Simon.
Venting of mainballast tanks 2 port and starboard
I don`t think the ventvalves operated in the engine room is an emergency venting. I would rather name it as a residueventing. The idea is when diving at a sharp angle, air would be trapped in the aft end of main ballast tank 2 port and starboard and the venting system takes care of this trapped air.
I guess you are right in removing the valvewheel d1
Tore