This plant solved the mystery of sudden, frequent pump failures.
by Stephen Bell
March 29, 2018

Resolution

Once these factors were considered, the manufacturer took measures to address the pump shaft deflection, and the facility maintenance team focused on addressing the standard operating procedures. Each group took action to resolve further shaft failures.

The pump manufacturer made a design change in the pump to incorporate a larger shaft. The shaft diameter was increased by 40 percent.

This thicker shaft helped to reduce the shaft deflection by 66 percent, thus reducing the potential for premature wear and rotor/shaft assembly failure. The larger shaft increased the L/D shaft deflection ratio and increased the maximum allowable discharge pressure to 200 psi.

This design change later became the standard for this size pump for all future builds by the manufacturer. There was no change in shaft diameter at the drive end of the pump, so existing power transmission equipment such as sheaves and couplings will still apply to the pumps.

The pump supplier placed process monitoring equipment on the asphalt flux pumps to measure process loop temperature, discharge pressure and motor amperage. Recordings were taken electronically every two seconds as well as visually monitored three times per day by the facilities maintenance team for three months.

The facilities maintenance team reviewed the standard operating procedures specifically related to shut down and start-up procedures regarding the asphalt flux temperature loop. Changes were initiated to prevent the occurrence of “cold” start-up conditions.

Image 4. Wear patterns in internal gear pumpsImage 4. Wear patterns in internal gear pumps (Courtesy of Viking Pump)

Conclusion

By determining the pumps were over-pressurized through the analysis of the wear patterns in the pump, the team was able to focus its attention on why the high pressures were occurring and what could be done to prevent future shaft breaks and unwanted downtime.

The facilities management team diligently looked at the product temperatures and viscosities during run conditions, shutdown times and start-up times. They determined that changes in operating procedures could be adjusted to prevent a cold start-up condition that could increase viscosities in the asphalt flux and cause high-pressure spikes. The pump manufacturer agreed that a larger shaft in this size pump was achievable and would allow for less shaft deflection and higher pressure capabilities. That design is now the standard product for the manufacturer.

The team effort of the facilities maintenance team, the pump manufacturer and the pump supplier resulted in reductions of pump failures due to shaft breaks by analyzing the root cause of the breaks.

Initiatives were taken by all parties to prevent similar failures in the future. Since the standardization of the larger pump shaft by the manufacturer and the attention to temperature loop conditions by the facilities maintenance team took place, failures due to shaft breaks were completely eliminated in these applications. Since 2012, there have been no shaft breaks causing unnecessary downtime in this facility.

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