UDK 621.772:621.791.052:620.179.1:620.193                                                                                                ISSN 1580-2949
Izvirni znanstveni članek                                                                                                  MATER. TEHNOL. 34(6)433(2000)
I. VITEZ ET AL.: THE CONTROL OF CRACKS IN PRESSURE VESSELS EXPOSED TO AGGRESSIVE MEDIA
THE CONTROL OF CRACKS IN PRESSURE VESSELS EXPOSED TO AGGRESSIVE MEDIA
KONTROLA RAZPOK V POSODAH POD PRITISKOM ZA AGRESIVNE MEDIJE
Ivan Vitez, Ivan Budi}, Slavko Sebastijanovi}
Strojarski fakultet Slavonski Brod, University of Osijek, Trg I. B. Mažurani} 18, HR-35000 Slavonski Brod, Croatia
Prejem rokopisa - received: 2000-07-14; sprejem za objavo - accepted for publication: 2000-11-13
In this article the parameters affecting cracking and the crack types that are detected at various stages of a pressure vessel’s fabrication and in-service use are described. The inspection for cracking with NDT methods, the repair procedure of pressure vessels (PVs) and the mitigation of cracking are also considered.
Key words: pressure vessels, cracks, aggressive media, non-destructive testing (NDT), welds
Opisani so parametri, ki vplivajo na pokanje in razpoke v različnih fazah izdelave posod pod pritiskom. Tudi preiskave pokanja po NDT-metodah, procedura popravil posod in blažitev pokanja so opisani.
Ključne besede: posode pod pritiskom, razpoke, agresivni mediji, neporušne preiskave, varjenje
1 INTRODUCTION
Pressure vessels (PVs) play a very important role in power and processing plants. The regulations relating to PVs include obligatory actions in design, production and exploitation. With welded PVs exposed to aggressive media the cracking that can occurr after long service times is a very serious problem. The growth of cracks during service results in leaks of corrosive media or even the fracture of a PV with very serious consequences. Especially dangerous media include wet H2S and liquid ammonia because they may cause sulphide cracking (SC) and hydrogen blistering in weldments. The first case of wet-H2S cracking of welds in Croatia was discovered in1982 onfour spherical storage tanks inthe oil Sisak refinery. Some of the cracks were through-thickness cracks causing the leakage of propane-butane gas. All four spherical tanks were scrapped. Many cases of cracking due to wet H2S and ammonia have subsequently been reported in a number of countries 1-5.
The mechanism of cracking was investigated in many countries and especially in the USA. In 1996 some important papers on the subject were pubblished by the NACE (National Association of Corrosion Engineers). In addition guidelines for the detection, the repair and mitigation of cracking for existing equipment in a wet H2S environment were printed in Europe and USA. In 1993 the International Institute of Welding prepared recommendations for avoidance of cracking 6-11.
Inspectio reports for a total of almost 5000 weldments in oil refineries operating in a wet-H2S environment were prepared and 1285 vessels were found to be cracked. Similar cracks were also observed in pressure vessels and piping for storage, transportation
and process equipment for aggresive media causing hydrogenor ammonia attack inthe process, pulp and power industries 1.
2 PARAMETERS OF CRACKING
The significant parameters which affect the cracking are:
– the presence of wet H2S,
– the partial pressure of H2S,
– pH value,
– operating temperature,
– H2S concentration. A H2S concentration of 50 ppm or more in the aqueous phase is considered dangerous for the integrity of structures inrefineries. Some authors, however, propose a threshold concentration even the presence of 10 ppm wet-H2S 2.
Results of investigations in oil- and gas-production environments have shown that 0,35 kPa and greater partial pressures of H2S inthe presence of free water may cause SCC cracks insusceptible steels 2.
The rate of the hydrogenpermeationflux insteels has been found to be the lowest in neutral solutions (pH 7) with increased rates observed at both lower and higher pH values 2.
