Loss of Valves Causing Flood

Can Loss of Valve Floats Lead to Premature Flood?

By Matthew Olsson and Henry Kister, Fluor

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. The missing valve floats were not found anywhere, probably washed away or corroded away. 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? In an endeavour to gain better understanding to the observation, we applied the Fluor Multipass Maldistribution Model (MMM) to see whether these observations can be modelled. In a previous article¹, we applied the same model to discover multiple steady state vapor/liquid distributions in 2-pass moving valve trays at turndown.

Due to the symmetry of 2-pass trays, a perfect split of both vapor and liquid between the passes is always one possible and well-known steady state distribution. 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.

1. Olsson, M., and H. Z. Kister, Multiple Steady States in 2-Pass Hydraulics of Moving Valve Trays” in Kister Distillation Symposium 2017, Proceedings of Topical Conference, AIChE Spring Meeting, San Antonio, Texas, March 26-30, 2017.