A Rupture in the High Pressure Drain Line for HP Scrubber in Urea Plant

This paper highlights a critical leak incident that occurred in one of the high pressure pipes of the urea plant during an emergency shutdown, and how the situation was analyzed to prevent re-occurrence of similar incidents in urea plants.

Mohamed Mostafa Shams Misr Fertilizers Production Company

Ashraf Mohamed Abdel-Hamid Misr Fertilizers Production Company

Introduction isr Fertilizer Production Company (MOPCO) operates one of the largest urea ammonia plants in Egypt (2 mil-lion tons of urea per year). On 17th

February 2016, an ammonia plant trip took place, and during the draining of the high pressure sys-tem of the urea plant a sudden rupture occurred on the high pressure drain line for the High Pres-sure Scrubber (HP Scrubber) resulting in major leakage of high pressure ammonium carbamate as illustrated in Figures 1 and 2.

Figure 1. Carbamate leak from drain line

Figure 2. Carbamate leak from drain

HP Drain Line Description A drainage network is used to drain liquid from the high pressure (HP) synthesis equipment to the recycle section in the urea plant. This system comprises 7 points at different elevations to en-sure complete drain: 1. Liquid line between HP Carbamate Conden-

ser (HPCC) and Reactor 2. Gas line between HPCC and Reactor 3. Reactor down comer line to HP Stripper 4. HP Scrubber overflow line 5. Downstream Ejector HV222602 to HPCC

M

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6. HP Scrubber heater recycle line 7. HP Stripper liquid outlet

During normal operation, the drain valves from the synthesis high-pressure lines are closed to-gether with the drain valve in the common drain line. The block valve in the urea discharge line of the HP Stripper is open and the HP Stripper level control valve controls the level in the strip-per. The pressure in the drain line is atmospheric, and the bleed in between the drain valves of the high-pressure synthesis lines and the drain valve in the common drain line are open. If one drain valve connected to one high-pressure synthesis line is open or corroded, this shall be detected by the vapor smell coming out of the bleed. During draining of the synthesis section, the bleed and the block valve in the urea solution line leaving the HP Stripper are closed. One by one, the drain valves from the HP synthesis lines are opened. The block valve in the common drain line is opened, and the stripper level control valve con-trols the flow to the recirculation section such that the pressure in that section is under control. In the urea plant, the drain system was kept under pressure during normal operation by a continu-ous injection of condensate from the HP flush water pumps. The reason for keeping the drain system under pressure during normal operation was to avoid leakage of urea or carbamate into the drain/flush system in case of a leaking block valve. The flow of the HP condensate injection pump is about 20 liters per hour (5 gal/hr).

Function of the Common Drain Line in the Urea Plant The HP synthesis section is provided with a drain circuit connecting all HP lines to the outlet of the HP Stripper, as illustrated in Figure 3. This drain line and other parts of the synthesis section are connected to the HP flush water circuit so as to enable them to be flushed. The HP flush water is delivered by the HP flush water pump 329P002A/B. The HP flush water line is pro-vided with a pressure gauge PI329273, required

for a special flush procedure. The injection pump 329P004A/B is connected to the common drain line, in order to keep it free of urea.

Figure3. Illustration of drain circuit (grey pip-

ing) for high pressure lines

The Incident On 17th February 2016 8 years after the urea plant commissioning, a sudden rupture occurred on a 1-inch branch off the common drain line (ac-tual thickness 3 mm, material AISI 310MoLN 1.4466) resulting in ammonium carbamate being relieved to atmosphere as illustrated in Figures 4. Figure 5 shows the ruptured drain line.

Figure 4. Ammonium carbamate being relieved

to atmosphere

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Figure 5. Drain line rupture.

Detailed Specification of the Pipe: Line Number: 50-ACA1-322005-EK19L-D50 Pipe Material: 1.4466 X 2 CRNIMON 25 22 Process Fluid: Ammonium Carbamate Line Size: 1” (OD 33.7 mm* thickness 2.6

mm) Design Temp: 200°C (392°F) Design Press: 167 barg (2420 psi) Insulation design is D50 insulation with steam tracing to maintain a certain temperature require-ment for process reason.

Investigation and Cause of Pipe Rupture The insulation was removed from the ruptured pipe and an investigation was conducted. The cause of the failure is as follows:

1. Visual inspection of the pipe indicated some minor surface defects in the pipe spool and major cracks near the rupture location as illustrated in Figure 5.

2. Thickness measurement for the ruptured spool indicated no reduction of wall thickness.

3. Die penetrant test indicated branched shape surface cracks as shown in Figure 6.

Figure 6. Illustrates branch-shaped surface cracks

4. Laboratory chemical analysis for insula-

tion material indicated high chloride con-tent in the insulation.

Conclusion of Pipeline Rupture

The inspection and laboratory results indicated that the cause of the line breakage was stress-corrosion cracking, which had originated on the outside of the pipe and was associated with the presence of chlorides. The austenitic stainless steel pipe ruptured as a result of stress-corrosion cracking. The presence of chloride ions, com-ing probably from the marine environment or other sources, caused chloride contamination of the pipe.

Repair and Rehabilitation The corroded section of pipe was removed, and a new pipe was welded in as illustrated in Figure 7.

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Figure 7. Pipeline after welding the new mate-

rial.

Chloride Stress Corrosion Cracking (Cl-SCC) Austenitic stainless steels suffer from stress cor-rosion cracking in hot solutions containing chlo-ride. A high chloride concentration is required, although relatively small amounts of chloride are sufficient at heated surfaces, where chloride con-centration can occur, or where chloride is con-centrated by pitting or crevice corrosion. Surface initiated cracks caused by environmental damage of 300 Series SS and some nickel base alloys can be caused under the combined action of tensile stress, temperature and an aqueous chloride envi-ronment. The presence of dissolved oxygen in-creases propensity for cracking. The temperature usually needs to be above 70°C (158°F)

Affected Materials

a) All 300 Series SS are highly susceptible. b) Duplex stainless steels are more resistant. c) Nickel Base alloys are highly resistant, but

not immune.

