Layup Practices for Cycling Units. Requirements, Issues and Concerns. By Michael Caravaagio, Electric Power Research Institute. Cycling units are those which frequently shutdown to zero power levels for short time intervals from as little as 8 hours or less up to 4. Typically these units operate on a system load demand and/or economic dispatch which may be tied to conditions such as time of day, availability of renewal generation or alternate fuel / generation sources. Cycling units are most often required to be in a state of readiness for rapid return to service, i. Accordingly, short term periods of 8- 4. These conditions all assist in the preservation techniques for the equipment. Certainly the layup and corrosion mitigation practices identified for cycling units are not limited to only those units of the foregoing description. Rather the layup practices and guidance are for those cycling units requiring the maximum flexibility for removal and return to service. It is recognized that there is no . Accordingly, the practices and recommendations for various unique operating/ shutdown conditions are presented for the water/steam touched circuitry that will require some effort on the part of the plant operators to discern the most applicable practice or methodology for the various components and sub- systems of the individual situations. Depending on numerous factors these practices may not be the same from outage to outage but should always focus on the most practical and beneficial techniques to minimize equipment damage associated with out- of- service and standby operations. The preservation and corrosion protection during shutdown (i. If several options are available certainly those providing the most practical and economic approach have advantages in situations of high frequency and often unplanned activity. Achievement of proper layup of the equipment and systems requires implementation of procedural steps during the unit shutdown and removal from service to eliminate and prevent introduction of corrosive conditions or environments. Accordingly, shutdown (and the subsequent startup) of equipment should be accomplished in a manner that does not subject the systems or components to an increased risk of corrosion damage; this would include such practices which induce increased localized stresses or increased concentration of contaminants or damage to the protective oxide which result in increased corrosion damage. Some of these unique events will be categorized. The goal of a lay- up program is the same as the chemical conditioning program during unit operation: to prevent and / or control and reduce corrosion and the accumulation of deposits in the water/ steam circuit of power plants. Optimization is most readily achieved when all conditions are at a steady state and equilibrium conditions can be established which are most favorable to corrosion and deposit prevention. Unit shutdown and startup by the very nature of these operations continually disrupt the established chemical equilibrium conditions within each circuit and between systems as a result of changes in the thermodynamic conditions of temperature, pressure, and flow, as well as numerous physiochemical properties. 1Point2 2G International 2isys 3-Phase Measurements AS 3D Production Multimedia 3D Systems 3M Corporation 3M Deutschland GmbH 3View.Com Inc. 7-Technologies A/S. Water and steam circuit corrosion during shutdown is defined by the simultaneous presence of water and oxygen. If one or both of these can be effectively excluded, corrosion during layup is not reasonably expected. The methods of dry preservation (excluding water) and / or wet preservation (excluding oxygen) are based on these conditions. ![]() If these conditions cannot be fully avoided, methods of active or passive inhibition are required. Principally, inhibition is enhanced by the application of alkalizing chemicals to elevate the p. H and provide the competing presence of hydroxide . Although technically inappropriate, practical economic factors of assumed risk and asset value may dictate the choices and practices employed for layup during shutdown. The economic viability of such choices should be prudently evaluated; units of low capacity factors or limited service life may initially appear to be non- economically viable for minimum measures of equipment protection, however if this means units are unreliable or unavailable for service when needed this could alter the assessment. As outlined, many of the practices for providing layup protection incur minimal costs; for example units with only seasonal demand stored following dry conditioning using methods of residual heat drying can require only procedural steps to preserve the greatest percentage of the water and steam circuits. SJP Properties is a privately held, vertically integrated real estate company specializing in the development, management and operation of Class A commercial and.Our company is an Equal Opportunity Employer/Affirmative Action Employer, and as such affirms the right of every person to participate in all aspects. Layup Practices. From the previous discussion it should be obvious that layup involves those practices which will contribute to