Piston pump with piston spring-biased to filter piston

A process for the conveyance and filtering of a fluid containing solids, using a piston pump comprised of a cylinder (2), an automatically closing intake valve (3) to a pump chamber (4), a sealing reciprocating piston (5) and a sealing upstream filter piston (7), wherein the piston (5) together with the filter piston (7) form an automatically closing discharge valve (8), the process comprising the steps of:

• enlarging the pump chamber (4), in an initial operating stroke of the piston (5), by an axial return stroke;
• creating an overpressure with a closed discharge valve (8) that causes the fluid (9) containing the solids to flow through the opened intake valve (3) into the pump chamber (4);
• reducing the pump chamber (4), in a second operating stroke of the piston (5), by an axial compression stroke;
• compressing the fluid (9) containing the solids;
• moving the upstream filter piston (7) towards the piston (5) against the spring-bias;
• filtering the fluid (9) through the filter piston (7);
• opening the discharge valve (8);
• discharging there-through the purified fluid (9) that was filtered by the filter piston (7) and compressed by the piston (5).