A Robust SRU Waste Heat Boiler Design - Brimstone …€¦ · 2012 Brimstone Conference • Limits...

A Robust SRU Waste Heat Boiler Design

Sean M. McGuffie, P.E. Porter McGuffie, Inc. Lawrence, KS, USA [email protected]

Dennis H. Martens, P.E. Porter McGuffie, Inc. Lawrence, KS, USA [email protected]

Michael J. Demskie Flint Hills Resources Pine Bend, MN, USA [email protected]

http://www.fhr.com/default.aspx�


2012 Brimstone Conference

• Michael J. Demskie – Flint Hills Resources - Reliability group

• Sean M. McGuffie – Porter McGuffie Inc. – Senior Engineer

• Dennis H. Martens – Porter McGuffie Inc. - Consultant and Technical

Advisor

Introduction of Authors




• Flint Hills Resources SRU # 5 WHB application – Pine Bend, Minnesota - 320,000 BPD Refinery

• An SRU WHB that was not reliable and the replacement with a reliable WHB

• What made the replacement WHB robust? – CFD analysis results for the replacement WHB

• State-of-the-art CFD analysis information

Introduction




• 525 LTPD SRU #5 initial startup in June 1993 – 2 pass in single shell kettle type – 600 psi sat steam – refractory and ferrule system

• May 1994 unplanned shutdown – tube leaks – Root cause – Burner vibration with refractory and

ferrule system failure – Replaced 73 of the 400 first pass tubes – refractory and ferrule system

WHB Chronological History




• May 1999 first planned shutdown – tube leaks – Root cause – refractory and ferrule system separation

from tubesheet face • Anchors burned up • chicken and the egg – cause and effect • Did the separation happen before or after the first tube leak?

– Replaced all 400 tubes in first pass – refractory and ferrule

Original WHB History




• Authors’ experience is for a >2500 °F operating temperature, the SS including 310 SS, anchors for a refractory and ferrule system always “burn off” to close to the tubesheet face or the face of the insulation for the tubesheet as the refractory temperature is above the ~ 2500 °F melting point for the SS anchors and sulfidation metal sulfide melting point of ~ 1650 °F.





• July 2002 unplanned shutdown – tube leaks – Root cause – undetermined - lack of consensus – Major corrosion on face of tube sheet (see picture) – Repair weld buildup on tube sheet – Replaced all 400 tubes in first pass – Significant down time and reduced SRU capacity – Two piece removable ferrules installed





Original WHB History 2002

Scale removed Sandblasted




• Life: installed in 1993 and replaced 2003 = 10 years • 1994 unplanned shutdown - replaced 73 tubes • 1999 first planned shutdown – replaced 400 tubes • 2002 unplanned shutdown – replaced 400 tubes

– significant shutdown and SRU capacity reduction The original WHB essentially had 3 failures in 9 years

with one failure occurring at a planned shutdown

Original WHB History Summary




• For Reliability FHR decided to replace Original WHB – Fit in existing plant plot space

• Don’t move furnace or condensers

– More robust design – better service reliability – Single pass kettle type – long tubes – Similar process conditions

• Consideration for O2 enrichment

• Sept 2003 - Second planned shutdown – new WHB

Replacement WHB History




• October 2006 - Unplanned shutdown due to furnace refractory problem - hot spot – Inspected WHB – removed some ferrules – Ferrules in good condition – No corrosion noted – SO FAR SO GOOD – 3 years of operation

• No O2 enrichment operation at this point • O2 operation begins July 2008





• September 2008 - First planned shutdown – Inspected WHB – removed all ferrules – Ferrules in good condition – No significant corrosion noted (see picture) – Two piece removable ferrules – same design – SO FAR SO GOOD – 5 years of operation

• O2 operation began July 2008 (2 months)





Replacement WHB 2008

5 years operation




• September 2012 status – 9 years service – No unplanned WHB shutdowns to date – SO FAR SO GOOD – Significant O2 enrichment service time to date – Next inspection plan is to remove all ferrules and

inspect • Tubesheet - tube ends – length of tube

Replacement WHB




• Installed 2003 and currently in operation 2012 • 2006 Inspection, unit down for furnace repair

– No corrosion or leaks found – no repairs • 2008 first scheduled turnaround - inspection

– No corrosion or leaks found – no repairs The replacement WHB had no unscheduled shut downs

or repairs in 9 years – SIGNIFICANT IMPROVEMENT COMPARED TO ORIGINAL WHB (3/9)

Replacement WHB History Summary




Please raise your hand if you have one or more SRU WHB’s in your custody

and care

POP Survey of Audience




Keep your hand raised if you have a WHB that has operated 5 years

without any corrosion or unscheduled shutdowns due to tube leaks

(not just fouling)





Keep your hand raised if you have a WHB that has operated 10 years

without any corrosion and unscheduled shutdowns due to tube leaks

(not just fouling)





