I am not sure what you mean by a “double weight valve tray.”  If you mean heavy weight (thicker gauge material) for the valves, the tray pressure drop will increase slightly because the force needed to lift the valves up will be greater.  If you mean a thicker tray deck material, it will change the orifice coefficient of the deck orifice.  The dry tray pressure drop will go down as the ration of tray thickness to hole diameter increases.  (See the sixth edition of Perry’s handbook figures 14-18). 

Also make sure that the curtain area of the valve when fully open is the same as the light weight valves.  Also the hole pitch and layout (square vs. triangular) also effects the pressure drop. (Refer to Henry Z. Kister’s book “Distillation Design” published by McGraw-Hill. Specifically, see table 6.9 “Coefficients for Valve Tray Dry Pressure Drop Equations” on page 312.)

Are you replacing exiting light gauge tray with a heavier gauge or are you scaling up from a small diameter tower to a large tower?  If you are scaling up from a small diameter tower to a larger one, there are a lot more parameters that need to be considered than just heavy valves vs. light ones.  If you have trays that failed due to vibration, just changing the tray thickness may not solve the problem.  I suggest you contact a well known tray supplier and discuss your situation with them.  You can find some of them under the “member companies” section of the Fractionation Research, Inc. Web site: www.fri.org.