Can 2-Pass Moving Valve Trays Experience Multiple Hydraulic Steady States?

Our study reveals that multiple steady state vapor/liquid distributions may be experienced 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 distribution. The others involve having uneven vapor distribution. Some of these steady states effect wide variations in L/V ratios from panel to panel. This maldistribution leads to efficiency reductions.

The maldistribution described in this paper is not due to poor distribution of vapor or liquid to the trayed section from an inlet, and it is also not caused by differences in tray geometry between one side and the other, though we have the capability to evaluate these situations.

The cause of the maldistribution is that at turndown, the valves close, and while they are closing, the dry pressure drop remains constant regardless of the vapor rate. This makes it easy for the vapor to swing from one pass to another, forming a number of alternative steady states.

Using our model, we investigated the effect of uniform weeping on the steady state multiplicity. While shifting the steady state some, uniform weeping does not largely impact the steady states. We also looked at the effect of weir loads on the steady state multiplicity. The multiplicity appears to exist both at the lower and higher weir loads. Our findings are that this is a vapor-driven phenomenon with little effect on or influence by liquid.

One strategy that appears to alleviate the effects of the multiplicity issue is using valves of different weight. This is a common practice used by valve tray manufacturers. Our study shows that this practice can significantly lessen the severity of maldistribution at turndown, but can also cause a wider range of loadings to experience some maldistribution.