(79h) On the Fluid Dynamics of Inlet Downcomer Seal at Low Liquid Rates

The primary function of downcomers on trays is to facilitate countercurrent liquid flow down the column with vapor flowing up through the tray openings on sieve and valve trays. Downcomer liquid seal prevents vapor bypassing up through the downcomer thus enabling the downcomer to fulfil the proper function of stable liquid entry on to the tray deck. Under certain conditions, when downcomer seal is lost, vapor bypasses up through the downcomer, which inhibits the liquid passage through the downcomer. Downcomer seal loss is detrimental to overall column performance, particularly at low liquid rates under vacuum conditions.

The paper comprises of two parts. Part 1, courtesy of FRI, is video footage of downcomer seal loss and vapor blowing on a 1.22 m diameter sieve tray operating at very low liquid rates. Observations of vapor bypass up through the downcomer and the sieve holes were made with the aid of directional flow pointers. The second part focuses on initial checks for a tray designer to determine whether downcomer sealing has been realized. Those checks include downcomer clearance, outlet weir height, minimum weir loading, spray/blowing factor, downcomer backup, tray pressure drop and tray liquid holdup. A practical solution for mitigating downcomer unseal and extend a tray’s operating range is by deployment of picket fence weirs.