Squeezing the most out of your Inspector – Part 1

POSTED :Mon, 10/02/2017 - 20:45 by Lynne

By Lynne Kaley

Seasoned and knowledgeable inspectors are becoming harder and harder to keep. But using some of the industry documents in a smart way, inspectors with less experience can perform like an inspector with many more years of experience and even develop a great materials specialty with practice.

Developing comprehensive equipment inspection plans is an important part of assuring the ongoing safety and reliability of plant fixed equipment. If you don’t have an experienced materials specialist to help identify potential damage mechanisms, the information provided in API RP 571 can serve as a systematic process for identifying potential damage mechanisms. This is essential for creating effective inspection strategies.  Let’s walk through a couple of examples to demonstrate.

Your supervisor gives you a list of equipment for you to write a detailed inspection plan.  The first is a crude column and associated equipment:

  • Crude column
  • Overhead exchangers, reflux drum and piping
  • Pumparound systems and piping
  • Reboiler system and piping
  • Bottoms product and piping to the pump

One of the first things you will probably do is review the inspection history for the equipment over the last 10 or more years with an eye for the potential damage mechanisms above.  The histories will indicate whether any of these types of damage have been observed in the past.  The inspection history should include thickness data looking for indications of corrosion, particularly localized thinning.

Potential Damage Mechanisms for all the pumparound systems, column bottom and associated piping and equipment:

  • 1 – Sulfidation
  • 6 – Naphthenic Acid

As API RP 571 shows, a large portion of the corrosion loops and equipment are susceptible to sulfidation, possibly in combination with naphthenic acid corrosion.  Sulfidation is most often generalized in nature but naphthenic is most often localized.  The localization is dependent on the concentration of sulfur, naphthenic acid and process stream velocity at temperatures from approximately 350OF and 750OF. Type 317L stainless steel is used to minimize or eliminate corrosion. With this in mind, the inspection plan should be concentrated on areas with carbon and low alloy, low chromium, high chromium steels and stainless steels operating at temperatures > 350OF.  The areas most vulnerable are high velocity areas such as turbulence and mixed phase streams.  These areas are typically near inlet nozzles and at tray downcomers or demister pads in pressure vessels.  For piping, velocities are highest near pumps and process flow changes such as elbows, tees and reducers. The damage descriptions for sulfidation and naphthenic acid will provide insights into where inspection should concentrate. All of this information is in API RP 571 and important for making inspection and NDE decisions, where and how to look for the corrosion.

In the next blog, we’ll talk about how collecting more specific process information and using the damage mechanism calculators in API RP 581 can help you better define the potential damage mechanisms of concern, provide a more quantitative assessment and calculate expected corrosion rates/susceptibilities.

MTech is an easy to use mobile app and web based damage mechanism screening tool based on API RP 571 logic for damage mechanism identification and API RP 581 methodology for corrosion rate/susceptibility calculations. Try it out for free here.