Risk-Based Inspection – Yanbu Gas Plant Case Study Module #4

4th Middle East NDT Conference and Exhibition, Kingdom of Bahrain, Dec 2007 For all papers of this publication click: www.ndt.net/search/docs.php3?MainSource=57

RISKRISKRISKRISK----BASED INSPECTION BASED INSPECTION BASED INSPECTION BASED INSPECTION –––– Y Y Y YANBU ANBU ANBU ANBU GGGGAS AS AS AS PPPPLANTLANTLANTLANT CASE STUDY CASE STUDY CASE STUDY CASE STUDY

MODULE #4 BUTANE MEROX MODULE #4 BUTANE MEROX MODULE #4 BUTANE MEROX MODULE #4 BUTANE MEROX & CAUSTIC REGENERATION & CAUSTIC REGENERATION & CAUSTIC REGENERATION & CAUSTIC REGENERATION UNITUNITUNITUNITSSSS Khalid Humaid Al-Hasani

Saudi Aramco, Yanbu, Saudi Arabia ABSTRACTABSTRACTABSTRACTABSTRACT In November 2006, Yanbu Gas Plant Engineering Inspection Unit had conducted RBI study at Module #4 Butane Merox & Caustic Regeneration Units under the supervision of the Inspection Department. Pertaining to the results of the study, it was a successful study for YGP. In fact, implementing the final recommendations of that study will grant for YGP a total saving of 11.277 MM per year. The key factor of that saving is extending the unit’s T&I interval from 5 to 10 years under some certain conditions.

INTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTION This report describes the findings of a Risk-Based Inspection (RBI) study on Module #4 Butane Merox and Caustic Regeneration Units at Yanbu Gas Plant (YGP). This study was performed by a team from NAGO departments under the supervision of the Inspection Department.

1. Objectives

The main objectives of this study are as follows:

o Identify the dominant damage mechanisms within the YGP Module #4 Butane Merox and Caustic Regeneration Units

o Establish relative risk levels for equipment and in-plant piping o Evaluate the possibility of increasing the T&I cycle and o Provide recommendations regarding an optimized future inspection and

assessment workscope

2. Scope

The scope of work consisted of static equipment and selected in-plant piping. Criteria for the selection of representative piping were established by the RBI Team, as follows:

1. Main process piping containing hydrocarbon, toxic or corrosive fluid. 2. Piping which failure could present a hazard to humans or to the environment,

or where such failure could not be repaired without disrupting operation.

3. Piping known to exhibit a high probability of failure, e.g. piping with injection point(s), dead leg(s) or vibrations.

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4. Piping known to be susceptible to Corrosion Under Insulation (CUI) and environmental damage such as Stress Corrosion Cracking (SCC) with failure consequences shown in item 2 above.

Important Notes:

o The above criteria were developed for use in conjunction with RBI studies. Piping not meeting the above criteria may not be exempted from being monitored in the OSI program.

o To optimize data gathering for in-plant piping, the following guidelines are also given:

i. Establish corrosion loops or circuits for the whole unit under study. Each

loop shall include all main lines and associated piping/branches attached to these main lines.

ii. From each corrosion loop, select one or more main (representative) line and include in the RBI study as a record. Include lines before and after equipment.

iii. Recommendations / inspection guidelines derived for each main line shall also be applicable to the entire corrosion loop piping, i.e. including associated piping/branches.

The main lines and associated piping relevant to this study are listed in Appendix 1.

The analysis shows more records than actual equipment and piping within the plant; this is because some equipment may be fragmented into several records, e.g. column top, column bottom, heat exchanger tube side, heat exchanger shell side, etc. The workscope items were further divided into 5 inventory groups based on process streams and isolation systems, as follows:

These inventories are shown diagrammatically in Appendix 2.

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This study does not include rotating equipment, valves, civil structures, electrical, instrumentation, and internals (e.g. trays, demisters).

