A laboratory image of droplet size analysis
A laboratory image of droplet size analysis
Credit: Pentair Valves & Controls

 

Martin-Jan Strebe   The new generation of desuperheaters must be able to address the complex engineering challenges and varied operational environments of today’s modern 50- and 60-Hz combined-cycle power generation markets, writes Martin-Jan Strebe

Global advances in steam and gas turbine technology are persistently pushing the envelope of metallurgy and steam cycle design efficiency, resulting in more diverse operational requirements for component manufacturers. Greater gas turbine mass flows, and ever increasing final superheated steam volumes and temperature requirements, coupled with multiple thermal cycles (per 24-hour plant operational cycle) now place substantial demands on critical and severe service products.

Precise control of steam temperature is a critical element for safe and efficient plant operation. The sustained emergence and demand for even larger and more super-efficient combined-cycle power generation facilities on the global market now drives the need for a new generation of heat recovery steam generator (HRSG) attemperator systems. These desuperheaters must be able to address the complex engineering challenges and varied operational environment of today’s modern 50- and 60-Hz combined-cycle power generation markets.

Desuperheaters control steam temperature by injecting water into the steam flow within the boiler circuit. This direct contact between the steam and water causes atomization and evaporation, resulting in a decrease in steam temperature. Due to the high temperature, pressure and mass density of the steam, there is substantial risk of component wear and thermal shock damage, which increases the risk of failure and impact on plant efficiency.

Rapid varying load conditions are typical of combined-cycle plants (CCPs) and place strenuous duty cycles on steam attemperation components and downstream apparatus. Depending on the boiler operating characteristics and the extent of load changes it is subjected to, a steam attemperator can experience extensive thermal cycling. Temperature differentials between the steam flow and spray water, intermittent desuperheater operation and low-load boiler operation all contribute to potential attemperator failures. This can lead to common problems, including cracks in material welds and older spray nozzle designs, cracks in the thermal lining (or in the attemperator steam line within a liner that is not used) which can cause pieces to break off, and broken spray nozzles which can become lodged in the venturi, causing a steam flow blockage and pressure drop.

Understanding the necessary desuperheater performance characteristics for effective steam temperature control requires exploration of the balance between design efficiency, component flexibility and system reliability.

Atomization is key

Water droplet atomization requires a precise methodology to improve understanding of primary and secondary atomization within the pipeline prior to evaporation and steam cooling. This process is ultimately predictable and measurable based on constant fluid dynamics. If atomization of spraywater into the steam system is negatively affecting the temperature probe’s ability to measure correct downstream steam temperature, this could create severe overspray and underspray conditions which result in increased thermal cycles and damage to system components.

Primary atomization of the cooling water is caused by the nozzle design and geometry within the desuperheater and the pressure differential between the cooling water and the steam. Pentair Valves & Controls has previously developed theoretical modelling of primary atomization using computational fluid dynamics (CFD) analysis and laboratory laser diffraction to analyze water droplet size upon discharge from the desuperheater. This testing examined two steam attemperator nozzle designs, spring-loaded and swirl nozzles. Results identified that when operating at 25 bar with a 0.05 mm lift and Kv 0.047, spring-loaded nozzles produce droplet sizes of 87 µm. The same calculation for swirl nozzles at 25 bar, Kv 0.043 resulted in droplet sizes of 27 µm – a factor of two to four times smaller than spring-loaded nozzles, depending on the operational pressure range.

Using this data, Pentair Valves & Controls analyzed the secondary atomization characteristics which occur when the speed differential and drag forces between the cooling water and pipeline media cause the droplets to split into smaller sizes. This occurrence is calculated by:

equation

By measuring the speed differential of the two nozzle designs, Pentair defined which nozzle achieved higher speeds and therefore faster secondary atomization. Optimum atomization will result in frictional forces breaking the droplet size, which results in complete mixing and true temperature control and measurement.

These results demonstrated that swirl nozzle designs offer enhanced performance and maximum use of water pressure DeltaP for atomization in the shortest possible length. Optimized spray injection angles of swirl nozzles allow equal temperature distribution within the pipeline and provide the highest turndown ratio due to mass flow, rather than pressure control. No springs or moving parts within the nozzle and no pressure drop and cavitation in the control valve maximizes the operational lifecycle of the swirl nozzle design compared to spring-loaded nozzles. Combining this analysis with Pentair Valves & Controls’ experience in steam attemperation provided an insight into desuperheater performance characteristics for engineering design criteria to support new product development.

Effective nozzle design

Operating at today’s higher temperatures and cycling ratios places high thermal stresses on critical components, and effective steam temperature control is needed to protect this vital equipment. Many plant operators are looking to maintain steam temperature control and the most effective ways to achieve this are through the design of the shortest possible evaporation length. Critical to achieving this is uniform and consistent atomization through small droplet sizes. Robust nozzle design offers plant operators an optimized solution that delivers effective performance and maximum component lifecycle to avoid unnecessary plant shutdown, maintenance and product replacement.

Optimum desuperheater performance requires evaporation of the spraywater to take place as quickly as possible in the shortest length within the pipe to avoid water droplet impingement. Low steam velocity and large droplet sizes create water fallout inside the pipe, creating cold spots and thermal stress points which risk pipeline failure. In developing the next generation of desuperheater technology to meet the evolving needs of plant operators, Pentair Valves & Controls, through its Yarway brand, wanted to identify which nozzle type offered the best performance in relation to water droplet size and spray pattern. Documentation to support this product development was limited, with little data to support an analysis of nozzle performance.

