SOLAR ENERGY SOLUTIONS |
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Parabolic and tower-based CSP plants
CSP plants typically employ two primary designs : parabolic troughs and solar towers . Most second-generation CSP plants use nitrate salt mixtures , usually comprising 60 % sodium nitrate and 40 % potassium nitrate . This mixture melts at 221 ° C and begins to crystallise at 238 ° C . In the operational cycle of a CSP plant , hot salt (> 400 ° C ) is heated by solar towers or troughs and stored in hot tanks , maintaining temperatures above 500 ° C . Cold salt (< 400 ° C ) is preheated to remain above its freezing point and is pumped to the tower , where it is further heated and cycled back , ensuring continuous energy generation .
Advantages and challenges
• The use of molten salts in CSP plants offers numerous advantages , including high thermal stability , high density , non-flammability and low vapour pressure , making them both efficient and safe .
• However , challenges such as salt crystallisation can pose significant risks to handle the equipment ’ s like pumps and valves , potentially leading to higher operational costs .
• CSP has higher costs compared to other energy sources due to technological limitations , with TES efficiency at 90 % but electricity transformation is only at 50 %.
Valves that can be used in CSP plants
Several types of valves can be employed in CSP plants for handling steam and air , but for molten salt applications , globe control valves and triple offset butterfly valves are preferred due to their unique design construction , ensuring efficient operation and control of the molten salt flow . Here are some common types of valves used in CSP Plants :
• Gate valves : Allow unrestricted flow of molten salt in large pipelines , minimising resistance .
• Butterfly valves : Compact design suitable for controlling flow in large pipelines , especially in spaceconstrained areas .
• Globe valves : Enable precise regulation of molten salt flow rate , essential for optimising heat transfer .
• Check valves : Prevent backflow , ensuring molten salt moves in the intended direction only .
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• Control valves : Actuator-equipped for precise control overflow , pressure and temperature .
• Pressure relief valves : Automatically release excess pressure to prevent over-pressurisation , ensuring safety .
• Thermal expansion valves : Maintain system integrity by accommodating thermal expansion and contraction of molten salt .
General requirements for valves
For valves handling molten salts , specific requirements must be met :
• Valves must have metal seating , and insulated graphite is recommended for wetted parts .
• Compliance with standards like ISO 5208 / API 598 / EN 13344-1 Rate-A for On-Off valves and FCI 70-2 Class V for Control valves is essential .
• Butt-weld end connections are preferred for secure sealing .
• Packing leak rates should meet ISO 15848 Rate-A standards .
• Valves should allow inline maintenance and packing replacement without actuator removal .
• Heat tracing systems are necessary for proper operation .
• Valves should withstand thermal cycles inherent in CSP operations .
• Packing and gasket materials should be selected based on valve type .
Valves must also meet plant-specific requirements , such as :
• Valves must withstand seismic loads .
• Noise and vibration should be within acceptable limits .
• Electrical equipment should be suitable for site conditions .
• Minimum design life of valves should comply to project and site-based requirements .
• Valves should endure thermal cycles of 20,000 to 25,000 cycles .
• Packing and gasket decisions should align with principal ’ s specifications .
• Compatibility with designated pipe schedules is necessary .
• Valves should function across a range of cold and hot salt temperatures .
• Compatibility with salt mixtures and chosen materials of construction is crucial .
• Adequate insulation thickness should be maintained .
• Optimisation of energy consumption and compliance with EHT requirements are essential .
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• Valves should undergo rigorous testing and operators should receive comprehensive training .
• Spares and special maintenance tools should be readily available .
Design considerations
Triple offset butterfly valve ( TOBV )
• TOBV designs must adhere to standards like ASME B16.34 , API 609 and ASME Sec VIII Div . 1 .
• They should feature butt-weld end connections as per ASME B16.25 , metal-to-metal sealing and efficient integration with heating systems to prevent salt freezing .
• Internal fasteners must be secured against thermal turbulence , and body necks / guides designed to accommodate effective EHT systems .
• Joint sealing , stem packing and seal rings must be designed to prevent salt from contacting graphite .
• Temperature sensors should be placed in the most adverse heat locations , such as the packing , bottom gasket and body flow region .
• The body stuffing box extension should be designed based on thermal FEA , site conditions , insulation accommodation and heat supply .
Triple offset butterfly valve ( TOBV )
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