3. Approach

The following phased approach was used based on the Saudi Aramco Engineering Procedure SAEP-343 “Risk-Based Inspection for Static Equipment and In-Plant Piping”:

o Phase 1: Pre-assessment Preparation – This phase involved the selection of process and inspection engineers, developing the work schedule and initial data gathering.

o Phase 2: Data Collection – This phase included a team kick-off meeting, plant walkthrough, interviews with Plant staff, completing the data gathering, review of PFDs, P&IDs, SISs, T&I inspection records and OSI data and finalizing the data entry in the software.

o Phase 3: Analysis and Inspection Planning – This phase aimed at producing an initial risk assessment, validating the study assumptions and data output and presentation of preliminary findings.

o Phase 4: Final Reporting – This phase involves the issuance of the final report of all findings and recommendations.

4. Methodology

RBI is a systematic approach that aims to minimize the risk exposure of plant equipment by optimizing the use of inspection resources. In this study, the API-RBI methodology was used based on API Publication 581, First Edition, May 2000 and API-RBI software version 7.0 (7.00.0012). The quantitative method used in this study prioritizes the process containing equipment, including piping, by calculating likelihood and consequence values for each piece of equipment. Full details of the RBI methodology are given in Appendix 3.

5. Corrosion Loops

Corrosion loops for the Butane Merox and Caustic Regeneration Units were established based on similarity in process, materials and operating conditions. In total, 7 loops were identified. Selected operating windows affecting mechanical integrity and the damage mechanisms for each loop were also identified in the API RBI software. Detailed descriptions for corrosion loops along with potential damage mechanisms are given in Appendix 4.

6. Assumptions/Comments

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It is customary in RBI studies to make certain assumptions and document the basis for the data used. Assumptions and comments relevant to this study are as follows:

1. A population density 1 x 10-4 was used based on 2 people being present at any time in the Butane Merox and Caustic Regeneration Units and a calculated process unit area of 20,000 square feet.

2. The production loss after commissioning 3rd Deethanizer system in 2009 is calculated as follows: In case of a shutdown of Mod-4 butane Merox unit, the produced butane (C4) will be diverted to Mod-3 butane Merox unit which can handle up to 50 MBD .This will result in reducing the butane from 80 MBD (based on normal rate of 490 MMBD) to 50 MBD. Reducing the total feed from 490MMBD to 320MMBD to maintain the 50MBD of Mod-3 butane Merox unit will result in a production loss equivalent to $ 4.5 MM/day.

3. Average T&I duration is 25 days per T&I.

4. T&I cost (inspection & maintenance) is $0.27MM per T&I.

5. Equipment cost is $650 per square feet; this is based on the Butane Merox and Caustic Regeneration Units replacement cost of approximately $13 MM over a plot area of 20,000 square feet.

6. A management score of 50 (neutral value) was used in this study. This is to ensure uniformity across Saudi Aramco facilities.

7. $5MM was used for the estimated cost of fatality per individual.

8. The default API BRD piping length of 50’ was used for all piping.

9. For Equipment, a specified Tmin was used from SIS.

10. For piping, a specified Tmin was used from SIS and OSI program.

11. Corrosion allowance for piping was calculated as Furnished Thickness – Tmin . For equipment, corrosion allowance was taken from SIS Sheet.

12. Software default structural Tmin for piping was overwritten by Tmin values from SAES-L-310.

13. Localized corrosion was used for all services where caustic is present.

14. For equipment that is internally cladded, the corrosion rate is taken as 1 MPY.

15. Corrosion Rate for external corrosion is taken as 0.5 MPY due to lack of appropriate data.

7. Study Findings The findings below cover the following:

o Inspection history review

o Risk analysis / Projection results

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7.1 Inspection History Review

7.1.1 General Plant Information

o Mod-4 Butane Merox and Caustic Regeneration Units were commissioned in 1982 and thus has experienced approximately 25 years of service.

o All equipment is currently on 5-year EIS. The inspection history was compiled from the last four post T&I reports.

o Detected stress corrosion cracking was found on C-462 (Extractor), D-464 (Caustic pre-wash), D-465 (Butane Water settler), D-466 (Sand filter) and C-463 (Oxidizer).

o All equipment and piping in caustic service was found to be not post weld heat treated.

o One pipe “8” – P – 4704 – 3A1” was replaced in-kind with post weld heat treatment due to detected CSCC.

o Cracking inspection has been performed consistently every two years for all non-stress relieved piping greater than 3’’ in caustic service.

7.1.2 Static Equipment A detailed review of static equipment inspection history was prepared. It is noted that the review centered on the last four inspections carried out to date, i.e. inspection history prior to the reviewed period was not undertaken.

