Valves for CCPPs

Louise Davis

Steam turbine bypass valve design overhaul needed to mitigate increased cyclic load on power plants.

The valve design for dispatchable gas-fired combined cycle power plants (CCPPs) must be adapted to the increased cyclic load due to the integration of intermittent renewable sources, says an industry expert.

With a growing global focus to decarbonise in line with COP27 emissions reduction goals, CCPPs are set to play a key role in the transition to clean power generation through their greater ability to cope with the demand for more frequent cycling. Yet with the accompanying grid instability meaning these plants are at risk of being used beyond their original design remit, existing steam turbine machinery components must be re-evaluated, says Jackie Hu, divisional managing director at IMI Critical Engineering.

“Steam turbines are a key component within a CCPP, as are the turbine bypass valves that match blade and superheater steam temperatures,” explains Hu. “However, renewables accounting for an ever-increasing proportion of the worldwide energy mix, cause fluctuation to power provision. Consequently, these systems are being started up and shut down more frequently than they were originally designed for which will lead to increased component wear.”

“The higher demand on these valves when generating power to support green energy sources during periods of peak demand is becoming a major cause for concern. This will become a critical issue as further action is taken to decarbonise, meaning that valves are required that can withstand even higher temperatures.”

A smart series

Flow control expert IMI Critical Engineering has developed the VLB3 angle-style steam conditioning valve series to safeguard operations in high-cycling power plants. Its uniform body thickness ensures its material expands and contracts at the same rate to reduce fatigue damage, and the more compact size and weight make it easier to install and lessen the weight load on pipework.

The valve body has smooth transitions, which are known to minimise thermal stress. Steam conditioning at the outlet now takes place with improved thermal separation of the steam and water flow paths, reducing thermal stress in the critical area of injection.

The VLB3 series also features the firm’s latest TwiNozzle injection technology, helping to ensure proper coverage across the overall pipe section downstream, alongside efficient droplet evaporation through primary and secondary atomisation. In turn, this leads to improved steam conditioning performance with an increased lifetime. Alongside this, the VLB3 series features an improved pilot plug springless design to reduce the required opening force, which facilitates the installation of electric drives, while still providing reliable valve shut-off.

Meeting future demand

“A stable grid is key to ensuring the best possible power provision, yet with the move towards more fluctuating renewable power sources, combined cycle power plants and the components that keep them operational will be extremely important to meeting baseload demand,” concludes Hu. “With the working environment for these valves becoming increasingly demanding, it is key that power plant developers and engineering procurement contractors select components that can withstand the stress and higher demand levels created by intermittent renewable energy provision.

“By contrast, not selecting components robust enough for steam turbine applications will, in turn, drive up the likelihood of equipment failures, and the ensuing unplanned downtime and expensive repairs. With rising energy prices set to continue in 2023 and beyond, any disruption to plant operations may result in unacceptable delays and further pressure on financial bottom lines. Clearly, this is a situation that must be avoided, and valve specification has a large role to play in this.”

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