Urea Reactor Explosion Accident Stress Corrosion Analysis

Zhonghe Chen1,a, Weiqiang Wang2,b, Mingda Song2,c ,Ningning Dong1,a

1School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China;

2School of Mech. Eng., Shandong University, Jinan 250061, China

awischen@163.com,

bwqwang@sdu.edu.cn,

csmdaazyf@163.com

Key words: urea reactor; explosion; fracture analysis; stress corrosion

Abstract: In order to find out the accident urea reactor explosion reasons, based on the accident

investigation, the reactor structure was anatomyed, the macroscopic and microscopic fractures were

sampled and observed, The results show that the trace sodium, potassium and other alkaline ions

content in the liquid of liquefied steam is the stress corrosion medium reason; the leak detection pine

and the 16MnR laminate connecting with screw joint is the structural reasons; typical stress corrosion

had occurred on the accident urea reactor cylindrical segment base metal and welding. Some

important suggestions were given.

Introduction

The urea reactor explosion accident occurred on March 21, 2005 in Shandong Pingyin Chemical

Fertilizer Plant, caused 4 persons to die, 32 persons of severely wounded, the direct economic losses

nearly 30 million Yuan[1]. This accident is extremely similar with the Hebei QianAn Chemical

Fertilizer Plant urea reactor explosion accident 15 years ago. Therefore, we try to carry on the analysis

and research to the accident,tofind out the explosion reasons, operating process, urea reactor structure

and other the correlation questions, will provide the model for the next similar accident prevention.

Urea reactor and accident survey

The accident urea reactor was made in December, 1999, put into the use in March, 2000.The service

examination carried in 2002, the security condition rank decided as 2 levels. This reactor design

pressure is 21.57MPa, the design temperature is 195℃, operating temperature is 188±2℃, actuating

medium is urea solution and ammonium carbamate [1,2]

Multilayer dressing for the reactor structure is made of 10 cylindrical segment and the upper and

lower head components. Multilayer structure is the cylinder, lining layer, blind lamellar, and

11-bandaging laminates. The lining material is the 8mm 316L stainless steel, the blind lamellar

material is 6mm Q235A carbon steel, the inner cylinder material is 12 mm 16MnR low-alloy steel

plate, the bandaging laminates material is 6 to 8 mm 15 MnVR, a total of 11 layers, the total wall

thickness is 110mm (refer with: Fig.1).The reactor length 26.2m, diameter 1400 mm, with a total

weight of 115,210 kg, as class III reaction vessel [1].

The urea reactor suddenly exploded without abnormal signs. The reactor body was exploded into

three sections, the first section is the lower head; the second section is the 9th cylindrical segment,

longitudinal rupture occurred , and torn along the 9th cylindrical segment link welding, infiltrated to

the south side main workshop; the third section is the upper head and the 1st to 8th cylindrical

segments, flew away the explosion area more than 90m (refer with: Fig.1-Fig.2).

Investigations shows that the accident occurred under the urea plant functioning properly and

found no abnormal pressure, temperature, media and other records[1].

Advanced Materials Research Vols. 239-242 (2011) pp 156-160Online available since 2011/May/12 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.239-242.156

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Fig.5 cracking cases

16MnR laminate link

welding

Fracture analysis

Macroscopic fractures analyses

All the fractures between the 9th and 10th cylindrical segment break along the link welding upper

side, also all occured above the link welding base metal area. The entire cross section is quite even

(refer with: Fig.1), many places have longitudinal cracks. The yawn bigger longitudinal crack passed

through the link welding, about 150mm long, the side has the obvious ballooning. The crack beneath

the same thermocouple takeover had a breakthrough before the bursting. This position Q235-A,

16MnR and the inside several 15MnVR steel plate all are the entire even fracture[3,4,5]

(refer with:

Fig.3). Full-face from outside can see the shear direction shear lips, large shear fracture southwest

direction, the radial (outward) shear cracking form.

