A Case Study on the Repair of a Vertical Knock-Out Drum

A 67” vertical knock-out drum suffered a system upset over three years ago resulting in a localized band of internal wall-loss near the entire circumferential dome weld. After performing initial defect assessments, it was determined that the pressure vessel could not operate at full capacity without suffering rapid through wall around the entire circumference. Additionally, the corrosion occurred beneath the lowest support meaning that a structural failure would result in the drum head detaching. To provide a short-term remediation, a composite repair system was custom designed for the operator to address the three primary concerns: through wall leaking, internal pressure containment and preventing dome separation. Three unique systems were combined and used to achieve a successful repair for over 3 years while a replacement drum was ordered for the next turn-around. This paper will highlight the flexible design capabilities of composite materials in addressing critical component repairs when Engineered appropriately.

By Casey Whalen, Global Engineering Manager, CSNRI

INTRODUCTION

In December 2015, CSNRI was contacted by a large international service company to review a vertical knock out drum with severe damage. The system had suffered an upset condition resulting in a ring of internal wall loss directly under the circumferential dome weld. After an assessment from the operator it was determined that the knock-out drum had to run at a greatly reduced capacity to prevent continued internal wall loss at an aggressive rate. Immediately, there were two primary modes of failures:

  • Leaking Vessel - localized areas were expected to quickly reach through-wall conditions.
  • Structural Failure - As can be seen in reference Figure 1, the support structure for this knock-out drum connects above the affected area. If sufficient wall loss occurs beneath this support, the bottom dome will detach causing severe structural failure.
Figure 1: Sketch and view of Vertical Drum prior to Repair

The scope of this potential repair scenario was initially deemed outside of any current composite repair standards, primarily ASME PCC-2 or ISO 24817, and would require a custom design. After exhausting all other known repair methods as well as a replacement drum, the end-user requested a custom design to be considered. The final proposed design consisted of three unique composite systems that would be applied atop one another in separate stages:

  • Inner system – tri-directional fiberglass weave designed primarily for leak containment.
  • Middle system – unidirectional carbon fiber composite system with the sole purpose of holding the dome in place in the event of complete dome separation.
  • Outer system – bidirectional carbon fiber composite system intended to minimize strain resulting from multi-directional stress forces.

The entire proposed surface area was mechanically prepped, cleaned and covered in an adhesive coating prior to the application of the 3 repair systems. Several unique challenges were present during the installation phase and some of the more significant issues will be discussed in further detail. Once applied and inspected a top-coat was applied. After approximately one year, the entire repair was repeated on top of the existing repair to provide further safety factors and increase the requested design life by an additional 2 years. To date, the repair has been considered a success with no structural failure or leaks detected. Although uninspected, based on previous wall loss rates it is expected that the entire ring under the weld has been removed and that the tank is operating entirely reliant upon the composite repair.

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