A jet nozzle and assembly for removing sludge deposits in oil storage tanks is provided. The jet nozzle can include a rigid conduit connectable to a pressurize liquid supply, a pivotal connection connected to the rigid conduit and a spray nozzle connected to the pivotal connection so that the spray nozzle can pivot around the proximal end of the spray nozzle. The assembly can include a manway cover attachable to a manway of a tank, a flange member provided on the manway cover with an aperture passing through the manway cover and inside the flange member, a valve connected to the flange member and a spool connected to the valve. A cover having an opening passing therethrough can be attached to the spool. The rigid conduit of the jet nozzle can be inserted through the opening in the cover so that the jet nozzle extends inside the assembly.