Vulnerable to Brittle Fracture

HydrocarbonProcessing.com | NOVEMBER 2014

®

PETROCHEMICALSFCC can be used

to produce olefins and aromatics on-purpose

MAINTENANCEProcess equipment may be

vulnerable to brittle fractures

REFINING DEVELOPMENTSUltra-fine solids need

aggressive treatment to protect heat-transfer networks

SPECIAL REPORT:

Plant Safety and Environment

Maintenance and Reliability

B. MACEJKO, The Equity Engineering Group, Inc., Shaker Heights, Ohio

Is your plant vulnerable to a brittle fracture?Much of the process equipment operating today was de-

signed to construction codes that did not require a formal evaluation for low-temperature considerations. Metal tem-perature highly influences the fracture toughness of construc-tion materials of plant equipment. At low temperatures, some materials tend to behave in a brittle manner, making it much more susceptible to fracture. The author discusses methods to identify potential brittle-fracture conditions in process equip-ment before failures occur.

BACKGROUNDThe majority of pressure equipment used in the refining and

petrochemical industry is constructed of carbon or low-alloy steel. Metal temperature highly influences the fracture tough-ness of these materials. At low temperatures, construction ma-terials can behave in a brittle manner (i.e., like glass), making them more susceptible to fracture. At high temperatures, the materials tend to behave in a ductile fashion. Pressure vessels and piping may experience low temperatures from the ambient environment or from operating and upset conditions.

The potential for auto-refrigeration from depressuriza-tion of liquefied compressed gases can be particularly con-cerning due to the extremely low equipment metal tem-peratures (–55°F and below). An engineering evaluation is typically required to assess whether low-temperature operat-ing or upset conditions could result in a brittle-fracture fail-ure of pressure equipment.

Brittle fracture. This is the sudden and rapid propaga-tion of a crack-like flaw under stress (residual or applied) where the material exhibits little or no evidence of ductility or plastic deformation.1 Although rare, the consequences of a brittle fracture are typically catastrophic. Brittle-fracture failures experienced within industry have resulted in costly unplanned repairs, extensive production downtime and loss of life (FIG. 1).

There can be a large variation in the way owner-users as-sess the risk for brittle fracture. Owner-users that have pre-viously experienced such a failure may have detailed and comprehensive programs in place to evaluate susceptibility to future issues. Alternatively, owner-users fortunate enough to have avoided such a failures tend to be less cognizant of potential risks. Just because a failure has not occurred yet, it does not mean it will not happen tomorrow.

WHY EVALUATE BRITTLE FRACTUREMuch of the plant equipment operating today was designed

to codes of construction that did not require a formal evalua-tion for low-temperature considerations. However, catastrophic failures and subsequent root-cause investigations have revealed deficiencies in code requirements. Present process equipment design and post-construction codes and standards have taken action to address the industry need to assess for brittle fracture.

OSHA 1910.119 process safety management requirements do not allow owner-users to ignore deficiencies in the original code of construction:2

• 1910.119(d)(3)(ii)—The employer shall document that equipment complies with recognized and generally accepted good engineering practices.2

• 1910.119(d)(3)(iii)—For existing equipment designed and constructed in accordance with codes, standards or practices that are no longer in general use, the employer shall determine and document that the equipment is designed, maintained, inspected, tested, and operating in a safe manner.2

Owner-users should consider a brittle fracture evaluation for any pressure-retaining equipment item where:

• Original design did not consider susceptibility to brittle fracture

• A change in process operating conditions increases the possibility that low-metal temperatures has occurred

• A PHA or HAZOP identifies process or ambient temperatures lower than anticipated in the original design, i.e., depressurization/auto-refrigeration potential

• The owner-user desires to rerate using a lower design margin

• The owner-user desires to optimize the timing of startups or shutdowns

FIG. 1. Ductile vs. brittle fracture failures.a

Originally appeared in:November 2014, pgs 68-76.Used with permission.

HYDROCARBON PROCESSING NOVEMBER 2014


• The owner-user intends to complete a hydrostatic pressure test at a temperature lower than the original test temperature.

