Standpipes are commonly used in circulating fluidised beds (CFB) to gravitationally transport solids from a region of lower pressure at the apex of the cyclone to the higher pressure region of the riser, Positron Emission Particle Tracking (PEPT) is applied to observe and study the real-time particle motion in standpipes of 2.5 cm and 4.5 cm ID respectively. Experiments use a hopper connected to the top of the standpipe while the base is connected to a free-discharging L-valve. PEPT results confirm that the solids velocity profile is asymmetrical along the vertical axis of the standpipe at any given height, The average voidage of the standpipe, epsilon(a) and solids flow rate, Q(s) are dependent upon the discharge rate of the L-valve, expressed by its fractional opening, f. An empirical equation to predict epsilon(a) has been derived: epsilon(a)/D-0.4 = 2.25 (C-d/C-ds)(-0.2). The drag force on an individual solid particle can then be calculated and determines the actual motion of solids. Upward solids motion will be detected when the drag force exceeds the particle weight, whereas the motion will be downward with some degree of particle acceleration if the particle weight exceeds the drag force. The use of the design equations is finally illustrated in assessing the operation of standpipes of different diameter. (C) 2009 Elsevier B.V. All rights reserved.