In this paper, experiments are carried out for three Geldart-D powders in three conical fluidized beds and the hopper-standpipe system. Based on these experimental data, the hopper-standpipe system, which is used to maintain the large pressure drop without the motion of particles when the flow of particles is cut off, is called "particle plug valve (PPV)" and the method for predicting the maximum strength factor of anti-pressure of PPV is given.