KEY COMPONENTSThere are three key components that drive a brittle-frac-

ture failure:• Stress (residual and/or applied)• Material toughness• Crack-like defect/flaws.A combination of metallurgical structure, residual stress

and inherent defects that act as stress intensifiers (lack of fu-sion, lack of penetration, porosity, and slag inclusions make welds and weld-heat-affected zones critical locations for po-tential brittle fracture.

Stress. Stress provides the energy necessary to drive a defect to fracture. Typical stress sources include pressure, weight and thermal loads, in addition to residual stress from the welding processes. A post-weld-heat treatment (PWHT) operation will significantly reduce weld-residual stress.

Material toughness. Metal temperature greatly influences material toughness in carbon steel (CS) and low-alloy steels. At low temperatures, the construction materials can behave in a brittle manner, and they have a high susceptibility to frac-ture. Conversely, the materials act in a ductile fashion at high temperatures. The fracture-appearance-transition temperature (FATT) is defined as the temperature corresponding to 50% shear. In FIG. 2, the FATT may be approximated as point D in the plot of Charpy impact energy as a function of temperature. Additionally, the material chemistry, grain size and heat treat-ment all affect toughness. The brittle-to-ductile-transition temperature will decrease (i.e., improve) with:

• Decrease in material carbon content• Increase in manganese to carbon ratio• Decrease in sulfur content• Decrease in material grain size• Normalization heat treatment of material through

rapid cooling.Partly due to chemistry control issues, older (or dirty) steels

often have a moderate brittle-to-ductile-transition slope rela-tive to clean steels (FIG. 3). Most of the common steels used to construct pressure equipment in the 1970s, 1960s and earlier (such as A70, SA-212, SA-201, SA-283, SA-285 and SA-515-70) have exhibited relatively high FATT (i.e., low toughness). A number of environmental issues exist that can degrade mate-rial toughness or result in embrittlement. The adverse effects of any such potential damage mechanisms should be consid-ered in a brittle-fracture evaluation.

Crack-like defect. Brittle fracture typically initiates at a crack-like defect. These defects can result from environmental damage (such as exposure to wet hydrogen sulfide or caustic), mechanical damage (such as gouges or dents), or from original fabrication (such as laminations, lack of fusion, lack of penetration, slag inclu-sions, and porosity). Performing a detailed inspection including both surface examination techniques (dye penetrant or magnetic particle examinations) and volumetric examination techniques (angled-beam ultrasonic methods) can be used to detect and cat-egorize any crack-like defects present in pressure equipment.

ASSESSING SUSCEPTIBILITY TO BRITTLE FRACTURE IN PRESSURE VESSELS

Industry codes and standards assess susceptibility to brittle fracture by comparing a critical exposure temperature (CET) to a minimum allowable temperature (MAT). The CET rep-resents the driving force for fracture and consists of the low-est-potential metal temperature from all operating, upset or atmospheric conditions. The MAT represents the material resistance to fracture, either calculated through engineering evaluation techniques or assigned based on destructive testing (i.e., Charpy impact testing). Both the CET and the MAT can consist of a single temperature and coincident pressure or an envelope of temperature and pressure combinations.

ASME Section VIII Division 1. Most pressure vessels at petro-chemical facilities were designed to ASME Section VIII Division 1 (ASME S8D1).3 The 1987 Addenda to the 1986 Edition in-troduced drastic changes to Part UCS-66 due to brittle fracture concerns with CS and low-alloy steels. Prior to this, ASME S8D1

Char

py im

pact

ener

gy

Lower shelf

Lower shelf

Upper shelf

Upper shelfNew steel

Old steel

Temperature

FIG. 3. Brittle-to-ductile transition for “old” steels.

Ductile

Brittle

Lower shelf:A - B

Temperature

Char

py im

pact

ener

gy

B/C

B

B

C C

D

A

Upper shelf:A + B

Transition zone

FIG. 2. Plot of Charpy impact energy FATT.



permitted operation of CS equipment to a temperature of –20°F without testing. Considering that no new refinery has been built in the US since the 1970s, the majority of pressure equipment operating today did not receive a formal assessment for suscepti-bility to brittle fracture. Currently, ASME S8D1 provides a series of impact-test exemption curves to assign a baseline MAT based on the material of construction and weld-joint governing thick-ness. ASME S8D1 also permits application of a temperature re-duction if the stress levels are below the design allowable stress.

