BRITTLE FRACTURE The Cold, Hard Facts

by Verne Ragle

Verne Ragle, P.E.45 years in the Petrochemical business with primary

emphasis on equipment integrity, inspection, materials, corrosion and failure analysis.

25 year member of NACE Active in numerous NACE and API Standards

Committees Worked in all areas of Process Safety Management

Mechanical Integrity PSM Compliance

Current job Support company operations worldwide on Corrosion

and Materials issues. Specific focus on Downstream Mechanical Integrity Issues.

Pressure Equipment Mech. Integ. Assessment Fitness for Service

Energy

Create and awareness of Brittle Fracture and the factors that cause it.

Notable Brittle Fracture Failures

Variables that Cause Brittle Fracture

Effect on Codes an Standards

API RP 579

Assessing Existing Facilities

Purpose of Presentation

Example: Brittle vs Ductile

(a) (b) (c)

(a) Highly ductile fracture in which the specimen necks down to a point.

(b) Moderately ductile fracture after some necking.

(c) Brittle fracture without any plastic deformation.

Notable Brittle Fracture Failures

• Great Boston Molasses Flood 1919

• Liberty Ships Breaking apart – 1943

• Oil Storage Tank Failure -1988

The Great Boston Molasses Flood

Boston Molasses Flood Data

Date: January 15, 1919

Location: Boston, Massachusetts

Temperature: -2 to 41°F (temp. rise over previous several day)

Construction: Riveted

Material: Steel- type unknown (one report said cast iron)

Significant Characteristics: Poor construction quality

Point of Origin: Manhole near the base of the tank

Commodity: Molasses

Amount Lost: 2,300,000 gallons ( 50ft tall by 90 ft diam.)

Deaths: 21

Injuries: 150

Significant event prior to rupture: Filled to maximum level

Boston Molasses Flood DataWitness Reports• Some say it collapsed, others say it exploded.

• Reported loud rumbling like a machine gun as rivets shot out of the tank.

• The ground shook like a train going by.

• Eight to fifteen foot wave of molasses at 35 MPH.

• Girders of Boston Elevated Railway broke – train lifted off the tracks

• Buildings swept off of their foundation

• Several blocks flooded to a depth of 2 to 3 feet with molasses.

• Moving masses investigated to determine if man or animal.

• Truck blown into Boston Harbor.

The Great Boston Molasses Flood

Boston Molasses Flood DataContributing factors reported and speculatedPoor construction and insufficient testing

• People reportedly filled their molasses jars from home from leaks

Filled to highest level (also filled to max on 8 other occasions)

• Cyclic stress and fatigue?

• Pre-stressed cracks?

Speculation of Carbon Dioxide pressure due to fermentation

• Vents Plugged?

Initiated from a manhole near the base of the tank

• Maximum hoop stress

• Stress riser

Liberty Ship

Failures

USS Schenectady

Liberty Ships Breaking Apart

Date: January 16, 1943

Location: Portland Oregon

Temperature: Water 29.2°F : Air 37°F

Construction: Welded

Material: Steel- type unknown

Significant Characteristics: Rapid construction, No Crack arresting plates, Inexperienced welders Poor construction quality

Point of Origin: Corners of Hatch opening,

Number of ships that failed; 1943 -20 1944- 120

Liberty Ships Breaking Apart

Significant contributors to failure:

• Poor quality steel

• New construction methods (welding)-thought to be an unsuitable method of construction

• Lack of knowledge of fracture characteristics of steel,

• Cold, North sea water,

• Overloading.

Add text

USS Ponaganset

Liberty Ship Failures

Oil Storage Tank

Failure

Oil Storage Tank FailureDate: January 2, 1988

Location: Floreffe , Pennsylvania

Temperature: 12 to 26°F (12 hours before to time of failure)

Construction: Welded

Material: Steel type: Carbon Steel Grade unknown

Significant Characteristics: Reconstructed Tank

Point of Origin: Flaw near a weld

Commodity: Diesel fuel

Amount Lost: 2,500,000 gallons

Deaths: none Injuries: none

Significant factor: Filled to highest level ever attained

* Photograph source: http://www.epa.gov/superfund/programs/er/resource/d1_07.htm

Oil Storage Tank Failure

Witness comments:

Eyewitness accounts of the failure indicated that there were no warnings.

