2.2.6 LP turbine blading
Steam passing through the stages of an LP turbine expands rapidly and accelerates to a high velocity, often supersonic. Steam at exit from the cylinder can have up to 10% moisture content. The flow then is complex and the design of LP blading reflects this. The blades are long and have a highly-twisted profile so that the inlet and outlet angles conform to the three-dimensional flow characteristics at all blade heights.
When large plant was being designed in the late 1950s and early 1960s there were no analytical tools available to confidently predict the three-dimensional nature of the flow in LP cylinders. Today, traverse tests on these machines reveal the significant radial components of velocity in the highly divergent flow. Techniques of through-flow analysis now employed permit the calculation in detail of the flow around each individual blade, using modern computational techniques, and have enabled the complex flow field in LP cylinders to be analysed effectively. These methods have been refined and validated by input of data from flow measurement probes on model turbines in manufacturers' works and on full-scale machines in service. The accuracy of prediction of flow is now high and blading can be designed to obtain the desired three-dimensional flow pattern. Modern final stage blades are twisted considerably, since the flow is at its most complex at the exit plane. The cross-sectional area at the tip is only around 15% of that at the root (Fig 1.12).
To illustrate the point let us assume, for the moment, that the final stage is designed to have a fairly low degree of reaction at the base diameter, near the periphery of the rotor disc. The fixed and moving blade profiles are then as shown in Fig 1.13.
Let us now move to the region half-way up the moving blade, i.e., to the stage mean diameter. Typically, on modern blading, the tip diameter is about double the base diameter so at the mean, the blade pitch (i.e., the circumferential distance between adjacent blades) is about 1.5 times the pitch at the base diameter. The peripheral speed of the blading is also 1.5 times as great as at the base. If the same moving blade section was used at the mean diameter as at the base, the flow passage through the moving blading would be rendered incorrect by the increased pitch; whilst the effect of increased blade speed would be to change the direction of the incident velocity of the steam relative to the moving blade, so the blade inlet angle would not line up with the direction of the oncoming steam flow. Furthermore, the leaving velocity would depart substantially from the axial direction, thus increasing the leaving loss and reducing efficiency. This is overcome by changing the shape of the moving blade section. The outlet angle is reduced to restrict the flow area so that a pressure drop develops across the moving blades, and the steam leaves the moving blades at a higher speed to offset the higher peripheral speed and arrive at an axial leaving velocity. This causes the pressure drop across the fixed blading to fall in order to maintain the same pressure drop over the stage as a whole. There is therefore a substantial degree of reaction at the mean diameter. Figure 1.13 shows the effect that increasing reaction from root to tip has on the profile and on the velocity vectors at root, mean and tip sections.
The effect of these changes leads to a more orderly streamline flow with substantial increase in the efficiency of LP turbines. The pressure losses after the last stage moving blades (proportional to the velocity squared) have also been reduced by achieving a reduced and substantially uniform velocity over the annular region immediately downstream of the last stage blades. A further beneficial effect of reducing crowding of flow at the blade-tip is that the excessive steam leakage over the moving blade-tips, which did no useful work, has been reduced.