Control valves are one of the key steam turbine components both in terms of operational safety and flexibility. It is hence fundamental to correctly predict the valve characteristics at the various working conditions to accurately estimate machine performance and control logics. The aim of this work is to develop a simple method to predict pressure losses within the partition system to be used at preliminary design stage.
Two types of partition valves typically employed in real industrial steam turbines of different power (from 1MW to 100MW) are analysed. The first type exploits a diffuser-like shape to maximize the dynamic pressure recovery before the discharge into the impulse stage. The second type, based on simple tube geometry, increases the allowable flow rate, for the same valve seat, at the cost of higher pressure losses. Geometrical dimensions have been varied to cover a wide range of configurations employed in industrial applications. An exception is made for the diffuser angle and the relative fillet radius which were fixed to guarantee product standardization among the various machine sizes.
The flow is supposed axisymmetric and upstream reference condition for the entire study is 140 bar and 540 °C which are typical working conditions for such steam turbines. The influence of the shutter is also considered to properly characterize regulation of the steam flow on the basis of valve lift.
Pressure losses are first modelled dividing the partition valve into singular homogeneous parts such as the intake, the straight pipe, the diffuser and the discharge, for which simple correlations are available in literature. The overall characteristic curve is validated using CFD computations conducted with the steady state RANS solver available in the commercial code CFX exploiting the SST turbulence model.
The development of the correlation permitted to rapidly cover the selected range of geometries and conditions highlighting that dynamic pressure losses are the major sources of losses. Minimal passage area to discharge section ratio is hence a dimensionless value able to describe characteristic curves insensitively to any other geometrical parameter.