After 60 years of operation, the intake trash racks at the Verbois power plant near Geneva were in need of replacement. Along with the replacement, the rather old two-part manual trash rack cleaning system was to be replaced.

With a modern, fully automated trash rack cleaner (TRC). Austrian-based specialist Künz submitted the most favoured solution and was awarded the contract. The new system with its redundant layout now ensures enhanced performance and security while reducing downtimes at the facility. Measuring 7.8 m in width, the TRCM with its portal-type layout offers more space for staff and improves plant security in previously dangerous areas.

Hydropower plant Verbois is situated on the River Rhône, around 10 km downriver past the city of Geneva. In 2012, after more than 60 years of use, the intake trash rakes had to be replaced due to natural wear and tear. The intake trash rack system measures 65 m in width and is almost 20 m high, which makes it one of the largest of its kind in Switzerland. As can be imagined, the replacement project was no small job. A year earlier, an official tender had been put out for the delivery of a trash rack cleaning machine. Provider Künz from the Austrian province of Vorarlberg submitted the economically most favourable offer and was awarded the contract. The two obsolete manual trash rack cleaning machines could now be replaced by a single TRCM based on redundancy supporting technology.

SYSTEM SWITCHOVER
The two old TRCMs are based on the concept of a so-called grab rake. As the name suggests, this type of rake has a claw-like shape that is used in a downward cleaning motion to grab the debris. During the hoisting phase, the rake remains closed, which prevents it from grabbing more debris until it is lowered again. Good cleaning efficiency also requires the application of a large amount of weight per meter. This original system was manufactured by long-established firm Jonneret of Geneva and proved very successful for manual cleaning. The reasons for installing two machines was their low cleaning width and the required redundancy. Unlike the old system, the new one has a motorised rake hoisting mechanism and uses a so-called pivot rake. “This rake has a greater cleaning width of 3 1/2 metres, and it’s heavier as well, which makes it easier to sweep across layers of debris,” explains Ing. Samuel Wolfgang, TRCM Project Leader at Künz.

GENTLE MOVING GIANT
The hoisting rig and carriage mechanism of the Künz trash rack cleaner are controlled electromechanically.Frequency-controlled drives keep the movements smooth and steady. To achieve optimum performance, the hoist mechanism is initially run at a low speed, which is then ramped up gradually to its maximum performance level.

HIGH SECURITY REQUIREMENTS
The power plant is designed to accommodate a public road, which passes across the premises at the highest point. This meant that the facility’s premises had to be fenced off along a 200 m stretch. For operational reasons, the area was divided into three sections, which allowed for automatic cleaning and maintenance work to be performed at the same time. The various access gates to the individual sections are monitored and integrated into the automatic control system. In addition to section monitoring, the TRCM was equipped with a new feature for detecting oversized debris objects such as tree trunks. There is a portal crane each to the left and to the right of the TRCM. These cranes are used for technical revisions. When the TCRM is in operation, they are usually parked in idle position. Whenever they are moved within the TRCM’s operating range, an anti-collision mechanism causes the TRCM to stop automatically. All these extra security measures were approved and released for automatic operation by Swiss industrial accident insurer SUVA.

LOW DOWNTIMES
From the beginning, the focus in this project was on minimising downtimes, as the new solution uses only one unit instead of the previous two. In case of a fault, the TRCM must be up and running again within at most 48 hours, as per requirement specifications. To address this need, a concept was developed based on system failure scenarios: the so-called MTTR (Mean Time To Repair) system concept. This provides an overview that specifies which structures, components and systems might fail and which repair measures would have to be taken in that case. It also keeps an up-to-date record of which replacement parts are already available. A stand-by package of these parts is kept on-site to ensure instant availability in case of a failure. This package consists mostly of parts that are normally subject to long delivery times or have an above-average likelihood of failure.

HUGE AMOUNTS OF DEBRIS
Hydropower Plant Verbois is the first major river dam structure past the Lake of Geneva on the River Rhône. In this area, the river tends to carry large volumes of debris and municipal waste. So far, the disposal concept was based on a channel system through which the collected debris was flushed into the tailwater. With the new solution, the idea of keeping the cleaning and disposal functions separate was preserved but improved further. As a result, the new TRCM is able to load more than 20 cubic metres of waste into a temporary storage container, from where it is dumped into a 30 cubic metre container situated on the left bank of the river. The temporary storage container is constructed in such a way that its headwater section diverts floating debris directly into the flushing channel so that the container is instantly ready for the next debris collection cycle. This construction helped to improved the cleaning capacity. However, the new rake has a large screen gap width. Larger debris objects would be likely to get caught in between the screen bars and be pushed towards the upper end of the trash rack by the rake. To prevent this, the steel lining is attached by means of slanted metal plates that run in between the screen bars. The entire steel lining is manufactured from tilted stainless steel elements.

RELIABLE DATA TRANSFER
To ensure seamless integration of the TRCM with the automation and control system, a communication system was to be put in place, which had to be able to transmit the various control commands and status messages. Künz decided to use optical waveguide (OWG) connections for this purpose. The OWG is integrated into a combined energy supply cable. This also carries the video signal of the cameras that are attached to the TRCM.

BROUGHT UP TO THE LATEST TECHNICAL STANDARDS
So far, the cleaning system has met – and in part even exceeded – the operator’s expectations. Equipped with its new intake trash rake, the tradition-steeped hydropower plant on the River Rhône is now ready for further decades of reliable service. With the installation of the fully automatic trash rack cleaner the facility has been upgraded to the latest technical standard, which ensures improved plant security and an unobstructed water flow.