The 9th cylindrical segment longitudinal fracture surface passed through the thermocouple

takeover hole, in addition to lining, all laminates fracture near the thermocouple takeover hole and

below were brittle fracture [3,5]., and the total cross-section is about 750mm high, and the main fault

near surface crack also can see a large number of vertical cracks cracking obviously, these vertical

cracks in existence before blasting. The main longitudinal fracture surface for the first half of the

ramps fracture toughness, fracture near every laminate did not see other cracks (refer with: Fig.4).

After bursting, the cylindrical segment along the wall outside the aforementioned profile stack

together, the laminates thickness there was no significant change. Around the leak detection hole, the

neighbor number layers plate have the obvious radiated dehiscence cracks [3] (refer with: Fig.5).

The fracture between the 8th and 9th cylindrical segment near the inside lining almost all to be

located on the link welding upper side , only a small area is situated at the lower side, while section

near the outer side is mainly at the lower side. Internal evener fracture is situated at laminates to the

southwest, nearly 10 layers, and from southwest to northeast gradually reduced, at least there are two

layers. The northeast outer fracture section is located at the weld lower side, in addition to the three

inner lining layers broken on the upper side of the link welds. After observation and survey, the cross

section and the nearby reactor diameter and various layers thickness has had no remarkable change.

The tray of 1st~8th and 10th cylindrical segment support welds on the inside lining mostly has

the distortion, the distortion direction all aims at the fracture to outside. Apart from the top cylindrical

segment tray stay in the in-situ, the others flew outside. Inspection on inner lining surface, we

discovered the welding and the head pile welded layer is the bright white color, the base metal surface

is brown or the ash black, the inner lining surface has no cracks. Except that nearby the 9th cylindrical

segment thermocouple hole partial and 8th, 9th cylindrical segment link welding, few macroscopic

even fractures, the other lining fracture all were the shear fracture. [4,5]

Sampling upward from 8th cylindrical segment bursting fracture, we found that there are obvious

laminates longitudinal crack, we discovered that similar serious longitudinal crack exists on the

laminates base metal, the crack penetrated 4 layers, the crack length up to 450 mm, the actual crack

continues from the sample place to the link welding direction to extend. At the same time, in 7th and

Fig.1 10th cylindrical

segment and its fracture

Fig.2 9th cylindrical

segment morphology

Fig.4 morphology of

tear into 8th cylindrical

segment

Fig.3 the radial cracks

around the leak detection

hole

Advanced Materials Research Vols. 239-242 157

Fig.6 the link welding

cracking case of 7th and

8th the leak detection hole

8th cylindrical segment, the link welding neighboring the sampling leak detection hole, also

discovered the parallel weld crack, cracks expanded from welding line to welding toe, including

Q235-A, 16MnR and 15MnVR four layers of plate[5,6] (refer with: Fig. 6).

Based on the above analysis and macro fracture anatomy test results, we found no obvious macro

manufacturing defects presence in the welding, there were serious longitudinal and the crosswise

cracks before the explosion, in 9th cylindrical segment near the thermocouple hole there were initial

longitudinal cracks long about 900 mm, penetrating all laminates except the inside lining,

longitudinal cracks in the cylindrical segment is the most serious.

Micro-fracture analysis

We carried analysis on the fracture and the crack microscopic fracture to the 8th~10th cylindrical

segment. Fig.7 to 9 is the 8th and 9th cylindrical segment sample fracture morphology. They are

obvious signs of corrosion and along crystal fracture morphology of these sites, only in the vicinity of

the crack or fracture near the obvious torn areas to see cleavage or dimples fracture, X-ray

spectroscopy analysis showed that corrosion products containing S, Si, K, Ca, Al and other elements [3].

