Martin RICHEZ. Franck ZANONCELLI

Transcription

1 DILLINGER PRESSURE VESSEL COLLOQUIUM Dillingen 16/17 th september 2009 TOTAL Requirements for equipment in severe wet H2S service. Examples of equipment made from Dillinger steel plates Martin RICHEZ Franck ZANONCELLI

2 Requirement for vessels in severe wet H2S service 2/7 TOTAL Requirements for equipment in severe wet H2S service. Examples of equipment made from Dillinger steel plates 1. Degradation of steel in wet H2S service H2S is present in many refinery process streams. Wet H2S service is referred in a process stream where both water in liquid phase and H2S are present. Carbon steel is commonly use in this environment, but it can be subject to degradation depending upon ph, H2S level, temperature, duration of exposure and nature of the material. Thickness loss is usually not the main problem, but hydrogen is the major concern. NACE and EFC have extensively studied this subject leading to different recommendations. Hydrogen is involved in most corrosion mechanism, but H2S presents the specificity to lead to high atomic hydrogen penetration in steel. This hydrogen tends to embritle the metal or to recombine to form Hydrogen molecule in the steel inclusions. Hydrogen pressure can get very high leading to high stresses. There are two main types of degradation: HIC (Hydrogen induced cracking). This is planar cracking that occurs in carbon and low alloy steels when atomic hydrogen diffuses into the steel and then combines to form hydrogen at trap sites. The driving force is the hydrogen pressure, helped by hydrogen embrittlement. No applied stresses are required for cracks to appear. Susceptibility of steels to HIC is strongly dependent upon cleanliness of the steel, and the size and shape of inclusions. These cracks are oriented in the plan of the steel plate. They can be clearly identified when they form bulges at the plate surface. SSC (Sulphide stress cracking). This is a form of hydrogen cracking due to the interaction of hydrogen and stresses (residual or applied). Cracks are perpendicular to the stresses. Hardness is known to increase susceptibility of the steel to SSC. NACE document MR deals with this subject. Other degradations can appear which are combinations of HIC and SSC: SWC (Stepwise Cracking). This term is used when closed hydrogen induced cracks (HIC), developed at different depths in the steel are connected by small perpendicular cracks. Theses cracks are due to hydrogen embrittlement and stresses induced by the bulging. Applied stresses can also play a role. SOHIC (Stress Oriented Induced Cracking). This term is used when staggered small cracks formed approximately perpendicular to the principal stress (residual or applied), result in a ladder like crack array linking some preexisting HIC cracks.

3 Requirement for vessels in severe wet H2S service 3/7 2. TOTAL practice for steel in severe wet H2S service 2.1 Severe wet H2S service Severity of Wet H2S service depends on the H2S content and ph of water in contact with the metal. During unit engineering, stream composition is provided by the process department. This data is the basis for determining the service severity. However this data may not be fully available and unusual or transient operations are often not considered. So any choice in H2S severity service has to be confirmed by a specialist during the material selection review to take into account these cases and company feed-back. ph H2S ppm 10<H2S<50 50=H2S= <H2S=10000 H2S>10000 ph<5 Low High High High 5=pH=7.8 Low Moderate* Moderate* High ph>7.8 Low* Moderate* High High * High severity if CN- (cyanide) > 20ppm When exposure to severe wet H2S service does not happen during normal operation but happens during upset or transient operation, it is considered that only SSC may occur and the service is considered as moderate. 2.2 Specification for plates Carbon steel is commonly used in low, moderate and severe wet H2S service, with a 3 or 6 mm corrosion allowance. In this case, steel components (plates, forged parts) and welding are subject to stringent specifications. For specific cases, when the environment is judged to be very corrosive, an internal liner made of a corrosion resistant alloy may also be used (cladded plates or weldoverlay). In this case, there is no specific requirement for the carbon steel base metal due to wet H2S service. The objective of these specifications is to: reduce the hardness, by limiting residuals elements leading to high hardness phase formation, and limiting the steel tensile stress, reduce susceptibility to HIC by limiting sulfur and by acting on the inclusion shape, limiting crack propagation by a low phosphorus and segregating element content. All carbon steel materials shall be delivered fully killed and normalized. The steel plates shall be fabricated from unalloyed carbon steels produced by an electric arc furnace or oxygen furnace process. The steel shall be vacuum degassed, deoxidized and produced to a fine grain structure, with a process leading low sulphur and phosphorus content. Steel shall be produced with thorough desulphurization, dephosphorization and deoxidizing treatments. Calcium treatment shall be applied for inclusion shape control, except that it need not be applied to plate with very low sulphur levels (below %). The calcium content shall not exceed 3 times the sulphur content.

