filtering apparatus including two or more passages acting in parallel , each having an extended filter which passes through inlet and outlet ports preferably sealed by substantially solidified plugs of the flowable substance such as polymer being filtered . the flow of such substance through an initially open passage can be terminated so as to permit filter advance under conditions of zero pressure drop whilst flow through the other passage or passages continues . in an additional or alternative mode , flow from a common inlet into a passage is terminated and a secondary outlet conduit in that passage is opened . this creates a counterflow caused by the pressure of substance from the other passage or passages which cleans the filter . either or any passage or conduit is sealed or valved by a substantially solidified valve plug made of the flowable substance being filtered , said plug being extrudable subsequently when the passage or conduit requires opening to flow . a valve suitable for use in a secondary outlet has a side passage in which a valve plug is formed of a flowable substance in a sufficiently solidified state , having also means for forcing the valve plug into the flow so that the flow of substance can be stopped . a diverter valve may be constructed from two or more such flow valves .