The highest incidence of cracking occurred in the operating range 65 to 95 °C, however cracking was found to occurr for all temperatures in the range 38 °C to 150 °C 2. In general the incidence of cracking increases with the increasing concentration of H2S inwater, nevertheless, 17% cracking was detected in weldments
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I. VITEZ ET AL.: THE CONTROL OF CRACKS IN PRESSURE VESSELS EXPOSED TO AGGRESSIVE MEDIA
for environments with less than 50 ppm H2S insolution inthe aqueous phase 2.
3 CRACK TYPES AT VARIOUS STAGES OF MANUFACTURING AND SERVICE
Four types of hydrogendamage canoccur: – sulfide stress cracking (SSC), which is a form of
stress corrosioncracking (SCC - figure 1); – hydrogen-induced cracking (HIC - figure 2); – stress-oriented hydrogen-induced cracking (SOHIC); – hydrogenblistering (figure 3).
All four types of cracking require the presence of nascent hydrogen atoms on the steel surface, these are usually produced through a corrosionreactioninH2S aqueous solution, since only atomic hydrogen can diffuse into the steel1.
Sulfide stress cracking (SSC) may cause rapid crack growth and a catastrophic vessel failure. It is defined as metal cracking under the combined action of tensile stress and corrosion in the presence of water and H2S. SSC usually occurs insteels over 600 MPa tensile strength or in hard areas of steel weldments. In the NACE standard MR 0175 for ferritic material the hardness maximum of 22 HRC (˜238 HB) is considered to be resistent to the occurence of SCC.
With hydrogen-induced cracking (HIC) internal cracks propagate stepwise and connect adjacent hydrogenblisters or isolated cracks inthe metal. HIC is generally found in steels with high impurity level and/or in regions with anomalous microstructure due to the segregation of impurities and alloying elements in the steel.
Stress-oriented hydrogen-induced cracking (SOHIC) is generally observed in the base metal adjacent to the heat-affected zone (HAZ) of a weld
Figure 1: SSC cracking in the heat-affected zone (HAZ) Slika 1: SSC-pokanje v toplotni zoni zvarov
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Figure 2: Hydrogen-induced cracking HIC-SWC (stepwise) Slika 2: Stopničasto inducirano vodikovo pokanje HIC-SWC
oriented in the through-trickness direction. Tensile stress or residual stress are required to produce SOHIC.
Hydrogen blistering is the formationof subsurface planar cavities ina metal. It results from anexcessive hydrogenpressure. Typical sites for the formationof hydrogen blisters are large nonmetalic inclusions, laminations or other discontinuities in the steel (more frequently found in lower strength carbon steels).
4 INSPECTION OF CRACKING
The required standard for a PV is obtained by a quality asssurance system. Different methods are used in practice to verify the reliability, e.g. stress-strength, fault analysis, failure rate etc. The cracks found during in-service inspection may originate at various stages of the PV’s manufacturing and use. The best reliability is obtained from the fault tree analysis-crack detected in service with five phases of cracking 1.
New cracks may initiate, propagate, and become detectable and non-destructive testing (NDT) should be performed after each phase of manufacturing. The following types of defects have been identified 7: – manufacturing cold cracks in welds, – pre-service hydro-test cracks inwelds,
Figure 3: Hydrogenblister Slika 3: Vodikov mehurček
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I. VITEZ ET AL.: THE CONTROL OF CRACKS IN PRESSURE VESSELS EXPOSED TO AGGRESSIVE MEDIA
Figure 4: General location and site of cracks: a - HAZ longitudinal cracks inwelds, b - WM longitudinal cracks in multipass welds, c -HAZ transversal cracks, d - WM transversal cracks, e - n ew cracks after hydrostatic testing
Slika 4: Lega inlokacija razpok: a - vzdolžne razpoke v toplotni zoni zvarov, b - vzdolžn e razpoke v večvarkovn ih zvarih, c - prečn e razpoke v toplotni zoni, d - prečne razpoke v zvaru, e - n ove razpoke po hidrostatskem preizkusu
– in-service low-temperature hydrogen damage (SCC,
HIC, SOHIC, blistering), – in-service hydro-test cracks in welds, – in-service weld-repair cracks. The locationand orientationof cracks detected inthe HAZ and welded material (WM) are shown in figure 4. The   majority   of   all   detected   cracks   were inner-surface cracks exposed to corrosion media. The number of revealed cracks varies from less than 10 to several hundreds 7. NDT methods have a major role in cracking control and are aimed at 9-14:
1.   the detecting of cracks during preventive cracking control;
2.   to confirm the completion of a welding repair and detecting possible new cracks caused by repair welding;
3.   to detect defects initiated or propagated during hydrostatic testing. Most of the detected cracks were small inner-surface
cracks which are difficult to reveal. It was recognised that radiography, ultrasonic, visual and dye-penetrant examinations were not sufficiently sensitive to small surface cracks and that the magnetic particle method should be used.
