Con Edison Replaces Mesh Laterals with Wedge-Wire Laterals in Ion Exchange

The East River Power Station

The East River Station is the largest plant in Manhattan. Electricity is produced with steam. Steam is made in boilers fueled by oil or natural gas (depending on the season), and sent from the boilers to a high-pressure electric turbine. From there, 30 percent of the remaining steam travels to another low pressure electric turbine, then to condensers to turn the steam back into water. The remaining steam is used in district heating, hot water and air conditioning in Manhattan. Feedwater to the plant is pre-treated to remove dissolved minerals, to prevent scale buildup. A "de-min" plant at the station treats the feedwater by activated charcoal filtration to remove suspended solids, and then ion exchange with both cationic (negative-charge) and ionic (positive-charge) exchange resin tanks.

The East River Station has three 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 allow water through the tank but keep the resin beads and broken pieces of resin inside the tanks, preventing them from leaking into the system. Until recently, these laterals were covered with mesh screening that acted as the filtration media.

On a typical winter day a train will process 700,000 to 800,000 pounds of water (approximately 1 00,000 gallons) an hour. Only two of the trains are used at a time, while the third is kept on standby or regenerated. When there is a high demand for steam because of severe cold weather, up to a million pounds of water per hour run through each tank, for short periods of time, when the third train is being regenerated, or is out of service.

A demin unit can process a certain amount of water before it becomes exhausted. At that point, the resin beads can no longer remove minerals from the water. The resin is then regenerated, the process of returning a positive or negative charge to the beads. The length of time before regeneration depends on the quality of the city water. This varies based on which city reservoir is supplying the station.

When a tank requires regeneration, it is removed from service and regenerated with sulfuric acid for the cation resin and sodium hydroxide for the anion resin. The acid replaces the H+ ion on the cation resin beads and the sodium hydroxide replaces the OH- ion an the anion resin beads. Regeneration requires 2 1/2 to 3 hours, approximately 4-5 times a day during the winter months.

Lateral Failure

One of the biggest concerns Con Edison has is lateral failure during a cold winter. A lateral failure (a forced outage) 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. When a resin leak is discovered, the entire train must be removed from service immediately to locate the problem. Since the leak may be coming from a line hole, this process can require several people and many hours to complete.

The Chemical Group performs a yearly inspection of the tanks and laterals. Slightly damaged screens are patched temporarily with a strong epoxy. When a screen is damaged badly, it is sent for repair or replacement. The difficulty of screen maintenance is actually finding damage. The screen mesh covering the laterals is so fine (.01 0 inches) that small holes are easily overlooked. But even tiny holes can be dangerous since resin beads are very small, and resin particles are smaller.

Although tank and lateral inspection is a careful process, parts of the laterals are difficult to inspect thoroughly, e.g., where the lateral is bracketed and at the tops of the laterals at the uppermost part of the tank. A small hole or tear in these areas could easily be overlooked during an inspection and impact the confidence of a total inspection.

When a resin leak is detected, the train is taken out of service immediately while maintenance crews work to get the tanks working. The resin is removed and the tank carefully inspected. Every lateral must be checked. If one leak is overlooked, the tank may have to be shut again. Laterals with broken screens are mended with epoxy if possible, or removed and sent for rescreening. While the laterals are being rescreened, the resin trap, coated with resin is also removed and cleaned. The pipes also require cleaning to ensure that no residual resin leaks into the system.

When the resin trap becomes coated with resin beads and particles, flow through the tank is reduced substantially, as a result of the pressure differential across the trap. In addition to shutting the tank with a resin leak, the other tank in the train is also rendered temporarily unusable, so the entire train (33% of the main plant's steam making capacity) is out of service. Although at that point two trains are still operational, they must be regenerated several times throughout the day, for 2 1/2 to 3 hours. This leaves only one train operating. Resin beads cost $75-$l 00 per cubic foot, or about $50,000 per tank. The amount of resin lost when a screen breaks on a lateral can vary, depending on the size of the hole. Other circumstances may lead to additional loss.

The last time the East River Station had an outage, 12 out of 34 laterals had to be replaced in one tank. More time is required to detect and correct a resin leak if there is a problem with the resin trap strainer, if there are holes in the mesh covering the strainer. Considerable resin can be lost before a pressure drop is noticed and the tank shut down. If a screen breaks on an inlet lateral, it may cause problems during regeneration.

The East River Station regenerates through the top and rinses through the inlet laterals (called counter-flow regeneration). More resin is put in a tank, and it supercharges the resin at the top of the tank, giving better quality water than conventional tanks. It also eliminates the need for an extra unmixed resin tank. A broken screen on an inlet lateral will allow resin particles to flow into the inlet pipe. Although there is a strainer in the inlet pipe, the mesh on it doesn't always hold. The last time this occurred a third of a tank of resin was lost costing more than $15,000. When epoxy is no longer effective, laterals have to be sent out for rescreening. This costs approximately $50 to $150 per lateral, depending on diameter and size.

Wedge Wire Replacement Provides Positive Results

Wedge Flow screens are made of triangular-shaped profile wires. These wires are wrapped around and welded to long, parallel support rods. Its design makes it strong and non-clogging; small particles either stay on the surface or pass through. The smooth walls of a "Wedge Flow" screen also help prevent the 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 (because it is less prone to blinding), so there is less breakage of the resin beads.

The reliability of the "Wedge-Flow" laterals provides no torn screens, no breaks, no shutdowns, no replacements. Each year mesh screen laterals in tanks are inspected for damage, especially before the upcoming winter. Inevitably, some have to be repaired or replaced. With the "Wedge Flow" laterals, this is not the case. In addition, laterals made with "Wedge Flow" are forgiving, not requiring extremely sensitive handling that a mesh lateral requires. A mechanic can put his wrench anywhere on the lateral, even on the actual filter medium, without causing damage.

More Output Without More Strain on the Tank

In the winter the East River Station normally runs 700,000-800,000 pounds of water an hour through its tanks. If a unit is out of service or for some reason extra steam is needed, up to one million pounds an hour can be pumped through the tanks. Unfortunately, that can cause problems.

At this higher rate the quality of water coming out of a tank decreases slightly. A million pounds of water travels through the tank so quickly that the resin may not remove all the minerals, and conductivity can go up. This is not a major concern if it is only for short periods of time, during emergencies. Still, pushing this much water through the tanks increases the pressure on the resin beads, leading to problems of higher pressure, such as cracked resin and clogged screens. These problems are compounded if they don't get a chance to clean the tanks before a surge in production. However, in the tank with the "Wedge Flow" laterals 1.2 million pounds an hour went through, well above the previous maximums.

When all the tanks are refitted with "Wedge Flow" laterals, the East River Station will have extra capacity during emergency conditions, and operate at a higher level without damage to the laterals.

Wedge FlowTm is a product of LEEM Filtration Products, Inc., Ramsey, NJ.

Author: John Forte, Performance Chemical Groups, Con Edison