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Investigation of Sulfur Condenser Tubesheet Failures in Complex Thermo-Mechanical Loading Using ASME Sec-VIII, Div-2 Stress Analysis and FEA Simulation

By Aslam Kittur, P.Eng., Static Equipment Engineer at Saudi Aramco. This article appears in the January/February 2022 issue of Inspectioneering Journal.
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Introduction

The ASME Sec-I flexible tubesheet design utilized in a sulfur condenser operating typically as a waste heat boiler is a common occurrence in sulfur recovery units (SRUs) of refineries. The differential pressures encountered in shell sides versus tube sides, combined with a significant temperature gradient across the tube-sheets causing high thermal stresses is typical. Additionally, the crevice-type configuration of the annulus space between the tube to the tubesheet joint is an important factor in geometric consideration. Moreover, repeated start-ups and shutdowns contribute to the accumulation of fatigue-related damage. This article demonstrates the application of a detailed design-by-analysis method using ASME Sec-VIII Div-2 that confirms field observed ratcheting and fatigue type failures in the tubesheet due to the complex cyclical thermal-mechanical loading encountered during operation. A linear static structural elastic stress simulation utilizing Finite Element Methodology was completed to capture the geometrical features and account for the loading parameters. The individual effect of each of the type of thermo-mechanical loading scenarios and their interaction with the tube-sheet design features was investigated to establish the root cause of observed field failures. The results of the analysis correlated with the tubesheet failures observed in the field, at the end of 20 years in service. The conclusions provided recommendations for establishing appropriate design specifications and operating parameters that are conducive to more reliable operation of a sulfur condenser.

In a sulfur recovery unit’s first sulfur condenser (waste heat boiler), the tube-side process gas transfers its heat (725°F to 375°F) to the shell-side high pressure 670 psig boiler feed water to heat it from 295°F to 440°F for producing #600 steam as shown in Figure 1. Shell-side high-pressure boiler feed water of the first sulfur condenser had leaked several times during normal operation into the sulfur process (tube-side) via the failed tube-to-tubesheet welds causing sulfur to solidify and clog the liquid sulfur process stream.

Figure 1. First sulfur condenser in SRU process.
Figure 1. First sulfur condenser in SRU process.

There were repeated failures, as the tube-to-tubesheet welds have failed numerous times during normal operations since the commissioning of this SRU in 1998, which subsequently led to several forced shutdowns of the SRU. Additionally, tubesheet failures such as surface cracks in the ligaments between tube to tubesheet welds and a 0.15 inch of permanent deformation of the tubesheet in the outward direction were observed as well.

Figure 2. First sulfur condenser field failures.
Figure 2. First sulfur condenser field failures.

Problem Statement

The existing tubesheet’s mechanical design complied with the code requirements at the time as follows:

  • The tubesheet of the first sulfur condenser had been designed to ASME Sec-I 1995 Edition, in accordance with the requirements of PART PFT-11 which considers tubes to act as stays.
  • The minimum thickness for stayed flat plates has been calculated as per PG-46.1, resulting in a 25 mm (1.00 inch) thick tubesheet.

  • The tube-to-tubesheet joints have been designed with strength welds without grooves as per PFT-12.2.1.3 and PFT-12.2.4 as shown in Figure 3.
Figure 3. PFT-12.1 attachment of tubes (type-g).
Figure 3. PFT-12.1 attachment of tubes (type-g).

However, an annulus space / crevice in the joint between the tube OD and tubesheet hole ID could exist, as these tubes were light rolled (expanded) and not grooved, and subsequently the effect of expansion had diminished over years in operation. This could result in the crevice witnessing a full internal operating pressure of high-pressure boiler feed water on the shell side. Additionally, the following actual loading scenarios existed which were not accounted for in the original design:

  • High pressure differential between the boiler feed water in the shell side when compared with the process gas on the face of the tubesheet.
  • High thermal gradient across tubesheet thickness due to lack of refractory on the face of the tubesheet.
  • Subjecting the flexible tubesheet to repetitive hydrotests.
  • Effect of the several start-up and shut-down events.

As a part of the failure investigation, an in-depth understanding of the individual effect of each type of thermo-mechanical loading scenarios and its interaction with the tubesheet design features was required to establish the root cause of the observed field failures. Only when the root causes were established could the appropriate corrective actions required for revising the design specifications and operating conditions be determined.

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Comments and Discussion

Posted by Ali Alshehri on February 24, 2022 (Edited on February 24, 2022)
Thanks Aslam for the very informative & useful... Log in or register to read the rest of this comment.

Posted by Mangesh Banale on March 15, 2022
Dear Sir, Please let me know for bonded... Log in or register to read the rest of this comment.

Posted by Dhananjay Rawal on May 2, 2022 (Edited on May 2, 2022)
Excellent application of a detailed... Log in or register to read the rest of this comment.

(Inspectioneering) Posted by Josh Means on May 3, 2022 (Edited on May 3, 2022)
Dhananjay, thank you for catching the inadvertent... Log in or register to read the rest of this comment.

Posted by Dhananjay Rawal on May 4, 2022 (Edited on May 4, 2022)
In order to read correctly Finite Element... Log in or register to read the rest of this comment.

Posted by Dhananjay Rawal on May 9, 2022
In order to read correctly Finite Element... Log in or register to read the rest of this comment.

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