Not much to add over what appears in my recent CP article. For trays, levelness is important. As the trays are being installed, one should also check for weir height, downcomer outlet opening clearance and fit-up of the deck panels. If there are manways, you should make sure that they are in place and secure. If you are working with sieve trays before installing, you should check the orifice size and count and for valve trays. Make sure the valve legs have been sufficiently expand that the valves will not come out when in use. If the tower contains trays with different percentage open area or valve counts, make sure that the correct trays are installed in the right location in the tower.