Remerciez-le!

Remerciez @Admin pour avoir partagé cet document gratuitement, de la manière la plus simple, en partageant sur les réseaux sociaux.

EPRI Licensed Material 5-1 5 FOULING Fouling of the condenser includes any orga

EPRI Licensed Material 5-1 5 FOULING Fouling of the condenser includes any organisms, organic or inorganic, that interfere with the circulating water in the tubes and, ultimately, with the heat transfer process. When there is an increase in condenser pressure and a decrease in cooling water flow, fouling is the most likely cause. Other indications of fouling include an increased pressure drop and a reduction in temperature change for the inlet and outlet cooling water. Fouling impacts the output of the plant as it affects the condenser backpressure. The plant availability can be affected during seasonal changes that produce annual fish runs, grass movement, seaweed deposition, accumulation of leaves, and so on. The deposits on the tube reduce heat transfer rates, decrease cooling water flow, and increase pumping costs. The formation of these deposits is a function of the cooling water environment, flow velocity, and the season. In addition, the solubility of certain compounds, such as calcium carbonate, decreases with increasing water temperature. Key Technical Point There are two main types of biofouling: macrofouling and microfouling. Macrofouling is defined as the blockage of condenser tubes by organic or inorganic debris such as sticks, leaves, fish, mussels, and so on. Microfouling is the accumulation of deposits (inorganic scales or organic growths) on the inside of the tubes. The following sections discuss macrofouling, microfouling, chemical treatment, water regulations, chemical application methods, and a fouling monitor. 5.1 Macrofouling [14] Organic or inorganic debris can occur from traveling screen carryovers or from growth of organisms on the condenser and water conduit walls. The organisms eventually dislodge and plug the tubes. Macrofouling upstream of the circulating water pumps can reduce the available net positive suction head. This results in pump cavitation and reduced flow. Debris, lodged at the entrance to or inside the condenser tubes, can increase flow velocities around the debris. This increased velocity will erode any protective film and, subsequently, corrosion at this part will occur at a higher rate. Copper-alloy tubes are more prone to this phenomenon while stainless steel and titanium tubes are less susceptible to this attack. Organic debris left in the condenser tubes during outages decomposes biologically. This process of decomposition can be highly corrosive and produces compounds that can promote pitting or EPRI Licensed Material Fouling 5-2 stress corrosion cracking in copper and brass alloy tubes. Pitting can also occur in copper nickel and in 300 series stainless steel under these conditions. Prevention of macrofouling depends on site-specific conditions causing macrofouling. If the source is debris such as seaweed or freshwater vegetation, then traveling screens at the circulating water intake might prevent the debris from flowing into the condenser. If the source is an organism such as clams, then a biocide is needed. Many plants have the capability to backwash the condenser to remove macrofouling. Strategies for backwashing depend on circulating water conditions and trends in condenser performance. To determine methods that control macrofouling problems, it is important to understand certain characteristics of various fouling agents. The freshwater and saltwater environments yield different organisms that can cause fouling problems. Some of these organisms are discussed in the following two sections. 5.1.1 Saltwater Organisms x Barnacles – Barnacles are one of the most common fouling mechanisms. Numerous species occur along the coastline of the United States. As adults, they become permanently attached to a substrate and are protected by hard calcareous plates. x Mussels – Mussels are common fouling organisms that create severe problems at power plants. The mussel shell is three-layered and the two valves of the shell are held tightly shut by a strong muscle. This tight closure of a thick shell and firm attachment to substrate by threads make the mussel extremely difficult to move. If the adult is killed, the three- to four- inch shells might remain attached until physically removed. In some instances, mussels accumulate in such dense aggregations on intake walls and in pipes that the weight pulls large mats of mussels, shells and accumulated debris down to the floor of the structure. These dense mats can clog downstream components and the resulting debris can clog smaller pipes farther along the system. x Oysters – The American oyster ranges along the Atlantic and the Pacific coast. It is commonly found in brackish water near the mouths of rivers or in bays and estuaries in shallow water. x Bryozoans – Bryozoans are colonial animals often mistaken for other organisms such as algae, hydroids, corals, sponges, and