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Due to this high ratio of expansion of steam, the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades has a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the carry over velocity or leaving loss.
The velocity diagram shown in figure 4 gives detail about the various components of steam velocity and Blade velocity. where, symbols have the same meaning as given above. An important point to note from the above velocity diagram is that the fluid exit angle (δ) is 90⁰.
Diagram of a de Laval nozzle, showing approximate flow velocity (v), together with the effect on temperature (T) and pressure (p) A de Laval nozzle (or convergent-divergent nozzle, CD nozzle or con-di nozzle) is a tube which is pinched in the middle, with a rapid convergence and gradual divergence.
The liquid particles have lesser velocity than that of vapor particles; hence, the liquid particles obstruct the flow of vapor particles in the last stage of the turbine, and therefore, a part of kinetic energy of the steam is lost. If the dryness fraction of steam falls below 0.88, then erosion and corrosion of blades can also take place.
The variation of the steam consumption rate ṁ (kg/h) with the turbine load during throttle governing is linear and is given by the “willan’s line”. [1] The equation for the willan’s line is given by: ṁ=aL+C. Where a is the steam rate in kg/kWh, 'L' is the load on turbine in KW and C is no load steam consumption.
The use of the flow coefficient offers a standard method of comparing valve capacities and sizing valves for specific applications that is widely accepted by industry. The general definition of the flow coefficient can be expanded into equations modeling the flow of liquids, gases and steam using the discharge coefficient.
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The velocity triangle [2] (Figure 2.) for the flow process within the stage represents the change in fluid velocity as it flows first in the stator or the fixed blades and then through the rotor or the moving blades. Due to the change in velocities there is a corresponding pressure change. Figure 2. Velocity Triangle for fluid flow in turbine