The observation analysis on the burst mouth fracture shows that the fracture morphology is a

typical stress corrosion cracking crack. We carried optical microscopy and scanning electron

microscopy analysis on the sites of link welding area between the two neighboring leak detection

holes 7th and 8th cylindrical segment, the link welding of 8th cylindrical segment and the fracture and

crack on the base metal. Fig.10 to12 shows that the same crystal crack and bifurcation phenomenon

appeared on Q235-A, 16MnR, 15MnVR materials and their welding. Apart from the Q235-A material

close to lining material, in the fracture along its weld fusion line containing Cr, Ni elements, other

materials inside the corrosion cracks forefront contains Na, K, Cl, P, S, Si, Ca, Mg, Al and other

elements, analysis with the X-ray spectrum. These fractures and the crack profile all have the crystal

dehiscence and furcation's characteristic, simultaneously in the crack has the corrosion product, is the

stress corrosion crack shape characteristic. Fig.13 is the analysis result of 7th and 8th cylindrical

segment link welding profile between the neighboring leak detection holes, it is the typical

morphology of stress corrosion cracking .

Fig.9 torn area and

crack morphology of

15MnVR

Fig.8 fracture

morphology of 16MnR

welding

Fig.7 fracture

morphology of 9th

thermocouple holes

Fig.10 fracture

morphology of 7th

~8th cylindrical

segment Q235-A

158 Advanced Materials, CEAM 2011

Analysis through the blast mouth fracture and the crack profile or the cross section far away the

burst mouth, had proven the urea reactor has already had the massive stress corrosion crack before the

explosion. The link stress corrosion dehiscence crack concentrates to the link welding on leans, but

the longitudinal stress corrosion dehiscence crack concentrates in the 9th cylindrical segment

thermocouple hole below. The stress corrosion dehiscence has caused the reactor body load bearing

cross-sectional area serious insufficiency or in the dangerous section place stress corrosion crack

approaches or surpass the material allowance size, and could easily have destroyed or exploded.

The stress corrosion dehiscence reason

The low stress brittle fracture which caused by the tensile stress and the corrosive medium joint action

is called the stress corrosion. The stress corrosion only under the following specific conditions could

occur: part withstanding tension stress role; has specific corrosive medium environment which

matches with the material category; the material contains impurities or alloy metal.

The stress corrosion medium condition. According to the structural characteristics of urea

reactor, we mainly carry on the analysis from three aspects: First, the stress corrosion crack and the

laminates crevice corrosion products; Second, vent hole exudation; Third, leak detection steam.

Stress corrosion crack and corrosion products in the laminates crevice. We takes samples this

place after wire cutting carries on the electronic exploration needle analysis. Fig.14 is one of the stress

corrosion cracking crevice electron microprobe analysis. The results shown in Tab.1.

Tab.1 analysis results of corrosion products in laminates crevice and stress corrosion crack slit

Analysis Spot Laminates crevice Crack slit

No. 1 2 Average1 2 3 4 5 Average

Na element content %

6.97 16.11 11.54 0 0.74 1.72 3.99 5.07 2.30

K element content % 4.90 17.95 11.43 1.30 0 0 2.59 0 0.78

Urea reactor body vent hole exudation. Accident investigation found that there was a lot of

exudation in the reactor body vent hole (refer with: Fig.15). We sampled and analyzed the 10th

cylindrical segment vent hole exudation. Analysis shows that, in the exudation Na+ content is 22.1%,

K+ content is 0.12%, Na

+ content is high. Laminates crevice exudation primarily from the

accumulation of leak detection steam, the liquid which discharges from the laminates crevice has the

very high caustic concentration, meets the stress corrosion medium requirement [3]

Leak detection steam

In order to analyze the basic ion ingredient content in the leak detection steam, we carried on the

sample analysis separately on the accident sealed desalting water after the explosion, boiler water, the

steam distribution of the former buffer cans condensed water as well as other 5 urea reactors leak

detection fluid. The analysis results shows that the accident sealed desalting water containing Na+ and

K+, processed by the reverse osmosis water treatment, the final desalting water pH value adjust by

sodium carbonate, therefore Na+ in the steam is understandable.

