Failure Of Double Wall Jacketed Piping In Refineries: Causes And Prevention
Chemical Engineering World|August 2018

The application of double wall jacketed pipeline in sulfur recovery units of refinery plays a major role in productivity of sulfur. The sulfur content in crude oil may contain 0.22 wt% as obtained by detailed assay report. This sulfur has been extracted from various hydrocarbon processing units as sour water. Further the sour water processed in sulfur recovery unit to produce sulfur in solid powder form. The molten sulfur condenses via sulfur condensers to produce solid sulfur. The molten sulfur transferred to double wall jacketed pipeline and its failure case study is discussed. An improper design results in internal and external corrosion of internal core pipe deteriorate the pipe within five years. Root cause is analyzed and possible ways to mitigate similar failures is suggested.

The double wall pipeline is utilized in petroleum refineries and chemical process industries for maintaining liquid sulfur in melting temperature or not below sulfur condense temperature within internal pipe or similar fluid services in similar piping. The low pressure steam supplied within external pipe transfers heat to internal pipe through its wall thickness. Heat transfer depend wall thickness, corrosion scale deposit on inner and outer surfaces of internal pipe, steam temperature, sulfur temperature and material of pipe. One such jacketed pipeline was failed during service is discussed in this short technical paper. The inner pipe contains molten liquid sulfur and outer pipe contains low pressure steam. The nominal pipe size of inner pipe and outer pipe is 50 mm and 75 mm respectively. Both the pipes were made up of ASTM SA106 Grade B carbon steel with design pressure of 0.9 kg/cm2 and design temperature of 250 deg C. The pipelines were operated at 0.11 kg/cm2 pressure and 160 deg C temperature. Frequent failures were observed on double wall piping system and new pipes were installed. It constitutes major breakdown of units and loss of revenue. The insulation was provided for complete double wall piping network. The failure was confirmed by pressure drop within internal pipe due to molten sulfur clogging resulted in reduction of pipe area for fluid to flow. No external steam leakage was observed. This short review paper discusses the causes, analysis and prevention of failure.

Failure Causes and Analysis

A 2 inch inner pipe weld with 3 inch outer steam jacket pipe carries liquid sulfur from coalescer gas vessel to sulfur locks. The location of failure is observed on cross connection of double wall pipe shown in figure1. A 2 inch inner pipe of 5.54 mm thickness has corrosion allowance of 3 mm. The inner pipe wall thickness reduces and fails due to corrosion. However, it is difficult to determine a failure due to internal or external corrosion. Both the corrosion may have occurred possibly for cross connection failure. The failure pattern is due to poor design subsequently resulted in metallurgical prone corrosion failure.

External Corrosion on Inner Pipe: The low pressure steam condensate traps within external pipe could not able to drain since non availability of separate drain line. The absence of wear plate on inner pipe further erodes eventually.

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