups resulting in damage to equipment and leading to large repair costs are now preventable in unmanned or partially manned treatment or pumping stations. in fact, after more than a year of using a new electronic monitoring and protection device at the Wetzel Road Waste Water Treatment Plant in Onondaga County, N.Y., damage resulting from jam-ups has been eliminated on primary and secondary treatment tanks.

Jamming can occur when buildups caused by ice, debris, or heavy sludge loads result in excessive stress 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 is the system out of commission, but the cost of repairs and replacements puts additional pressure on tight maintenance department budgets.

Problems of this type are much more common since plastic chain has come into widespread use in settling tanks because of its light weight and non-corrosive characteristics.

Shear pins, commonly installed in drive motor hubs, are supposed to be the weakest points in any drive system, protecting expensive equipment from damage. However, when a jam occurs, and motors strain to overcome the unexpected load, the plastic chain often breaks before a shear pin fails.

The 3.5 mgd Wetzel Road plant was experiencing this kind of problem with unreliable shear pins in its primary and secondary treatment tanks, which are about 120 feet long and 20 feet deep.

The plant is manned only during regular working hours, a.m. to 3:30 p.m., five days per week. Periodically, the daytime crew would come in and discover that all the flight boards had broken and gathered at one end of a tank due to a chain failure. It could take a couple of days to drain the tank, clean up the mess and return to normal operation.

Like the Wetzel Road Plant, most of the treatment facilities in the Onondaga County wastewater treatment system operate with daytime crews or are unmanned pumping stations or small package plants. In the north section, there is just one 24-hour-per-day plant with a central control room to monitor the operation of the other facilities in the section. It is possible for a breakdown to occur on a Friday evening and go unnoticed throughout the weekend.

To avoid such problems, a new type of protective device that monitors a motor’s output current was investigated. When the motor works to overcome an obstruction, drawing an excessive amount of electric current in an effort to overcome the load, this 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, Wheeling, Ill.-based U.S. Tsubaki’s Shock Relay protective device trips and shuts off the power and prevents damage to the treatment plant.

Continuously adjustable trip points provide the sensitivity and precision that are missing in mechanical protective devices such as shear pins.

Since August 1994, when four of the Shock Relay units were installed on drive motors in the Wetzel Road Plant’s treatment tanks, damage due to a broken chain or any kind of overload has been totally eliminated.

The system reacts immediately, tripping when the established current and time limits are exceeded.

All movement stops and damage to equipment, including the plastic chain, is prevented. Removing any obstructions and resetting the instrument is all that is necessary. This is much easier and less costly than taking a tank out of service to repair damage, which was often necessary in the past.

By preventing just one breakdown requiring a shutdown for a day or two, the device pays for itself many times over. Eliminating problems of this sort also eliminates headaches for the operations section of the system because any breakdown affects many aspects of the downstream operation.

One of the real benefits of these devices is that they are installed in enclosed electrical panels, isolated and protected from the treatment plant environment. Maintenance personnel can easily reset them with the push of a button if they trip to protect a drive system.

The Shock Relay units also work when the plant is unmanned and no one is around to shut off a drive motor if a problem occurs. They can be wired directly to a central control computer.

Two additional Shock Relay units have been purchased for installation on screen rakes, which tend to jam when heavy flows bring large objects into the plant. These protective devices would also be valuable on submersible pumps, which can bum out easily if they run dry.

Paul Backes, maintenance coordinator, Omondaga County (N.Y.) Department of Drainage and Sanitation