BCJ,
We have had experience with both types of operator-swung blinds (Hamer and ONIS) in an ethylene plant.
I agree with the comments of Williams21034 in that overall, the ONIS design is better than the Hamer from a safety and leakage point of view. On both types it is really important that...
The steam line to the bottom of the firebox apart from being used for smothering, can also be used to purge the firebox before light off of the first burner (ensures no fuel present, provided sufficient time is allowed for purging)