Critical Factors for Chloride Stress Corrosion Cracking (Cl-SCC)

The critical factors that result in CI-SCC are as follows:

a) Chloride content, pH, temperature, stress, presence of oxygen and alloy composition are critical factors.

b) Increasing temperatures increase the suscep-tibility to cracking.

c) Increasing levels of chloride increase the likelihood of cracking.

d) No practical lower limit for chlorides exists because there is always a potential for chlo-rides to concentrate.

e) Heat transfer conditions significantly in-crease cracking susceptibility because they allow chlorides to concentrate. Alternate ex-posures to wet-dry conditions or steam and water are also conducive to cracking.

f) SCC usually occurs at pH values above 2. At lower pH values, uniform corrosion generally predominates. SCC tendency decreases to-ward the alkaline pH region.

g) Cracking usually occurs at metal tempera-tures above about 140oF (60oC), although ex-ceptions can be found at lower temperatures.

h) Oxygen dissolved in the water normally ac-celerates SCC but it is not clear whether there is an oxygen concentration threshold below which chloride SCC is impossible.

i) Nickel content of the alloy has a major effect on resistance. The greatest susceptibility is at a nickel content of 8% to 12%. Alloys with nickel content above 35% are highly resistant and alloys above 45% are nearly immune.

j) Low-nickel stainless steels, such as the du-plex (ferrite-austenite) stainless steels, have improved resistance over the 300 Series SS but are not immune.

k) External Cl–SCC has also been a problem on insulated surfaces when insulation gets wet.

l) Drains in liquid processing units are suscep-tible to cracking during startup/shutdown if not properly purged.

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Appearance or Morphology of Damage a) Surface breaking cracks can occur from the

process side or externally under insulation (Figures 5, 8).

Figure 8. External cracking of Type 304SS in-

strument tubing under insulation b) The material usually shows no visible signs

of corrosion. c) Characteristic stress corrosion cracks have

many branches and may be visually detecta-ble by an irregular cracked appearance of the surface (Figure 9, 10).

Figure 9. Close-up of the tube showing tight cracks with a spider web appearance.

Figure 10. Close-up showing cracks in MOPCO

HP pressure drain line. d) Metallography of cracked samples typically

shows branched transgranular cracks (Figure 11).

Figure 11. Photomicrograph of a cross-section of a cracked tube illustrating the transgranular

mode of cracking initiating on the surface.

e) Sometimes inter granular cracking of sensi-tized 300 Series SS may also be seen.

f) Welds in 300 Series SS usually contain some ferrite, producing a duplex structure that is usually more resistant to Cl–SCC

g) Corrosion-resistant nickel-based alloys are also susceptible to cracking under severe conditions. Figure 12 shows severe cracking, including branching trans-granular cracks, of a finned Alloy C-276 tube in a reboiler after 8 years in service due to ammonium chloride carryover.

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Figure 12. – Severe cracking of a finned Alloy

C-276 tube in a reboiler after 8 years in service.

Prevention / Mitigation of SCC a) Use resistant materials of construction. b) When hydro testing, use low chloride content

water and dry out thoroughly and quickly. c) Properly applied coatings under insulation. d) Avoid designs that allow stagnant regions

where chlorides can concentrate or deposit. e) A high temperature stress relief of 300 Series

SS after fabrication may reduce residual stresses. However, consideration should be given to the possible effects of sensitization that may occur, increasing susceptibility to polythionic SCC, possible distortion prob-lems and potential reheat cracking.

Inspection and Monitoring of SCC a) Cracking is surface connected and may be de-

tected visually in some cases. b) Penetrant testing or phase analysis eddy cur-

rent techniques are the preferred methods. c) Eddy current inspection methods have also

been used on condenser tubes, as well as pip-ing and pressure vessels.

d) Extremely fine cracks may be difficult to find with die penetrant testing. Special surface preparation methods, including polishing or high-pressure water blast, may be required in some cases, especially in high pressure ser-vices.

e) Ultrasonic testing (UT) is an inspection method which uses high frequency sound

waves, well above range of human hearing, to measure geometric and physical properties in material. Ultrasonic testing may be an-other alternative to find Cl-SCC.

f) Often, radioactive testing is not sufficiently sensitive to detect cracks except in advanced stages where a significant network of cracks has developed.

Lessons Learned 1. Inspecting and monitoring operating condi-

tions of the high pressure lines periodically are critical to ensuring safe operation.

2. Periodically inspect insulation on high pres-sure pipes to ensure it is in good condition and has good sealing.

3. Avoid any source of chloride in urea high pressure lines to avoid stress corrosion crack-ing.

4. Time and money spent in inspection and maintaining the vessel in healthy condition will always be paid back and achieve the goal of preventing accidents and improving safety at the plant.

5. Emergency preparedness and Emergency Re-sponse Team (ERT) should be available. • Hazmat equipment should be found at the

plant for containing small leakage until the unit can be quickly shutdown and de-pressured.

• Emergency equipment should be distrib-uted at all levels in the urea building and inside elevators (SCBA, escape cylin-ders, Full face masks, etc.) to protect em-ployees’ respiratory, eyes, and skin.

• Be fully aware of the escape routes - es-cape lateral / upwind direction

• Training and drills should be regularly performed both for hazardous materials response and rescue system.

References 1. Recommended Practice API 571 Second

Edition, April 2011 2. UreaKnowhow.com

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