Spain is a world leader in innovation, technology and construction of solar thermal power plants by taking part in most of the CSP (Concentrated Solar Power) projects that are currently being developed worldwide.
CADE has taken part actively in many of these projects by the side of some of the leading companies in the sector, supporting them with its capabilities and experience in the fields of thermal and process design, mechanical design, thermal-structural and fluid-dynamics analysis; mainly on the design and evaluation of critical equipment, upgrade and optimization of operating plants and CSP structures.
Within CADE references, works carried out on heat exchangers are most relevant, both in the steam generation train and salts train, in those plants which comprehend thermal storage based on molten salts.
In-depth knowledge of processes and operation procedures of CSP plants altogether with the powerful simulation and analysis tools allow CADE to assess the performance and behavior of critic equipment during the different operating , start-up and shut down cycles. As a result of this assessment, operating failures can be predicted and/or prevented which means a relevant cost-saving measure for owners and operators.
In that sense, the fatigue analysis, that measures the effect of the thermal and mechanical cyclic loads, is a key part on the design and validation of heat exchangers, as indicated in the different codes of design for pressure equipment (ASME, EN 13445, etc.).
Damage caused by cyclic loads on specific areas or components mainly depends on two parameters: amplitude of variation of stress results along a cycle (stress amplitude) and the number of cycles.
Damage generated by a determined value of alternate stress value (amplified by certain coeficients according to the design code applicable) will be relation between of actual number of cycles and maximum allowable cycles according to the fatigue curve for each material.
We can identify two types of Pressure-Temperature cycles in these exchangers:
- Start-up cycles, or long-term cycles, which occur every day following the daily solar cycle (start-up – operating – stop), or which are initiated by a start-up after breakdowns, maintenance tasks, reductions in solar radiation for a long period of time, etc.
- Short-term cycles, which are due to alterations of the level of solar radiation during the normal operating performance of heat exchangers. These variations, which cause thermal fluctuations in heat transfer fluid (HTF) are usually caused by the effect of the weather, short stops, etc.
Start-up cycles are characterized by considerable thermal jumps, which generate significant variations in the structural stresses (stress range), although with a reduced number of repetitions (less than 1 or 2 cycles per day).
Short-period cycles, on the other hand, are characterized by a high number of fluctuations, several times higher than the daily start-ups or long-term cycles. This causes that, even with relatively low thermal jumps, may create damage because of fatigue phenomenon. The stress amplitude of these cycles will depend on both thermal jump (faster ramps generate larger stress jumps) and the velocity of thermal jumps (ΔT/time) faster or sudden ramps lead to larger stress gradients in certain components of the exchanger. This fact involves differential expansion between adjacent areas and, therefore, local stresses that may result relevant.
Summarizing, based on CADE’s experience conducting fatigue analyses on this type of equipment, it can be concluded that the effects of short-term cycles must not be discarded.