7.1.1   Design for high temperature operation

 

The increase in size of large turbines over recent years, together with the higher steam inlet conditions often used, has made the design of turbine components in contact with high temperature steam more critical. Fundamental considerations include creep and thermal fatigue, both requiring control of the temperature distribution in components during steady and transient operation.

At the design stage of high temperature turbine components, temperatures and thermal stresses are assessed to ensure that creep, which occurs as a function of time, temperature and stress levels do not exceed specified values and that thermal stresses due to cyclic operation are not of sufficient magnitude to cause thermal fatigue during the life of the turbine.

Creep can be limited by close attention to detailed geometry in critical areas, such as blade fastenings, in order that stress-concentrating features are controlled. Rotor stresses generally are kept low in relation to the high temperature mechanical properties of the creep-resisting steels employed.

Thermal fatigue is associated with changes in operating conditions, such as occur during two-shift operation, due to the cyclic reversals of thermal stresses resulting from the transient temperature gradients established during the heating and cooling periods. The thermal fatigue problem is accentuated by the increased scantlings on larger machines which result in higher thermal stresses occurring within components during transient conditions.

The inlet sections of HP and IP rotors are the critical areas for thermal fatigue, demanding careful attention to design detail and operational practices.

Typical stress-strain cycle at rotor surface

A typical cycle of events undergone by material in the critical region of a turbine rotor is illustrated in Fig 1.107. The material undergoes plastic strain compression during the heating period to point 1 on the diagram, at which point the thermal stress has reached its maximum value. The stress then falls as the temperature of the rotor interior increases towards that of the surface, causing the stress and strain at the stress concentration to move along the line 1-2. At point 2, the nominal thermal stress is virtually zero and stress at the concentration is the residual stress arising from the plasticity which occurred at 1. During the period of full-load running, the temperature of the rotor is such that this residual stress relaxes to point 3, due to creep. The shutdown or load reduction which follows, causes cooling of the rotor surface and consequently tensile thermal stress in the surface layers. The stress-strain condition at the concentration then follows the line 3-4 with possibly some further plasticity, in tension, at 4. As the rotor temperature becomes more uniform at the part-load or shutdown condition, the material stress-strain moves along the line 4-5 to point 5. At this point, the rotor temperature will generally be too low for significant creep to occur and there is no stress relaxation, such as occurred between 2 and 3.

When the material at the point of strain concentration is held at reasonably high stress (point 3), there is a damaging effect of the hold-time due to irreversible creep strain. The effect of this is shown on Fig 1.108.

Typical strain range plotted against a number of thermal cycles to crack initiation at 540 C showing the effect of dwell time

Thermal fatigue, which shows as surface cracks, can occur when there is a net amount of plastic strain in the thermal cycle. Material property curves are derived for combinations of plastic strain range, number of thermal cycles and hold-time at the peak of the tensile part of the cycle.

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