4 Requirement for vessels in severe wet H2S service 4/7 The supplier shall exercise special care in the selection and supervision of the conditions of fabrication and normalizing heat treatment, in order to eliminate heterogeneous structures, such as pearlite structures in very pronounced bands and aggregates of bainite and unannealed martensite. Chemical analysis shall be in accordance with material norm/specification requirements. Materials with a minimum specified tensile strength value greater than 415 MPa are not allowed (according to the material norm or specification). Hardness shall be lower than 200 HB. The chemical composition of carbon steel plates (heat analysis) shall be restricted as followed, except where the material norm/specification is more restrictive: Single Elements Maximum wt. % Carbon (C) 0.18 Manganese (Mn) 1.20 Phosphorous (P) Sulphur (S) Silicon (Si) 0.40 Copper (Cu) 0.20 Nickel (Ni) 0.40 Chromium (Cr) 0.30 Molybdenum (Mo) 0.12 Vanadium (V) Niobium (Nb) Titanium (Ti) 0.02 Boron (B) Multiple Elements Cr + Mo 0.30 Ni + Cu + Cr + Mo 0.70 Nb + V + Titanium (Ti) 0.12 V + Nb 0.03 Carbon Equivalent (Note 1) 0.38

5 Requirement for vessels in severe wet H2S service 5/7 Note 1: Carbon Equivalent (CE) shall be calculated using the following formula: Plates shall be subjected to an ultrasonic lamination check in accordance with EN Class S2 or ASTM A 578, level C, supplementary requirement S Forged components and piping The chemical composition (product analysis) shall be restricted as follows, except where the material norm/specification is more restrictive: Single Elements Maximum wt. % Carbon (C) 0.21 / 0.20 (note 2) Manganese (Mn) 1.05 (note 2) Phosphorous (P) (note 3) Sulphur (S) / (note 2 note 3) Multiple elements Maximum wt. % Vanadium (V) + Niobium (Nb) 0.20 Nb + V + Titanium (Ti) 0.12 Carbon Equivalent (Note 1) 0.43 / 0.40 (Note 2 Note 3) Nickel (Ni) 0.40 Note 1: Carbon Equivalent (CE) shall be calculated using the following formula: Note 2: the higher limits are for forgings the lower ones for pipes. Note 3: for forgings: Mn = 1.35 % max provided that P %, and S % 2.4 Specification for manufacturing All pressure vessels in wet H2S Service shall be Post Weld Heat Treated in accordance with the code requirements. 1 hour per 25 mm of thickness (minimum holding time of 1 hour) and 600 C shall be applied as a minimum Weld hardness testing of production welds shall be carried out in accordance with the NACE International Publication 8X194 with a maximum value of 200 HB (hardness of 210 HV10 max.). In heat affected zone (HAZ), higher values not exceeding 248 HV 10 are acceptable. All accessible inside surfaces of welds of pressure retaining parts shall be 100 % inspected by Wet Magnetic Fluorescent / Particle Test (WFMT) after PWHT. For thickness > 40mm, a 100% radiography and ultrasonic examination is required before and after PWHT. 2.5 Hydrogen induced cracking test. The HIC test and crack evaluation shall be in accordance with NACE Standard TM0284. The test is performed at ph = 3. The test shall be performed per heat on the thickest plate in a simulated PWHT condition.