API 579-1/ASME FFS-1 Fitness-For-Service. The API 579-1/ASME FFS-1 (API-579) Fitness-For-Service (FFS) standard provides procedures and criteria to evaluate equipment in post-construction service.4 The Part 3 Level 1 and Level 2 methods used to screen for the propensity for brittle fracture are consistent with the ASME S8D1 design philosophy. The Level 3 method (which references Part 9 of API-579) includes a detailed fracture mechanics evaluation. Characteristics of the three levels include:

Level 1:• Typically completed by an inspector or a plant engineer• Table or chart exemption curve lookup based on material

of construction and joint governing thickness.Level 2:• Typically completed by a plant engineer• Screening method extension of the Level 1 approach

with more detailed and prescriptive calculations• Three analysis method options are provided and

described here. However, typically, Method A is the only practical and reasonable option:o Method A. Calculation of a safe envelope based on

an evaluation of actual stress compared to design allowable stress (i.e., trade lower stress levels for lower permissible temperatures).

o Method B. Determine the MAT based on the metal temperature at time of hydrotest. This method is not practical or feasible for many instances. Substantial risks exist for fracture during the hydrotest.

o Method C. Grandfathering approach based on a proof-test argument. This method assumes that if it can be shown that the combination of worst-case applied stress, temperature and flaw size has already occurred and did not result in a failure, then other less-severe conditions will not result in a failure. This method requires detailed historical operating, inspection and repair information that is not available in many instances.

Level 3:• Completed by an engineering specialist with extensive

experience in FFS.• Most reliable and accurate method for establishing

a MAT because it involves a detailed fracture mechanics evaluation

• Required when the equipment operates in a service where brittle fracture is a legitimate concern or if a crack-like flaw is known to exist

If the equipment has undergone PWHT at original construc-tion and all subsequent repairs or alterations have not adversely compromised the effectiveness of the PWHT, then the FFS anal-ysis calculation procedures allow credit for the positive benefit from the stress-relief operation on the calculated MAT. PWHT

is often critical for low temperature (below –55°F) acceptability.

CURRENT CODES AND STANDARDS DEFICIENCIES

A number of deficiencies and inconsistencies exist with the ASME S8D1 and API-579 Level 2 FFS methods. All of these items enforce the requirement to complete a Level 3 Part 9 fracture me-chanics evaluation if brittle fracture is a legitimate concern.

Applied stress threshold and impact testing of weld-ments. Both ASME S8D1 and the Part 3 Level 2 procedure in API-579 assign a MAT of –155°F to a component if the calcu-lated stress ratio falls below a certain threshold (that depends upon the original code of construction factor for safety). How-ever, Part UCS-67(c)(3) of ASME S8D1 requires mandatory impact testing of welds to qualify metal temperature below –55°F. This weldment impact test requirement applies regard-less of the stress ratio. It could be argued that the API-579 Level 2 procedure should also require the weldment impact test.

Additionally, API-579 implies that, for a component thick-ness less than 2 in. and a general primary membrane tensile stress less than 8 ksi, brittle fracture will not occur. Although this may be true for equipment that has had PWHT performed, failures of non-PWHT weld joints due to weld-residual stress alone have occurred. Therefore, the 8-ksi primary stress thresh-old may not be appropriate in all instances.

PWHT temperature credit. For P-1 Group 1 and P-1 Group 2 materials, both ASME S8D1 and API-579 (Level 1 or 2) al-low a temperature reduction of 30°F from the component MAT if PWHT was completed and the component thickness does not exceed 1.5 in. The origin and engineering justification for the 30°F reduction are unknown. As noted in WRC 528, esti-mates of the weld-residual stress were directly considered in the ASME Section VIII Division 2 (ASME S8D2) exemption curves development.5, 6 Thus ASME S8D2 provides different exemp-tion curves for PWHT and non-PWHT materials. The ASME S8D2 approach appears more technically appropriate when compared to the general 30°F reduction.

