Screw feeder into the rubber hopper - flexible walled hopper. Discharge is via butterfly valve, air blown into the top to assist the discharge. ■

It sounds as if you are discharging most of the material and then when the material levels falls, the gas you are adding to the top breaks through and becomes in effective in assisting discharge.

This may be made worse by your use of a butterfly valve, which effectively gives you two smaller outlets not one large one. If the material tends to preferentially discharge through one orifice, you can get a bridge over the second one leaving material stuck in the hopper.

Changing to a slide valve or similar which gives a full unrestricted outlet may help.

Another little trick is to rapidly close and open the butterfly valve again, the faster the better. The movement of the valve disc can sometimes dislodge a build up.

The normal way to discharge rubber walled hoppers is to external massagers to the wall. If the material sticks to the wall in a particular place, put a pneumatic cylinder in that location and use it to push in the wall when you lose feed, hopefully that will dislodge the build up. There are more sophisted massaging systems available but they are normally supplied as a package including the hopper. ■