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The Evolution of Steam Attemperation. The fundamental design principles and process for modern steam desuperheating, or the attemperation of superheated steam in the power generation industry, have been evolving since the early 1. Meeting the requirement for steam quantity, quality, and temperature consistency is the foundation of traditional attemperator component design, particularly for fast- response combined cycle plants. Increases in steam and combustion turbine operating temperatures and capacity that are inherent in the quest to increase steam cycle efficiency are advancing metallurgy technology.
At the same time, diverse operational requirements—including cycling and low- load and load- following operations—have added complexity to the design of today’s combined cycle (CC) plants. Increased final superheated steam volumes and temperatures coupled with these diverse operational modes are, in turn, challenging many other vital plant components and systems, particularly the steam attemperator system.
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- The fundamental design principles and process for modern steam desuperheating, or the attemperation of superheated steam in the power generation industry, have been.
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Attemperator Design Overview. An excellent attemperation system for a modern CC plant requires a balance of design efficiency, component flexibility, and system reliability.
Rapidly varying load conditions place strenuous duty cycles on steam attemperation components and downstream apparatus. On average, the attemperator system will experience 7. The thermal cycles can double in a cycling unit. Most modern heat- recovery steam generation (HRSG) superheated steam attemperator component designs can be characterized as either circumferential, probe, or a combination of both technologies. As with many complex engineering components, designs evolve from functional requirements derived from expected plant operations.
Steam exiting the turbine flows into condensers located underneath the low pressure turbines where the steam is cooled and returned to the liquid state (condensate).
Each of these design categories has a unique set of requirements that must be met to achieve expected levels of plant performance and efficiency. One of the more common superheater attemperator designs used in the HRSG CC market today is a circumferential spray design (Figure 1). The primary function of this design is to inject water perpendicular to the steam flow through multiple fixed or floating spray nozzles via a penetration in the main steam pipe wall and the attemperator’s inner spray liner or protective shield located inside the pipe. The nozzles produce mechanical atomization of the water droplets into the superheated steam flow. This design will often utilize external circumferential piping to the main steam pipe for water supply to the individual spray nozzles in conjunction with a remote spraywater control station. Circumferential in- line attemperator.
In this design, water is injected perpendicular to the pipe steam flow through spray nozzles to desuperheat steam. Source: Tyco Valves & Controls. An alternative design for steam temperature control integrates a probe unit within the pipe. This design is divided into two major categories: integrated units (IU) and separated units (SU). Integrated probes incorporate the spraywater control valve function within the component (Figure 2). SUs offer a probe- style spray for water atomization with a remote spraywater control valve and external water supply piping (Figure 3). The probe application, whether of IU or SU design, employs single or multiple spray probes into the superheated steam flow, spraying water droplets parallel with the steam flow.
Probe- style IU desuperheater. In this design, an integrated flow control valve is inserted into a pipe through which water is injected into the flowing steam.
A downstream probe measures the downstream temperature and is used to control the water flow. Source: Tyco Valves & Controls. Whether an attemperation system is circumferential or probe style in design, it must be supported by robust integrated control components and control functionality. The placement, design, and function of temperature probes are critical. A spraywater control valve or valves must enable “bubble tight” shutoff, and manual valves required for component and system isolation should be routinely inspected. Most current HRSG steam attemperator systems are designed for minimal to zero water flow at maximum steam flow. CC plants that are dispatched through automated load- following management systems or automatic generation controls will see constant superheated steam attemperation as load is increased or decreased to meet fluctuating megawatt demand.
This mode of operational dispatch will stress existing design limitations of the attemperation system. Common system and component failure issues associated with extreme cycling conditions are: Spraywater control valve packing leaks or packing blowout. Wetting or droplet impingement of downstream thermal probes. Nozzle spring failure. Nozzle cracking or erosion. Linear weld attachment (pin) cracking or complete line failure.
Main steam pipe cracking. Foreign object damage to the steam turbine. Engineering and Design Considerations. Attemperator system components are designed and engineered to an expected life span, based on detailed 3- D finite analysis computer models, operational case histories, material composition, and expected thermal cycles associated with each component. Some shortened component life in the steam attemperator system can be attributed to supporting operational systems, such as feedwater or condensate supply conditions, water chemistry, distributed control system (DCS) settings, or response times. These support systems are usually designed for no or minimum spray conditions at design or baseload conditions for maximum efficiency. The attemperator system installed at a plant designed for baseload may exhibit much different operation when cycled.
A functional field test often proves prior factory test settings to be inaccurate. The following is a minimum list of supporting systems and parameters associated with the attemperator that should be reviewed and/or inspected to minimize the chance of downstream damage: Feedwater or condensate supply pressure, flow rate, and temperature at the spraywater control valve during various load conditions, or at the attemperator probe if an integrated design is present. Thermal probes, operational temperature, and location specifications should be verified and/or tested. DCS logic settings should be consistent with plant operation. The dead band of the control signal should be within the required tolerance. Water chemistry should be known throughout the steam and condensate systems under various load conditions.
This equipment, if not originally designed for cyclical operation, can be redesigned or modified to better suit current operational conditions. Often, a presumed shortage of feedwater or condensate spray capacity can be attributed to a logic setting in the DCS for valve position, or for response at a predetermined load condition. Additionally, if the plant infrastructure has been in service for a period of years and has experienced a series of routine control valve preventative and corrective maintenance actions, operators may observe a minor, incorrect spraywater control valve stem position setting. Thermal probes are often placed incorrectly during unit construction, resulting in probe wetting or water droplet impingement, which will result in inaccurate steam temperature measurement. Locating the Attemperator. In addition to mechanical design and field operations, accurately predicting water droplet atomization is very important. However, measuring droplet atomization in the field is difficult.
If atomization of spraywater into the steam system is negatively affecting the ability of the temperature probe to measure downstream steam temperature correctly, then severe overspray and underspray conditions can produce increased thermal cycles and component damage. Thermal probe location is a first step in verifying or eliminating probes as a possible contributor to poor steam attemperator system performance. Here are two general rules for measuring and verifying the proper location of upstream and downstream attemperator thermal probes (in a straight pipe): The upstream thermal probe should be a minimum of five pipe diameters from the attemperator location. The downstream thermal probe should be a minimum of 2.
These rules of thumb should be used as a quick check of an existing installation in straight pipe and are useful in determining if a gross error was made in thermal probe placement. Droplet atomization calculations can be used to determine the exact requirements and dimensions for the piping arrangement and thermocouple locations. Advances in Steam Temperature Control.
Precise steam temperature control has been a challenge for steam plant operators since coal was first shoveled into a furnace. Today’s superheat temperatures and daily plant cycling place extraordinary stresses on critical components. Effective steam temperature control is needed to protect expensive downstream equipment, such as the steam turbine. In a typical CC plant, precise steam temperature control often conflicts with compact steam piping design. That makes it difficult to select an attemperator that can operate in the shortest possible straight length of pipe yet with an effective evaporation rate. This is particularly difficult when short pipe length is coupled with a high turndown ratio and the desire for a flat temperature distribution across the steam pipe. Primary atomization of the feedwater used to attemperate the steam is produced by the nozzle design and geometry within the desuperheater and the pressure differential between the cooling water and the steam.
Together with the University of Eindhoven in The Netherlands, Tyco Valves & Controls commissioned a joint research project to develop theoretical modeling of primary atomization using computational fluid dynamics (CFD) analysis and laboratory laser diffraction to analyze water droplet size upon discharge from the desuperheater. The study examined two nozzle designs, spring- loaded and swirl nozzles.