7.1.3 In-Plant Piping Selected in-plant piping was covered to represent other pipes within the same corrosion loop as describes in the Scope section above. Associated piping related to each representative line are listed in Appendix 1. The piping is monitored via OSI program and full thinning history is documented in this program. Corrosion rates used in this study were partly derived from the OSI program output which is entered as “measured” and partially from expert opinion which is entered as “estimated.” 7.2 Risk Analysis (Current Status-2006)

The current risk summary of all equipment and piping (42 components) is given in Appendix 6. The current risk (November 2006) matrix for all components is shown in Figure1. High and medium-high risk components are listed in Tables 1 and 2 respectively.

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Current Risk (2006)

Consequence Category

Likelihood Category A B C D E

5 0 0 0 0 0 0

4 0 0 0 0 0 0

3 0 0 0 0 3 3

2 0 0 3 1 4 8

1 3 3 12 4 9 31

3 3 15 5 16

Total Components = 42

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High 3 7.41%

Medium High 13 30.95%

Medium 20 47.62%

Low 6 14.29%

Figure 1 – Current Risk Matrix (March 2007)

Table 1 – High Risk Components

Component (Risk) Description Maine Driver

C- 462 Bottom (3E) Merox Butane Extractor Caustic Stress Corrosion Cracking (no PWHT) +

Consequence

C- 462 Middle (3E) Merox Butane Extractor Caustic Stress Corrosion Cracking (no PWHT) +

Consequence

C- 462 Top (3E) Merox Butane Extractor Caustic Stress Corrosion Cracking (no PWHT) +

Consequence

Table 2 – Medium-High Risk Components (Selected)

Component (Risk) Description Maine Driver

C-464 (2E) Merox Butane Pre-wash

Drum

Caustic Stress Corrosion Cracking (no PWHT) +

Consequence

C-466 (2E) Merox Butane Sand Filter Caustic Stress Corrosion Cracking (no PWHT) +

Consequence

C-465 (2E) Merox Butane Water Settler Caustic Stress Corrosion Cracking (no PWHT) +

Consequence

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The main observations to note from the above figures and tables are as follows:

o The risk distribution for this plant shows 7% of components at high risk and 31% at medium high risk category.

o The Butane Merox Extractor (D-462) exhibited a High Risk category driven by both likelihood and consequence. The main driver of the likelihood is the high susceptibility to caustic stress corrosion cracking (CSCC) resulted from detected caustic cracking and lack of post-weld heat treatment (PWHT) in these vessels. The driver of the consequences is the nature and large volume of the fluid handled by this equipment coupled with the low rating of the isolation system.

o 68% of components are either medium or low risk. Accordingly, some relaxation of the inspection requirements for these components is possible.

o The high and medium- high risk components showing high likelihood of failure are driven by the susceptibility to CSCC coupled with high consequence.

o The concern with CSCC is related to the lack of post-weld heat which resulting in detected caustic cracking. It is noted that this type of cracking has already been detected on some equipment at previous T&Is.

7.3 Risk Projection (Future 2010-2016)

A risk projection was carried out to evaluate future risk for 5 and 10-year T&I cycles. For comparison, the risk matrix for the current risk and a 5-year T&I cycle are shown in Figure 2 below.

Current Risk (2006) Future Risk 2010 (5 year T&I)

Consequence Category Consequence Category Likelihood Category A B C D E


5 0 0 0 0 0 0 5 0 0 0 0 0 0

4 0 0 0 0 0 0 4 0 0 0 0 3 3

3 0 0 0 0 3 3 3 0 0 3 0 3 6

2 0 0 3 1 4 8 2 0 0 1 1 1 3

1 3 3 12 4 9 31 1 3 3 11 4 9 30

3 3 15 5 16

3 3 15 5 16

Figure 2 – Current vs. Future (5-Year or 2010) Risk Projection

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By comparing the above matrices, it is apparent that the risk level for ten (10) components has increased, as follows:

o D-466, Merox Butane Sand Filter, from category 2E to 3E

o C-463, Oxidizers bottom , from category 2C to 3C

o C-463, Oxidizers middle, from category 2C to 3C

o C-463, Oxidizers top, from category 2C to 3C

o C-462, Merox Butane Extractor bottom, from category 3E to 4E

o C-462, Merox Butane Extractor middle , from category 3E to 4E

o C-462, Merox Butane Extractor top, from category 3E to 4E

o D-465, Merox Butane Water settler bottom, from category 2E to 3E

o D-465, Merox Butane Pre-wash top, from category 2E to 3E

o D-467, Disulfide Separator, from category 1C to 2C

Further risk projection to a 10-year T&I cycle yielded the following results (Figure 3).

