A steam turbine system which includes a steam generator and a steam bypass path for bypassing steam around the turbine. The outlet throttle pressure of the steam generator is controlled by controlling admission of steam into the bypass path by means of a bypass valve. A desired throttle pressure set point is generated which is independent of steam flow and this set point is compared with the actual throttle pressure for governing the bypass valve during turbine start-up. When the turbine is fully operational the bypass valve control is effected by a comparison of the actual throttle pressure with the desired throttle pressure set point plus some bias.