Application38Filtration+Separation June 2008

Screenwashing:

Filter achieves re-usable water for screenwashing A sewage treatment site and a plastic piping factory were looking

to produce higher quality water for use in two entirely different day-to-day operations. However when the two sites decided to install the same filtration product from Cross Manufacturing Company, both problems were solved.

At the Wessex Water Bridport Sewage Treatment Works in Dorset, UK, the drum thickeners demand a constant supply of clean water to maintain efficient operation by keeping the drain holes in the rotating drums free from the build-up of debris. The water is sprayed under pressure through nozzles, which themselves will become blocked if the water is not of a good, clean quality.

Looking to solve this problem, the site decided to install a Cross automatic backwashing water filter from the Cross Manufacturing company, a designer and manufacturer of liquid filters, heat resisting wrought alloy sealing rings and associated products. Wessex Water hoped that by installing a Cross automatic backwashing water filter, the site’s final effluent could then be used as a virtually free-of-charge alternative to expensive clean water for the screen washing operations on the site’s drum thickeners. By filtering out any solids with a size in excess of 200 microns, the Cross filter ensures that final effluent can be safely used for this application without the risk of interruption to normal operations.

The potential for savings

With a flow capacity of up to 18 litres/second, Cross manufacturing says, the compact Cross Phoenix filter installed at Bridport has the potential to make dramatic savings in terms of reduced clean water

consumption. There are no additional running costs since the pump which supplies the water to the thickeners creates enough pressure in the system to backwash the filter. The Phoenix is designed to backwash without interrupting the forward flow of filtered water and using only minimal quantities of backwash water. Backwashing is triggered by an adjustable pressure differential switch, with a set time lapse back-up.

Wessex Water hoped that by installing a Cross automatic backwashing water filter, the site’s final effluent could then be used as a virtually free-of-charge alternative to expensive clean water for the screen washing operations on the site’s drum thickeners.Central to Cross filtration technology is the ‘zero-gravity’ stainless steel coil with raised nodes on the surface which define a precise filter rating. Available in seven ratings between 12 and 400 microns, the coil is designed to open slightly and evenly along its total length when the flow across it is reversed during backwashing, ensuring that the build-up of particles is completely washed away. Cross filter systems are able to suit any size of duty by adding enough ‘zero-gravity’ coils to meet the required

flow capacity. In the case of the Phoenix, six coils are fitted in a specially designed housing that creates a cyclonic flow path for the water passing through, helping to keep suspended solids away from the surface of the coils and therefore extending the length of time before a backwash is required.

From Bridport to Huntingdon

In addition to the installation at the Wessex Water Sewage Treatment Works, Cross Manufacturing were also called on for the automatic backwashing water filter, this time by producer of plastic piping products, GPS PE Pipe Systems. The company was looking for a way to protect the manufacture of its composite piping systems from interruptions caused by poor quality cooling water and decided to install the Cross System 2000 automatic backwashing water filtration equipment at its factory in Huntingdon, Cambridge, UK. The Cross System 2000 performs exactly them same function as the Cross Phoenix filter installed at the Wessex Water site, however the two products differ in size.

Specialising in polyethylene pipe systems for gas and water used in the industrial and utilities markets, the GPS factory in Huntingdon manufactures 25,000 tonnes of pipework annually in diameters ranging from 20 mm to 1000 mm. During manufacture the extruded pipework is sprayed with recirculated water to cool it. It is essential

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Application 39Filtration+Separation June 2008

that this water is kept in a clean condition to prevent any blockages in the spray nozzles that would affect the quality of the finished product and to maintain the efficiency of the overall system.

However, use in the process inevitably introduces debris from the product and other sources that must therefore be removed. The cooling plant comprises a closed loop network containing 200 tonnes of water. The process raises the water temperatureand by the time that it reaches the ‘hot tank’ it is approximately 5° to 21°C. From there it is pumped through four cooling towers at a rate of 300 m3/h and returned to the ‘cold tank’ at the start of the process.

The replaced filter was failing to backwash efficiently, resulting in a build-up of debris that demanded dismantling and manual cleaning at regular intervals, which often disrupted the manufacturing process.The 75 micron-rated Cross System 2000 filter has been installed between the ‘hot tank’ and the cooling towers, replacing a mechanical backwashing filter that was originally used. The replaced filter was failing to backwash efficiently, resulting in a build-up of debris that demanded dismantling and manual cleaning at regular intervals, which often disrupted the manufacturing process. Cross backwashing technology employs the properties of the ‘zero gravity’ coils that are central to all Cross filters to eliminate dependence on mechanical devices that can create reliability problems.

Simply reversing the flow forces the coils to open evenly along their entire length, releasing all trapped particles which are immediately and completely flushed away. Backwashing is triggered automatically and the frequency is usually determined by the increased pressure differential across the filter coils. Backwashing is performed sequentially on individual filter pods, ensuring that forward flow through the filter is never interrupted and eliminating any requirement for redundant filter capacity. Combined with the all-stainless steel construction, Cross Manufacturing says these features have a proven track record of providing efficient and long term service in arduous applications with the minimum of maintenance. •Contact:Mark ClarkTel: +44 1225 8370 00E-mail: filters@crossmanufacturing.comwww.crossmanufacturing.com/filters

A service engineer records the backwash count on the Cross Phoenix filter installed at Wessex Water Bridport Sewage Treatment Works.

The water is pumped through four cooling towers before it is returned to the cold tank, at the start of the process.

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