NBIC alternative weld methods. The National Board In-spection Code (NBIC) provides a series of alternative welding methods without PWHT.7 These include various combinations of preheat temperatures, weld-interpass temperatures and tem-per-bead procedures. Although these welding methods may help to slightly temper the material, they do not significantly reduce weld-residual stress when compared to a proper PWHT stress-relief operation. Therefore, without further study, it is difficult to justify credit for a full PWHT in a brittle-fracture evaluation if one of these alternative welding methods were used.

CASE STUDY 1This example illustrates the use of the Level 2 FFS method-

ology for brittle-fracture screening of typical “pots and pans” pressure vessels that meet these limitations:

• Do not operate in cyclic service• Not susceptible to any shock-chilling event• Not susceptible to any environmental cracking• Do not operate in a service that may result in loss



of material toughness• Have no known crack-like flaws• Where excessive low-temperature operation

(below –55°F) is not feasible.In this case history, a service review identified that the

knockout (KO) drum has the potential under ambient condi-tions to reach a minimum metal temperature of –10°F. Because the Level 1 analysis for the vessel indicated a baseline MAT of 53°F, a Level 2 FFS evaluation was completed. TABLE 1 summa-rizes the construction criteria for this KO drum.

Analysis. Level 2 FFS calculations were performed in accor-dance with Part 3, Method A of API-579 using proprietary soft-ware. The Level 1 MAT for each component was obtained us-ing the component governing thickness and respective material curve. In the Level 2 calculations, credit was taken for any ad-ditional plate thickness, above the minimum required thickness, through the use of a stress ratio. The evaluation then assigned a temperature reduction based on the calculated stress ratio. The final MAT was determined as the Level 1 MAT minus the tem-perature reduction.

Results. A Level 2 FFS evaluation established the maximum permissible pressure of 174 psig at the client-specified CET of –10°F. Additionally, the evaluation provided an envelope of per-

missible pressure-temperature combinations that the owner-user could compare against all operating and upset scenarios (FIG. 4). This enabled the owner-user to develop operational procedures to ensure satisfac-tory protection from a potential brittle-fracture event.

CASE STUDY 2This example illustrates the use of the Level 3

FFS methodology for brittle-fracture evaluation of a deethanizer column that operates in a service with auto-refrigeration potential. TABLE 2 summarizes

the operating conditions for the deethanizer column.

Analysis. A rigorous, fracture mechanics-based assessment in accordance with the Level 3 procedures and criteria in Part 9 of API-579 was performed to establish the allowable pressure-temperature curve using proprietary software. The analysis performed assumed a detectible reference flaw size that could be compared to the results of detailed inspection for cracking. A comprehensive, 3D finite-element (FE) model was used to characterize operating stresses (FIG. 6).

The failure assessment diagram (FAD) fracture mechan-ics methodology presented in Part 9 of API-579 (FIG. 4) was used to calculate the allowable pressure-temperature curve. A postulated semi-elliptical surface-breaking flaw with a depth of 1⁄4 of the nominal shell thickness and a length of six times the depth (i.e., 6:1 aspect ratio) was analyzed in various orien-

TABLE 1. Material construction background for a KO drum

Code of construction ASME S8D1

Year of construction 1965

Material SA-212-B

Design pressure 300 psig

Joint efficiency 100%

PWHT No

Inside diameter 60 in.

Thickness 0.75 in.

Metal temperature influences the fracture toughness of construction materials for plant equipment. At low temperatures, construction materials can behave in a brittle manner, thus making them more susceptible to fracture.

TABLE 2. Material construction background of the deethanizer column



Material SA-515-70



PWHT Yes


Thickness 1.5625 in.

Stress intensity factorsolution, KI

Kr =KI

KMAT

Material toughness,KMAT

Failure assessmentdiagram envelope

Mixed mode – brittlefracture and plastic

collapse

Plastic collapse

Material yield stress,�ys

Reference stresssolution, �ref

Flaw dimensions Stress analysis

Assessmentpoint

Brittle fracture

Load ratio

Unacceptableregion

Acceptableregion

Lr =�ref�ys

Toug

hnes

s rat

io

Flaw dimensions Stress analysis

FIG. 4. API-579 Part 9 FAD diagram.



tations at critical locations. The presence of such large flaws, while unlikely, should be reliably detected by modern inspec-tion techniques (including surface and ultrasonic inspection techniques). The calculated operating and weld-residual stress, along with the material strength and material fracture tough-ness, were used to determine a toughness ratio and a load ratio. These two quantities represent the coordinates of a point that was plotted on a 2D FAD to determine acceptability.

