What does a Multiport Relief Valve do? Why do I need it? The use of a Multiport Relief Valve dampens the pressure swings within deaerators, steam lines and associated components. The Multiport Relief Valve dash pot vapor action provides smooth relief of excess steam to the atmosphere. By limiting operating pressure variances within the deaerator, it eliminates the need to re-stack trays, and provides protection to the trays, boiler feed pumps and other equipment.