Future Risk 2010 (5 year T&I) Future Risk 2016 (10 year T&I)

Consequence Category Consequence Category Likelihood Category A B C D E


5 0 0 0 0 0 0 5 0 0 0 0 0 0

4 0 0 0 0 0 0 4 0 0 3 1 7 11

3 0 0 3 1 7 11 3 0 0 0 0 0 0

2 0 0 0 0 0 0 2 0 0 2 0 0 2

1 3 3 12 4 9 31 1 3 3 10 4 9 29

3 3 15 5 16

3 3 15 5 16

Figure 3 – Future (5-Year or 2010) vs. Future (7-Ye ar or 2016) Risk Projection

With the exception of thirteen (9) components showing an increase in the risk category, the risk level for the rest of components remains unchanged. The thirteen subject components are:

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o D-466, Merox Butane Sand Filter, from category 3E to 4E

o C-463, Oxidizers bottom , from category 3C to 4C

o C-463, Oxidizers middle, from category 3C to 4C

o C-463, Oxidizers top, from category 3C to 4C

o D-467, Disulfide Separator from category 2C to 3C

o D-465, Merox Butane Water Settler bottom, from category 2E to 3E

o D-465, Merox Butane Water Settler top, from category 2E to 3E

o D-465, Merox Butane Water Settler Middle, from category 3E to 4E

o D-464, Merox Butane Pre-wash Drum bottom, from category 2D to 3D

Overall, it is apparent that there is an increase in the risk for some components when projecting from a 5 to a 10-year T&I cycle. The risk category for these components which are prone to caustic stress corrosion cracking may be significantly reduced by performing post-weld heat treatment at the next available opportunity. Based on the above, it is concluded that an increase in the T&I cycle to 10 years is permissible provided the following recommendations are implemented:

a. Perform PWHT at the next available opportunity for all none stress relieved

equipment in caustic service: C-462 (Extractor), D-464 (Caustic Pre-wash), D-465 (Butane Water Settler), D-466 (Sand filter), and C-463 (Oxidizer).

b. Perform WFMPI on the internal welds for all stress relieved equipment on 2016 utilizing Debutanizer Column T&I opportunity.

c. Replace all non stress relieved piping in caustic service with stress relieved piping. As a minimum, replace the 3” diameter and below piping and perform 100% shear wave inspection on the welds of piping 4” diameter and greater.

d. Incorporate critical parameters that could affect RBI results in the existing Management of Change (MOC) program.

8. Financial Benefits

Based on an increased T&I interval from 5 to 10 years, a saving of approximately $11.28 MM per year over 20 years is derived based on the following information: o Mod-4 Butane Merox and Caustic Regeneration Unit is shutdown for approx. 25

days every 5 years

o T&I Maintenance/Inspection Cost approx. $0.27MM per T&I

o Business Interruption Cost = $4.5 MM / day x 25 days = $112.5 MM (Based on 490 MMBD)

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o Over 20-year timeframe, based on proposed T&I interval of 10 years, Number of T&Is reduced from 4 (5-Y) to 2 (10-Y), i.e. 2 x ($112.5 MM+0.27 MM)

o Total Savings per year (Reduced number of T&Is) = ($225 MM + $.54MM)/20 = $225.54 MM/20 = $11.28 MM / year (over 20 years)

The production loss after commissioning 3rd Deethanizer system in 2009 is calculated as follows: In case of a shutdown of Mod-4 butane Merox unit, the produced butane (C4) will be diverted to Mod-3 butane Merox unit, which can handle up to 50 MBD .This will result in reducing the butane from 80 MBD (based on normal rate of 490 MMBD) to 50 MBD. Reducing the total feed from 490MMBD to 320MMBD to maintain the 50MBD of Mod-3 butane Merox unit will result in a production loss equivalent to $ 4.5 MM/day.

