Dec 09, 2003

Outcome Reversed

Florida Water Utility incorporates detection system to halt unwanted reverse rotation in pump

Electro-Sensors, Inc.

6111 Blue Circle Drive * Minnetonka, MN 55343

Phone 952/930-0100

George Osterberg of the Palm Beach County (Fla.) Water
Utility related a situation that occurred in his facility recently. It involved
unwanted reverse rotation of a high service pump.

"Reverse rotation occurs because of a backpressure
condition that can develop in the piping system after the pump is shut
off," said Osterberg. "Seepage back into the pump is always a
possibility. At our water utility in Palm Beach, reverse rotation occurred unexpectedly
in one of our pumps.

"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.

"Our emergency generator kicked in and continued to
close the hydraulic valve" Osterberg continued. "At the same time,
water demand increased and the pump was called back into service. When the
800-hp 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-in. diameter
pump shaft to snap rendering the pump out of commission. The time and expense
of repairing the pump was considerable."

Answer found

The solution involved developing or acquiring a low-cost system
to detect shaft reversal that would safeguard pumps from potentially damaging
reverse rotation. This system needed to be simple to install, operate
automatically and, above all, be reliable.

After an extensive Internet search, a system made by Electro-Sensors, Inc., was found.

The company offered the ideal shaft reversal detection
system, the UDS1000. This system uses a non-contact, bi-directional rotation
sensor combined with a signal generating magnetic collar installed on the pump
shaft. It continuously monitors incoming pulses from the rotation sensor and
magnetic collar. The unique circuitry of this product employs a failsafe switch
output--an isolated relay--that 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, as in this case, would de-energize the
relay. This means the starting circuit to the pump motor would still 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 and
eliminating the problem.

All components of the detection system are corrosion
resistant and designed for water utility usage. The Palm Beach County Water
Utility discovered reliable protection for their critical high service pump
after installing the UDS1000 Shaft Reversal Detector System.

About the author