Total Requirements

Dillinger Pressure Vessel Colloquium – Dillingen – 16 & 17 September 2009

DILLINGER PRESSURE VESSEL COLLOQUIUM

Dillingen – 16/17 th september 2009

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Martin RICHEZ

Franck ZANONCELLI

Requirement for vessels in severe wet H2S service 2/7


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1. Degradation of steel in wet H2S serviceH2S is present in many refinery process streams. Wet H2S service is referred in a process streamwhere both water in liquid phase and H2S are present.Carbon steel is commonly use in this environment, but it can be subject to degradation dependingupon pH, H2S level, temperature, duration of exposure and nature of the material. Thickness lossis usually not the main problem, but hydrogen is the major concern. NACE and EFC haveextensively studied this subject leading to different recommendations.Hydrogen is involved in most corrosion mechanism, but H2S presents the specificity to lead tohigh atomic hydrogen penetration in steel. This hydrogen tends to embritle the metal or torecombine to form Hydrogen molecule in the steel inclusions. Hydrogen pressure can get very highleading to high stresses.

There are two main types of degradation:

HIC (Hydrogen induced cracking). This is planar cracking that occurs in carbon and low alloysteels when atomic hydrogen diffuses into the steel and then combines to form hydrogen at trapsites. The driving force is the hydrogen pressure, helped by hydrogen embrittlement. No appliedstresses are required for cracks to appear. Susceptibility of steels to HIC is strongly dependentupon cleanliness of the steel, and the size and shape of inclusions. These cracks are oriented in theplan 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 ofhydrogen and stresses (residual or applied). Cracks are perpendicular to the stresses. Hardness isknown to increase susceptibility of the steel to SSC. NACE document MR 0103-2007 deals withthis 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. Thesescracks are due to hydrogen embrittlement and stresses induced by the bulging. Applied stresses canalso play a role.

SOHIC (Stress Oriented Induced Cracking). This term is used when staggered small cracksformed approximately perpendicular to the principal stress (residual or applied), result in a “ladderlike” crack array linking some preexisting HIC cracks.



2. TOTAL practice for steel in severe wet H2S service

2.1 Severe wet H2S serviceSeverity of Wet H2S service depends on the H2S content and pH of water in contact with themetal. During unit engineering, stream composition is provided by the process department. Thisdata is the basis for determining the service severity. However this data may not be fully availableand unusual or transient operations are often not considered. So any choice in H2S severity servicehas to be confirmed by a specialist during the material selection review to take into account thesecases and company feed-back.

H2S ppmpH

10<H2S<50 50=H2S=2 000 2000<H2S=10000 H2S>10000pH<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 happensduring upset or transient operation, it is considered that only SSC may occur and the service isconsidered as moderate.

2.2 Specification for platesCarbon steel is commonly used in low, moderate and severe wet H2S service, with a 3 or 6 mmcorrosion allowance. In this case, steel components (plates, forged parts) and welding are subject tostringent specifications.

For specific cases, when the environment is judged to be very corrosive, an internal liner made of acorrosion resistant alloy may also be used (cladded plates or weldoverlay). In this case, there is nospecific 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 arcfurnace or oxygen furnace process. The steel shall be vacuum degassed, deoxidized and producedto a fine grain structure, with a process leading low sulphur and phosphorus content.

Steel shall be produced with thorough desulphurization, dephosphorization and deoxidizingtreatments. Calcium treatment shall be applied for inclusion shape control, except that it need notbe applied to plate with very low sulphur levels (below 0.002 %). The calcium content shall notexceed 3 times the sulphur content.



The supplier shall exercise special care in the selection and supervision of the conditions offabrication and normalizing heat treatment, in order to eliminate heterogeneous structures, such aspearlite structures in very pronounced bands and aggregates of bainite andunannealed 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) 0.18Manganese (Mn) 1.20Phosphorous (P) 0.010Sulphur (S) 0.005Silicon (Si) 0.40Copper (Cu) 0.20Nickel (Ni) 0.40Chromium (Cr) 0.30Molybdenum (Mo) 0.12Vanadium (V) 0.015Niobium (Nb) 0.015Titanium (Ti) 0.02Boron (B) 0.0005

Multiple ElementsCr + Mo 0.30Ni + Cu + Cr + Mo 0.70Nb + V + Titanium (Ti) 0.12V + Nb 0.03Carbon Equivalent (Note 1) 0.38



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 10160 Class S2or ASTM A 578, level C, supplementary requirement S1.

