implementation of the api rp 584 integrity operating

Implementation of the API RP 584 Integrity Operating

Windows Methodology at the Gibson Island Ammonia

Manufacturing Plant

A process gas leak outside of the primary reformer caused a serious fire incident at the Gibson Island

Works (GIW) ammonia manufacturing plant in Brisbane, Australia. As part of their global reliability

improvement strategy, Incitec Pivot Ltd. (IPL) selected GIW as a pilot for the pending API RP 584

Integrity Operating Windows (IOW) standard. An all-inclusive review of plant performance was

conducted, including a creep remnant life assessment and stress analyses of the primary reformer.

This paper presents a safety overview of the GIW facility and presents a case study of how the IOW

standard was implemented at the facility. The steps used to mitigate risk and improve the methods

used for additional facilities are also covered.

L. Bateman, D. Keen

Incitec Pivot /Dyno Nobel

Q. Rowson, B. Fletcher, O. Kwon, C. Thomas, A. Saunders-Tack, A. Karstensen

Quest Integrity NZL Limited

failure led to a significant fire outside of the primary reformer at Incitec Pivot Ltd. (IPL) Gibson Island ammonia plant (GIW) in December 2010. The

impact of this incident on the plant reliability and safety management processes led to the decision to pilot the implementation of the API RP 584 Integrity Operating Windows standards (IOW) best practice [1]. This paper describes the implementation process for this standard for the reforming section of the plant.

Introduction & Background

To properly understand the nature of the IOW implementation process, a basic understanding of the equipment involved is required. The

process loop in question is the fired section of the front end steam reformer at GIW in Brisbane Australia. The front end is made up of a generic type fired pre-heater up-stream of a typical 1960’s vintage Foster Wheeler terrace-wall fired radiant section reformer, complete with a mixed feed coils and an auxiliary fired convection bank. The items of equipment and their configuration are shown in Figure 1 in the simplified process flow diagram excerpt. The items of equipment addressed in the IOW process include the preheat furnace (EF602) and the steam reformer (R601). The reason that this loop is a focus for the IOW process is because it is the highest risk loop in the plant in addition to it having experienced the incident in December 2010.

A

772011 AMMONIA TECHNICAL MANUAL

73 AMMONIA TECHNICAL MANUAL2012

Figure 1. P&ID excerpt of the reforming loop, EF602 furnace left, R601 right

Outline of the Incident Leading to

IOW Program

The outlet pigtails and manifold are contained within a “coffin box” located at the bottom of the furnace. The pigtail connections to the catalyst tubes are located approximately 900 mm (35.43 in) above the bottom of the tubes which extend outside the bottom of the “coffin box” and are fitted with a bottom flange as shown in Figure 1. An outlet pigtail weldolet suffered a creep failure in December 2010 [2]. The partially reformed process gas leaked into the “coffin-box” section of the reformer and directly impinged on the fabric seal at the bottom of the tubes. The jet of leaking gas quickly pierced the fabric seal as it is only designed to seal the slightly negative-pressure furnace. The instant

that the H2-rich gas (at 760°C (1400 °F)), met the oxygen-rich atmosphere outside the furnace, it immediately ignited. The sudden increase in temperature at the bottom of the tube caused the stud bolts on the bottom flanges to soften and stretch, leading to another leak at the flanges in the vicinity of the failed pigtail. This also ignited, causing the next tube flange studs to soften and leak, resulting in a chain reaction along the bottom of the reformer. The fire was clearly visible on the outside of the furnace box. The instant the condition was noted by the control room, the feed was shut off immediately and the plant safely ramped down to allow further investigation. The root cause analysis clearly indicated a creep failure was responsible for the original leak at the pigtail. Micrographs of the failed pigtail showed that the material of a single pigtail contained extensive creep voids, while nearby

Coffin box section

78 2011AMMONIA TECHNICAL MANUAL


pigtails were in good condition. The root cause of the incident was extended operation at high temperatures meaning operation had been above a safe “Operating Window”. The pigtails were installed in 2007, had under gone 10 thermal cycles in that time and had a design pressure and temperature of 2.75MPa (0.4 ksi) and 815˚C (1500 °F) respectively. This conclusion led to the development of an IOW program involving analysis of all the components and pipework within the front end of the ammonia plant.

