While chain and scraper systems are a solid investment for removing solids, even the best clarification systems don't last forever. To prevent your chain and scraper system from failing, follow these helpful tips.
Carry Chain tension is paramount to avoid the chain “skipping" a sprocket tooth or worse, falling off the sprocket and ending up in “spaghetti junction” with the chain and flights wrapped around the shafting. Chain tension should be checked annually to prevent these problems. Chain tension can be easily measured by having a ~160 lb person stand on the chain midway between the third and fourth flight back from the head shaft. The chain should not touch the ground with this weight. If it does, then links need to be removed. It is important to perform this test during the coolest part of the day. Thermal expansion is prevalent in non-metallic applications, and in the hot sun on a summer day the chain will naturally become slack when the tank is drained.
To adjust tension use a chain fall or “come-along” of 2,000 Ibs (908 kg) maximum capacity and heavy-duty straps looped around the chain barrels (do not use metal chain or cable). Make all adjustments at the tank’s floor level and approximately midway between the effluent and influent ends of the tank. Remove all visible slack from all sections of the chain, excluding the catenary. Remember to remove the same number of links from each strand and to keep the flight attachments opposite one another. After adjusting, the mechanism should be run dry at least once around the tank and the tension rechecked in the same manner prior to putting the tank in service.
Shear pins are a simple and inexpensive tool that are the last line of defense for your drive unit. They are also the item that is most often neglected or missed. When inspecting chain and scraper systems, we often find shear pins out of alignment - where the neck is not centered with the split line in the shear pin sprocket. This causes the pin to shear at the thinnest point, instead of at the full diameter, which compromises the performance and safety of your system.
Another common issue is when the pin has been replaced with a bolt. This should absolutely not be done, as it will effectively disable the safety of your drive. Shear pins from Evoqua are the exact length of the bore of the sprocket, flush on both ends to ensure effective performance and safety. If cast iron shear pin hubs are used, please grease regularly to prevent hub seizure.
Wear Shoes and wear strips are designed to absorb the friction of the unit when it touches the tank floor or return rails. They have an excellent coefficient of friction and can last many years, even in high grit applications. However, wear shoes and strips inevitably need to be replaced and the placing of wear strips is critical to chain and scraper system performance.
When installing polymeric wear strips, all strips should be positioned and spaced per the drawings before attempting to field drill and install the anchors. Once the strips are positioned, locate and drill the holes for the anchors in the center of the countersunk hole or slot in the wear strips. This will allow maximum expansion of the strip and prevent curling or overlapping if the tank is drained and thermal expansion takes place. There should be a gap in between the strips in the ambient temperature condition. Gap measurement depends on the type of wear strip, so refer to your Operation and Maintenance (O&M) Manual for the correct spacing.
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