Perforated-pipe distributors are normally of the ladder type (spider pipe) or the perforated-ring type. Perforations are located on the underside of the pipes. The ladder type is usually easier to fabricate and therefore less expensive than the perforated-ring type.
The quality of distribution achieved with perforated-pipe distributors is generally somewhat inferior to that achievable with orifice-type distributors. The higher liquid pressure drop available in perforated-pipe distributors (compared to gravity orifice-type distributors) induces a greater liquid flow per unit area; this in turn restricts the numbers of drip points. In an effort to reduce the liquid pressure drop, a gravity trough replaces the pressure header in some high-performance variations. If it is practical to provide a sufficient number of evenly spaced drip points per unit of column cross-sectional area, the perforated-pipe distributor can provide a distribution as good as orifice-type distributors. To improve irrigation evenness, some high-performance variations have some additional perforations drilled at an angle to the vertical.
The perforated-pipe distributor is best suited where vapor mass velocities are high and where an open area in excess of 70 percent is needed to avoid localized flooding. Together with the spray type, the perforated-pipe distributor offers the highest vapor flow area. However, the maximum liquid flow recommended for this type of distributor is relatively low and should not exceed 10 gpm/ft2 of column area with standard designs.
Another advantage of the perforated-pipe distributor is its low cost. Its construction is simple, it is easy to support, and it generally consumes less vertical space than most other distributors. Guidelines for selection, design, and operation of perforated-pipe distributors are given below.
- The perforated-pipe distributor is suitable for liquid feeds only and should be avoided when vapor is present. This distributor also needs to be running full if uniform distribution is to be achieved. Lessons learned where this type of distributor performed poorly with a partially vaporized feed have been reported.
- It is generally recommended that perforated-pipe distributors be located 6 to 8 in above the top of the packed bed to permit vapor disengagement from the bed before passing through the distributor. Distributor designer feels that this distributor may be embedded directly in the packing, thus using the packing layers above it as a mist eliminator. It is not recommend using the latter practice, as it may cause premature flooding of the tower.
- Guidelines, with the exception of guidelines 10 and 12, mentioned in Reflux and Intermediate Feed Inlets for Tray Columns also apply to perforated-pipe distributors. In addition, it has been recommended that for perforated-pipe distributors, liquid velocity through the perforations should not exceed 4 to 6 ft/s.
- The perforated-pipe distributor is best avoided in services where plugging may occur, such as when solids are present or when the liquid is close to its freezing point. A partially plugged distributor may perform worse than no distributor at all. If it is still desired to use this distributor with a solids-containing stream, adequate filtration is mandatory.
- The perforated-pipe distributor is best avoided when the liquid may corrode, erode, or otherwise expand the orifices, because some orifices may expand more than others, resulting in mal-distribution.
- Excessive liquid pressure drop through the distributor should be avoided, because this may restrict the number of drip points. The liquid line to the distributor should contain a control valve or a restriction orifice to let down any excessive pressure. A more satisfactory alternative is to use the high-performance perforated-pipe distributor variation which lets out liquid by gravity instead of pressure.
- The perforated-pipe distributor has a relatively low turndown ratio, roughly 2:1 to 2.5:1. Excessive liquid flow rates may generate fine mist, while deficient liquid flow rates may generate uneven irrigation. The turndown can be enhanced by using a dual liquid distributor.
* Reproduced from Distillation Operation by Henry Kister