Major Failure Review and Remediation:

Rotating Drum Screens

Summary

The service life of rotating drum screens used to remove solids from municipal waste water systems can be significantly increased by the application of principals of continuous improvement during routine maintenance.

Background

The  customer has two pairs of rotating drum screens used to removed solids from the municipal waste water at sewage treatment plants. The two plants had been significantly upgraded and modernised. Two new drum screens were installed at each site as part of the upgrade.

After the plants were commissioned the performance of the drum screens quickly degraded requiring servicing. The service interval had reduced to as little as four weeks. On some occasions servicing was triggered by mechanical failure.

The expected major service interval for the drums screens was three months.

Problem

The significantly shorter service interval was increasing the cost of ownership of the drum screen asset and the low reliability was increasing the risk of breaching environmental protection conditions.

Solution

At each occasion the drums screens where removed for service and/or repair failure mode analysis was performed on broken parts and failure mode prediction was performed on worn parts. From the analysis improvements to prevent failure modes were suggested to the customer and approved for implementation.

The following failure modes were identified and improvements made:

Failure: Retaining screws for perforated screen vibrating loose. Caused by lack of locking washers.

Improvement: Serrated locking washer installed on all retaining screws.

Failure: Perforated screen blocking with fine fibre debris. Caused by nozzle alignment on two spray bars used to clean perforated screen from outside of drum.

Improvement: One spray bar was offset by half the nozzle pitch to increase coverage of spray.

Failure: Bearings support rollers seized due to seized bearings. Bearings are submerged in waste stream and had corroded to failure.

Bearings in support rollers replaced with 316 stainless bearings.

Improvement: Bolts on drive ring sheered.

Replace bolts with high tensile custom made fasteners.

Failure: Drive sprocket  coming out of mesh with drive chain. No fine adjust present on gearbox mount.

Improvement: Fine adjust bolts installed beneath gearbox so that mesh can be accurately set.

Failure: Screw conveyor nylon support bush wearing the steel shaft of the screw auger. Nylon impregnated with grit from waste water.

Improvement: Screw shaft protected with nylon cap and nylon support bush modified to suit.

Waste solids collecting at discharge of screw conveyor and blocking discharge. 

Relocated baffle plate to eliminate lipping.

Outcome

The service life of the drum screens has been increased from an average of weeks between failures to six months between scheduled servicing. Cost of ownership of the assets has been significantly reduced and performance of the assets is now predictable.

Cost Saving Estimates

• Maintenance costs were reduced by a factor of 6.

• Downtime was reduced by a factor of 9.

Reflection

The customer went on to purchase a spare drum screen for each site. The requested that the new screen be modified to include the suite of improvements so that they could function as a drop in replacement to be used during scheduled servicing.