Explanation of the IOW Program

IOW Definition:

The definition, monitoring and control of key process as well as operational parameters, commonly referred to as the plant Integrity Operating Window (IOW). An essential step in

ensuring optimal reliability, availability and profitability of critical plant equipment.

Purpose:

To provide sustained operational reliability and understanding of the relationship between campaign life, production rates, and opportunities for improvement.

Deliverables:

A clear understanding of likely failure modes, remaining life and integrity of components under controllable process variables (e.g. temperature, pressure, flow, etc.) and operational guidelines outlining the upper and lower bounds of these limitations. The end goal of the IOW process is illustrated in Figure 2 below.

Figure 2. Integrity Operating Window limits

IOW Process as Executed by

IPL/Quest Integrity

The IOW process consists of series of steps used to limit operational variables that govern the most likely failure mechanism for a given item of equipment.

The actual process begins similarly to the Risk Based Inspection (RBI) program [3] as outlined in Figure 3. A team of experts assesses the risk of failure with respect to a given mode and mechanism of failure. RBI then branches off to create Inspection Test Plans (ITPs) and inspection schedules to reduce these risks. The IOW program uses the initial work of the RBI process of defining and ranking the risk of particular failure modes and mechanisms. This



part of the analysis was already completed in the form of root cause analysis and updated RBI risk assessments following the serious incident in December 2010. The condition of the vessels was assessed by Non-Destructive Examination (NDE) and inspection, and the remaining life was assessed with respect to the process variable that governs the failure mechanism (e.g. temperature for creep failure). This analysis is carried out in a

variety of ways, from API579 [4] level 1 screening (e.g. go/no-go assessment) through to complete computational fluid dynamics and finite element models encompassed by an API579 level 3 assessment. Consumed life based on past operating history is subtracted from the total life, and the IOW limits are agreed upon by all the original stakeholders.

Figure 3. RBI and IOW Process Flow Charts

Why IOW was Chosen by IPL

An IOW program was chosen to ensure the safety, reliability, availability and profitability of critical plant equipment via the prevention of unexpected failure.

The entire program is a synergistic multi-discipline approach used to establish the plant IOW, as an essential prerequisite and integral part of RBI implementation. There are many stakeholders in the IOW process as it affects numerous aspects of the operation. How this strategy fits into IPL’s systems is shown in Figure 4.



Figure 4. How IOW fits into the plant operational groups

IPL identified that a failure to properly define, monitor and control the IOW may have severe or even catastrophic impact on plant integrity. In addition, it can discredit the gains in safety, reliability and availability offered by RBI. The pigtail failure in December 2010 was an example in point. A well-executed IOW and RBI work process is essential to increase the flexibility and profitability of plant operations.

Analysis Performed at Gibson Island

Facility

In order for IPL to implement the IOW standard, first a comprehensive understanding of actual operating conditions was required. Detailed Computational Fluid Dynamics (CFD) modeling of the radiant and convective sections of the ammonia front end were preformed. This determined actual temperature variations along the pipe work, dead spaces and bends. CFD

findings were then used as the input conditions to develop Finite Element Analysis (FEA) models of the front end. The FEA Models were used to determine the state of stress of the unit under creep conditions, taking into account all system load and supports of the unit. Results from this stress analysis were then used as input into API579 fitness-for-service assessments which were used to determine the safe operating conditions and later defined the IOW to API RP 584 draft standard.

Computational Fluid Dynamics (CFD)

Computational Fluid Dynamics (CFD) is a simulation tool that is used to numerically solve and analyze problems that deal with fluid flow in detail. CFD modeling was used to understand observed uneven temperature distributions in the radiant and convection sections of the primary reformer. The main objective of the CFD modeling was to provide the steady state flow field distribution inside the steam reformer while operating under typical firing conditions.



In this study, three-dimensional steady state CFD models were developed to determine the flow field solution distribution (pressure, temperature, velocity, etc.) throughout the primary reformer. The CFD modeling included the combustion of hydrocarbons as well as radiation heat transfer in an effort to accurately predict the flue gas distribution in the convection and radiant sections of the reformer. The overall geometry of the CFD model is presented in Figure 5.

Figure 5. Isometric view of the CFD model.