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CONCLUSION

1. The risk distribution for this plant shows 7% of the components at high risk and

31% at Medium-High risk category.

2. The Butane Merox Extractor (D-462) exhibited a High Risk category driven by both likelihood and consequence. The main driver of the likelihood is the high susceptibility to caustic stress corrosion cracking (CSCC) resulted from detected caustic cracking and lack of post-weld heat treatment (PWHT) in this drum. The driver of the consequences is the nature and large volume of the fluid handled by this equipment coupled with the low rating of the isolation system.

3. 62% of components are either medium or low risk. Accordingly, some relaxation of the inspection requirements for these components is possible.

4. The Medium- High risk components showing high likelihood of failure are driven by the susceptibility to CSCC coupled with high consequence.

5. The susceptibility for (CSCC) is driven by the lack of post-weld heat treatment of components.

6. Risk projection to 2016 (10-year T&I cycle) showed that risk level has increased in comparison with the 2010 (5-year T&I cycle) due to the susceptibility of (CSCC).

7. Based on the results of this study, it is concluded that an increase in the T&I cycle from 5 to 10 years is permissible provided the following recommendations are implemented:

o Perform PWHT at the next available opportunity for all none stress relieved equipment in caustic service: C-462 (Extractor), D-464 (Caustic pre-wash), D-465 (Butane Water settler), D-466 (Sand filter), and C-463 (Oxidizer).

o Perform Wet Fluorescent Magnetic Particle Inspection (WFMPI) on the internal welds for all stress relieved equipment on 2016 utilizing Debutanizer column T&I opportunity.

o Replace all non stress relieved piping in caustic service with stress relieved piping. As a minimum, replace the 3” diameter and below piping and perform 100% shear wave inspection on the welds of piping 4” diameter and greater.

o Incorporate critical parameters that could affect RBI results in the existing Management of Change (MOC) program.

8. Based on initial estimates, the anticipated savings derived from an increase in the T&I cycle from 5 to 10 years is $11.28 MM per year.

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APPENDIX 1APPENDIX 1APPENDIX 1APPENDIX 1

REPRESENTATIVE AND ASSOCIATED PIPINGREPRESENTATIVE AND ASSOCIATED PIPINGREPRESENTATIVE AND ASSOCIATED PIPINGREPRESENTATIVE AND ASSOCIATED PIPING

Representative Piping Corrosion Loops Associated Piping 10-P-4312-1A1 10-P-4701-1A1 2-S-4701-1A1C

3-C-4701-3L1 4/6-C-4702-3A1

6-P-4702-1A1 6-BD-4701-1A1

10-P-4703-1A1 2-S-4702-1A1C

2-C-4706-1A1 2-BD-4702-1L1

6-BD-4703-1A1 3-C-4705-1A1

4-C-4705-1A1 10-P-4704-1A1 8-P-4707-1A1

3-P-4712-1A1

3-P-4706-1A1 2-BD-4705-1L1

4-P-4705-1A1 6-BD-4704-1A1

2-C-4707-1A1 2-C-4706-1A1

3-C-4658-1A1

10-P-4707-1A1 2-P-4708-1A1

2-BD-4705-1L1 4-P-4709-1A1 6-BD-4701-1A1

3-C-4658-1A1 2-P-4709-1A1 6-BD-4701-1A1

2-C-4709-3L1 4-C-4754-1A1 2-BD-4753-1L1

2

2-C-4753-1A1

3-S-4751-1A1C 2-SC-4751-1A1C

3 1-SC-4753-3L1

4-C-4751-1A1 1/1/2-A-4751-2D3

4-C-4755-1A1 5

2-C-4769-1A1 6-C-4764-1A1 2-C-4763-1A1

2-C-4763-1A1

6-C-4765-1A1

2-C-4767-1A1

1-C-4766-1A1 6-RL-4751-1A1

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1/1/2-S-4755-1A1C

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INVENTORY GROUP DIAGRAMINVENTORY GROUP DIAGRAMINVENTORY GROUP DIAGRAMINVENTORY GROUP DIAGRAM

