Large mechanical drive steam turbines used in the oil & gas industry are operating at increasingly higher inlet pressure, generating higher shaft power. Those higher power requirements result in larger disk diameters and surface areas. High thrust forces can be a result, due to both the high inlet pressure and large disk surface area. Industry standards require oversizing of thrust bearings to handle uncertainty in thrust predictions. These factors make improvement in thrust prediction accuracy and mitigation strategies important.
A full-size, axial flow steam turbine test rig capable of measuring turbine thrust, and static pressure in the upstream rotor-stator cavity was built and commissioned. The test rig was operated in single stage configuration for the tests reported here. The rotor disk had balance holes and stationary axial face seals near the disk rim. The face seals divide the upstream rotor-stator cavity into inner and outer circumferential cavities. The rotor-stator cavity upstream of the rotor disk was instrumented, on the stationary wall, to measure the radial and circumferential pressure distribution. Bearing thrust was measured with load cells. Tests varied nominal pressure ratios (1.2, 1.5, 2.0 and 3.0), velocity ratios (0.35–0.6), admission fractions (0.25–1.0) and shaft leakage flow rates.
Circumferential pressure asymmetry, due to partial admission operation, was confined to the outer cavity. The inner cavity pressure coefficient was circumferentially uniform at all operating points. The average pressure coefficient in the upstream rotor-stator cavity generally decreased as the shaft leakage flow rate coefficient increased. Increased leakage flow rate coefficient also increased the magnitude of the upstream directed or negative thrust.