Con Edison experiments with wedge-wire laterals
By Ann Chambers, Associate Editor
Con Edison, New York`s largest utility company, took a new approach to ion exchange in the water treatment tanks at its East River Station, replacing the screen-covered outlet laterals in one of its anion tanks with laterals covered in stainless steel wedge-wire instead of mesh screen. The wedge-wire laterals performed better than expected, eliminating problems typically associated with mesh screen and increasing maximum flow through the ion exchange train by about 15 percent–an advantage during high-flow emergencies of short-term duration.
The East River Station is the largest plant in Manhattan. Electricity is produced with steam from boilers fueled by oil or natural gas, depending on the season. The steam is sent through a high pressure turbine, with 30 percent of the steam traveling on to a low pressure turbine and on to condensers, while the other 70 percent of the steam is sent to various steam mains supplying heat in area buildings. East River is considered a swing station because of its location in Manhattan–it can be called on to send steam either uptown or downtown if another station is experiencing difficulty meeting demand.
The station has three sets, or trains, of ion exchange units in the main plant, each consisting of a cation tank and an anion tank. Inside each tank are 34 inlet and outlet laterals which let water pass through the tank but keep the resin beads and broken pieces of resin inside the tanks. Until recently, these laterals were covered with mesh screening that acted as the filtration media. On a typical winter day, one train will process approximately 100,000 gallons of water an hour. Only two trains are used at a time, with the third either on standby or in regeneration.
One of Con Edison`s biggest concerns is lateral failure in the middle of winter (see figure). A lateral failure occurs when the mesh screen on one or more of the laterals in a tank has eroded or broken, allowing resin to flow into the system. East River had a forced outage on a cation tank during the extremely cold winter of 1993-1994, and it was this incident which motivated staff to find a more reliable lateral.
The Chemical Group performs a yearly inspection of the tanks and laterals, patching slightly damaged screens and repairing or replacing more damaged ones. The difficulty in screen maintenance is finding damage. The mesh covering of laterals is so fine–0.010 inch–that small holes are easily overlooked. Inspection is a careful process, but parts of the laterals are difficult to inspect thoroughly. A small hole or tear in these areas could easily be overlooked during an inspection. Erosion occurs naturally due to the enormous amount of water flowing through the screens. Mesh wears away and eventually resin can pass through.
After 15 years of working with screen mesh, John Forte, East River`s results supervisor for the Performance and Chemical Groups, decided to seek out alternatives. It was then that he found Wedge-Flow, an industrial filtration grade of wedge-wire. Wedge-wire is a long-lasting, stainless steel filtration media specifically designed to be clog-resistant. Wedge-wire screens are made of triangular-shaped profile wires, wrapped around and welded to long, parallel support rods. The design makes them strong and non-clogging as small particles either stay on the surface or pass right through. The smooth walls of a wedge-wire screen also help prevent cracking of resin in a tank. There is less grinding against the screen and less plugging. During backwashing, the water maintains a more even velocity, with less breakage of the resin beads.
Forte elected to test wedge-wire in one ion exchange tank, replacing 34 mesh wrapped laterals with 0.010 inch slot opening, 0.060 inch wrap Wedge-Flow laterals. The laterals were installed in June 1994. Forte reports that the laterals have proven 100 percent reliable to date.
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Con Edison`s East River Station