De Laval Impulse Turbine-Single Stage
Optimum efficiency occurs when the blade is moving at half the speed of the jet stream. To achieve this very high rotational speeds would be required ( in the order of 15000 rpm). High centrifugal stress, high journal speed and excessive gearing requirements prohibits the use of such system for propulsion by itself.This system is often found as the first stage of a HP turbine were a large pressure drop is required to allow for a smaller turbine. Only the nozzle box has to cope with full boiler pressure and temperatures simplifying design especially of gland boxes. Special material requirements are again restricted to nozzle box. Reduced pressure within the following stages reduces tip leakage
The steam leaving the blades has a high kinetic energy indicating high leaving loss.
Pressure Compounding (Rateau)
The overall heat and pressure drop is divided between the stages. The U/Ci ratio is 0.5 for each stage. By careful design the rotor mean diameter may be kept to a minimum. Excessive number of stages produces an overly long rotor, these leads to problems of critical vibration, increased rotor diameter, increased stage losses due friction and windage and increased gland leakage both at the main glands and the diaphragm plate glands. This due to the increased number of glands and the increased rotor diameter.
Stage mean diameter and nozzle height are increased at the LP end as the steam expands to the limits of centrifugal stress. Nozzle and/or blade angles may be altered to accommodate the increase in volume reducing the requirement to increase blade height excessively.This is referred to as taper-twisting
The blade height increase towards the LP end means that the rotational velocity also increases. Hence for the same value of U/Ci they can deal with higher inlet steam velocities and hence higher enthalpy drops p>The design produces a short lightweight turbine used where size, weight and strength are more important than efficiency. E.G. feed pumps , astern turbines and the inlet portion of HP turbines where it provides a large initial drop in temperature and pressure lightening the rotor and reducing the need for high grade alloys for remaining stages