Create constant mass flow

Hey guys,
I'm new to LIGGGHTS and of course have a question. I am trying to set up a case with sand flowing verticalle through a duct. I want it to flow at a constant mass flow rate, ideally by using periodic boundaries in z-direction to reduce the no. of particles. Now I don't seem to be able to fix a velocity or mass flow so that it stays constant. The particles just accelerate more the more often they repeat their fall (logically, due to gravity), but how can I fix this problem (I want to system to stay as "normal" as possible as I will have to couple it with CFD later on)? I've tried the fix pour command, but only got an error message "invalid fix command"... Is there anything I'm missing? I added my case, if it should help. It's a bit of a mess, sorry for that, I'm still experimenting. My first idea was to fill the volume and then set it in motion by removing a stopper and fixing a velocity to the particles.

Any help or pointers in the right direction are greatly appreciated :)

Regards, Jacqui