The inspection of weldments is mandatory. It includes circumferential, longitudinal, nozzle and internal attachment welds. Inspection areas should include repair or vessel-alteration welds and areas of the vessels that exhibit visible blistering, significant corrosion, or other defects. The surface of the weld and the adjacent area base metal to a distance of about 150 mm on both sides should be cleaned of all scale and residue.
Visual examination (VT) must be performed in suitable conditions, however, fine cracks may remain invisible.
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Magnetic particle examination (MT) is the best method for the detectionof fine surface cracks. This method is the most sensitive and it reveals many cracks that are not detected by other methods with a lower defect sensitivity. AC yoke WMFT is preferred over DC or prod methods. DC methods are not as sensitive and prod methods may leave arc strikes that, if not ground out, canserve as crack initiators.
Dye-penetrant examination (PT) is used only occasionally, is less sensitive than MT, it is however, easier to perform.
Ultrasonic examination (UT) with straight and angle-beam probes is the main method for the detection of subsurface and deeper surface defects of sufficient size. UT shear waves, longitudinal waves, and crack-tip diffraction ultrasonic inspections are used. UT may also be used for the examination of a vessel under pressure without discharging of the liquid gas from the vessel.
Radiography (RT) is used incases of volume defects revealed by ultrasonics as an additional method for the characterisationof the defects. Incases of weld repair it is always recommended to use RT.
Acoustic emission (AET) monitoring during some vessel pressure tests is recommended for the detection of possible crack propagation. AE will detect any crack propagation at a sufficient stress level. Existing cracks, which are stable, will not be detected.
Eddy-current testing (ET) is used for crack detection and depth measurement.
Sequence and volume of NDT. Magnetic and ultrasonic examinatioust, are used for a 100% testing of welds for the life-cycle phases whencracks may initiate or propagate. The use of other NDT methods depends on the type of defects expected and other influencing factors such as the availability of instrumentation, location and positionof defects.
Besides NDT methods for crack control other methods are used for inspection and control: – metalographic and SEM examinations; – hardness   measurement  (SCC/SSC  is  generally
avoided for a hardness below 250 HV10); – Ch V and FM parameters (CTOD, JIc, KIscc); – Measurement of dimensional and shape deviations.
The use of plates of “cleansteel” is recommended to improve the resistance to blistering, HIC, and SOHIC. The contents of sulphur and phosphorus are limited up to 0.002% and up to 0.01%, respectively. A maximum carbon equivalent value of CE = 0,43 is recommended for carbon-steel pressure-retaining components for ensuring an acceptable microstructure, HAZ hardness, and tensile strength of the base material.
In the absence of surface corrosion, hydrogen cracking (SSC, HIC, SOHIC) or blistering cannot occur. Various practices are recommended for manufacturing new pressure vessels and for the operating conditions of existing units, such as:
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Liners separating the carbon-steel wall from the process environment to prevent corrosion, (e.g. alloy weld overlay, alloy integral lining, alloy strip lining, thermal spray processes).