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RBI METHODOLOGYRBI METHODOLOGYRBI METHODOLOGYRBI METHODOLOGY

RBI is a systematic approach that attempts to minimize the risk exposure of plant equipment by optimizing the use of inspection resources. In this study, the API-RBI methodology was used based on API Publication 581, First Edition, May 2000 and API-RBI software version 7.0 (7.00.0008). The methodology is both qualitative and quantitative. The qualitative technique is a quick way of prioritizing the various work scope items based on risk. The quantitative method prioritizes the process containing equipment, including piping, by calculating separate likelihood and consequence values for each piece of equipment. This method was used in the study and involved the following steps:

o Prepare a suitable database

Equipment items were identified from the P&IDs and the PFDs. The data associated with the equipment in the study was taken from the available piping specifications and line lists with the process stream compositions and operating conditions emanating from other sources. The preparation of the database for the first time is an extensive process. However, once completed, re-analyses and “What-if?” scenarios are relatively straightforward. As far as possible, electronic data sources are used, both to speed the data assembly process and to minimize data entry errors. Where equipment identification in the Safety Instruction Sheet (SIS) and the actual equipment identification are found to be different from one another, the database has been prepared to provide a cross reference.

o Identify the main active damage mechanisms

A key step for this study is the identification of the possible progressive damage mechanisms within the plant under study and a prediction of the likely rate of thinning processes and severity of other mechanisms such as cracking. This step requires input from Corrosion and Materials experts. Refer to the attached damage mechanisms marked up on PFD drawings.

The rate of attack by corrosion under insulation (CUI) is usually calculated by the software based on the average rainfall, the operating temperature, and whether or not the item has been coated. Subsequently, the inspection history is assessed to determine how well the actual state has been characterized based on the inspections performed.

o Calculate the likelihood of failure for each piece of equipment as a function of the different damage mechanisms to whic h it is exposed

The likelihood of failure of a piece of equipment or pipe is a direct function of the nature and rate of the degradation mechanisms to which it is subject. The key steps were to:

1) Identify the damage mechanisms.

2) Predict the rate of degradation.

3) Assess the inspection history.

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4) Consider the age.

5) Calculate the likelihood of failure. Years of operating experience in process plants worldwide have produced databases of failure frequencies for different types of equipment items (e.g. for heat exchangers, pressure vessels, etc.). For a given type, the average frequency from these databases is known as the “generic” frequency. If an item of equipment is operating under “typical” conditions, then it is assumed that its likelihood of failure is given by the generic frequency. The RBI program contains a database of generic failure frequencies for different types of process equipment.

o Calculate the consequence of failure associated wit h each piece of equipment

Based on the P&IDs, the location of the isolation valves and discussion with the Team’s senior plant engineer, each item of equipment was assigned to an inventory group. This group represents the fluid that could escape in the event of a leak at any one of the items in the group. Each item of equipment is then associated with two inventories, its own and the group’s inventory. Please refer to the attached inventory grouping marked up on a PFD drawing.

The consequences were calculated taking into account the nature and amount of the fluid released. The amount and rate of fluid released depends upon factors such as the size of the hole, the fluid viscosity and density and the operating pressure. The rupture of a large diameter high-pressure pipe or vessel obviously has a different consequence than a pinhole leak at a small diameter low pressure pipe and this method quantifies that difference.

Each piece of equipment or piping has a certain generic (industry average) probability of failing either by a pinhole type leak, a medium size hole, a large hole or a rupture. The consequence of each type of failure is calculated and combined with the probability for that failure; to calculate the overall risk associated with each piece of equipment.

o Combine the likelihood and the consequence numbers to calculate the risk associated with each piece of equipment and ra nk the equipment according to the risk results

The risk associated with each piece of equipment is essentially given by the formula:

RISK = (Sum of damage factors) * (Generic failure Frequency) * Consequence of Failure

The risk is the combination of two key terms:

• Likelihood of failure and

• Consequence of failure

• Calculate the reductions achievable in the likeliho od of failure through a suitable inspection program (this is achieved by re moving the uncertainty in the actual rate of degradation or co ndition of the item and thus reducing the chance of failure)

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• Develop inspection-planning guidelines based on ris k

Additional methodology details are given in Documents API 581 and API 580.

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1. Corrosion Loop 1 (CL-1)

Loop Description

This loop starts after Butane Coolers “E-433 A/B” and finishes before the Merox Butane Caustic Pre-wash Drum “D-464” inlet, at the pressure control valve “PCV-601. The pre-wash drum is not included in this loop.

