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Inspection engineers require new FFS competencies: Illustrations from a pressure vessel failure (January 2005)

   
Julian Speck 1 and Bridget Hayes 2

1 Julian Speck ( julian.speck@twi.co.uk) is TWI's structural integrity group manager, responsible for FFS and RBI activities.

2 Bridget Hayes ( bridget.hayes@twi.co.uk) is a principal engineer at TWI, undertaking a wide range of FFS assessments and failure investigations.

Paper published in Inspectioneering Journal, January 2005.

The number of FFS assessments carried out by inspection engineers is expected to increase in future, as operators 'sweat' their ageing process equipment. The parameters required for assessments are uncertain. Therefore, a multidisciplinary peer review (involving stress analysts, NDE experts and materials engineers) is often necessary before acting on the findings of the even the most regular FFS assessment. Operators and inspection engineers using FFS assessments would do well to learn from previous failures.

Amine absorber column failure

On 23 July 1984, the Union Oil refinery near Lemont, Illinois, suffered an explosion and a fire. Seventeen people working at the refinery were killed and the damage was estimated to be over $100 million. The explosion was caused bythe ignition of a propane and butane cloud that had leaked from a ruptured amine-absorber vessel.

Prior to the explosion an operator at the column noticed gas escaping from a horizontal crack near the bottom of the vessel. The crack grew and he initiated evacuation of the area. As the company fire fighters arrived, the column cracked further and a large amount of gas was released. The gas ignited and the explosion sent the upper part of the tower into the air, landing over a mile away, Fig.1.

Fig.1. The amine unit after the fire
Fig.1. The amine unit after the fire

The cause of failure

The column was commissioned in 1970 and the shell was of 1" (25mm) thick ASTM A516 Grade 70 carbon steel with full penetration submerged arc welds (SAW), in the as-welded (non-PWHT) condition. The vessel was design and built to ASMESection VIII Section 1. Its purpose was to strip H 2 S from the propane/butane gas mixture in counter-current mono-ethanolamine (MEA) flow. The operating conditions were 1.4MPa (14bar) internal pressure at 38°C (100°F).

The investigation into the failure found that the tower fractured at the circumferential weld between a replacement course and the lowest course, Fig.2. Four large cracks in the heat affected zone (HAZ) had been present prior to the explosion, originating at the inner surface of the tower and extending almost through the wall thickness.

Fig.2. Section through the tower
Fig.2. Section through the tower

Microhardnesses measurements in the HAZ near the surface pointed to the cracks initiating by hydrogen cracking and then progressing by hydrogen-induced stepwise cracking (HISC). Tests according to a NACE standard procedure confirmed that the material was susceptible to HISC.

Surprisingly, impact toughness tests of the replacement course material and the weld between this course and the next one showed the weld metal and HAZ to have superior notch toughness to the base material. However, fracture toughness tests on the HAZ measuring crack tip opening displacement (CTOD) on hydrogen charged specimens revealed much reduced CTOD values at 38°C. The effect of hydrogen on CTOD is illustrated in Fig.3.

Fig.3. Effect of hydrogen on toughness
Fig.3. Effect of hydrogen on toughness

Taking all of these findings into account, it was concluded that this failure occurred because the welding procedure used, when replacing a shell course, caused the formation of a hard HAZ microstructure. This hard region was therefore susceptible to hydrogen assisted environmental cracking, resulting in the growth of large cracks in the vessel. The uncracked material in the vicinity of these cracks had low fracture toughness due to hydrogen embrittlement. The vessel subsequently failed under normal operating pressure and the residual stresses at the weld.

Fitness-for-Service (FFS) assessment

The API 759, BS 7910, ASME B31G, etc. assessment procedures are used to make run/repair/replace decisions that reduce unnecessary repairs, avoid unplanned shutdowns, etc. The fitness for service of damaged equipment is always determined by three characteristics, namely:

  • Component Loads: Pressure, thermal, etc. acting on the damaged region;
  • In-service Damage: Size, position, etc. of cracks, local corrosion, etc; and
  • Material Resistance: Toughness, tensile, etc. properties of the damaged region.

The triangle of integrity, Fig.4, illustrates the quantitative relationship between these three aspects. If these they are in equilibrium, an engineering assessment can demonstrate that the equipment is fit (safe) for continued service. In other words, for a known extent of damage (eg. corroded depth) and for a known level of applied stress (eg. internal pressure), an engineering assessment can demonstrate that the equipment has sufficient material resistance (ie. tensile properties)to avoid failure (rupture).

 

Fig.4. Parameters required for a FFS assessment
Fig.4. Parameters required for a FFS assessment

What would you have done?

Assume for a moment that the initial cracks were detected during a normal shutdown inspection of the amine column, and a FFS assessment was carried out to determine the safety of the cracks in the column; would you have:

  1. correctly identified the initial damage mechanism as environmental cracking, in the presence of amine and wet H 2 S corrosion;
  2. correctly identified the significance of welding residual stress, as driving force for failure by fracture; and
  3. used the correct toughness value, rather than assuming so-called 'virgin' material properties in your assessment?

Inspection engineers must always ensure that the chosen FFS input parameter (eg. stress, flaw size, toughness, etc.) correctly matches the environment. For example, for ferritic steels, the material toughness input value should bemeasured at the minimum service temperature. However, beyond this it is often incorrectly assumed that parameters will remain constant, despite a number of factors that can progressively alter their value, such as hydrogen charging from corrosion.

Engineering disasters

A study conducted at the Swiss Federal Institute of Technology in Zurich analyzed 800 cases of structural failure in which 504 people were killed, 592 people injured, and millions of dollars of damage incurred. When engineers were at fault, the dominant cause of failure was found to 'insufficient knowledge', Table 1.

Table 1. Cause of failures for engineering disasters

Insufficient knowledge 36%
Underestimation of influence 16%
Ignorance, carelessness, negligence 14%
Forgetfulness, error 13%
Relying upon others without sufficient control 9%
Unknown situation 7%
Other 5%

Organizations have trouble learning from their failures. They have strong tendencies to explain away failures as being idiosyncratic (ie. that 'one-off' event). And when new technology is involved, many organizations have trouble with learning.

They may actually have trouble learning from their technological successes! They over-learn successful behaviour and become over-confident, which limits their awareness of environmental changes. So failures become inevitable.

Sensibly under-confident about FFS

For many inspection engineers, FFS assessment is a 'new technology', used to demonstrate the safety of damaged process equipment. Operators need to realise that their inspection engineers will need a broad understanding of the three associated disciplines: materials (resistance to failure), NDE (damage extent and size), and stress (driving force for failure). This is likely to require training and a demonstration of competency through examination, which industry must address.

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