Process Engineering: Inspect with your mind, not just your eyes

By Andrew Sloley, contributing editor

Equipment upgrades often are attractive options for plants. However, tiny installation details can count a great deal. This makes equipment inspection, which is the final chance to catch installation errors before operations begin, all the more important. A proper inspection involves more than making sure the equipment in the field matches the manufacturer’s or construction company’s drawings. Effective inspectors leverage knowledge of the equipment, the process and how they work together. Consider the following example:

During a plant revamp, normal distillation trays were being replaced with high-capacity trays. Most high-capacity-tray designs use hanging or aggressively sloped downcomers to achieve an optimal split between active tray area and downcomer volume. High-capacity trays often have very tight restrictions on the downcomer outlet.

The new trays were installed and the vessel was ready to be closed up. The installation crew reported that all trays were installed per drawing specification. A follow-up check confirmed this. However, the inspectors had fallen into a trap for the unwary and missed one important point. Because of a simple nozzle detail, the equipment might fail to meet its new performance expectations.

The external, 24-in.-diameter manholes were not flush with the inner wall of the tower. Instead, they had internal projections. For one tray, a manhole lined up nearly perfectly with the downcomer. The projection thus blocked a large portion of the downcomer-liquid escape area. The figure shows the contrast between the required clearance across the downcomer’s minimum width for a correctly installed high-capacity tray and that of the tray that was incorrectly installed. This was a tower failure waiting to happen. Quick modifications were made to the downcomer on that tray. After startup, the tower operated without problems.
Always inspect equipment during maintenance turnarounds. This should be a key responsibility of plant engineers. Checking that the equipment is per drawing or design specification is only the start. When inspecting, go the extra step — ask yourself how the equipment is supposed to work and whether what you see makes sense.