When partially closed, steam turbine governing valves rely on flow separation from the valve head and seat to generate loss and throttle the flow. The aim of this type of valve is to avoid separation and therefore eliminate loss when the valve is fully open, and to have stable and controllable separations at all other valve lifts. Any significant unsteadiness in the valve flow can result in unacceptable mechanical vibration of the valve, which in extreme cases can lead to failure.;Results will be discussed from work that has been undertaken into valve flow instabilities at Leicester University Engineering Department in collaboration with Alstom Energy Ltd. At high lifts, the Mach number of the steam flow between the head and the seat is sufficiently low for the fluid to be considered as incompressible. Water was therefore used as the working fluid in the tests at Leicester to model accurately the flow in a fifth scale acrylic model valve under high lift conditions. Results from laser light sheet visualisation, Particle Image Velocimetry and transient pressure measurements of the valve flow are presented.;Laser light sheet illumination and high-speed Cine photography have been used to visualise the highly three dimensional valve flow. A range of valve head geometries has been tested. The results of the flow visualisation show the presence of stable and unstable separation zones and their influence on the valve flow. Particle Image Velocimetry has provided quantitative information on these features. Methods for stabilising the separation zones by modifying the valve head and seat have also been investigated and the results from these tests have shown improvements in reducing valve exit pipe unsteadiness.
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