2.4.7 Partial admission
Partial admission gives a loss in turbine efficiency if the steam supply to the turbine is governed by the number of nozzles in use. The alternative means of steam governing is throttle-control, where admission is over the complete circumference of the first stage, and control is achieved by reducing the main steam supply. Partial admission is therefore not experienced by throttle-controlled turbines.
There is also an additional factor to consider. As the idle blades come under the influence of the nozzles, the steam in the blade passages has to be accelerated rapidly, so causing a loss in power. The magnitude of this loss increases with the work done per row, the heat drop, the blade speed, blade width and height, and the number of steam admissions. This factor, known as the steam inertia loss, can account for 15-20% of the total windage loss in a stage with single arc admission.
In the UK, there are many interconnected units in service so that each can be kept in efficient operation at high load. System load reduction is achieved by shutting down complete units, so that there is no benefit from the adoption of nozzle control for individual units. Alternatively, load reduction is accomplished by reducing the boiler pressure, so that the turbine valves are maintained wide open over most of the load range up to 100%. Part-load efficiencies obtained in this manner are comparable to nozzle-governed machines, whilst the full-load efficiency is better.