Vapor Line Inspection by Thermography - Dist. Unit

Using thermography in a petroleum refinery to supplement ultrasonic testing of a crude unit overhead vapor line Bill Davis ChevronTexaco Inc.

ABSTRACT

This paper describes the value of utilizing infrared (IR) thermal imaging as a supplement to ultrasonicthickness (UT) gauging for predictive maintenance of a crude unit overhead vapor line. UT gauging was performed on a line during scheduled downtime, utilizing UT points set up in areas where corrosion or erosion was thought most likely to occur. No signs of significant metal loss were found. However, the line failed only a week after reactivation of the process, causing an unscheduled and costly plant shutdown.  A new line was fabricated and installed. Subsequent inspection with an infrared camera clearly showed showed cool spots in the l ine — areas of sour water condensate — that had been previously undetected by the UT gauging. This line was added to the infrared monitoring program, and at least once a year it is inspected with the IR camera to monitor these areas. Keywords: Infrared thermography, ultrasonic thickness, predictive maintenance, vapor line, thermal imaging, infrared

INTRODUCTION

During our last major plant turnaround the uni nsulated overhead vapor line on #2 crude unit C-301 was ultrasonic-thickness (UT) gauged to determine its thickness (see Fig. 1). Thinning of the walls of the vapor line occur as the result of corrosion and erosion, and constitute areas of weakness and potential failure. UT inspection points were set up at locations on the various lines throughout the refinery where corrosion or erosion was determined most likely to occur (see Fig. 2).

Figure 1. The overhead vapor line on # 2 crude unit C-301

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Figure 2. UT thickness measurement location points were established throughout the refinery, on the basis of engineering expectations of where problems were most likely to occur.

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Based on the UT readings, there was no indication of any significant metal loss. However, only a week after the plant was recommissioned, the line failed, but not in the vicinity of the UT reading points, causing an unscheduled and costly plant shutdown. PROBLEM IDENTIFIED WITH IR THERMOGRAPHY

Subsequent visual investigation detected small areas on the bottom of the horizontal section of the line that had thinned due to the pooling of condensate sour water. The unit was shut down for about one week while a new line was fabricated and installed. The cost to ChevronTexaco, including the lost profit opportunity, was about $1.2 million, plus the maintenance cost of the repairs.  After the unit was restarted, failure analysis identified the problem as low velocity and poor design, which allowed sour water to condense and pool where we had found it. Subsequent investigation using an infrared thermal imaging camera clearly showed cool spots on the line, providing an environment in which the condensate had likely formed (see Figs. 3, 4, and 5).

Figures 3 and 4: Underside of the uninsulated vapor line shows the cool areas where liquid condensed.

Figure 5. Infrared mosaic of the entire length of the uninsulated vapor line. Note the cool spots where pooling of condensate and subsequent thinning of the vapor line wall occurred.

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This overhead line was added to the infrared camera monitoring program. At least once a year it is inspected with the IR camera to monitor these areas. Subsequent inspections indicate that the water seems to accumulate in one general area that varies in size depending on feed rates and overhead temperature, but always seems to remain wet. With this information, we can now make better decisions on where we need to inspect when UT readings are taken. SUMMARY

The ability of the infrared camera to instantly reveal the thermal characteristics and anomalies in large refinery pipelines enabled the Reliability Group at ChevronTexaco to locate thin portions of vapor line walls that were not identified by ultrasound thickness gauging. The growing IR inspection program at the refinery now includes vapor lines, supplementing the UT inspection effort. ABOUT THE AUTHOR

Bill Davis is a level II Thermographer who initiated the infrared inspection program at ChevronTexaco in 2002. He has 19 years of refinery experience with the company in a wide range of roles. For 13 years, he was assigned to Operations, including 4 years as a Head Operator in the Delayed Coker. In 1996, he transferred into the Reliability Group as a fixed equipment inspector, as the plant inspector for the Delayed Coker. Bill has worked many plant turnarounds, in numerous different operating refinery plants, as an operations coordinator and inspector. During 2002 his efforts eliminated the company's an nual infrared contractor cost for furnace tube inspection of $200,000. His goal for 2003 is to eliminate the remaining $20,000 annual IR contractor cost for inspection of electrical systems.

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