The CFD analysis predicted flue gas distributions containing large vortex structures spinning at low velocities in the radiant section of the unit [5]. Low flue gas velocities were expected since the heat transfer mechanism is predominately governed by radiation rather than forced convection in this region. The modeling predicted cooler flue gas temperatures for the catalyst tubes adjacent to the end walls in the radiant section; these lower temperatures were also observed by IPL through the use of infrared imagery. Figures 6 and 7 shows significantly lower flue gas temperatures predicted at the end walls at the lower portion of the radiant section. The CFD modeling indicated the flow emanating from the lower radiant burners closest to the end walls were slowed down and pushed away from the wall by standing vortices

located at the bottom of the radiant section. These vortices were caused by the flow being slowed down due to the presence of the catalyst tubes.

Figure 6. Flue gas temperature distribution in

the radiant section.

Figure 7. Flue gas temperature distribution in

the radiant section.

Similar insights for other flow distribution issues within the unit as well as burner performance observations were obtained from the CFD modeling. The use of the CFD analysis has provided IPL with understanding on how the reformer tube and burner layout and configuration affected the temperature distribution within the unit. This model can be used to make future operational decisions increasing the safe and economic operation of the unit, such as investigating the remaining life for a given flow-embedded component using CFD-predicted heat loads when updating the design or operating conditions. The added value of having an existing CFD model of the unit is that it can be readily used to understand the



implication on the flue gas heat distribution for possible design modifications during upcoming shutdowns as well as modifications to the unit’s operating conditions (such as: increasing the burner firing rates, upgrading the fan at the stack, etc.).

Finite Element Analysis (FEA)

The temperature information obtained from the CFD analysis was used as input in the Finite Element Analysis (FEA) to determine the critically stressed areas, which will be the life limiting locations. FEA can determine the state of stress in components, due to external and

internal loads and is invaluable when the problem to be solved is statically indeterminate. The front end of the ammonia plant required the creation of finite element analysis (FEA) global 1D piping-element model of the radiant section. This was then broken out into detailed FEA 3D solid-element sub-models of critically high stressed areas where the simplified global 1D piping model was unable to satisfactorily resolve the state of stress in a specific location. Figure 8 shows an example of the mixed feed coil outlet manifold.

Figure 8. Maximum von Mises stress location in mixed feed coil outlet manifold prior to stress

relaxation of 250.9 MPa (36.4 ksi), units of contour plot in MPa.



Fitness-For-Service (FFS) Assessments

The stress determined from the FEA was used as input to FFS assessments which were undertaken to the requirements of API579/ASME FFS-1. Due to the high metal temperatures that the radiant section and associated pipework operates under, creep was the predominant failure mechanism of concern. For the example of the mixed feed coil outlet manifold [6], the high level of stress (250.9 MPa (36.4 ksi) von Mises stress) was mainly caused by the system loading and throughout one year of operation, the initial maximum stress, 250 MPa (36.3 ksi) was predicted to relax to 41 MPa (5.9 ksi) due to creep. It is worth noting that the stress had already relaxed below 50 MPa (7.3 ksi) after 3000 hours. The evolution of the stress at the peak stress location from start-up has been shown over an uninterrupted 40,000 hour time period of operation in Figure 9 and the evolutionary changes in the stress was used in the remnant life calculations.

Figure 9. Maximum von Mises stress location

in mixed feed coil outlet manifold

during stress relaxation during

operation.

This component has operated for 231,108 hours and has undergone 46 thermal cycles. Installed in 1982, its first 18 years of life were at an operating and design temperature of 560˚C (1040 °F). Operating conditions changed in the year 2000. Based on thermocouple

measurements its remaining 11 years in service have been at an average operating temperature of 573.6˚C (1064.5 °F) a temperature well above the original design temperature. These thermocouples are located in the mixed feed coil outlet manifold and inlet header. It was assumed that the average operating hours between each thermal cycle as represented by a start-up and shut-down sequence of 5522 hours (7.5 months), with 29 thermal cycles occurring prior to the year 2000 and 17 thermal cycles occurring post the year 2000. This has been summarized in Table 1.

Operation Temperature

(°C)

Pressure

(MPa)

Number of

Start

(thermal

cycles)

occurred

Duration

of

operating

hours

between

starts,

hours

pre 2000 (18 yrs.)

560 (1040 °F)

2.93 (0.4 ksi)

29 5522

post 2000 (11 yrs.)

573.6 (1064.5 °F)

2.93 (0.4 ksi)

17 5522

Table 1. Assumed historic use for start

frequency (thermal cycles) and time

between starts.

