Providing a manway or nozzle above the support plate of each packed bed in the tower can ease packing removal. While the packing can be scooped out from the top, this is slow and labor intensive. Vacuuming the packing out of a tower is another option, particularly for large towers. It’s the fastest way to remove the packing but can pose the risk of damaging the packing, depending on its shape and gauge. In most cases the damage is slight and the packing can be reused. Consult with the packing supplier before using the vacuum method, though.
Structured packing. Metal structured packing comes in blocks that are normally sized to allow installation through a manway. It’s very helpful to lay out each layer of structured packing before installing it into the tower to make sure the configuration is correctly understood. For large towers where this isn’t practical, it nevertheless is worthwhile to take the time to lay out one layer of each layer design outside of the tower to check it has the correct dimensions. This also helps in understanding how the layers are to be rotated. Take care not to introduce dirt and particulates into the tower during installation as they may plug up the liquid distributors after startup.
The first layer on the support grid must be correctly oriented to obtain proper support. Rotate subsequent layers per the vendor’s instructions. The orientation from one layer to the next is usually 90 degrees. Place the bundles of packing so they don’t have space between them and so they correctly meet the tower wall. If the bundles have a wall wiper, it should touch the wall. Some vendors may supply extra sheets of structured packing to fill any gaps that occur between bundles. Check the bed height occasionally as the bed is built up to see if the height is as expected for the number of layers installed. Especially for large installations it’s better to find out sooner rather than later whether the installed bed is going to come to the correct height.
It’s good practice not to walk directly on structured packing. Use sheets of plywood or other methods for spreading the load of the person. Consult the packing supplier about how to move around on the surface of the structured packing during installation.
To help spot a poor installation, ask yourself: “Does the installed packing allow liquid or vapor to short circuit between packing bundles or between packing bundles and the tower wall, or does it cause liquid and vapor to channel and bypass one and other?”
Packing retainer. A packing retainer is a device that holds the random or structured packing in place during upset conditions. (Figures 4 and 5.)
Figure 4. Mesh retainer prevents random packing from moving around during an upset.
Two conditions can move the packing around in a tower. One is a vapor surge and the other is a high liquid level in the bottom of the tower so high that liquid enters the vapor inlet or even the bottom of the packed bed.
Figure 5. Bars keep structured packing in place when a column upset occurs.
A vapor surge can fluidize the random packing and blow it into the liquid distributor and, in the extreme case, dislodge the support plate. A high liquid level can knock the support plate off its ledge and damage packing even if the support plate isn’t displaced. If either condition can exist in your tower, it’s best to use a packing retainer and attach the support plate to the support ring or wall clips.
The packing retainer may be separate from or integral with the liquid distributor. The structural elements of the retainer should never interfere with the liquid leaving the liquid distributor and cause splashing. Watch out for this when installing separate retainers.
Liquid distributors. Correctly designing and installing a liquid distributor is critical to obtaining the best performance from the packed tower. The distributor’s purpose is to provide an even flow of liquid across the entire top of the packed bed. We will limit discussion to properly designed gravity head distributors and will cover only those installation items that can affect the distributor’s performance.
There are three basic types of gravity orifice liquid distributors: pan (Figure 6), trough arm (Figure 7), and deck.
Figure 6. This FRI pan distributor design is commonly used for packing tests.
The performance of all of these distributors can be negatively affected by being installed:
- with poorly sealed joints;
- out of level;
- with construction dirt left in the tower; and
- with poorly located feed-pipe discharges.
Figure 7. Unit undergoes flow testing before installation.
Industrial-size liquid distributors come in sections small enough to pass through a manway and are assembled in the tower with gasketed joints. Take care to ensure that the joints are sealed. It’s possible to have leaks in low-flow distributors with small orifices where a single leak flow rate can exceed the orifice flow. (See References 1 and 2 on the effects on performance of orifice layout patterns.) Whenever possible, we recommend water testing the distributor after assembly . FRI installation personnel find it very useful to become familiar with a liquid distributor by laying it out before starting installation. This is especially true for distributors made up of many pieces. Be sure to check the liquid orifice size before installing the distributor.