Spiral wound membrane element and a process for preventing telescoping of the filter element

Filter assembly for ultrafiltration comprising in a pressure vessel one or more filter elements with antitelescoping devices (ATD) located upstreams and downstreams for each filter element, wherein the filter elements comprises one or more membranes, each consisting of a central permeate spacer covered on both sides by separating membranes, connected at one edge with a permeate pipe and blocked at the three other edges, wound around a central permeate pipe with a concentrate spacer allowing fluid from the space between the wound filter element and the pressure vessel to flow into the wound filter element in a direction tangential to the cross-section of the filter element, so that the membranes and concentrate spacers are lying alternating in the wound element;

• wherein the inlet to the space between the wound filter element and the pressure vessel is free of obstruction and the outlet from said space is restricted so that no flow or only a limited flow is allowed from said space to the space between the respective wound filter element and the pressure vessel, characterized in that, the wound filter elements are provided with means for securing that the pressure inside retentate channels of the filter element is equal to or lower than the pressure in the space between the filter element and the pressure vessel at the same longitudinal position over the whole length of the filter element, and wherein means for securing that the pressure at the inlet of the filter element is equal to or lower than the pressure in the space between the filter element and the pressure vessel at the same longitudinal position is a flow restrictor placed at an inlet to a spiral wound filter element.