Analysis assuming mean creep properties Using mean Omega creep parameters from API579 under the historic conditions specified in Table 1, the critical location in the mixed feed outlet manifold was assessed to have already reached 85% of its life. With the 15% life remaining, various scenarios of pressure and temperature were considered. This has been summarized in Table 2. The future on-line time for the unit was assumed high, close to 8760 hours of operation are obtained each year. This means that even if the operating temperature and pressures were reduced to design conditions, the mixed feed coil would have a remaining life of approximately 6 more years (52,540/8760) before failure is predicted.



Remaining life

(hr.)

Temperature ˚C (°F)

Pressure: 2.69 MPa (0.39 ksi)

Pressure: 2.96 MPa (0.43 ksi)

560 (1040 °F) 52,540 50,747

573.6 (1064.5 °F) 26,223 25,023

578.6 (1073.5 °F) 19,764 18,794

583.6 (1082.5 °F) 14,759 14,027

588.3 (1091.5 °F) 11,147 10,582

593.6 (1100.5 °F) 6,208 5,777

598.3 (1109.5 °F) 6,096 5,777

603.6 (1118.5 °F) 4,416 4,183

Table 2. Mixed feed coil outlet manifold

remaining operating hours versus

operating temperatures and

pressures with mean creep

properties.

As a result of the IOW process the FFS analysis led to the repair of the mixed feed coil outlet manifold, with a schedule redesign and replacement prior to the next major outage. Items considered for the redesign of this manifold are: Materials selection Replacement of a fixed mounting with a

spring hanger so that it reduces the system loading

Change of position at which the stub tubes enter the manifold.

These changes will reduce the peak stresses that led to the short predicted life of the original manifold for the operating conditions considered. However, a detailed assessment of the extent of life increase due to these changes has not been completed.

How IOW Results were Implemented

at GIW

The intent of an IOW study is not necessarily to produce a lower set of programmable limits within the plant’s distributed control system (DCS). Rather, the IOW results create a range of operating set points with regard to the governing process variable that influences the failure mode/mechanism. All stakeholders were invited to an IOW workshop where these process variable ranges are presented and the best limit is agreed upon. The best way to illustrate this is by example such as the mixed feed coil outlet pass tube row (hottest tubes). Using the Level 2 creep analysis of Part 10 in API-579/ASME-FFS-1 [4], a table of metal skin temperatures vs. remnant life was created (much like Table 2). This table was presented to the stakeholders as ‘the IOW results’, and the implementation of these results were agreed upon in a stakeholder workshop. For example, operations process engineers ruled out the lower and upper-most temperatures because they were unachievable in the current plant firing configuration. Maintenance engineers/managers preferred the lower temperatures as they resulted in longer equipment life and therefore greater campaign life between change-outs. Process development engineers preferred the higher operating temperatures because hotter mixed feed gas before the primary reformer catalyst tubes means a greater rate of conversion i.e. greater steam reforming of CH4 to produce more H2 per unit volume. Plant management and business analysts (accountants) calculated the optimized operating temperature based on the trade-off between extra production output and the cost of increased maintenance/change-outs, (i.e. 10-15 year life was optimal as the cost of the more frequent change-outs was exceeded substantially by the increased production output).



In this instance the IOW limit as implemented was actually a higher temperature than current operating temperatures because the understanding of the condition and equipment life expectancy was enhanced and well documented after completion of the IOW. The other type of IOW result is capital works required for sustained operation. The study found on two occasions that the serviceable lifespan of the equipment in question had either expired or was close to failure. The intended course of action is then further inspection, modification or replacement. This is illustrated by way of the three following examples: 1) Further Inspection: The inlet header tee IOW result indicated that the stress at a particular location would have resulted in a consumption of creep life in order of 30% had the metal exhibited minimum creep properties. If this was the case the microstructure would have contained some creep voids/fissuring on the grain boundaries and the item would require immediate replacement. Replication was carried out on the area of high stress plus three other locations for comparison and all showed acceptable microstructure with no voids or fissuring on the grain boundaries. Shear wave UT was also conducted to ensure there were no crack-like flaws. Hardness testing was completed to restore confidence in the material’s strength. The item was added to the change-out list for the 2015 turn-around, with the new design possibly using a better material metallurgy. No further action was required, other than limiting the operating temperature at a nearby temperature indicator. 2) Modification: The mixed feed coil outlet manifold was found to have an extremely high stress relaxation magnitude that may have exceeded yield in the one location of highest combined stress as illustrated in Figure 8. This area was un-inspectable due to its location on the intrados of the branch connection. The risk was unacceptable to the stakeholders, so a

