Brittle Failure Assessment (chap 10 / I- #8)

Question 1

What is fracture?

Answer 1

when separation of an object into pieces due to stress, at a temp lower than melting point

Question 2

Ductile vs Brittle?

Answer 2

Brittle shows no plastic deformation. when temp is low, tensile stresses are applied and if there are stress concentrators as gouges or flaws.
Thus sharp cracks and large defects would lower the fracture strength.

Question 3

When does brittle more susceptible??

Answer 3

when T>1/2” and T<60F

Question 4

When is brittle more common?

Answer 4

• shortly after erection during hydro testing,
• first filling in cold weather
• change to lower temp service
• after repair/alteration

Question 5

When to enter decision tree?

Answer 5

When not built to API 7th edition or API 650 appendix G (5th and 6th ed) OR tank will be put in a more severe service.

Question 6

Decision Tree

Answer 6

1. Prior hydro test good?
2. Shell thickness .5” or below?
3. Tank operated at metal temp above 60F?
4. Membrane stress below 7000Psi?
5. Tanks steep exempt from impact testing API650 table 4.1 or API653 Fig 5.2?
6. Tank full at lowest one day temperature per API 650 Figure 4-2?

Question 7

How to calculate membrane stress?

Answer 7

S=2.6HDG/t 
where 
D = ft
H = liquid level ft
t= inches (area of interest)
G=specific gravity

Question 8

What does API figure 4.1 shows?

Answer 8

When impact testing is required for plates thickness 1.5” and less. Impact testing only needed if below the design metal temperature in figure 4.1. If at or above DT, than impact test is not needed.

Question 9

How is impact test performed?

Answer 9

Using 3 specimen, taken from a single test coupon or test location. The average value of the specimens shall comply with the specified minimum value. If more than one value is less than the specified minimum
value, or if one value is less than two-thirds the specified minimum value, three additional specimens shall be tested, and each of these must have a value greater than or equal to the specified minimum value.

Question 10

What to do when deciding upon using a existing tank from CS of unknown materials?

Answer 10

Use figure 5.2, thicker than .5” and T>60F will need additional assessment,.

Question 11

Impact testing for reconstructed tanks?

Answer 11

each individual plates for which adequate identification DOES NOT exist shall be subjected to chemicals analysis and mechanical tests including Charpy Vnotch. Impacts test too

Question 12

Is impact test required for preheated welds

Answer 12

May be required if apply preheating as an alternative to PWHT.

Question 13

How to avoid hydrotest?

Answer 13

Can include impact testing in PQRs

Question 14

How to know when impact test can be exempted?

Answer 14

Obtain group # in 4.4, and use table 4.1 in API650.

Question 15

If need impact test, how to determine if ok?

Answer 15

Use table 4.5, to obtain avg min impact value of longitudinal/transverse and max thickness.

If more than one value is less than the specified minimum value, or if one value is less than two-thirds the specified minimum value, three additional specimens shall be tested, and each of these must have a value greater than or equal to the specified minimum value.

Question 16

What is rolled/killed/normalized?

Answer 16

Killed - completely deoxidized by the addition of an agent before casting. No evolution of gas during solidification.
Rolled - when customer heat treat their own plates , cooled in static airs (not heat treated)
Normalized - heated to aprox 55C above Ac3 or Acm for 1 hours, this ensure the steel completely transform to austenite. Then is air cooled which is a cooling rate of 38C/100F per minute. This results in a fine perlitic structure and a more uniform structure.