so on. The bryozoan colony consists of numerous box- like compartments arranged in characteristic patterns. Each compartment contains an individual animal having a tubular gut, a well-organized nervous system and other anatomical features that distinguish the bryozoans from the other groups they resemble. x Coelenterates – The coelenterates include relatively simple animals such as hydras, jellyfish, sea anemones, and corals. Because of the size and abundance of jellyfish, they are often responsible for damage to traveling water screen panels at power plants. x Tunicates – Tunicates are soft, sac-like animals growing either singly or flat, spreading forms that grow in colonies. When growing as individuals, they can grow in large, denser masses. When these masses become large and heavy enough, they can break off and clog screens, tubesheets, and other intake apparatus. EPRI Licensed Material Fouling 5-3 x Tube Worms – Tube worms are small, segmented marine worms that live in white tubules that are an inch (2.54-cm) or more in length. They can grow in such masses that they cover large areas and can be more of a fouling nuisance than the barnacle. x Seaweeds – Seaweeds inhabit intertidal and subtidal areas attached to the substrate by specialized features for anchoring. Seaweed can cause clogging problems by attaching to structures such as trash racks or as debris when ripped from their attachments by ocean storms. x Fish and Miscellaneous Invertebrates – Along the coastal United States, the herring and anchovy families, as well as various crabs and shrimp, cause occasional and sometimes severe problems. Crabs can be particularly troublesome because of their attraction to traveling screens and their ability to cling to the screen mesh. They pass through spraywashes and carry over the screens and into the circulating water systems. x Debris – Mussel and barnacle shells are a common blockage problem in condensers. The carryover of grasses, leaves, shell fragments, and other waterborne debris can result in severe plugging of condenser tubes. 5.1.2 Freshwater Organisms Some organisms found in freshwater are: x Corbicula – Corbicula organisms include clams. Any water system operating within areas currently populated by this organism and utilizing raw freshwater is vulnerable to clogging by the Asiatic clam. The larvae can be carried through all standard screening equipment in power plants. x Algae – Various types of freshwater algae also create operational problems in circulating water systems. Being photosynthetic plants, algae species of concern at power plants only grow in areas exposed to light. The major problem caused by freshwater algae is the massive influx of mats or clumps that occur seasonally or after storms at many sites. The debris matting can become so severe that spray washes cannot remove it. Manual brushing or even burning of the debris might then be required. x Hydrilla – The hydrilla plant has the ability to spread rapidly and dominate natural aquatic vegetation. Water depth indirectly controls hydrilla by affecting light levels in the bottom few feet of the water column. x Fish and Miscellaneous Invertebrates – Many power plants experience periodic influxes of fish in large enough quantities to create operating problems. Particularly affected are traveling water screens that have been known to collapse due to rapid blockage. Condensers might be blocked if carryover or carry-through of fish occurs. x Debris fouling – Grasses, leaves, trees, branches, rocks, sand, and silt can cause problems in intake lines. EPRI Licensed Material Fouling 5-4 5.2 Macrofouling Control Technologies [14] Key Technical Point A variety of macrofouling control technologies are used in power plants. These technologies can be categorized as: mechanical control, flow reversal, thermal backwash, hydraulic control, materials control, chlorination and alternate biofouling control methods, and manual cleaning. The following is a discussion of these control technologies. 5.2.1 Mechanical Controls Mechanical equipment in the intake and along the pipeline has traditionally been used to protect the condenser and the circulating water system from macrofouling. A conventional power plant intake is shown in Figure 5-1. EPRI Licensed Material Fouling 5-5 Figure 5-1 Power Plant Intake Schematic [14] Effective intake screening will help minimize some of the macrofouling in the condenser. Trash racks and rakes are the first line of defense and screen out a significant portion of debris larger than 3/8 in (9.5 mm). Conventional through-flow screens can be modified to provide additional macrofouling control and reliability. Some utilities have incorporated dual-flow, center-flow and fine-mesh traveling screens at intakes to meet environmental requirements or to achieve additional condenser protection from debris carryover. These screen designs have features that can aid in macrofouling control. EPRI Licensed Material Fouling 5-6 uploads/Ingenierie_Lourd/ condenser-maintenance-guide 1 .pdf