Evans Water Engineers


+44 1566 782323 · www.evans-engineering.co.uk · email sales@evans-engineering.co.uk

EVANS Screening systems



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.

Rotary Screener from Evans EngineeringNo 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.