Results. A Level 3 FFS evaluation established the envelope of permissible pressure vs. temperature curves, as shown in FIG. 7. This enabled the owner-user to develop operational procedures to ensure satisfactory protection from a potential brittle-frac-ture event. The bottom curve in this figure shows the boiling point curve for the process fluid, ethane.

CASE STUDY 3This example illustrates the difference in results from a Level

2 FFS vs. a Level 3 FFS for brittle-fracture evaluation of a pres-sure vessel that did not receive a PWHT, but operates in a service with an auto-refrigeration potential. TABLE 3 summarizes details for a pressure vessel subject to auto-refrigeration with no PWHT.

Analysis. The assessment included a Part 3 Level 2 FFS brit-

tle-fracture screening evaluation in addition to a detailed Part 9 Level 3 FFS fracture mechanics evaluation using proprietary software. The methodologies for the Level 2 and Level 3 evalu-ations were consistent with those detailed in Case Studies 1 and 2, respectively.

Results. The results from the Level 2 FFS evaluation and the Level 3 FFS evaluation (for the recommended postulated flaw depth of 1⁄4 of nominal thickness) both indicate that the vessel does not satisfy the requirements for protection against brittle

FIG. 6. FEA stress results plot at local discontinuity.

0-160 -140 -120 -100 -80 -60 -40

Temperature, °F-20 0 20 40 60 80 100

20

40

60

80

100

Pres

sure,

psi

120

140

160

180

200

220

240

260

280

300

Acceptable regionPressure-temperature curveUser operating curve

FIG. 5. Level 2 MAT curve results for Case Study 1.

0255075100125150175200225250275300325350375400425450475500525550

-160

-150

-140

-130

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20 -10 0 10 20 30 40 50 60 70 80 90 100

Inter

nal p

ressu

re, p

sig

Temperature, °F

Level 3 (Part 9) MAT curve w/o PWHT

Ethane boiling point curve Part 3 Level 2 - shell Part 9 Level 3 - shell (t/4 flaw) Part 9 Level 3 - shell (t/8 flaw) Part 9 Level 3 - shell (t/16 flaw) Part 9 Level 3 - shell (t/4 flaw) Annex E

FIG. 8. Level 2 and Level 3 MAT curve results for Case Study 3.

0255075100125150175200225250275300325350375400425450475500

-160

-150

-140

-130

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20 -10 0 10 20 30 40 50 60 70 80 90 100

Inter

nal p

ressu

re, p

sig

Temperature, °F

Level 3 (Part 9) MAT curve (t/4 flaw)

Ethane boiling point curve Level 3 - shell away from nozzles Level 3 - shell @ M1/M2


TABLE 3. Details for a pressure vessel subject to auto-refrigeration with no PWHT



Material SA-515-70



PWHT No


Thickness 1.1875 in.



fracture, as shown in FIG. 8. However, the evaluation shows that the Level 2 results are more favorable than the Level 3 results for pressures below 350 psig. Even if the Level 3 analysis uses a flaw depth of 1⁄8 of nominal thickness, the Level 2 results remain more favorable than the Level 3 results below 230 psig. This ap-parent contradiction occurs because weld residual stress greatly influences the permissible MAT, and it completely drives the MAT at lower applied stresses (i.e., low pressure). These results highlight the importance of completing a detailed Level 3 eval-uation when brittle fracture is a legitimate concern.

However, if the system was re-evaluated to the Level 3 FFS using a postulated flaw depth of 1⁄16 of nominal thickness, as op-posed to the 1⁄4 or 1⁄8 of nominal thickness, then the calculated MAT curve is above the process boiling point curve, thus indi-cating the acceptability of the 1⁄16 thickness flaw depth. There-fore, with a much more extensive and comprehensive inspec-tion plan, qualification for protection against brittle fracture may be justifiable so long as the inspection techniques are sen-sitive and thorough enough to ensure that no defects greater than 1⁄16 of the nominal shell thickness exist.