2.3 Forged components and pipingThe chemical composition (product analysis) shall be restricted as follows, except where thematerial 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) 0.025 (note 3)Sulphur (S) 0.020 / 0.010 (note 2 – note 3)Multiple elements Maximum wt. %Vanadium (V) + Niobium (Nb) 0.20Nb + V + Titanium (Ti) 0.12Carbon 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 ≤ 0.015 %, and S ≤ 0.010 %

2.4 Specification for manufacturingAll pressure vessels in wet H2S Service shall be Post Weld Heat Treated in accordance with thecode requirements. 1 hour per 25 mm of thickness (minimum holding time of 1 hour) and 600°Cshall be applied as a minimum

Weld hardness testing of production welds shall be carried out in accordance with the NACEInternational 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 byWet Magnetic Fluorescent / Particle Test (WFMT) after PWHT.

For thickness > 40mm, a 100% radiography and ultrasonic examination is required before and afterPWHT.

2.5 Hydrogen induced cracking test.The HIC test and crack evaluation shall be in accordance with NACE Standard TM0284. The testis performed at pH = 3.

The test shall be performed per heat on the thickest plate in a simulated PWHT condition.



The following acceptance criteria shall be met:

% (maximum)CLR CTR CSR

Average 5 1.5 0.5Single 10 2 0.7

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 orhydrodesulfurization.

• Since H2S is not usually present in the crude, H2S content is limited in crude distillationunit. 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 isusually 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. Lowtemperature 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 alot 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 H2Sservice.

• Thermal cracking units including Visbreaker, Coker, or catalytic cracking unit such as FCCare also classified as severe wet H2S service on the cold light end cut (mainly LPG). Onthese units cyanides can strongly increase degradation.

• In amine units (absorber or regeneration), all rich amines streams, and regenerator overheadare 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 Dillingersteel plates



Dicrest steels have been developed by Dillinger Hutte GTS to be used in wet H2S service.Dicrest 5 is the most elaborate of these steels. Its effective composition and characteristics are veryclose to TOTAL requirements. These steels are also immediately available due to a permanentstock maintained by Dillinger Hutte GTS.

These two advantages lead to the regular choice of Dicrest 5 for equipment made of thin tomedium thickness steel plates. This is the case in maintenance work where delivery is a primaryconcern, 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 dedicatedto remove separately H2S and ammonia from process water. In the first step H2S is removed in acolumn operating at 10 bars, in a second step ammonia is removed in a second column operating at0.5 bars.Most of the equipment are in wet H2S service (moderate or severe). Dicrest 5 has been the materialchosen by most of the manufacturers with TOTAL agreement.

Unit Item Function H2Sservice

Operating

temp.

Operatingpressure

Thick. Steel

WaterStripper

45 B 753 Condensatedrum

Severe 212°C 19 bars 15 mm A 516 Gr 60Dicrest 5

WaterStripper

45 B 754 Condensatedrum

Severe 200°C 3 bars 15 mm A 516 Gr 60Dicrest 5

WaterStripper

45 E 751 Feed/Effluent Severe 163°C 9.5 Chan.14.5 Tube

13 mm8 mm

A 516 Gr 60Dicrest 5

WaterStripper

45 E 752 Feed/Effluent Severe 200°C 9.1 Chan.13.8 Tub

14 mm8 mm


WaterStripper

45 E 753 Reboiler Severe(Channel)

200°C380°C

9.1 Chan.18.8 Tub.

15 mm(Channel)


WaterStripper

45 EA 754 Air cooler Moderate 136°C 7.8 bars 25 mm A 516 Gr 60Dicrest 5

WaterStripper

45 EA 755 Air cooler Severe 109°C 1.5 bars 25 mm A 516 Gr 60Dicrest 5

WaterStripper

45 EA756 Air coolerStripped water

Moderate 84°C 3 bars 18 mm A 516 Gr 60Dicrest 5

For thick equipment, steel plates are usually not available on stock. In that case a special heat canbe performed in accordance with TOTAL requirements.

As an example, a cold high pressure separator (R-4206) for an HDS unit has been fabricatedrecently 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 professionaldocuments 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 recentlyfabricated carbon steel equipment.

However, as for any complex recommendation, it is regularly updated to better suit the needs. Forexample, the use of higher steel grade (grade 65 or 70) is under consideration and some adaptationof the chemistry will be necessary in the near future.

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