Trash racks are metal structures installed in the intake system of hydroelectric power plants to prevent the entrance of large debris, which can damage turbine parts and hinder the power plant operations.
In this article, we discuss the types, selection, inclination, construction and maintenance of trash racks.
Contents:
Types of Trash Racks
Trash racks are classified into three types based on their constructional features and the methods of installation:
Type 1: Removable Racks
The racks installed between side guides or grooves are provided in the trash rack structure so that they can be easily removed by lifting them from guides. Type 1 racks shall be used for all major trash rack installations where a portion of the rack is deeply submerged.
Type 2: Removable Racks
In this type, the individual sections are placed adjacent to each other laterally and in an inclined plane to obtain the desired flow area. The rack section is secured in place with bolts located above the waterline for easy removal.
Type 2 racks shall be used for canal headworks and pumping plants where a single rack section extends from the water surface to the bottom of the rack.
Type 3: Fixed Racks
In fixed racks, the rack sections are bolted in place below the waterline. Type 3 racks shall be used with power-driven cleaning rakes that are required for cleaning them. These types of racks are commonly used for intakes that are submerged.
Selection of Type of Trash Rack
The selection of the type of trash racks depends upon the following parameters:
- Accessibility for maintenance or replacement
- Size and quantity of trash expected
- Mechanism available for raking.
Inclination of Trash Racks
- The trash racks must be installed in slanting positions except for guided racks, which can be installed in a vertical position.
- For manual cleaning of the racks, the slope should be 1 vertical to 1/3 or 1/2 horizontal.
- For mechanically cleaned racks, the slope shall be 10° to 15° with the vertical.
Construction and Installation of Trash Racks
- The trash bars shall be fabricated from flats with rounded edges.
- The lateral support to the bars shall be provided intermediately between the end supports.
- If spacers are required, they should be placed as far away from the upstream face of the bars as feasible to avoid interfering with the rake's movement.
- The panel bars should be directly in line with the corresponding bar above or below so that the cleaning rake operates satisfactorily while passing up and down the screen.
- The trash rack sections must be manufactured in light enough parts to be removed and replaced manually.
Design Requirements for Maintenance of Trash Racks
- Cleaning of racks (automatically or manually) at regular intervals should be arranged in a suitable manner.
- The frequency with which the racks are cleaned would be determined by the rate at which trash accumulates.
- At any given moment, no more than 33% of the trash rack space should be permitted to clog the racks.
- In the case of Type 1 racks located in reservoirs, a platform at an appropriate level above the trash rack structure should be built to be utilized as a maintenance platform.
- The platform's level should be set so that the water level in the pond or reservoir falls below the platform's level at least once a year.
- This level can be set arbitrarily at the height of at least 5 meters above the minimum drawdown level (MDDL).
- Each rack should include hooks to allow for lifting the rack in Type 1 for cleaning, if necessary.
- For the easy raising of racks, chains may be linked to these hooks, and the end of the chains can be tied to the top platform.
FAQs
Trash racks are installed in the intake system of hydroelectric power plants to prevent the entrance of large debris, which can damage turbine parts and can hinder the power plant operations.
1. The trash racks must be installed in slanting positions except for guided racks, where these can be kept in a vertical position.
2. For manual cleaning of the racks, the slope should be 1 vertical to 1/3 or 1/2 horizontal.
3. For mechanically cleaned racks, the slope shall be 10° to 15° with the vertical.
1. Cleaning of racks (automatically or manually) at regular intervals should be arranged in a suitable manner.
2. The frequency with which the racks are cleaned would be determined by the rate at which trash accumulates.
3. At any given moment, no more than 33% of the trash rack space should be permitted to clog the racks.
4. In the case of Type 1 racks located in reservoirs, a platform at an appropriate level above the trash rack structure should be built to be utilized as a maintenance platform.
5. The platform's level should be set so that the water level in the pond or reservoir falls below the platform's level at least once a year.
6. This level can be set arbitrarily at the height of at least 5 meters above the minimum drawdown level (MDDL).
7. Each rack should include hooks to allow for lifting the rack in Type 1 for cleaning, if necessary.
8. For the easy raising of racks, chains may be linked to these hooks, and the end of the chains can be tied to the top platform.
Read More
Difference Between Pelton, Francis and Kaplan Turbines
Factors Affecting Selection of Hydraulic Turbine