The leak detection fluid analysis shows that the leak detection solution and the concentration of

ions is very complex, some reactor hazardous ion concentration is still quite high. The factories are

difficult to ensure that the steam cleaner under current technical conditions, which eventually led to

stress corrosion cracking.

Advanced Materials Research Vols. 239-242 159

The ammonia leak testing steam saturation pressure is 1MPa, the temperature is 182°C.

Considering the wall radiation, the water between laminates exists at the liquid state. To each

cylindrical segment the condensation liquid is above link welding upper side; to the entire body, the

condensation water will stress to the lower cylindrical segment.

The above detection and analysis shows that, the stress corrosion cracking cracks and cracks in the

laminates contain higher Na+ , K

+ and other ions, especially in the vent hole exudation Na

+ content is

astonishing high, which meets the stress corrosion medium requirement. At the same time, the final

desalting water pH value adjusted by sodium carbonate, a trace amount of Na+ and K

+ in the steam

enriched in the laminates gap is very shocking[3,6].

The structural conditions caused stress corrosion

Traditional steam leak detection hole structure was designed by hand welding methods to prevent the

leak detection steam leaking to laminates [2].In order to overcome inter-deep hole and laminates

difficult to detect and weld, the screw joint connection way between leak detection pipe and 16MnR

laminates were used, but it is difficult to guarantee seal reliability under the vibration loads and wind

loads， this makes ammonia leakage detection media and the detection steam easily into the

laminates, the appearance of vent hole exudation had proven the detection steam divulges to various

laminates [6].

Conclusion.

Macro-fracture and micro-fracture sampling analysis showed that the explosion reactor existed a lot

of stress corrosion cracks before explosion, and a lot of stress corrosion cracks in base metal zone.

Longitudinal stress corrosion cracking crack beneath the thermocouple hole has exceeded the

capacity limits of size, is a possible explosion fracture origin.

The ingredient analysis on the corrosion product, vent hole exudation, the leak detection fluid and

desalting water indicate that the stress corrosion medium reason is the trace sodium, potassium and

other alkaline ions content in the liquid of liquefied steam. Such enrichment is mutual anastomosis

with the scene of the explosion. The best solution is to abolish the steam leak detection.

The leak detection pine and the 16MnR laminate connecting with screw joint is difficult to

guarantee sealing reliability under the vibration loads and wind loads, which have made a lot of stress

corrosion cracking on laminates, resulting laminate materials serious stress corrosion cracking, this is

the structural reasons of the explosion. The problem of connecting and sealing between leak detection

pine and laminates is still a problem need to study.

References

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Fertilizer Plant urea reactor explosion accident investigation report .Jinan,2005. (In Chinese)

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[4] Liu Su-e, Zhu ZiYong, Ke Wei. Stress corrosion cracking of a low alloy steel in hot caustic

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160 Advanced Materials, CEAM 2011

Advanced Materials, CEAM 2011 10.4028/www.scientific.net/AMR.239-242 Urea Reactor Explosion Accident Stress Corrosion Analysis 10.4028/www.scientific.net/AMR.239-242.156

DOI References

[2] L. Vandebroek, F. Verplaetsen, J. Berghmans. Auto-ignition hazard of mixtures of ammonia, hydrogen,

methane and air in a urea plant. Journal of Hazardous Materials, 2002, 93(1): 123-136.

http://dx.doi.org/10.1016/S0304-3894(02)00043-2 [4] Liu Su-e, Zhu ZiYong, Ke Wei. Stress corrosion cracking of a low alloy steel in hot caustic aluminate

solutions. Scripta Metallurgica et Materialia, 1994, 31(4): 427-432.

http://dx.doi.org/10.1016/0956-716X(94)90013-2 [5] Novotny Radek, Sevini Filippo, Luigi Debarberis. Testing environmentally assisted cracking of reactor

materials using pneumatic servo-controlled fracture mechanics device. International Journal of Pressure

Vessels and Piping, 2006, 83(10): 701-706.

http://dx.doi.org/10.1016/j.ijpvp.2006.08.001