A protecting device is provided, having two plates forming a rectangular protecting body for sandwiching, constraining and protecting a flexible pouch. The two plates are attached together at a peripheral margin. Each plate has an outer surface with ribs provided in a peripheral annular region of a covering part receiving the pouch, along the peripheral margin. The peripheral margin is mounted in supporting parts of a frame and sliding positioning members may be secured to the peripheral margin, so that the peripheral margin is guided and allowed to be displaced inwardly during filling of the pouch, while the protecting body extends generally planar, at least at panel parts surrounded by the ribs. This allows for progressive conformational change of the protecting body when filling the pouch with biopharmaceutical product, while facilitating reverse displacement of the peripheral margin during draining operations.