We consider preventive maintenance of a continuously operating system, whose real-life prototype is a rotating chemical reactor for production of phosphorous acid. The drum, in which the reaction takes place, has 42 rollers (elements), which are subjected to a heavy load and to chemical corrosion. The components are organized in a ring-type structure. The system failure is defined either as the failure of 2 adjacent elements, or as a failure of any three elements in a set of 6 adjacent elements. The existing servicing policy prescribes replacing only the failed elements at the instant of system failure occurrence. The operational conditions permit the opportunistic replacement of non-failed components at the instant of system failure. In this paper, we propose a cost-effective policy of preventive maintenance: at the same time the system fails, several of the oldest non-failed components are replaced by new ones. The application of the above optimal preventive maintenance policy results in a reduction of the average cost per unit time by 15-30%.