A throttling valve assembly has an outer housing that is sealed. An actuator is connected to a pressure balanced shaft that extends into the housing to operate a plurality of linkages that can rotate blades in tandem. With the surrounding housing sealed, the shaft penetrations of the blade support shafts no longer need seals or a purge system to supply process gas during times of reduced pressure in situations with the blades nearly closed. A pressure equalization system has balance passages internal to the housing to allow flow from higher pressure zones to lower pressure zone of the process gas so as to avoid creation of a condition that can draw air into any zones of reduced pressure in the housing.