• Reoccurring issue of tubesheet and tube end corrosion–leaking tubes – Temperatures > 600 °F for corrosion to occur

• July 2002 failure had severe ferrule ID deposits that reduced flow path to pencil size

• High pressure drop – hot gas bypassing – heating tubesheet • Observed average localized corrosion rate from1999 – 2002

(3 years) was in excess of 50 mil/yr indicating > 850 °F metal temperatures (see corrosion graph)

Original WHB Design/Failure Review





0.001

0.01

0.1

1

10

500 600 700 800 900 1000 1100

Temperature (F)

Mol

% H

2S

1 mil/year2 mils/year3 mils/year5 mils/year10 mils/year15 mils/year20 mils/year25 mils/year30 mils/year40 mils/year50 mils/year

No Corrosion

f

@ 4% H2S 50 mil/yr 850 °F




• Why the high temp at the tubesheet and tube ends? – Refractory and ferrule system possible issues:

• Improper design -heat flux – thickness of refractory • Improper installation – QC - materials and workmanship • Improper dryout procedures • Burner vibration damage to refractory and ferrules • Inadequate operational parameter control • Chicken and the Egg – tube leak or refractory/ferrule • 2002 FHR Root cause – undetermined - lack of consensus





• Tubesheet-to-tube inlet end joint possible issues: – 2” thick tubesheet with two grooves – rolled - seal weld

• concern for high tube compression/tension loads on joint?

– Chicken and the Egg – tube leak or refractory/ferrule • Obviously there are a number of possible issues any

one of which could initiate the problem and result in high corrosion rates and leaking tubes





• 5 year turnaround intervals • 20 year life • Avoid unplanned shutdowns due to WHB by:

– Avoiding high temperatures at tubesheet (corrosion) – Avoiding tube-to-tubesheet joint leakage – Avoiding tube damage downstream of ferrules

• Not a significant problem for the original WHB

Robust Replacement WHB Criteria




Item Original WHB - First Pass Replacement WHB Type 2 Pass - 1 shell - kettle 1 Pass - 1 shell - kettle

Diameter of tubes 2 ¼” OD 2.03” ID 3” NPD pipe 2.9” ID

Number of tubes 400 479 Tube length 15’ – 0” 36” – 0”

Tube specification SA 213-T11 - 12 BWG A 106B Sch 80 Mass flux lb/sec-ft2 4.4 1.75

Tube sheet thick 2” 1 ¼ “ Tube sheet material SA 516-70 SA 516-70

Inlet TS joint Roll and seal weld Full penetration weld Tube projection 7/16” No projection

Original and Replacement WHBs




• Avoiding high temp corrosion – tubesheet/tubes – Used FHR two piece removable ferrule experience

• Installation, inspection and maintenance procedures

– Limit mass flux – limited ferrule inlet pressure loss – Establish/manage max process temperature and flow

• Startup and shutdown considerations

– No tube projection past face of tubesheet – Minimize tubesheet thickness





• Avoiding tube-to-tubesheet joint leakage – Utilized full penetration tube-to-tubesheet weld – Minimized tubesheet thickness – Used thick wall pipe - accommodates FP weld – No tube projection past face of tubesheet – Startup and shutdown considerations (tube stress)

• Rigid tube sheet considerations (tube-to-shell temperatures)





• Avoiding tube damage downstream of ferrules – Limited mass flux

• Limiting turbulence at ferrule outlet • Limiting max heat flux

– Establish/manage max process temperature and flow • Startup and shutdown considerations





• The operational history of the replacement WHB indicates a robust design was achieved

• FHR decision to analyze the robust design – Determine how design features affect performance

• Maximum heat flux • Tube/Tubesheet operating temperatures • Pressure drop in ferrule assembly

CFD Analysis of Robust WHB




• To establish the necessary process input, PMI partnered with KPS Technology & Engineering to: – Data mine 6 years of DCS data to determine periods of

interest and maximum WHB duty – Process combustion calculations using Sulsim 7 for

the periods of interest and development of gas composition data

• Consensus selection of representative process conditions for CFD analysis





• Develop process-side model • Fully coupled heat transfer problem

– Requires the inclusion of all assembly geometry





• Initially developed 3 process conditions of interest, based on DCS data





• Velocity results were typical – Recirculation occurs downstream of ferrule – Increased turbulence at this location increases

maximum heat transfer • Primary use of CFD is to estimate the flux multiplier at

this location





• Analyses indicated large margin to established limits – Maximum heat flux was below 50,000 BTU/hr-ft2

– Maximum tubesheet temperature was below sulfidation limit

– Pressure drops were acceptable





• Abandoned remaining cases from DCS data • Performed analyses at maximum established firing

temperature (refractory limit) • Performed analyses at greater mass flows than

recorded in operation • Used results to provide guidance for future operation





• Limits must be based on measurable process parameters – Zone 1 and 2 temperatures – Mass / Volume flow

