Failure of Double Wall Jacketed Piping in Refineries: Causes and Prevention
S Chidambaram
Officer Inspection and Metallurgy
Inspection Department
Numaligarh Refinery Limited

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.

Internal Corrosion on Inner Pipe: The forged cross connection pipes were not used during erection and instead cross connection fabricated locally from sub-standard pipes by unqualified welders or fabricators was observed initially for fabricating T-joint from records and documents. This may lead to non uniform inner surface of internal pipe particularly at weld joints location conducive for stagnant molten sulfur deposits. This sulfur deposited in stagnant locations further build up deposits and alters the localized electrochemical reactions of internal pipe at those sulfur deposited area locations. This localized electrochemical alterations attack the inner surface wall on or near the weld joint. Further reactive process gas agents like hydrogen sulfide present in liquid sulfur increases deterioration of internal pipe. After internal pipe perforates, the steam from outer pipe enters into internal pipe through perforated area, mixed with molten sulfur. A mixing cools liquid sulfur suddenly and solid sulfur formed subsequently clogged the internal pipe completely. These have been indicated by pressure gauges installed in those piping as action of sudden pressure drop which was noted.

Internal Corrosion of Outer Pipe: No leakage of steam from outer jacketed pipe was reported. Prior history confirms no such leakage observed till now from construction.

Welding Procedure and Non Destructive examination during and after Fabrication

The welding of failed cross connection was fabricated as mentioned below. All the weld joints were fabricated by fillet weld and no other joint design (single v groove) were used.

1. Welding one end flange into inner pipe
2. Guide strip of equal circumference were divided for fillet joint between inner and outer pipe
3. Welding wear plate into inner pipe where steam impinges an external surface of inner pipe
4. Welding remaining section of inner pipe to outer pipe
5. Welding outer pipe to inner pipe on remaining three flange connections
6. Performed liquid penetrant test and hydro test on inner pipe at 3.0 kg per sq.cm
7. Liquid penetrant and hydro test were conducted on outer pipe at 12.0 kg per sq.cm

It was observed that no defects were found from fabrication during initial welding construction. No significant observations were recorded during initial fabrication.

The fillet root pass welded with E70XX series welding rod and no post weld heat treatment was done. The pre heating before welding was maintained at 150 deg C to avoid hydrogen dissolution in welded joints. This may cause hydrogen embrittlement of fillet welding. The hot and subsequent passes were welded by E-60XX series welding rods.















Figure 1: Shows cross connection of jacketed double wall piping in sulfur recovery units and its geometric features

Discussion

The phenomenon of failure is due to inadequate condensate draining of steam on external jacketed pipe and sharp edge of inner core pipe at cross junction. Inadequate draining causes cavitation damage on external surface of inner core pipe where condensate impinges. Also inner cross connection having sharp edges acting as stress raisers and it leads to sudden changes in flow pattern of molten sulfur resulted in localized erosion corrosion. Cumulative effect of aforementioned mechanisms may results in failure of jacketed pipe. The condensate drain line was not provided separately in an external pipe design and it is considered design defect. Therefore both internal and external corrosion contributes metallurgical prone effect on material corrosion in inner pipe due to design deficiency. The failed portion did not confirm to any standard specification. The standard cross connection is shown in figure 1 which has design for smooth laminar molten sulfur flow pattern in internal pipe at cross connections. However, right shown figure were not constructed as per design shown in left figure and pressure drop was found in the right shown cross connection figure which was failed.

Recommendations to avoid Failure
  • An inner bar shall be fixed during fit up fabrication for maintaining proper concentricity between inner core and outer jacketed pipe. It may lead to uniform load distribution on effective area of welded joint between inner and outer pipe.
  • The wear plate shall be installed on inner core pipe. This certainly avoids the inner pipe from erosion since steam impinging on external surface of inner core is severe.
  • The condensate drain lines shall be provided on outer jacketed pipeline as a provision for draining the condensate available in steam. Otherwise condensate trapped within jacketed pipe gradually settles inside shell and corrode further an inner pipe.
  • Before taking shut down of steam jacketed lines, the condensate trapped within shell shall be drained out completely.
  • Proper guide strip shall be provided on inner pipe for uniform steam distribution to avoid process fluid concealing.
  • The venting holes shall be provided on outer pipe to empty out residual water.
  • The cross connection edge corners shall be provided with smooth surfaces such that smooth laminar flow may occur on cross connections. The maximum the smooth minimum the damage on inner core pipe.
  • Hydro test shall be carried out for every 5 years on internal pipe and outer jacketed pipe which assure structural integrity
  • The inner pipe shall be flushed properly to remove stagnant sulphur deposits if any present which further avoids under deposits like active sulphur agents and prevent from under deposit corrosion.
  • Low pressure steam in outer jacketed pipe shall be shut off when the jacketed lines are shut down.
  • Construction of steam jacketed lines shall be conformance international and national code standard specifications
  • Epoxy internal coated inner pipe shall be used for inner core pipe which exhibits corrosion resistance at 150 deg C for molten liquid sulfur.
Conclusions

The cross connection pipe clogged with sulfur deposits were analyzed and it is observed inner core pipe failed at fillet weld joint between inner core pipe and external jacketed pipe. It is concluded as design defect and further recommendations were provided to improve the design of those pipelines. Improved design pipe cross connection were replaced after this failure and author experiences no leakage was obtained even after 5 years. No pressure drop was observed in those piping systems.

References
1. ASME Boiler and Pressure Vessel code, American Society of Mechanical Engineers ASME Sec IX, USA
2. SME Boiler and Pressure Vessel code, American Society of Mechanical Engineers ASME Sec II A, USA
3. ASME Boiler and Pressure Vessel code, American Society of Mechanical Engineers ASME Sec V, USA
4. ASME Code for Pressure Piping, American Society of Mechanical Engineers ASME B31.3, USA