Failure analysis of burner piping
Investigation of the causes of a severe refinery fire leads to recommendations for burner piping design to prevent such incidents
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Typically, huge damage is involved when a fire occurs at a refinery. However, the industry keeps repeating the same old mistakes with the same results: leaks, fires and process upsets. In order to help reduce the risks of fatal or catastrophic incidents, this article will describe failure analysis investigations of burner piping. It also discusses the corrosion assessments and remedial measures such as burner piping design criteria including the recommendations in specifications. The focus is to learn lessons from a prior incident, which has been thoroughly investigated and the results shared.
The incident whose causes are analysed here occurred during the start-up of a fired heater immediately after a turnaround. There was a severe fire around the burner piping which led to catastrophic incidents involving neighbouring equipment and huge unscheduled downtime, mainly due to fire damage of instrumentation cables. Investigation after the fire was put out showed that flexible hoses for fuel gas were leaking. Just like most refinery fires, the incident started with a single problem of piping leakage.
Specifications of flexible hose and burner tip
A flexible hose is specially designed to achieve several objectives in pipework design. These include avoidance of absorption or vibration, to operate effectively under high pressure, and to adjust or correct for misalignment. Flexible hoses are generally used in burner piping (fuel oil, atomising steam, fuel gas), specially for the purpose of positional adjustment of the burner gun. They permit a very economical installation against the rigid piping in difficult locations, such as complicated burner piping. It is easy to adjust the elevation or orientation of a burner gun without any mechanical modifications to the burner piping. The inside tube is made from stainless steel strip, an annularly corrugated tube manufactured by continuously processing the material on a high speed automatic forming machine. The geometry of the corrugations gives a metal hose excellent hoop strength, providing superior resistance to collapse when exposed to high pressure. After this processing step, it is annealed in a furnace without oxidation to completely eliminate residual stresses. The outer covering is made from stainless steel wire braid which provides the necessary protection from abrasion.
The burner typically combines oil and gas firing and is designed to operate both liquid and gas fuels. The burner in question had been operated with either oil or gas during normal operation. The fuel gas burner tip is made of a higher temperature metal alloy casting, because it is typically exposed directly to heat (flame) in the radiant box of the fired heaters. The burner components, including the burner tip, are designed in accordance with the minimum requirements shown in Table 13 of API standard 560 (Fired Heaters for General Refinery Service).1
The detail specifications of flexible hose and burner tip are described in Table 1.
Morphology of damage
Many small pin holes were found in the flexible hose bellows (corrugated stainless steel tube), especially at the bottom side (see Figure 3). Corrosion is characterised by localised metal losses in the form of pits.
Additionally, the top of the fuel gas tip was melted and the gas tip holes were severely plugged and eroded (see Figure 4). Following a review of the maintenance history, it was found that the fuel gas burner tips had been frequently replaced during operation, as a result of severe damage. In general, as burner tips are custom designed in number, size and angle of tip holes (see Figure 2) for specific applications, the damaged burner tip will result in undesired flame characteristics (length, size and so on), as well as poor performance in operation, including environmental problems (NOx emission levels).
The process details for the burner are as follows:
• Operating fuel gas pressure: 1.2-1.5 kg/cm2
• Operating fuel gas temperature: 30-60°C
• Fuel gas composition: H2 69%, C2H6 10%, C3H8 8%, others 13% (no critical toxic components)
• Operating scheme: designed to operate either oil or gas (not to operate on both fuels simultaneously).
Probable reasons for failure
The failure of the bellows tube may have occurred due to any one of, or a combination of, the following:
• Wrong material of construction: an incorrect material (abnormal composition) was selected
• Improper installation: due to kinking or excessive bending, the corrugated tube was damaged
• Corrosion: even if the correct material is used, corrosive chemicals could have been present
• Process upset: a higher temperature or pressure than design conditions.
A review of the material composition of the corrugated tube was made in accordance with the ASTM specification. Visual inspection was carried out in order to identify the main clues behind material failure. Since the corrosion pits were found on the corrugate tube, it was necessary to carry out energy-dispersive X-ray spectroscopy (EDX) to indentify the components of corrosion from the scale deposited on the inside of the tube.
• The chemical composition of the corrugated tube was reviewed
• Metallurgical analysis of the failed tube satisfied the ASTM specification (AISI Type 304).
• Many small pin holes were noticed on the surface of the bottom of the corrugated tube
• Liquid stagnation marks were noticed at the bottom of the corrugated tube
• Severe thinning and pitting were noticed at the bottom area of the internal surface of the corrugated tube
• The region near the pits, which was thinned due to corrosion, showed layers of deposits over the surface
• The failed portion of the tubes contained pit holes, which had perforated the tube, initiating from the inside surface. The perforations made holes of 1-2mm diameter, surrounded by thin pits (see Figure 3)
• The top part of the gas tip was melted out and there was severe coke build-up inside the gas tip (see Figure 4)
• The gas tip holes were melted, plugged and eroded.
Energy-dispersive X-ray spectroscopy
EDX is an analytical technique used for the elemental analysis or chemical characterisation of a sample. EDX studies were carried out to determine the elemental compositions of the matrix and the deposits/scales on the failed tubes:
• The EDX profile of the failed tube (see Figure 6) shows iron, sulphur and chromium in very high concentrations. Sulphur and chloride cause corrosion, while Cr, Fe and Ni were released as a result of corrosion
• The results of EDX studies indicate that there is substantial incorporation of sulphur comp-ounds in the corrugated tube during operation
• Pitting corrosion is caused by the effects of sulphur and chloride, especially when they are present in hydrous solution.
A number of key issues arose from the EDX analysis including the presence of a lot of sulphur, although the fuel gas contains no sulphur at all. This implies that the main cause of the corrosion may not be related to the fuel gas itself. In this regard, engineering failure analysis is needed to identify the main causes of corrosion. There was a need to go further and ask ourselves how and from where the sulphur is generated. Engineering failure analysis has two major objectives: to determine the failure mechanism; and to determine the failure’s root cause. The failure mode is the basic material behaviour that results in failure, for instance pitting corrosion. Root cause is the fundamental condition or event that caused the failure –material defects, design problems, or improper use.
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