Reverse Rotation Detector Protects High Service Pumps

High Service Pumps are widely used at thousands of water utilities worldwide. These pumps typically have enormous pumping capacity and are driven with high horsepower motors. While they normally operate reliably, unchecked backpressure can lead to severe pump damage when a pump is started during a reverse rotation condition.

To help prevent this problem, Electro-Sensors Inc. developed the UDS1000 to detect shaft reversal in pumps. The system features a failsafe reversal switch that constantly monitors a pump shaft to assure that it's turning in the correct rotational direction.

Relay contacts in the unit can be used to electrically lock out the starting circuit of the pump to prevent accidental start-up during a condition of reverse rotation. The device is supplied with a custom-machined split collar magnetic pulser wrap that is designed to clamp tightly around the pump shaft.

The magnetic pulser generates an alternating magnetic field that is picked up by a large gap non-contact sensor. The sensor transmits this speed signal as a digital pulse (frequency) to the UDS1000 switch via a 4-conductor shielded cable.

The system continuously monitors incoming pulses from the rotation sensor and magnetic collar. A failsafe switch output (isolated relay) is electrically interlocked directly with the motor starter circuit for the pump. If the slightest reverse rotation of the pump shaft is detected, it disables the pump starting circuit, thus preventing start-up during reverse rotation.

When the pump stops turning in reverse, the system automatically resets itself to allow a normal, safe start-up.

Because the UDS1000 is failsafe, a power failure would de-energize the relay. This means the starting circuit to the pump motor would be disabled. Upon restoration of power, the UDS1000 relay resets but would instantaneously switch out again if it detected a shaft reversal, thus preventing the pump motor from starting during a reversal condition.

The Palm Beach County, FL, Water Utility installed the system after it experienced problems with a pump.

"The problem began to develop when we experienced a power failure from our primary electrical supplier, Florida Power and Light. Normally our pumps are protected from backpressure with an electrically actuated hydraulic valve system. Because of the power failure, this valve did not close completely after the pump was shut off and backpressure seepage caused the pump to begin rotating in reverse," said George Osterberg, Maintenance Supervisor of the Water Utilities Department.

"Our emergency generator kicked in and continued to close the hydraulic valve. At the same time, water demand increased and the pump was called back into service. When the 800 horsepower motor that drives the pump was started, it delivered full torque to the pump shaft that was still turning in reverse. This caused the 4-inch-diameter pump shaft to snap, rendering the pump out of commission. The time and expense of repairing the pump was considerable."

The utility installed the Electro Sensors system as a protective measure. The cost for the complete system was about $650 per pump and included all components needed plus a complete installation and operating manual with wiring diagrams. All components are corrosion resistant and designed for water utility usage.

Electro-Sensors, Inc., based in Minnetonka, MN, manufactures motion monitoring and speed control systems for industrial machinery. More information on its products may be found at www.electro-sensors.com.