modification plan was initiated. An FEA was done to assess the feasibility of jacketing the header with thick rolled plate to reduce stress by increasing cross-sectional area; and to provide a secondary containment in the event of a creep/fatigue failure at the original location. The management of change procedure was initiated and the modification was completed. The item was added to the change-out list for the 2015 turn-around, with the new design possibly using a better material metallurgy 3) Replacement: The outlet header was installed new in 2011; however, the old spring support system was reused. The IOW results indicated that the support was inadequate at the ends and the manifold ‘drooped’ somewhat in service. This consequently put excess bending strain on the end pigtails as well as the outlet manifold to Bull Tee welds, reducing the pigtail’s serviceable lives. The upgrade of the support system and modification was added to the capital list for the 2015 turn-around, with the new design intent to take load off the pigtails and re-establish their lives. The IOW study revealed some points of weakness under the current operating conditions. When these weaknesses are addressed and the plant is run at higher rates, the IOW results should also show the next level of weakness. The IOW study is therefore a useful tool for operations and development groups who will be making up-rate improvements overtime. As a result, operations and development groups should fully understand the effect their changes will have on the upstream and downstream items of equipment.

Benefits of the IOW program

The root cause analysis for the fire incident highlighted a number of systemic issues relating to the management of pressure equipment that required attention. Although RBI and Reliability Centered Maintenance (RCM) practices were in



place and implemented, there was opportunity for improvement on the integration and implementation of individual integrity management practices within the plant environment. In particular, processes which drove a holistic approach to integrity management integrating the respective plant disciplines around operations, process technology, maintenance and inspection were largely missing within the procedural framework. Draft best practice API RP 584 IOW was reviewed and found to provide a key procedural driver for improving plant processes in this respect. Implementation of the API RP 584 IOW meant an improved appreciation of the consequences of operating the plant in a particular fashion. It also defined key parameters through a multidisciplinary approach that allowed the implementation of the shared supervision of plant integrity and performance management. In some cases, it meant being able to run plant components beyond their original design life or above their original design pressures and temperatures in a controlled, safe and monitored manner. This is due to the conservatism inherent in the original design. One of the great successes that highlighted the value of implementing the IOW program internally was the determination of the remaining life expectancy of the mixed feed outlet manifold based on historic operational and support settings. Determination that this was the life limiting component, and that a repair was needed in order to get to the next major shutdown, so that a re-design manifold could then be fitted, brought tangible appreciation for the IOW program, as well as having better operational control limits on the plant, allowing it to be run in a more optimized state, with increased production.

When the repair of the manifold was being carried out, everybody, from the CEO to the trades assistant helping the welder, knew that this repair was necessary due to the findings of the IOW and without it there was a possibility of another catastrophic event before the next major shutdown. Another notable outcome was the hip coil outlet tubes. In 2007, these tubes were replaced with 304 material. The CFD study completed as part of the IOW project revealed that the temperature on these tubes was in excess of the design allowable temperature, because of a concentration of flow (channeling) through the Dietrich arch. The following FFS analysis showed the tubes were very close to failing by creep fatigue and they were scheduled for change-out with Incoloy 800H tubes. Inspection of the removed 304 tubes showed some creep damage from only 3-4 yrs. operation, meaning the change-out was very well warranted.

Final Words

Based on the successful implementation of the IOW process at the GIW plant, IPL is in the process of completing the IOW process at two of their other plants. The current API579 creep assessment failure criteria were found to be wanting and the R5 code [7] was used in its place. Efforts are being made to improve the creep assessment methodology as it’s rolled out at other plants/facilities. The main damage mechanisms at each plant that need to be given the highest consideration and are not all the same. For example, one of the plants for which analysis is still being performed shows that metal dusting of key components is likely to be the life limiting factor of safe plant life.



References

[1] API RP 584 Integrity Operating

Windows standard [2] Private communication [3] API 581 Risk Based Inspection

Standard [4] API 579/ASME FFS-1 Fitness-For-

Service Standard [5] Quest Integrity CFD report number

101317-Rev02 issued April 2010 [6] Quest Integrity FEA/FFS report

number 101303.01 Issued October 2011

[7] British Energy’s R5 code, “Assessment procedure for the high temperature response of structures”



implementation of the api rp 584 integrity operating

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