If I have a stream of water flowing upwards in a pipe how do I go about calculating the velocity required for the particles to be stationary?

Spherical particles of density 2500 kg/m3 and in the size range 20-100 μm are fed continuously into a stream of water (density 1000 kg/m3 and viscosity 0.001 Pa s) flowing upwards in a vertical, large diameter pipe. Assume 𝑔 = 9.81m/s2. What maximum water velocity is required to ensure that no particles of diameter greater than 60 𝜇𝑚 are carried upwards with the water? Assume that the upward velocity of the water is uniform across the cross section of the large pipe and that the pipe walls have no effect. Ignore the interactions between the particles. State all of your assumptions.