German company Muhr, in partnership with its US distributor Lakeside Equipment Corporation, has developed and installed the world’s largest trash rack cleaning system at Rock Island dam in the US. Florian Kufner gives more details
Rock Island project was the first dam to span the Columbia river in the US. The project is located near the geographical centre of Washington State, about 19km downstream from the city of Wenatchee. It consists of two units. Powerhouse I was constructed in 1933 and includes 11 generators. Since 1979 Powerhouse II has eight additional horizontal shaft generators with a common nameplate capacity of 410MW. Both powerhouses achieve a total power plant capacity of 62MW. The 1022m wide dam also contains 31 spillway gates and three fishways.
Trash rack cleaning system
At Rock Island, the Columbia River mainly contains two types of floating debris: coarse materials like wood, trees etc that float on the water surface; and fine material (mainly grass) dispersed in the water.
Coarse materials accumulate in masses before the intakes, where they have to be removed. Without a trashrack cleaning system (TRCS) this had to be done by one or more truck-mounted cranes. The disadvantage of this method was that every time debris had to be removed, the crane and a truck onto which the lifted debris was discharged had to be prepared. Another problem was the quite limited operating range of the crane. To clean all intakes it had to be displaced several times.
A more frequent and even worse problem is the grass content of the river. Although the bar-width of the trash racks is quite amply dimensioned, some grass enlaces the rack-bars and reduces the flow rate. Because the grass could not be removed by a crane, divers had to clean the trash racks several times a year. This laborious procedure was extremely expensive because not only did the divers have to be paid but the generators also had to be switched off.
To improve matters the operator of Rock Island hydro project decided to install a specifically customised TRCS to help increase efficiency of the plant and simultaneously reduce labour costs.
The TRCS had to be capable of:
• Cleaning all intakes at Powerhouse II.
• Removing all debris and discharging this onto a truck or a container to be placed on the dam.
• Travelling on the existing rail system that is also used by the local gantry cranes.
• Offering enough height clearance to avoid passing traffic – between the rails of the rail system there is a road which is used by vehicles, including large trucks.
• Operating in manual and automatic modes.
After reading the tender specifications and visiting the site, German company muhr worked out a TRCS-concept based on the Muhr Hydronic M-5000 series.
The Muhr team also had to cope with extraordinarily high stream velocity at the intakes (2m3/sec). Despite the streamlined design of the boom and rake, straight and lateral flow forces are so high that countervailing these forces takes more power than retracting the telescope with maximum load.
Another interesting feature of this machine is the cleaning heads. Catching huge and bulky debris from the water surface and exactly raking out grass from the trash racks is not possible with one rake. As a result, Rock Island’s Hydronic M-5000 is equipped with an interchangeable flange that allows easy changing between the cleaning and gripper head.
Development of the first idea to the first dry test run at Muhr’s headquarter in Brannenburg, Germany only took 13 months. After intensive consultations with the client and a detailed planning process, the system was at first assembled in the Muhr factory in Brannenburg, Bavaria. This is also where the acceptance by the client took place.
Acceptance in the Branneneburg factory was advantageous for both client and manufacturer. Adjustments and additions could be made quickly and easily, saving both time and money for all involved.
Right after the acceptance test the machine was disassembled, packed and prepared for shipping. The journey started from South Germany to Bremerhaven. After containerisation the TRCS parts went across the Atlantic Ocean through the Panama Canal to the harbour of Vancouver and then on trucks to Rock Island dam.
The assembly on site started in July 2008 and was finished in the middle of August. The time required for assembling only amounted to two and a half weeks. Including site-specific additional work and final inspection it resulted in a total commissioning period of only five weeks. This was achieved due to the skilled assembly manager and site supervisor, who not only performed the on-site commissioning but also the first assembly in Germany.
Since installation the Hydronic M-5000 has been working very well. According to the plant operator the TRCS increases energy output by 8MW.
Only a few months after the Rock Island project, Muhr delivered one of the world’s largest cable operated TRCS’ to Kol dam in India, which is currently under construction. This specially designed machine is equipped with a 12.3m wide cleaning rake and is expected to clean 14 intakes. Due to the curved dam construction, the machine has to travel on a curved rail system. The TRCS is already setting 1200 tons of trash rack panels.
TRCS type: Mobile, hydraulically driven