Process Description

The sweet hydrocarbon fluid, (350 PPM RSH, 7 PPM H2S), flows from the butane coolers of Debutanizer column at module four (4) toward the non-return valve before entering fore pre-wash drum at an average pressure of 170 psig controlled by the pressure control valve (PCV-601) and temperature of 110°F.

Potential Damage Mechanisms

Materials of construction are carbon steel.

Based on the gas composition, (i.e. RSH, H2S, CO2 and water), the following potential damage mechanism was identified:

• Generalized Thinning

Key Operating Windows

Refer to the software database. 2. Corrosion Loop 2 (CL-2)

Loop Description

This loop starts from the pressure control valve “PCV-601” before the pre-wash drum D-464 and finishes at the inlet piping of treated butane to dehydrator “C-472 A/B”. This loop includes pre-wash Drum, Extractor, water settler and sand filter associated with their inlet and outlet piping. Also the loop includes the regenerated and contaminated caustic piping.

Process Description

The butane product, (350 PPM RSH, 5 PPM H2S, COS), at an average pressure of 160 psig and temperature of 110°F splits into two streams and enter the “D-464” pre-wash drum. In the butane Meroxprewash drum “D-464”, the butane product is prewashed with a 10 Be° caustic solution to remove any (H2S < 5 PPM) and COS in the feed gas. The H2S reacts with the 10 Be° caustic to form sodium sulfide. The COS reacts with the 10 Be° caustic solution to form sodium sulfide and sodium carbonate. The sodium sulfide and sodium carbonate stay in the 10 Be° caustic solution. The 10 Be° caustic strength is gradually spent (contaminated) and it is replaced when around 75% of spent. Spent caustic is sent from “D-464”, via LCV-600, to the return unit caustic degassing drum”D-784” and fresh 10 Be° caustic is made up to the pre-wash drum from the chemical area in

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the utilities side, when required. The pre-washed fluid is then flow from “D-464” to the butane extractor “C-462” at 159 psig and 110o F. The velocity of the fluid is 5 FPS and 0.23 psi pressure drop for every 100 ft. The butane extractor “C-462” has 9 trays. In the “C-462” the C4 product enters “C-462” above the 1st tray level and flows upward through the extractor column. Here, it is met by a countercurrent flow of 25 Be° caustic Merox solution where the 25 Be° caustic enters the “C-462” above the 9th tray level and flows downwards through the extractor and it is controlled by FCV-601. In the extractor”C-462”, mercaptan sulfur (RSH) is removed from the C4 product during contact with the 25 Be° caustic Merox solution. The mercaptan sulfur is catalytically converted to mercaptides. These mercaptides stay in the 25 Be° caustic Merox solution. The contaminated 25 Be° caustic Merox solution flows from the bottom of “C- 462”, via LCV-601, to the caustic Merox regeneration section. The C4 product flows at an average pressure of 148 psig and temperature of 110°F from the top of “C-462” to the butane water settler drum “D-465”. In the water settler drum any entrained caustic Merox solution in the C4 product settles out and collects in D-465’s boot. Then it goes, under level control of LCV-603, to the caustic Merox regeneration section. The C4 product flows at an average pressure of 145 psig and temperature of 110°F from the top of “D-465” to the butane sand filter “D-466”for final cleaning. Finally, the treated C4 product flows at an average pressure of 138 psig and temperature of 110°F downwards through “D-466” and to the C4 dehydrator section. In this drum “D-466”, any remaining entrained caustic Merox solution in the C4 product is collected in the bottom of the drum and dumped, under level control of (LCV-608), to the return unit caustic degassing drum “D-784”.



Based on the gas compositions, (i.e. RSH, H2S, NaOH, NaRS, and water), the following damage mechanism was identified.

• Caustic Stress Corrosion Cracking


Refer to the software database. 3. Corrosion Loop 3 (CL-3)

Loop Description

This loop comprises steam inlet / outlet piping and the shell side of Heater “E-461”.

Process Description

The 60#, 330o F steam condensate is used to provide heat media for contaminated caustic. The condensate will be send to utilities area or caustic degassing drum depending on the condensate conductivity reading. If there is a tube leak in “E-461”, a high conductivity meter opens ZV-650B which will dump the contaminated condensate to degassing drum “D-784”. Otherwise, the condensate will be send to condensate system. Potential Damage Mechanisms

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Based on the feed compositions, (i.e. Oxygen, water), only one potential damage mechanism was identified.

