The pumps with plunging pistons have been studied for feeding the spray-driers, and for trasporting ceramic slips and thick liquids. The pump is formed by: two pistons, run by two hydraulic cylinders; suction and delivery valves can be easily disassembled for rapid inspection or for cleaning the pump; an air accumulator; a control pressure gauge; a max. valve to adjust the working pressure at any value (within the fixed limits), and a safety valve.