the elimination of corrosion mechanisms prevalent during periods of unit shutdown. While the optimum conditioning for each component in the water/ steam cycle is achievable using methods of nitrogen (or other inert gas) blanketing, p. H adjustment, and/or humidity control (dehumidification) these techniques often require special steps and equipment isolation that preclude having optimum flexibility of unit operation. For cycling operation there are some critical conditions that should be considered to improve the layup practices and lower the risk of damage. Greater details for proper layup are given in EPRI reports 1. Cycling, Startup, Shutdown, Fossil Plant Cycle Chemistry Guidelines for Operators and Chemist, 2. Cycle Chemistry Guidelines for Shutdown, Layup, and Startup of Combined Cycle Units with Heat Recovery Steam Generators, 2. Shutdown Protection of Steam Turbines Using Dehumidified Air, 2. Addressing the necessity to maintain optimum unit availability and responsiveness to generation dispatching requirements while optimizing operations to provide layup protection to cycling units requires some practical and innovative methods which differ from the . Wet layup in the feedwater and condensate system equipment consists of filling the components and connecting piping with treated demineralized water with low dissolved oxygen (DO) (less than 1. The equipment is completely filled (water solid) with the treated water to avoid pockets of trapped air, and is not open to the atmosphere. For cycling units layup of the preboiler circuit is straight forward. The p. H of the water in the circuit is the same as during operation or slightly elevated using the same chemicals. The oxygen is reduced to levels of less than 1. The system is kept water solid to preclude any introduction of air. None of the other methods of lay- up are practical or plausible for cycling units – nitrogen capping, or draining dry are not amenable to the circuit configuration. The challenges faced by cycling units, as with all units, with this scenario is that as simple as it sounds, the achievement is quite complex. During the shutdown and coast down of the unit, the condenser performance for air removal and deaeration declines such that dissolved oxygen levels in the condensate escalate. Once steam flow to the condenser is discontinued the vacuum conditions and air removal is virtually loss and condensate is fully aerated. Similarly, following depressurization of the unit the deaerator in the circuit ceases to function and sometimes acts as a source of aeration. Flow through the circuit is still required to fill the boiler or maintain the liquid volume as a result of the contraction during shutdown and cool down of the components. The conditions lead to unacceptably high oxygen levels for shutdown and unit storage in the preboiler circuit (even units practicing oxygenated feedwater treatment require low oxygen for wet layup storage). Chemically reducing oxygen with the addition of reducing agents (inappropriately referred to as oxygen scavengers) is ineffective and for all- ferrous circuits can be detrimental to the protective oxide. With mixed- metallurgy units the use of excess reducing agents promotes unacceptably high ammonia concentrations on the subsequent startup and dangerously high corrosion of steam side copper components. H control of the preboiler circuit is frequently lost during unit shutdown as a result of increased levels of carbon dioxide from air entrainment and increased make- up to the cycle with air saturated water. Make- up water is untreated (no p. H adjustment) and aerated. The preboiler circuit serves as the conduit to transfer make- up water to the boiler or evaporator to supply the void created by the thermal contraction of the water. Recognizing the importance of layup and stabilization of the iron oxides (corrosion products) in the preboiler circuit of cycling units takes into consideration that these units spend a disproportionate amount of time in shutdown and startup operations. Consequently the opportunity for excessive transport of corrosion products to the steam generating equipment is greatly enhanced leading to excessive deposition and associated damage. Approaches to layup and preservation of the pre- boiler circuit to address these challenges (and possibly those of the subsequent startup) include: Hotwell bubbler for oxygen removal incorporates a steam (possibly nitrogen) sparging/bubbling system near the hotwell outlet to strip non- condensable gases from the condensate. Steam sources during/after shutdown include LP heater extraction (prior to shutdown), steam drum as unit depressurizes, or steam header from adjacent unit or auxiliary system. Nitrogen can similarly be used but the consumption rate may be excessive. Steam or nitrogen sparger in the deaerator storage tank. This option offers great advantages on startup not only for deaeration but for pre- heating the boiler feedwater to minimize thermal differentials at the economizer inlet or boiler water downcomer. Minimum flow circuit from the economizer inlet or deaerator outlet to the condenser hotwell or condensate pump's suction.
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