An extensive experimental program was carried out on group B and D powders, in L-valves of diameter between 20 and 40 mm. The particle size influences both solids＇ flux and pressure drop across the L-valve. The length of the horizontal transfer section has only a marked effect on the pressure drop. The maximum solids＇ Awe achievable is determined by the hopper outlet. Correlations are given from which the aeration gas flow and the pressure drop across the L-valve for any solids＇ flux and also the critical hopper solids＇ flux can be calculated. A design procedure is given for systems using group B or D powders with air as the carrier gas.