Trash Rakes - Advantages of Automated Trash Rack Cleaning

Trash rakes have existed since the first hydroelectric power plants were developed. Used to clean the intake racks that prevent debris from entering the plant, trash raking can be done as either a manual or automated process. While manual cleaning is still commonly used, automated systems are becoming more widely used because of their increased efficiency and lower operational costs.

Challenges in Cleaning Trash Racks

Trash rack cleaning poses many challenges to the operators of hydroelectric plants. As outlined in Civil Works for hydroelectric facilities, published by the American Society of Civil Engineers (ASCE) in 2007, these problems include:

• Siltation, which occurs when twigs, branches and leafy materials are not removed properly, causing an increase of sedimentation on the trash rack.
• Lower efficiency, caused by a damaged raking arm or misaligned trash rake.
• Damaged cables or pulleys.
• Inadequate lifting capacity.
• Inability to reach the bottom of the trash racks. (Hand raking is limited to depths of 6 to 8 feet below the water's surface.)

One of the key solutions suggested in this book, beyond repairing damaged parts, is to upgrade manual operations to an automated system. The book cites an example from the Racine Hydroelectric Project on the Ohio River. It notes the benefits of changing to an automated system: "... it reduced [the] overall operating costs of raking trash, trash disposal, and lost generation."

Design of Automated Trash Rack Cleaning Systems

The book Guidelines for Design of Intakes for Hydroelectric Plants talks about the design of automated trash rakes. There are a couple of designs available:

• A hydraulically driven chain system pulls raking fingers through the trash racks.
• A fully hydraulic arm trash rake consists of a boom that lowers the rake head into the water. In advanced models, a hydraulic cylinder tilts the boom to apply the rake at a constant pressure to ensure thorough cleaning.

Automated trash rakes can handle debris with great efficiency, either by dumping it on the forebay or intake deck, or through a conveyor or trough. Automated raking systems can be initiated based on time or a head differential. A buildup of trash creates an increased differential head across the trash rack; the rake begins operations when a specific level is reached. Automated systems can also be initiated manually by an operator who can watch for certain conditions and trigger the rake when necessary.

The Benefits of Automated Trash Rakes

Automated trash rack cleaning has been proven to improve efficiency and reduce costs. The Racine experiment, cited above, is just one example. In Europe, Verbun upgraded its hydroelectric plants along the Mur River with fully automated systems in 2005. The plants have been unmanned ever since and working very efficiently.

Operator safety is another benefit of automatic systems. Manual trash raking is inherently risky, and automated systems reduce that risk.

Overall, automated trash rakes offer many operational and financial benefits. Evidence of these benefits can be seen in the upgrades being done to hydroelectric plants around the world, and in the ACSE recommendation that upgrades from manual to automatic systems are one of the best ways to overcome the challenges of trash rack cleaning.