Can Loss of Valve Floats Lead to Premature Flood?

An Atmospheric Crude Tower functioned well for a few years. Towards the end of run, the performance of this tower became poor, with high naphtha end points. A gamma scan confirmed flooding taking place in the upper four 2-pass fractionation trays. It was expected that the trays were plugged. Upon shutdown and inspection, the trays were found clean, but missing most of the valve floats. This increased the open area on the trays and should have moved the trays further away from flood, but the flood only occurred near the end of the run, and did not occur in the early years of the run, when the valve floats are believed to have been in place.

This raised the question: could losing the valve float have promoted flood? We applied the Fluor Multipass Maldistribution Model (MMM) to see whether these observations can be modelled. In a previous article, we applied the same model to discover multiple steady state vapor/liquid distributions in 2-pass moving valve trays at turndown.

Our analysis shows that uneven or complete float removal on the trays can channel the vapor and liquid in different directions, which may account for the premature flood and above observations.