• Thinning


Refer to the software database 4. Corrosion Loop 4 (CL-4)

Loop Description

This loop covers contaminated caustic to Heater E-461, the tube side is only component included in this loop. Process Description

The contaminated caustic enters the caustic Merox regeneration section at an average pressure of 160 psig and temperature of 105° F. The contaminated caustic Merox solution’s temperature is raised to 110° F as it flows through the tube side of the caustic heater “E-461”. The heating medium passing through shell side of “E-461” is steam condensate at an average pressure of 60 psig and temperature of 330° F. A self-activated temperature control valve controls the temperature of caustic Merox flowing from tube side outlet.



Loop Description

This loop starts from the “E-461” tubes outlet piping including the Merox Oxidizer ”C-463” and finishes at Merox Disulfide separator drum “D-467” inlet piping to the dome. The D-467 is not included in the loop.

Process Description

After preheating the contaminated caustic in “E-461” tube side, the caustic Merox solution is injected with process air at a ratio between 5% and 10% by volume. This air injection is determined by oxygen analyzer. The air comes from utilities plant at an average pressure of 100 psig and temperature of 890 F and it is injected into the caustic Merox solution via FCV-602. The contaminated caustic Merox solution then flows to the Oxidizer column “C-463” at an average pressure of 80 psig and temperature of 1100 F where the mercaptides (NaRS) are catalytically converted to insoluble disulfide oils (RSSR). The Oxidizer column “C-463” is packed with carbon raschig rings. The mixture leaving the top of”C-463” consists of caustic Merox solution, disulfide oil, disulfide vapor, and air.

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Based on the gas compositions, (i.e. NaOH, RSSR, O2), the following damage mechanisms were identified:

• Oxygen pitting

• Corrosion under insulation



Loop Description

This loop covers the Merox Disulfide separator “D-467” and the Merox Water Knockout drum “D-470” only.

Process Description

The mixture solution “Caustic Merox solution, Disulfide oil, Disulfide vapor and Air” flows at an average pressure of 75 psig and temperature of 1170 F from the oxidizer “C-463” enters the dome section of the disulfide separator “D-467”. In the “D-467”, the Disulfide oil, Disulfide vapor and Air, not used up during catalytic oxidization, are separated from regenerated caustic Merox solution. This solution settles down in the bottom of “D-467” and form an interface with the disulfide oils. The disulfide oils are level controlled by “LCV-650” from the top section of “D-467”. These oils flow to water knockout drum “D-470”. In the “D-470” the disulfide oils are pumped by knockout pump G-362. The KO pump is started automatically on a high “D-470” level signal from level switch “LS-653” and stopped automatically on low “D-470” level signal from level switch “LS-654”.



Based on the gas compositions and material of construction, (i.e. NaOH, RSSR, O2,

Water), the following potential damage mechanisms were identified.



Refer to the software database

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7. Corrosion Loop 7 (CL-7)

Loop Description

This loop incorporates piping from Disulfide oil Drum dome “D-467” to KO Drum “D-470” inlet piping and from Disulfide oil Drum “D-467” to burn bit and D-503.

Process Description

The disulfide oil flows at an average pressure of 46 psig and temperature of 1200 F to KO drum “D-470” from the top section of “D-467” and level controlled by “LCV-650”. The disulfide oils are pumped from the “D-470” by KO pump “G-362”. The “G-362” is started automatically on a high “D-470” level signal “LS-653” and stopped automatically on low level signal “LS-654”. The outlet flows from “D-467” dome is controlled by “PCV-651” to maintain the pressure on the disulfide oil drum “D-467” at 46 psig. This valve vents excess pressure to the water KO drum “D-470”.



Based on the gas compositions, (i.e. Oxygen, RSSR, NaOH), different types of damage mechanisms were identified.


• Oxygen Pitting


Refer to the software database

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CORROSION LOOPSCORROSION LOOPSCORROSION LOOPSCORROSION LOOPS

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DAMAGE MECHANDAMAGE MECHANDAMAGE MECHANDAMAGE MECHANISMSISMSISMSISMS

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