Several methods of mitigationinoperationmay be employed, for example:
– water washing,
– polisulfide injection,
– corrosion inhibitor injection. The assessment of the degradation of the material and of the cracking revealed by in-service inspection requires a careful analysis, and the consideration of specific details for each vessel and application are required. The generic procedure for failure analysis and reliability/remaining-life assessment should cover:
1.   the collection and review of manufacturing and construction records;
2.   the review of ISI records;
3.   the review the operating history;
4.   the stress analysis and fracture mechanics parameters evaluation;
5.   the estimation of the tensile properties and of the fracture toughness for the material (HAZ, WM, BM) at the en d of the next operating period (EONOP);
6.   the evaluationof the vessel reliability;
7.   the establishment of actions to be taken;
8.   the establishment of the plan for the next ISI.
5 REPAIR PROCEDURE FOR THE PRESSURE VESSELS
The following steps are used for the repair procedure of PVs:
– defect removal and repair by grinding only. If the depth of cracking is less than the corrosion allowance, the careful removal of the crack and the blending of the cavity with the surrounding is recommended;
– repair by welding. Incase of deep cracks arc-air gouging and grinding are used for the preparation of the weld groove. Manual arc-air welding is mostly used for repair welding, (also TIG and SAW are used inspecial cases);
– welding qualifications and welding variables have to be strictly implemented and inspected;
– weld reinforcements should be flush ground, at least for mainseams, to improve the detectability of weld defects by NDT, to remove stress raisers and to relieve residual tensile stresses, that are higher in the last layer or the last pass;
– new weld cracks have oftenbeenrevealed by NDT after HT test and then the complete sequence of NDT and weld crack repair was to be repeated.
6 CONCLUSIONS
The cracking of pressure vessels that operate in aggresive media was a serious problem over the last 35 years in many countries. Wet-H2S and liquid-ammonia cracking of exposed equipment are especially dangerous in service. Sulphide cracking (SCC) and hydrogen blistering occur in the presence of wet-H2S in weldments, which are not heat treated.
The basic parameters affecting the cracking and four types of crack that occur at various stages of manufacturing and service are described. NDT inspectionplays a very important role inthe detectionof cracks. The monitoring and control of aggressive constituents in the vessel medium and the vessel purging during service contribute to prevention of SCC and to the reliable pressure-vessel operation.
7 REFERENCES
1 I. Vitez, I. Budi}, L. Magli}: The Preventionof the Cracks inwelded Vessels exposed to aggresive Media at Exploitation, 2. International Conference on Revitalization and Modernization of Production, Tehnički fakultet Biha}, 1999, 627-632
2 Z. Lukačevi}, I. Vitez, L. Magli}: Metalurgija 38, 4, 1999, 251-256
3 S. Sebastijanovi}: The Degree of Safety and Reliability of Pressure Vessels, CIM’97, Opatija, 1997, C81-C90
4 S. Sebastijanovi}, M. Vujči}, N. Sebastijanovi}, Designed and Remaining Useful Life of Appliances in process Reactors. MARCON’99, vol.2, Ten., USA 1999, 71.01-72.09
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6 NACE Standard RP 0296-96: Guidelines for detection, repair, and Mitigation of Cracking of Existing Petroleum Refinery Pressure Vessels inwet H2S Environments, NACE International, 1996
7 G. Gnirss, I. Hrivnak, Z. Lukačevi}: IIW DOC. XI-609-93 and IIW DOC. IX-1711-93 Recommendations for Reliable Operation of Pressure Vessels Exposed to Aggressive Media like Ammonia and Sour Gas.
8 S. Yukawa: Guidelines for Pressure Vessel Safety Assessment. NIST Special Publication780, Boulder, Colorado, 1990.
9 ASME B&PV Code, Section XI. Rules for Inservice Inspection of Nuclear Power Plants Components, 1989.
10 I. Vitez, S. Sebastijanovi}, P. Baličevi}: The Regulations for the Assurance of the Reliability of Pressure Vessels; Materiali in tehnologije 34, (2000) 5, 223-227.
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12 S. E. Mahmoud et. al.: Overview of Hydrogen-Induced Cracking (HIC), The NACE Annual Conference 1991, Cinncinati, Ohio, 1991.
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