Inspecting Distillation Towers Part 2: Revamps and Other Inspections

December
,
2018

This article is based on a presentation given at the Kister Distillation Symposium at the AIChE Spring 2017 meeting in San Antonio, TX.

Process engineers are responsible for inspecting distillation columns. Follow this guide to prepare for a column revamp and other inspections.

Revamps of distillation columns involve replacing or modifying the internals. These changes are usually done on a planned basis and are implemented during shutdowns or turnarounds. In addition to revamps, some other inspections must also be conducted during turnarounds because of operational or material balance issues. Columns that include random and structured packing have their own inspection requirements.

Part 1 of this article (1) describes the role and responsibilities of process engineers for distillation column inspections during standard turnarounds. Here, Part 2 covers distillation column revamps and other inspections.

Process revamp review

You need to be able to answer the following questions before revamp activities get underway:

  • What will be different, e.g., trays, distributors, tray open area, reboiler, condenser size?
  • What is the objective of making the change, e.g., to increase capacity, to accommodate different liquid or vapor properties?
  • Will the changes meet the objectives and is there an economic justification for the change?
  • What is the impact of the change on the process, e.g., increase in capacity or quality, new product draw or recycle? Any expected process changes may require new calculations. For example, if higher-capacity trays are being installed, the condenser and reboiler should be evaluated to ensure they can handle the new tray vapor rates.
  • Are the changes consistent and based on valid engineering, not just driven by plant politics?
  • Does the mechanical design match the process design?

Revamp field inspections

Once the design of the revamp is complete and the parts are delivered to the plant, the process engineer is responsible for ensuring everything is correct. Use these examples to guide your revamp field inspection.

Verify the parts. A debutanizer tower had tray capacity limitations so the feed was redesigned and the trays were changed to new high-capacity trays. The trays were staged close to the tower for installation. While checking the valve clearance height, we found that valves in one section of the tray had a lower clearance than the others. The discrepancy appeared to be more than random manufacturing tolerance. A follow-up with the vendor revealed a manufacturing error that would have limited the capacity of the tower and negated the benefits of the new trays. The tray pieces were remade and installed within the time constraints of the shutdown.

Conduct a field process check. The top section of a main fractionator was redesigned with smaller tray spacing and a new draw tray. The trays and draw were installed according to the drawings. Operation at normal rates produced heavy oil in the overhead distillate. A gamma scan indicated that the chimney tray was operating while flooded, which carried the flood up the column. A review of the drawings showed that the redesign of the draw tray did not include replacing the original downcomers for liquid to flow to the trays below. The process review of the drawings or process inspection at installation should have revealed the missing path for the liquid reflux. A temporary redirection of the liquid bypassing the chimney tray through an unused nozzle allowed operation until a fix could be installed at the next turnaround opportunity.

Account for mistakes. A large two-phase feed pipe was to be installed during a turnaround. The process engineer arrived at the site after the pipe had been installed and realized that the pipe had been welded in place upside down. The turnaround schedule afforded no extra time to cut out the pipe and reinstall it. The engineer had to redesign the baffles on-site to make the new orientation work. A proper process inspection before and during installation would have prevented the mistake and secured the original best design.

In another instance, a side reboiler draw was redesigned to include a water draw. Before installation, the process engineer had the contractor assemble the tray parts on the ground. The new weirs and water sump all fit properly, but the tray deck appeared upside down. Stiffeners on the top of the tray deck would interfere with the intended wash water flow into the collection sump. The drawing that was given to the contractor was wrong, and modifications were made in the field to correct the problem. The process engineer was able to understand the intent of the tray modification and prevent the installation of the dysfunctional modification.

Other inspections

Feed and return pipe distributors. Distributors vary in design from open pipes to pipes with holes or slots. Even if internal distributors are made of a type of stainless steel, upstream piping and vessels will likely be carbon steel. Distributors need to be checked for scale or other foreign materials that can cause them to become plugged. Ensure that all pipes and distribution holes are clear and functional.

Some carbon steel distributors and distributors that process liquids or vapor laden with particulates may show signs of erosion-corrosion, which wears the holes unevenly and may affect distribution (Figure 1). Assess whether the damage affects the process, requiring the distributor to be repaired or replaced.

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Figure 1. Assess damage to distributor holes to determine whether it will affect the process and require the distributor to be repaired or replaced.

Spray distributors. Spray distributors suffer from many of the same issues as feed and return pipe distributors, but may also have hole plugging and bolting failure problems. Spray distributors have relatively small openings, which are prone to plugging. They also tend to vibrate, which can loosen flange and support bolting (Figure 2).

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Figure 2. Spray distributors are subject to high vibration that can loosen flanges and support bolting that is not secured with two nuts.

Nozzles must be removed for inspection because plugged spray nozzles on the upstream side cannot be identified from below due to their internal baffles. If the liquid being distributed is clean, removing the nozzles for inspection may be sufficient. Inspection of the piping is not necessary if there is no indication of plugging in any of the nozzles. Plugged nozzles indicate that piping needs to be removed, inspected, and cleaned. Nozzles need to be cleaned to bare metal or replaced.

Because nozzles must be special-ordered, it is good practice to have spares or a complete new set on hand. Check all reinstalled nozzles, flanges, and piping support bolting for tightness using a wrench. I recommend using two nuts for fastenings as a precaution against loosening due to vibration. Before startup, check that any filters on the inlet line to the spray distributors are clean.

False downcomers and distributor...

Author Bios: 

Doug Bouck

Doug Bouck is the Technical Consultant at Separation Solutions, Ltd. (Email: SeparationSolutions@Bouck.com). He entered the consulting field in 1999 after 32 years with SOHIO, BP, and BP Amoco, where his assignments included prestartup, startup, and lead technical service for a 150,000-bbl/day refinery integrated unit and process lead engineer on the prestartup and startup team for a thermal hydro­dealkylation (THDA)/benzene unit. He served as BP’s Process Technology Manager for distillation, treating, and energy from 1990 to 1993 in...Read more

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