Abstract

Industrial steam turbines are frequently operated under part load. Therefore, a partially admitted control stage is usually located upstream of a reaction stage. To obtain high flexibility and efficiency, the power output is controlled by individually throttling the separated sectors of the control stage. Operation under partial admission generates additional losses. For industrial steam turbines, the admission rate is adjusted to control the power output, while on the contrary, a completely different application of partial admission for small turbines is for the reduction of losses. Modern control stages have slightly higher reaction degrees in comparison to older ones.

Subject of the paper presented is the investigation of the performance, efficiency and partial admission losses of a control stage. The measurements take place under a large range of velocity ratios, pressure ratios and admission rates. Additionally, the stator of the test rig has two rotationally symmetric stator end wall designs: a contoured and a plane end wall. There are two different staggering angles available for the contoured end wall design to investigate the influence of the reaction on partial admission turbines.

For the experiment, a single-staged axial-flow turbine with a diameter of 0.71 m is operated within a rotational speed in the range of 1000 to 6000 rpm. The pressure ratio is varied from 1.5 to 2.5. Individual sectors are separated by flow blockages which are directly in contact with the stator, while additional annular blockages are placed further upstream to simulate closed sectors. A variation of the partial admission degree ranging from 13.9% to 100% is thereby obtained. However, the amount of sectors is varied simultaneously. Hence, the measurements are complemented by only varying the admission rate or the amount of sectors at once, in an attempt to isolate pumping and mixing losses. Only variants of mentioned flow blockages are used in this particular loss isolating setup. Additionally, the influence of the arc length of a closed section on the overall partial admission loss is investigated.

The turbine with higher reaction has a slightly higher efficiency. This gain is also seen at low admission rates. Classical loss equations for predicting partial admission losses do not consider the reaction of the stage. Isolating the losses reveals a higher overall magnitude of the pumping loss, as compared to the mixing loss for the test rig. The comparison between two different comparatively long blockage arcs does not indicate significant deviations on the partial admission loss. The influence on the partial admission loss by varying the length of comparatively long blockage arcs is proven to be insignificant.