|The TempLowHT is a novel integralted probe style desuperheater, specifically designed to meet the steam control needs of modern CCPPs|
With the demand for energy expected to double by 2050 and alternative resources on the rise, the power industry is facing a multitude of new challenges in order to keep up with the soaring energy use of the industrialised world and deal with fluctuation in supply.
Across the world, power plants have started operating at levels previously unforeseen. New operating procedures, innovations in steam turbine technology and advances in plant efficiency have started challenging existing designs and materials of steam attemporation technology.
As operating temperatures approaching 621°C (1150°F) and severe duty cycling are rapidly becoming industry standards, demands for more effective and durable steam attemporation technology are growing, driving recent advancements and innovation in desuperheater design.
Driven by the ever-increasing demand for energy, operational temperatures in supercritical (SC) and ultrasupercritical (USC) power plants – as well as in combined-cycle power plants (CCPP) – have climbed significantly in the last two decades. Testing in Europe is currently at 700°C for boiler systems and plants in the United States will soon be operating above 600°C. While these temperatures allow plant operators to maximise steam generation and increase plant efficiency, they also put an immense amount of stress on system components. Increasing steam volumes and higher pipeline velocities can cause considerable erosive damage to equipment and increase the risk of thermal shock and equipment failure.
Diversified energy sources
Higher operating temperatures are only one of the many factors affecting desuperheater technology. Across the world, governments are looking to diversify their energy sources in order to cater for their countries energy needs and secure future supplies.
Although, nuclear and coal-fired power plants still account for around 60-70 per cent of the world’s power supply, energy from renewable resources, such as solar, wind or geothermal heat, is growing in all sectors and is now supplying 17 per cent of overall global energy consumption.
The diversification of fuel sources has required conventional power plants to operate on a much more flexible basis in order to fill the energy gaps and deal with a fluctuating supply. As a result, the power industry is gradually moving away from baseload operations towards higher cycling units, in which plants are switched on or off on a regular basis, depending on the demand at that time.
Varying load conditions and multiple thermal cycles have accelerated the emergence of heat recovery steam generators and gas-fired power plants because these systems are able to cycle more quickly than conventional coal power plants. This trend towards higher cycling units and aggressive turndown rates is adding further thermodynamic strain on equipment. Every time a power plant is cycled, steam flow and temperature fluctuate, amplifying the need for precise temperature control in order to guarantee optimal plant efficiency and avoid damage to components.
With predicted rises in steam temperature and high-load cycling becoming more widespread, component manufacturers are under immense pressure to deliver technology that can handle these complex engineering challenges and demonstrate operational reliability, even in such a demanding environment.
Effective temperature control
Initially developed for process plant applications, desuperheater technology is now found in a wide variety of power generation applications. The emergence of larger and more efficient CCPPs amplified the need for more precise steam temperature control and transformed desuperheaters into an indispensable system component in most modern power plants.
Although different types of desuperheaters are now available, the primary function of this technology is to reduce the temperature of superheated steam by bringing it in contact with cooling water. Once the water droplets have been dispersed into the pipe, any excessive heat will be absorbed during evaporation. Steam temperature is accurately regulated by the amount of water that is injected into the pipe.
While effective steam attemporation is critical for the protection of equipment and overall efficiency, inaccurate and inefficient steam temperature control can have severe implications on system performance and equipment stability. Common issues associated with these types of CCPPs, such as ‘wet’ steam and overspray, under spray and leakage can result in erosion of the blades, increased friction, water build-up in the pipe and in some cases even equipment failure. Incorrectly designed or faulty desuperheaters can have costly consequences. Cold spots created by the incorrect geometry of the cooling water nozzle will eventually cause the pipe to crack because of thermal stress. These malfunctions not only result in expensive repairs or replacement of the affected part, but can also lead to a partial plant shutdown, stressing the importance of quality equipment and proper installation.
One of the more common desuperheater designs uses a circumferential spray concept (Figure 1). The primary function of this design is to inject water at an angle perpendicular to the steam flow through multiple fixed or floating spray nozzles, providing a mechanical method for atomisation of the water droplets into the superheated steam flow. This design often utilises external circumferential piping to the main steam pipe for water supply to the individual spray nozzles. A remote spraywater control valve provides control of the water flow to the system.
|Figure 1: Schematic of an In-line desuperheater design|
An alternative design for steam temperature control integrates a probe unit within the pipeline. This design is divided into two major categories – integrated (IU) or separated unit (SU). Integrated probes incorporate the spraywater control valve function within the component (Figure 2). The second probe design offers a probe style spray component for water atomization with a remote spraywater control valve and external water supply piping, as shown in Figure 3.
|Figure 2: Schematic of an integrated probe desuperheater design|
|Figure 3: Schematic of a fixed probe desuperheater design|
The global trend towards more efficient plant operations, and with it the emergence of high temperatures and thermodynamic stresses, highlight the need for a more innovative approach to steam temperature control.
Extensive research into effective steam attemporation and testing by component manufacturers demonstrate that these high-efficiency power plants require a much more robust desuperheater technology, comprising tougher materials and new design concepts that can resist the effects of corrosion and operate efficiently in the adverse conditions of severe power generation applications.
Narvik-Yarway’s TempLowHT represents a major advance in steam attemporation technology. Part of the successful TempLow range of desuperheaters, the enhanced TempLowHT incorporates a number of specific features that guarantee effective operation, even in the challenging environments of today’s modern CCPPs.
The TempLowHT is an integral probe style desuperheater with isolation (stop) and water proportioning (control) in response to a temperature feedback control signal. A fully-forged body with stainless steel internals allows it to maintain the integrity of all components, even under the most extreme conditions, effectively eliminating the risk of thermal deterioration.
One of the most innovative design aspects of the TempLowHT is that all moving and welded components, including the spraywater control valve, have been moved away from the steam flow path and out of the ‘hot zone’. As a result, parts that are essential to an effective operation of the desuperheater are less exposed to temperature fluctuations, allowing it to deal with higher cycling units and maintain high service reliability even in temperatures of up to 621°C.
The upgraded design not only extends the lifespan of the equipment during cycle service operations, but it also facilitates the maintenance of key components. All moving parts are easily accessible for maintenance, while thorough inspection can be carried out during shutdown. Installation of the TempLowHT only requires minimal headroom for mounting and it can be fitted in straight, vertical or horizontal pipes. Furthermore, the desuperheater is highly adaptable to changing needs and can be easily customised with different spray nozzles to suit specific requirements without having to change the stem/disc or seat.
The upgraded TempLowHT incorporates the latest spray nozzle technology. Each nozzle is served by individual feed holes in the cylinder wall. Water is proportioned evenly through each nozzle, while a piston inside the control part guarantees maximum water pressure. Through a combination of splitting the feed flow, increasing velocity and rotating effect, water is injected into the system in a fine symmetrical hollow cone spray, ensuring rapid evaporation and minimising the risk of water accumulating inside the pipe work at slow steam flow rate.
Its sophisticated design allows for precise temperature control. For example, should the control system indicate that a reduction in steam temperature is required, the actuator will force the stem/disc further downward in order to expose more nozzles and allow for more water to be injected into the steam flow.
As water flow is controlled by the desuperheater itself, so it does not require an external control valve, reducing the risk of equipment failure. The nozzle head design;s unusually high turndown capacity of 50:1 and higher makes it suitable for systems with wide fluctuations in their steam flow, allowing it to deal effectively with the high flows required at start-up and low flows required at load swings.
Martin-Jan Strebe is director of Global Product Management Control Valves at Pentair Valves & Controls. For more information, visit www.pentair.com.
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