A dynamic seal on hanging downcomers or truncated downcomers is important for high capacity trays to function properly. Unseal of hanging downcomers reduces tray capacity and efficiency. Experimental results show that the liquid head and liquid waves in the downcomer affect the downcomer seal. A new model is proposed to predict the liquid wave amplitude. The downcomer seat experiments were carried out in a 300 x 200 mm simulator with the air-water system. With a seal definition of bubbling once per second for the vapour to rise up the downcomer, the seal line was experimentally determined for various loading conditions. From the experimental data, a seal correlation was obtained. A transition point occurs on the seal line because of vapour-liquid two phase oscillations on the tray. Consequently, the correlation presents a minimum value for the liquid rate for the dynamic seal.