Because the flow of liquid through an orifice is square root function of the liquid height above the orifice, doubling the liquid level causes a 41% increase in liquid flow. A liquid distributor designed for a minimum liquid rate at a 1-in. head has a 12% larger flow-rate on one side of the distributor than the other if it’s ¼ in. out of level from one side to the other. A way to minimize the effect of out-of-levelness at the minimum required flow rate is to increase the liquid head to 2-in.
Leveling a distributor to a tight tolerance is difficult. It’s best to make sure that the liquid distributor has a built-in leveling mechanism. FRI’s orifice pan distributors are hung on rods, with the length of the rods adjusted to hang the distributor the correct distance above the packing and assure levelness. With trough arm distributors, the individual arms aren’t exactly level with each other, so making sure they are level on average is the best that can be done, but keep in mind the effect of head variation on the flow through the liquid orifice. Any un-levelness can reduce tower performance especially at low flow rates.
Construction dirt left in the tower can plug up the orifices. This is especially true if the orifices are in the bottom of the liquid pool. The flow unevenness caused by dirt-plugged orifices can lower tower efficiency if the number of plugged orifices isn’t small and random. Plugged orifices also depress tower capacity by decreasing the capacity of the liquid distributor. FRI’s crews are careful not to introduce dirt into the tower during equipment installation and they inspect and clean the liquid distributor before closing the tower. Always check the cleanliness of the piping leading into the tower and clean the line if necessary. (References 4, 5 and 6 cover the effect of plugged liquid distributor orifices and their impact on performance.)
Unlike trough arm and pan liquid distributors, deck distributors are supported and sealed to the 360° tray ring. The tower wall retains the liquid on the distributor instead of the wall of a pan or trough arm. So, it’s essential that the deck distributor panels be sealed to the ring as well as to one another with gasketing. The leaking of a deck distributor at the tray ring allows liquid to flow down the tower wall, totally bypassing the packed bed below and resulting in poor apparent packing efficiency.
Feedpipes. Reflux or feed introduced onto a liquid distributor via a feed pipe must not splash, upset the liquid pool or impinge directly onto the distributor orifices. Any of these occurrences can increase the liquid flow variation out of the distributor and decrease tower efficiency. Also make sure that the liquid leaving the feedpipe is discharged at least 1 in. below the top of the gas-riser discharge to prevent the liquid being fed onto the distributor from entering the gas riser or the vapor leaving the gas riser from entraining the liquid up the tower. Ensure that the liquid is single phase and doesn’t flash as enters the tower — otherwise, a flashing-feed liquid distributor is required. The liquid velocities in the feed pipe should be kept low in the range of 3 ft./sec to 5 ft./sec. Reference 7 covers this and other parameters of a good liquid feedpipe design.
Total collector tray. These trays are used for a number of reasons in a tower, but we will limit our discussion to their use with liquid distributors. In this case the liquid passing down the tower is totally collected and then fed to the distributor below. This is done to have a totally mixed liquid entering the distributor or to collect all of the liquid raining from the bed above (including liquid that may be running down the tower wall) to make sure it enters the liquid distributor below.
When a total collector is used above a trough arm or pan distributor, it’s critical that it’s very well sealed with gasketing or seal-welded because any leaking liquid can entirely miss the liquid distributor below and lower the apparent packing efficiency due to the maldistribution the leaking creates. FRI crews have successfully gasketed total collectors in the 4-ft.-diameter section of our column but have been less successful in the 8-ft.-diameter section. In addition to making sure that the tray panels are gasketed correctly at the tray ring and between panels, ensure that the bolt holes at the tray clamp are sealed.
Tips for trays
Much of what has been covered in this article also applies to trays. FRI’s crew has put in many different types of trays and always starts by installing the expandable tray rings to make sure they are level and sealed at the wall with gasketing. It’s essential that the tray ring be sealed so the liquid on the tray does not leak down to the tray below. In commercial towers, except for those with cartridge trays , tray rings usually are seal-welded to the tower wall and don’t require gasketing. For both normal and cartridge trays, it is important that the trays are level, so liquid flows evenly across the tray deck — otherwise the tray efficiency and capacity will suffer.
It also is very useful to lay out the trays outside the tower before starting installation to make sure of fit-up and how the trays are to be assembled (Figure 8). Be prepared to make minor adjustments during installation.
Figure 8. Laying out tray provides a check on fit-up and clarifies how its assembly.
In addition, it’s critical that the downcomer outlet clearance and tray weirs are set to the specified height and leveled. The valves on the valve tray deck must be free to move and the valve legs must be the correct length and spread properly so the valve isn’t blown out of the deck by the vapor. Check sieve-tray orifice size and deck thickness to confirm that they match the design values.