EVANS Screening systems

INTRODUCTION

The design of the screening system for hydro plants is often an afterthought but its proper functioning is vital to the success of the installation. There are also a number of misconceptions that often get incorporated into designs that at best limit the performance and at worst, make the whole plant unusable.

Few screens are self-cleaning, and simply angling the screen to the flow, makes little or no difference to the amount of debris collected. If you are abstracting a significant percentage of the river flow, material will adhere to the screen. Materials like wedge-wire are not self-cleaning, and only a ‘coanda’ or over flow type of screen is normally self-cleaning, but they have the disadvantage that significant head is lost across them, making them less suitable for medium and low head applications.

No screener will do everything perfectly and be cheap, so it is important to follow the logic of each design. All screening systems are a compromise between first cost, maintenance cost, running cost, environmental cost and turbine performance. The type of debris being removed, the size of fish and the percentage of the flow being abstracted, will all affect the design and performance. The hydro engineer will want to minimise the head loss across the screen for a particular flow, while environmental legislation usually determines the mesh size or bar spacing to keep fish out. If you are using an existing mill leat, even a small head loss across a screen can severely limit the water flow.

SCREENER TYPES

• A ‘Coanda’ type or flat bar screen that slope steeply down the front face of a weir can be used where head loss is not an issue and are commonly used on large hydro schemes where side streams enter a conduit. This system is not applicable for low head sites where the head loss would have a major impact on the output of the plant.

• A fixed screen that is hand raked or has a mechanical raking system can operate where labour is cheap, the bar spacing is wide or the amount of debris is small. Even with large schemes such as the ‘Three Gorges Dam’ screen cleaning can become a serious problem. Cleaning systems that employ a mechanical arm or moving scrapers have the inherent problems associated with mechanical components operating in water.

• A back-washing fixed screen, works by reversing the flow of water through all or part of the screen to remove the debris. The debris must then move directly or in steps towards a waste channel. The reversal of the flow can be done with a water jet, suction system or by reversing the differential head. If the system involves the whole screen, there will be a dip in power output while the screen cleaner operates.

• A moving screen of the conveyor or band type require twice the amount of screen material compared with a simple flat screen, and the head loss will be doubled because the water passes through it twice. In addition, there are the mechanical problems talked about earlier. Some advantage can be gained by only operating the cleaner when a head difference is detected.

• A drum screener is mechanically simple and can be very fine mesh if required, but it needs a constant downstream water-level if the back washing is done with a weir or an auxiliary water supply if the cleaning is done with a water spray.

• Other types include ‘walking’ and ‘rotating disk’. The latter are also used as macerators for smaller flows