Dry Risers are normally installed in Low Rise buildings (those less than 23 M in height). The riser pipe is empty and does not have any water in them. In the event of a fire incident, water is pumped into the riser from Fire tenders using the Fire brigade inlets provided at the bottom of the riser.
Wet risers provided in high rise buildings (more than 23 M in height). In these systems, the riser pipe is always full of water, and is pressurized using gravity tanks, or dedicated fire pumps. Hence water is always available for fire fighting. Fire brigade inlets are also provided on wet riser systems, to provide water in case of failure of the system fire pumps.