At the time of failure the tank was nearly full.

There was no explosion.

An operator was on the roof of the tank to verify that it was nearly full just five minutes before the tank ruptured.

Sounds like thunder were described as emanating from the tank for about 30 seconds at the time of the failure.

Oil Storage Tank Failure

R.M. Keddal & Assoc., Library, PA

The AftermathObservations of the failure site revealed that the tank had moved about 120 feet.

The roof of the tank was still attached to portions of the tank wall.

The bottom of the failed tank remained intact.

Collateral damage included a fifty ft high adjoining tank that had oil on its roof and another tank some distance away that had oil all over it and was physically damaged

The tidal wave effect of the sudden release of a column of diesel oil 120 ft in diameter and 50 ft high caused the oil to flow over the dike wall, into storm drain at an adjacent power plant that flowed directly to the Monongahela River.

An estimated 500,000 gallons of oil went into the river.

Oil Storage Tank Failure

R.M. Keddal & Assoc., Library, PA

Contributing Factors to Tank Failure

• Tank was built in 1940

• Poor quality steel

• Welding Technology was not what it is today

• Tank was cut apart and rewelded

• Flaw existed

• From original Welding

• Service Change

• Old service required Heating and Insulation

• New Service did not required heating and insulation

Contributing Factors to Tank Failure

Flaw in bottom shell course from original construction.

Battelle; Columbus ,Ohio

Factors Contributing to Brittle FractureCommon factors that are very consequential.

• All of the failures were associated with cold weather

• All of the failed structures were subjected to high stress levels.

• The tanks were at their maximum fill height • The ships were subjected to the stresses of

the pounding of waves and, in many cases overloading.

• They were fabricated during times that very little was known concerning fracture mechanics and the effect low temperature could have on the toughness of steel.

Factors Contributing to Brittle Fracture. • Stress risers were present

• The molasses tank was noted to have many flaws

• Revealed by the leaks• Initiated at a lower manway

• The oil tank had a flaw that was attributed to be the triggering mechanism for the failure.

• Many of the ship failures initiated in corners of hatches or other locations that are know now to be points of high stress concentration

Similar Traits of FailuresMolasses

TankOil Tank Ships

Low temperature

-2 to 41°F 12 to 26°F 29/37°F

Flaws, Leaks Yes Stress Risers

Stress Maximum fill Maximum fill Movement and overload

Susceptible Metal

Yes Yes Yes

New Technology (welding)

NO Yes Yes

Common FactorsThree things are necessary for brittle fracture to occur:

1) A material that is susceptible to brittle fracture• High NDT• Low Charpy Values

2) Stress• Uniform stress • Concentrated Stress due to flaws or discontinuities

3) Low metal temperature• Below or near the NDT

Effect on Codes and StandardsMolasses Flood Era

• No active organization such as API-AME• Minimal failures • Lack of attention

Liberty Ship Era• New technology• War Effort• Early Refineries• No significant incidents

Early ASME Codes• 1951 API-ASME • Listed allowable stress down to -20°F

Effect on Codes and Standards1980s & 90sAPI – In response to industry needs was In a period of unprecedented development of documentsRP 570 Piping Inspection Code: RP 571 Damage Mechanisms Affecting Fixed Equipment RP 572 Inspection of Pressure Vessels RP 573 Inspection of Fired Boilers and Heaters RP 574 Inspection Practices for Piping System Components RP 575 Inspection of Atmospheric & L P Storage Tanks RP 576 Inspection of Pressure-Relieving Devices RP 577 Welding Inspection and Metallurgy RP 578 Material Verification Program Std 579-1/ASME FFS-1 Fitness-For-Service RP 580 & 581 Risk-Based Inspection

Effect on Codes and Standards

ASME data on Brittle Fracture and Low temperature In UCS 65, UCS 66

• ASME 1988 -- 3” by 8 “ column

• ASME 1989 -- 6 pages

• API Std 650

Extensive section on Low Temperature

• API 620 Std

Appendix Q and R related to Low Temperature

Considerations for Existing EquipmentThe brittle fracture resistance of the material of construction is fixed for any existing piece of equipment and cannot be altered .

