April 2011, Vol. 23, No.4

Problem Solvers

Water and wastewater authority takes steps to prevent pump failure

Problem: Inability to monitor pumps resulted in pump failure.
Solution: Installation of continuous vibration monitors that interface with SCADA system.

Loudoun Water, a water and wastewater authority in Ashburn, Va., experienced a catastrophic pump failure that led to lost revenue and customer complaints. The failure occurred at a single impeller-between-bearings pump in a sump pump arrangement.  

Because the authority did not have monitoring equipment installed on its pumps, it did not receive notice of the impending pump failure. Preventing this failure could have avoided thousands of dollars in expenses from extended downtime, lost revenue, extensive repairs, and parts replacements.

To prevent future failures like this, Loudoun Water decided to implement simplified continuous vibration monitoring to monitor its equipment. The authority also decided to connect the vibration monitors to its existing supervisory control and data acquisition (SCADA) system.

The authority installed vibration monitors on four separate motor and pump combinations at a single remote pumping station. Three of the motor–pump combinations were impeller-between-bearings pumps; the fourth unit was an overhung impeller pump. For maximum protection, Loudoun Water permanently installed accelerometers in the horizontal position on the inboard and outboard bearings of the motors and pumps.

The authority chose Wilcoxon Research 786A general purpose industrial accelerometers, manufactured by Meggitt Sensing Systems (Germantown, Md.). Ancillary equipment included Wilcoxon Research iT122 Intelligent Transmitters because of their 4–20 mA output signals for simple trending data, as well as front-panel connectors to access dynamic data when additional vibration analysis is needed. The DIN rail-mountable vibration transmitters, housed in an enclosure local to the machine, provide a termination point for the vibration signal conditioners, power supply, and cables, as well as a means of determining present condition at the machine.

Each of the 16 accelerometers capture unique signals from one bearing location and forward it to a dedicated transmitter, where the acceleration signal is electrically converted to velocity and processed into a single 4–20 mA signal. The 4–20 mA signal, which now represents the overall vibration velocity, is then tied into the SCADA system.

The vibration monitors operate continuously with both local and remote monitoring capabilities. The SCADA system monitors the 4–20 mA signal for significant upward or downward changes in the vibration levels. When such a change is detected, the SCADA system sends an alarm message to the appropriate plant personnel via a local control panel and e-mail messaging. In turn, authority personnel are able to initiate corrective action. Personnel also can use the front-panel connector of the vibration transmitter to view the raw accelerometer signal to conduct advanced vibration analysis. After identifying any problems either by visual inspection or vibration analysis, plant personnel will have the necessary information to determine if immediate shutdown is required or if maintenance can be postponed until a scheduled shutdown period.

If a vibration monitoring program had been in place to detect the original failure, the water authority may have saved thousands of dollars on pump repair and parts replacement, as well as downtime and lost revenue.

Loudoun Water implemented vibration monitoring for condition-based maintenance as an investment in reliability to help detect and prevent problems and better serve customers. As a result of implementing this system, Loudoun Water has maximized uptime and minimized unscheduled maintenance. The authority is including continuous vibration monitoring requirements into planned station upgrades.

 

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