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Inspectioneering Journal

HTHA Inspection Detection Confidence Improves Basis for New API Recommendations

By Michael Nugent at Equity Engineering Group, and Timothy Armitt at LINDT, UK. This article appears in the September/October 2018 issue of Inspectioneering Journal

Introduction

The refining, petrochemical and ammonia industries have been processing hydrogen at elevated temperatures and pressures since 1933. In 2010, there was a significant incident with the loss of seven lives. This incident was attributed to a damage mechanism known as high temperature hydrogen attack (HTHA). In addition, there are other reports of ‘damage’ below the then established Nelson Curves, prompting the most significant revision of these curves in the last 20 years.

The curves graphically represent “safe” operating zones for common materials of construction as related to operating exposure temperatures and hydrogen partial pressures. The “safe” operating zones are considered to be some distance below those curves as most practitioners/operators assign a safety margin or cushion below the curve.

Figure 1. An area of HTHA damage in the heat affected zone of a weld. HTHA is usually highly localized.
Figure 1. An area of HTHA damage in the heat affected 
zone of a weld. HTHA is usually highly localized.

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