• Process parameters are used to develop pseudo-duty (product of mass flow and Zone 2 temperature) for each case analyzed

Determining Operational Limits




• Perform curve fit for pseudo-duty versus maximum flux





• Relate pseudo-duty to operational limits – Maximum flux – Maximum metal temperature

• Algebraically solve for allowable mass flow at a given temperature

• Derate curve based on process measurement limits and unknowns with CFD model





• CFD is a method of solving the Navier-Stokes Equations

• Equations are nonlinear and require a numerical procedure to solve all but the most trivial problems

Unknowns in CFD




• Due to computing requirements, techniques have only been in use recently – Limits for physics models implemented – Limits on applicability based on experience – Primary simplifying assumption

Unknowns in CFD




• Consideration of turbulence requires grids with a large number of cells with transient integration

• Not suited to engineering applications, only for laboratory environments

• Steady-state solutions incorporate turbulence models – Methods to account for the transient nature of the flow

Primary Simplifying Assumption




• Turbulence models can be sensitive – Grid topology – Model selected – Flow conditions

• Adverse pressure gradients • Recirculating flow • Detached flow





• Recirculation occurs downstream from ferrule

• Known problem with turbulence models





• Consider this graph prepared for ASME Tutorial Primary Simplifying Assumption




• DNB – Most limits developed by nuclear industry • Sulfidation – Temperatures on Couper-Gorman curves

are estimates • Gas bypass – Bypassing can be highly sensitive to

geometry of assembly as installed

Other Limitations in CFD use for WHBs




• Detached Eddy Simulation (DES) and Large Eddy Simulation (LES) are available

• Reduce computational expense from Direct Numerical Simulation (DNS)

• Transient analysis techniques that avoid some of the major limitations of turbulence models

What if More Accurate Information is Needed?




• Dissimilar time-scales between flow phenomena and thermal time constants require difficult procedure

• Two-models must be used and boundary conditions exchanged

• Statistical analysis of output required • Orders of magnitude greater effort





DES Analysis of WHB Ferrule

Temperatures with Turbulence Model

Temperatures with DES Model




DES Analysis of WHB Ferrule




• Through the use of proper design, successful WHBs are possible – Process design and operating limit parameters must

be considered and evaluated for impact on reliability • Mass flux • Operating temperatures • Maximum heat flux

Conclusions




• Guidance for these parameters is not well addressed in any industry consensus document

• Consideration of parameters is, instead, typically left to the SRU licensor

• Evaluation of capital costs versus reliability – Impacts of unscheduled shutdowns are very significant – Repairs often extend shutdowns

Conclusions




• Three Critical areas to consider for WHB designs

1. Avoid high metal temperatures at the tubesheet

2. Avoid tube-to-tubesheet joint leakage

3. Avoid tube damage downstream of the ferrules

Conclusions




• Seven design guidance features to consider and incorporate to address the 3 critical design areas:

1. Ferrule system design – Limit maximum tube end temperature to not more than 615 °F

2. Mass flux – Limits maximum heat flux at end of ferrule and pressure drop through ferrule. Authors suggest 2.5 lb/sec-ft2 for waste heat boilers at higher operating temperatures

Conclusions




3. Establish and limit process temperature – design the WHB accordingly and manage operations consistent with these limits

4. Minimize tube projection past the front face of the tubesheet – flush is best practice - no fin

5. Minimize tubesheet thickness – both conductance through the tube sheet and the heat input through the area of the tube holes are important

Conclusions




6. Use full penetration welds - or strength welds and rolling – but do not use seal welds for high pressure steam applications or tube end loads

7. Limit pressure drop through the ferrule – associated with # 2 above. Process gas can be driven through a refractory and ferrule or removable ferrule systems which increases the metal temperature and likelihood of corrosion

Conclusions




• CFD can be used to qualify designs – Determine operational temperatures and fluxes – Compare to limits

• DNB – 50 – 70 kBTU/hr-ft2 - evaluate turbulence multiplier

• Temperature – 600 °F – avoid corrosion • Pressure drop – 0.15 psi at inlet of ferrules - can be a problem

if a significant gas path through the ferrule system is present

– Limitations of CFD results must be understood

Conclusions




• Common to evaluate failures to “not make the same mistake again”

• Uncommon to evaluate a WHB to determine why it is reliable

• Incorporating knowledge and experience can help prevent unscheduled shutdowns

Conclusions




• There is not one set of numerical values that are applicable to all WHB applications – For instance, mass flux for large ID tubes may be

unsuitable for small tubes – Understanding how all design parameters influence

the WHB’s performance is critical for reliable designs

Conclusions




• “The height of stupidity is doing the same thing the same way and expecting a different result”

• “The height of good engineering is determining what resulted in high reliability and doing the same thing the same way and expecting the same results.”

Conclusions




Thank you for your attention

Questions


A Robust SRU Waste Heat Boiler Design - Brimstone …€¦ · 2012 Brimstone Conference • Limits...

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