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POWER STATION SPRAYS IMPROVE EFFICIENCY

Engineers at Longannet power station on the Firth of Forth recently completed a re-fitting programme on drum screen spray cleaning installations that from an early stage resulted in a marked improvement in downstream operating efficiency. Their main task had been to strip out the old spray nozzles, which had performed poorly, and replace them with high-impact FlatJet nozzles manufactured by Spraying Systems.

Cooling water for steam condensers at Longannet, Scottish Power's generating facility on the north bank of the Forth, is drawn directly from the estuary. Four 100ft diameter revolving drum screens filter out seaweed, leaves and other debris that would otherwise foul condenser tubes and plates.

Should these screens become blocked or partially blocked, there is a chain of cause and effect. Water flow to the condensers will be restricted, reducing cooling efficiency, which in turn means loss of vacuum in the condenser and consequent lowering of efficiency in the steam turbine. So for maximum efficiency the drum screens must always be kept clean.

They work continuously 24/7. As they revolve, they are cleaned with filtered estuary water pumped at 3-4 bar pressure to spray nozzles mounted in manifolds just above each screen.

However, said Donald Adrain, the engineer responsible for the refurbishment project, the original nozzles were not performing well and did not seem suited to the flow rates of the pump. Despite close nozzle spacing, the jets had too little impact to clean the mesh thoroughly. Moreover, the nozzles were extremely expensive to replace when they were worn or damaged.

Consulting with the area engineer from spray technology specialists Spraying Systems, Scottish Power decided to replace the whole spray installation with high-impact nozzles carefully matched with pump performance and capacity.

The nozzles recommended were QuickJet FlatJets - 2-part stainless steel nozzle assemblies in which only the spray tip needs replacement. The body stays permanently mounted in the manifold while the tip (with integral seal) is quickly removed and re-installed by 1/4 turn. No tools are needed and correct alignment is automatic. Important features are uniform high impact across the flat fan spray pattern and a large unobstructed flow passage that minimises clogging.

The first drum screen was converted in July 2005 and it was immediately apparent that less debris remained on the screen. During the winter months two more screens were converted. That is a period when more debris is visible in the estuary, but since the conversions there has been no trouble with choked screens.

Donald Adrain sums up. "We are seeing cleaner screens with less debris caught in the mesh. Cooling water is flowing to the condensers at a higher rate; there is less loss of back pressure and the result is that downstream efficiency is increased."