The steps needed to improve reliability are (1) make the equipment stronger, (2) change upstream equipment and operation to keep wet feeds out and (3) improve level control, especially at start-up. Changing upstream conditions and improving level control often involve process changes. However, they are necessary. If at every shutdown, the same damage is found, fundamental questions should be asked about the process.
It is impossible to build equipment that cannot be destroyed or damaged. Nevertheless, vast improvements can be made. One clue to possible improvements is the situation where the same type of damage is consistently found at every turnaround. This is a clue. The question should be asked ‘how do we improve this?’ Do not just replace the equipment in kind. Enriching equipment vendors by repeated replacement is not the primary mission of the maintenance department. Minor mechanical changes, costing little, can have a large impact on the strength of tower internals.
Internals fail either by forces pushing up (lifting) or forces pushing down (crushing). Most internals, especially trays, have much higher resistance to crushing failure compared to lifting failure. Tower internals rarely fail due to crushing. Lifting is the major mechanical damage mode. With lifting failure the edges of the equipment are bent down because the equipment is being pushed up while the edges are being held. With crushing failure the edges of the equipment are bent up because the equipment is being pushed down while the edges are being held. Figure 1 shows what greets an inspector when major uplift damage has occurred. Looking through the manhole, the trays are obviously still held at the tray ring and bent up.
Mechanical damage inside a tower may range from severe to relatively minor. Figure 2 shows severe damage from water in a tower feed. Figure 3 shows minor damage. In Figure 3, just one side of a tray manway has lifted.
While mechanically minor, the damage in Figure 3 has a major process impact. The damage shown in Figure 2 and Figure 3 can be easily diagnosed with basic temperature, composition, and pressure measurements . ‘Advanced’ (and expensive, difficult to use, and difficult to interpret) techniques such as gamma scanning are not required to diagnose problems resulting from tray damage. In fact, the manway damage shown in Figure 3 will virtually never be found in a gamma scan.
Of course, having the technical competence to use the basics
such as temperature, pressure, and composition requires that engineers
truly understand their complexities and how to use them. Advanced
attempts to look inside towers are rarely either useful or necessary.
In addition they are expensive. Given today’s reengineering
rush, it’s unfortunate that more searching questions are
not asked about the value of such methods. This (reducing the
number of scans being done) is a cost-cutting opportunity waiting
to be grabbed.
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