6 Requirement for vessels in severe wet H2S service 6/7 The following acceptance criteria shall be met: % (maximum) CLR CTR CSR Average Single Note: Average value. Arithmetical mean of the values obtained on the total number of studied sections. Single value. Arithmetical mean of the values obtained on the 3 studied sections for each specimen. The maximum individual crack length on any section shall not exceed 5 mm. 3. Refining unit and area of concern Many refining units are concerned by wet H2S service. H2S rarely comes from the crude itself, most of it is provided by thermal degradation of the feed, catalytic cracking or hydrodesulfurization. Since H2S is not usually present in the crude, H2S content is limited in crude distillation unit. However some H2S is added from thermal cracking or cuts from other units (HDS, Coker ) that are introduced into the feed to be reprocessed. Distillation overhead is usually considered as a moderate wet H2S service. Some equipment, dedicated to off gas, may be classified as severe. Hydrotreatment of light cuts like gasoline often lead to a limited amount of H2S. Low temperature areas of the unit are often classified as moderate wet H2S service. Hydrodesulfuration units (that operate from 50 to 100 bars) or Hydrocracker units contain a lot of H2S. Low temperature areas, where water can be present, are classified as severe. Overhead of down stream separator and amine absorbers are considered as severe wet H2S service. Thermal cracking units including Visbreaker, Coker, or catalytic cracking unit such as FCC are also classified as severe wet H2S service on the cold light end cut (mainly LPG). On these units cyanides can strongly increase degradation. In amine units (absorber or regeneration), all rich amines streams, and regenerator overhead are considered as severe wet H2S service. Units downstream of Hydrotreatment units or amine absorbers, such as Reforming, Alkylation, Isomerisation, Propylene units are not concerned by wet H2S service. 4. Recent examples of equipment fabricated from Dillinger steel plates

7 Requirement for vessels in severe wet H2S service 7/7 Dicrest steels have been developed by Dillinger Hutte GTS to be used in wet H2S service. is the most elaborate of these steels. Its effective composition and characteristics are very close to TOTAL requirements. These steels are also immediately available due to a permanent stock maintained by Dillinger Hutte GTS. These two advantages lead to the regular choice of for equipment made of thin to medium thickness steel plates. This is the case in maintenance work where delivery is a primary concern, but also for projects such as new units or major debottlenecking. At this time a two stage water stripper is being built for the Feyzin refinery. This unit is dedicated to remove separately H2S and ammonia from process water. In the first step H2S is removed in a column operating at 10 bars, in a second step ammonia is removed in a second column operating at 0.5 bars. Most of the equipment are in wet H2S service (moderate or severe). has been the material chosen by most of the manufacturers with TOTAL agreement. Unit Item Function H2S service Operati ng temp. Operating pressure Thick. Steel 45 B 753 Condensate drum Severe 212 C 19 bars 15 mm A 516 Gr B 754 Condensate Severe 200 C 3 bars 15 mm A 516 Gr 60 drum 45 E 751 Feed/Effluent Severe 163 C 9.5 Chan. 13 mm A 516 Gr Tube 8 mm 45 E 752 Feed/Effluent Severe 200 C 9.1 Chan. 14 mm A 516 Gr Tub 8 mm 45 E 753 Reboiler Severe 200 C 9.1 Chan. 15 mm A 516 Gr 60 (Channel) 380 C 18.8 Tub. (Channel) 45 EA 754 Air cooler Moderate 136 C 7.8 bars 25 mm A 516 Gr EA 755 Air cooler Severe 109 C 1.5 bars 25 mm A 516 Gr EA756 Air cooler Moderate 84 C 3 bars 18 mm A 516 Gr 60 Stripped water For thick equipment, steel plates are usually not available on stock. In that case a special heat can be performed in accordance with TOTAL requirements. As an example, a cold high pressure separator (R-4206) for an HDS unit has been fabricated recently from 65 mm thick steel plates, made by Dillinger Hütte. 5. Conclusion TOTAL requirements for wet H2S service have been developed on the basis of professional documents issued by NACE or EFC, with the help of steel manufacturers and TOTAL feed-back. These requirements give satisfactory results since no failures have been encountered on recently fabricated carbon steel equipment. However, as for any complex recommendation, it is regularly updated to better suit the needs. For example, the use of higher steel grade (grade 65 or 70) is under consideration and some adaptation of the chemistry will be necessary in the near future.