CASE STUDY 4This example illustrates the use of a Level 3 FFS brittle-frac-

ture evaluation to optimize the cool-down rate of a hydrodesul-furization (HDS) reactor.

Background. To expedite the cool-down process and limit downtime for an HDS reactor, owner-users often inject cold nitrogen into the reactor upstream piping system. To deter-mine the rate limits for the cool-down process, a formal FFS evaluation was conducted.

Analysis. The study of the cooling process used a rigorous Part 9 Level 3 fracture mechanics assessment to establish the allow-able pressure-temperature curve. The analysis was performed for a reference flaw size that could be readily detected in de-tailed inspections for cracking. A comprehensive, 3D FE model determined the stresses due to internal pressure and thermal transient effects during the cooling process, as shown in FIG. 9.

Results. The results of the Level 3 FFS evaluation (FIG. 10) were used to optimize the cool-down procedure and enable the owner-user to minimize downtime without adversely affecting the mechanical integrity of the equipment.

OVERVIEWThere have been a number of catastrophic brittle-fracture

failures in the petrochemical industry. Deficiencies in histori-cal codes of construction and discrepancies in present codes and standards have been identified. Pressure equipment must be properly assessed to qualify for low-temperature service. In many instances, a Level 2 FFS evaluation may not be sufficient or appropriate. If a legitimate concern exists for brittle fracture due to the potential for cracking, or if metal temperatures below –55°F are achievable, a detailed Level 3 Part 9 FFS fracture me-chanics evaluation should be completed. The Part 9 Level 3 eval-uation, coupled with PWHT and a detailed inspection plan, can be used successfully to qualify low-temperature acceptability.

ACKNOWLEDGMENT a Source for Fig. 1 is: Callister, W. D. and D. G. Rethwisch, Fundamentals of Materials

Science and Engineering: An Integrated Approach, 4th Ed., and Callister, W. D., Fundamentals of Materials Science and Engineering, 5th Ed., pg. 257, Fig 9.3.

LITERATURE CITED 1 API Recommended Practice 571, Damage Mechanisms Affecting Fixed Equipment in

the Refining Industry, Second Ed., April 2011. 2 Occupational Safety and Health Administration, Part Number 1910, Subpart

H, Standard Number 1910.119, Process Safety Management of Highly Hazardous Chemicals.

3 ASME B&PV Code Section VIII, Division 1, Rules for construction of pressure vessels, ASME, July 2013.

4 API 579-1/ASME FFS-1, Fitness-For-Service, June 5, 2007 (API 579 Second Ed.). 5 WRC Bulletin 528, “Development of Material Fracture Toughness Rules for the

ASME B&PV Code, Section VIII Division 2.” 6 ASME B&PV Code Section VIII, Division 2, Alternative Rules for Construction of

Pressure Vessels, ASME, July 2013. 7 National Board Inspection Code , Part 3 Repairs and Alterations, 2013.

BRIAN MACEJKO is the head of the pressure vessel group within the mechanical engineering business unit of The Equity Engineering Group, Inc. (E2G). He is also a member of the ASME/API Joint Committee on Fitness- For-Service. He has experience as both an owner-user and, as a consultant providing engineering support to oil and gas and petrochemical facilities. The primary focus of his

experience has been in the design, maintenance/repair, failure analysis, and fitness-for-service activities for fixed equipment.

FIG. 9. Thermal and stress plots at reactor quench nozzle.

150 °F/hr - quench nozzle 100 °F/hr - quench nozzle 75 °F/hr - quench nozzle Prior limit Recommended limit

0200

400

600

800

1,000

1,200

1,400

1,600

1,800

-20020406080100120140160180200220240260

Pres

sure

, psi

Temperature, °F


Eprinted and posted with permission to Equity Engineering Group, Inc. (E2G) from Hydrocarbon ProcessingNovember © 2014 Gulf Publishing Company

Vulnerable to Brittle Fracture

Documents

Transcript of Vulnerable to Brittle Fracture