To understand the design rules for desuperheaters to achieve optimized steam temperature control, Pentair Valves & Controls embarked on a research project with the University of Eindhoven in the Netherlands. The goal of this cutting-edge research was to undertake an assessment of existing desuperheater nozzles, focusing on spring-loaded and swirl nozzle designs. This assessment would help to identify the design correlations of spray characteristics, which could then support a product development roadmap for improved nozzle design.

In spray generation, a viscous liquid sheet breaks up and becomes unstable due to capillary, aerodynamic and liquid viscous forces. Primary break-up of the cooling water typically takes place in the wake of the spray nozzle during the first 10 mm, with complete atmomization taking place at around 25 mm downstream of the nozzle. The pressure differential between the spraywater and steam is vital for both water atomization and the rangeability between maximum and minimum water flow. Along with spraywater temperature and nozzle design, the maximum pressure differential directly affects atomization at the smallest droplet size. Spraywater pressure is ideally 150-1000 psi (10-70 bar) greater than the steam pressure to provide optimum vaporization speed and maintain controllable low flow levels. While many desuperheaters are capable of operating at much lower levels, there is a direct correlation between pressure differentials and component performance.

To better understand the impact of spraywater characteristics on steam attemperator performance, Pentair Valves & Controls studied the functional operation of pressure swirl nozzle and spring-loaded nozzle designs. Both nozzles produce a hollow cone spray with a spray angle between 80° and 120°. The hollow cone inside a swirl nozzle is created by enforcing a natural swirling action where the spraywater is injected through tangential or helical inlet ports into a swirl chamber. This nozzle design creates strong rotational and axial velocity components and a thin conical sheet forms at the exit of the nozzle due to the large centrifugal force. In spring-loaded nozzles this hollow cone is mechanically created. When the pressure increases the valve opens with a certain lift and the spraywater exists via a small circular slit.

Pentair Valves & Controls and the University of Eindhoven defined set research parameters for both the pressure swirl nozzle and spring-loaded nozzles. Experiments used demineralized water in temperature range 5°C-85°C and set pressure between 8-70 bar, with a maximum of 8x nozzles. The characteristics measured during the tests were spraywater pattern, drop size distribution and spraywater velocity. A Malvern Spraytec laser diffraction system with a 100 mm diameter enabled measurement of spray particle and droplet size distribution in real-time. Using a Dantec Laser Doppler Anemometry (LDA) optical technique, accurate measurement of velocity and turbulence distribution could be measured in order to gain a clearer understanding of fluid mechanics of the spraywater characteristics of both nozzle types.

Applying research results

Many plant operators are looking to maintain steam temperature control. The most effective ways to achieve this are through the design of the shortest possible evaporation length within the attemperator system, together with equal temperature distribution within the steam and high turndown ratio. Combining the results of the computational modelling and this latest research from Pentair Valves & Controls, further understanding can be gained to achieve more effective desuperheater performance, particularly in combined-cycle and heat recovery steam generator (HRSG) applications. Using this data enables the effective application of theoretical models to identify the nozzle design and engineering considerations for successful primary and secondary atomization, evaporation and defined spraywater characteristics. By examining the results of Pentair Valves & Controls’ research, it is possible to identify the design correlations of these parameters and understand how each nozzle type impacts on desuperheater performance.

Spring-loaded nozzle

The research results defined the spraywater characteristics as not uniform and not steady, which causes potential water fallout. Uneven and inconsistent distribution of droplet size affects atomization within the pipe and results in longer and slower evaporation, affecting precise steam temperature control. These characteristics are typically created by restrictions in the nozzle design caused by tolerances engineered into the nozzle, which creates gaps and impact on the spraywater pattern.

Pressure swirl nozzle

Under performance testing as part of Pentair Valves & Controls’ research, this nozzle design delivered a uniform and steady spray pattern. In comparison to the spring-loaded nozzle design, this nozzle delivers much smaller droplets to achieve evaporation more quickly and in a shorter pipe length. The uniform, hollow cone spray pattern ensures even droplet size distribution within the pipe and minimizes impingement and water fallout. These characteristics demonstrate faster response times and more accurate steam temperature control to help plant operators improve the precision and operational efficiency of the boiler circuit.

Higher performance solution

The research results conclude that spring-loaded nozzles deliver a spray pattern and water droplet distribution that is both non-uniform and unsteady. This indicates that this nozzle design is best suited to less critical process conditions or unit operations. The robust pressure swirl nozzle provides a higher performance solution for critical and severe applications, such as steam attemperation.

The future design of higher efficiency and high cycling duty CCPs will lead to higher steam temperatures and flows. Addressing the requirement for more efficient power generation will drive forward material development and innovation and combine these properties with the need for continued operation, scheduled maintenance and repair. Improving the design and engineered performance of critical components is key to meeting the power generation industry’s future requirements and ensuring maximum plant uptime. Investment into sound research using modern methodologies and techniques is one solution to delivering new, advanced technology to plant engineers and operators.

In collaboration with the University of Eindhoven, Pentair Valves & Controls has identified new correlations which have been generated for the discharge coefficients, sheet velocities and drop size distribution parameters of various types of pressure swirl nozzles. As the operational and functional design requirements of steam attemperator systems continue to evolve, this engineering research and design understanding will be critical in the development of new technology to meet these new challenges. Component performance data, advanced detailed 3D computer modelling, and material and industry design experience are providing the tools for this next generation of attemperator systems. The work that manufacturers such as Pentair Valves & Controls undertake is an integral step in this process, using qualified research and testing to improve product design and push the industry towards higher efficiency, reliability and safety.

Martin-Jan Strebe is Global Product Manager, Control Valves at Pentair Valves & Controls

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