API 579-1/ASME FFS-1, JUNE 5, 2007

Part 3 - based on ASME Section 8 Div 1, Para UCS-66Screening tool for determining propensity for Brittle Fracture• Variables

• Material Type• Thickness• Stress

• Applied Stress• Known flaws

• Credit for PWHT• Temperature -Limit Exposure

Assessment Considerations

Three Levels of Assessment

Level 1 Can be satisfied based on:

• Impact test results or impact test exemptions curves from the code

• Accomplished by a scrutiny of existing equipment data

• Comparing the CET (critical exposure temperature) to the MAT (minimum allowable temperature).

The methodology of RP 579 is quite thorough in the guidelines provided for determining the CET and the MAT. Equipment that has a CET equal to or greater than the MAT are exempt from further brittle fracture assessment unless conditions change.

.

Assessment Considerations

A good Management of Change program should be in place to trigger an action item should changes occur that might affect the CET.

One level 1 assessment of a plant resulted in 15% of the equipment being exempt from further assessment.

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Assessment ConsiderationsLevel 2 assessment takes into consideration:

• Operating pressure/temperature envelope

• Compared to the component design stress and MAT.

Adjustments are permitted to the MAT providing proper impact test documentation is present.

Credit is also given for fabrication conditions such as PWHT (post weld heat treatment).

Assessment ConsiderationsLevel 2 assessment (cont’d):

When determining the stress conditions, consideration is given to:

• Excess material above the required minimum thickness

• The effect of joint efficiency

• Wall thickness

In the aforementioned assessment, 51% of the equipment met the required criteria after a level 1 and level 2 assessment..

Assessment ConsiderationsLevel 3 AssessmentNormally involves more detailed determinations of one or more of the three factors that control the susceptibility to brittle fracture:

• stress• flaw size• material toughness.

Many factors affect the outcome.Significant amounts of inspection data may be available and other problems may be on record that must be considered in the brittle fracture assessment. Example--Equipment that was in amine service --possibly susceptible to cracking or blistering.

Many parts of RP 579 specifically address many of these issues and can be effectively utilized to enhance the brittle fracture assessment.

Assessment ConsiderationsThere are many ways to present the results of the brittle fracture assessments.

A very effective way is to provide a graph of each component showing the minimum allowable temperature as a function of percent of design pressure.

This method provides:

• A rapid assessment of the permitted pressure for all temperatures

• Permitted temperature for all pressures within the limits of the design pressure of the equipment.

Nature of Brittle Fracture & Assessment• Most variables are not exact

• Stress levels are based on overall stress

• No accountability for stress concentrations such as residual stress in welds, stress at connections

• Concentrated stresses act as crack initiators that cannot be arrested

• Hydrotest in ductile range can blunt cracks and flaws to resist BF

• All three components must be present at the same time

• Susceptibility- Cannot be changed

• Stress – Must be controlled

• Temperature—be aware of sources of low temperature

Sources of Low Temperature• Weather--can’t be controlled; must provide protection

• False sense of security in warm parts of the country.

• Process related situations

• Autorefrigeration due to Relief Valve

• Relief valve open- Cool down below CET

• Relief valve close- repressurization while cold.

• Depressurization for other reasons

• Mixed phase flow- cooling of piping from Vessel stream

• Cold start-up or repressurization procedures must be considered

• Shock chilling

Summary• Older equipment is more likely to be susceptible.

• Failure is usually catastrophic with no warning

• Stress and Temperature are only controllable factors

• Stress from applied pressure or flaws

• Fabrication Practices

• Temperature from weather

• Low temperature sources can come from process even in warm weather

• Codes and Recommend Practices provide Guidance

• Continually being revised

Questions??

Verne Ragle, P.E.Mechanical Integrity Consultant Siemens Energy Oil & Gas DivisionEngineering Consulting Business Unit4615 Southwest Freeway, Suite 900Houston, TX 77027Tel.: (281)-220-1701Fax: (713)-570-1230Mobile: (850) 398-7097Email: verne.ragle@siemens.comhttp://www.sea.siemens.com 

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