Recovery equipment covers a wide range; crystallizers, centrifuges, dryers, towers and so on. Many of these have moving parts similar to those of pumps and compressors. As such, they have been the subject of standard machinery reliability programs. Non-moving equipment such as distillation and extraction tower internals has not had nearly the same effort put into its reliability. We will be filling part of that gap today by concentrating on distillation, stripping, absorption, and extraction internals.
What is the meaning of reliability when applied to product recovery equipment? Reliability means that the equipment functions, as was intended. This implies that product rates, purity, and energy consumption are as expected or designed. Reliability problems arise from both process problems and mechanical problems. Inherent process problems, while very important to reliability, are outside our current scope. But one thought needs to be kept in mind. If nothing you do seems to help the mechanical reliability of a unit, has as much effort gone into checking the process as has gone into checking mechanical issues? If not, then the effort to check the process should be made.
Mechanical problems are today’s emphasis. Mechanical problems with recovery equipment are different from mechanical problems with machinery. Tower internals have no moving parts (with a few exceptions that we won’t consider today). Standard failure modes with moving equipment result from wear and vibration induced by movement. Standard failure modes from stationary equipment are different. The main reliability feature required from stationary equipment is to keep it intact through the length of the run.
Distillation reliability is an all-or-nothing thing for a given item. A tray is either intact and working or damaged and not working. Their is no gradual drop off in performance for a single tray. For a collection of trays in a tower it may look like the tower is gradually failing to work, but what is happening is that trays are failing completely, just one-by-one.
As an example take an atmospheric crude column with wet reflux and poor upstream desalter operation. Chloride salts in the charge heater hydrolyze to form hydrogen chloride. The hydrogen chloride combines with the moisture to generate hydrochloric acid inside the tower. This occurs in the coldest section of the tower. Lower down, the higher tower temperature vaporizes the water, preventing corrosion. However, as the corrosion progresses, the cold trays disappear. When that occurs, the temperature profile in the tower moves down, and the hydrochloric acid goes to work on the next lower tray. While the tower’s performance degrades over time, the performance of any one tray is all-or-nothing. It either has sufficient metal remaining to function, or it does not.
Failure by improper materials is best solved by using proper materials in replacement equipment. Cyclic operating modes, while possible , are very rare in standard services. Improper fabrication and installation can usually be caught by careful review of equipment before, during and after installation , . This leaves the affect of operating events.
Sudden changes in operation can severely stress equipment. This includes both startup and shutdown procedures as well as operating upsets. Sudden changes in operation are the major cause of recovery equipment damage. The major causes of damage result from:
Of these, the vast majority of equipment damage comes from
wet feed and loss of level control.