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Pipeline code requirements for fracture toughness of line pipe steels are addressed in ANSI B31.4 for liquid lines and ANSI B31.8 for gas lines. Pipeline Safety Regu-lations 49 CFR Part 195 (Liquids) and 49 CFR Part 192 (gas) incorporate the ANSI Codes by reference. The intent of both codes is to prevent any type of crack or leak in the pipeline (such as a fatigue crack, or mechanical damage from a backhoe) from initiating a major fracture in the line. This section provides some additional background on fracture toughness, and explains the reasoning behind the recom-mended fracture toughness testing requirements summarized in Figure 300-3. For
additional help in specifying adequate fracture toughness, consult CRTC Materials and Equipment Engineering.
(1) Critical service should include the following:
All offshore lines, and onshore lines in populated areas
Large diameter, high pressure gas lines (particularly lines greater than 14 inch or 1480 psi) Gas or liquid lines where the lowest expected operating temperature is below 32°F LPG lines where the lowest auto-refrigeration temperature is below 32°F
(2) Test temperature should be 32°F or the lowest expected operating temperature, whichever is lower.
For buried lines, the lowest expected operating temperature is seldom below 20°F.
For LPG lines, use 32°F or lower. Test temperature should be based on the lowest auto-refrigeration temperature, but may be higher in some cases. Consult CRTC Materials and Equipment Engineering for specific recommendations.
(3) Shear Area: 50% minimum average of all heats, 35% minimum average for each individual heat (4) Absorbed energy: calculate requirements according to the equations given in ANSI B31.8 Section 841.
The specified minimum average energy should the the highest value calculated or 20 ft-lbs, whichever is greater. If all calculated values are below 10 ft-lbs, see discussion in Section 313. Also note that the equations are based on methane; see discussion regarding the effect of gas mixtures.
Fig. 300-3 Fracture Toughness Requirements for Pipelines
Fluid
Pressure General Service Critical Service(1)
Liquid other than LPG
All sizes and grades All pres-sures
LPG All sizes and grades All
pres-sures
No tests recommended if lowest auto-refrigeration temperature is
4-inch maximum Grade B or X-42 and maximum hoop stress does not exceed 72%
of SMYS or lower and maximum hoop stress does not exceed 72%
of SMYS hoop stress greater than 72%
of SMYS
16-inch and larger All Grades All pres-sures
Ductile to Brittle Transition
At low temperature, steel can fracture in a brittle manner like glass or ceramic. The fracture surface has a crystalline appearance, and the amount of energy absorbed is low. As the temperature increases, the steel undergoes a transition from brittle frac-ture behavior to ductile tearing (also called shear), with a significant increase in the amount of energy required for fracture. This ductile to brittle transition can be char-acterized using the Charpy impact test, as illustrated in Figure 300-4. The transition temperature can be defined as the temperature where either the absorbed energy for a full-size Charpy specimen exceeds 15 ft-lbs, or the appearance of the fracture surface of the specimen is at least 50% shear. Brittle fracture can be prevented by insuring that the minimum operating temperature of the pipeline is well above this transition temperature.
Liquid Lines (ANSI B31.4)
For liquid lines, we are primarily concerned about preventing brittle fracture. Since most pipeline steels have adequate toughness to prevent brittle fracture at tempera-tures above freezing, fracture toughness testing for liquid lines operating above 32°F is generally not required. For liquid lines in critical service, such as a large diameter offshore crude oil line, fracture toughness testing is recommended as an extra guarantee that the steel will be operating above its transition temperature. For these lines, Charpy impact testing should be required according to API 5L SR5, and a minimum average energy of 20 ft-lbs should be specified. The standard test temperature is 32°F, which is acceptable for all lines which operate above this temperature.
For liquid lines which operate at temperatures below 32°F, Charpy impact testing should always be required. Specify a minimum average energy of at least 20 ft-lbs at the lowest expected operating temperature of the line. This will insure that the Fig. 300-4 Schematic Drawing Showing Ductile to Brittle Transition Behavior in the Charpy
Impact Test
transition temperature of the steel is below the minimum operating temperature, and the steel will have adequate resistance to brittle fracture. Note that for buried lines, the lowest expected operating temperature is seldom below 20°F due to warming from the earth.
LPG Lines
LPG lines are a special case, because auto-refrigeration can cause very cold temper-atures in the area of a leak as the line depressurizes. Brittle fracture of the line could occur if the temperature falls below the transition temperature of the steel while the line is still under substantial pressure. Although the ANSI B31.4 Code does not require special treatment of LPG lines, we recommend fracture toughness testing if the lowest auto-refrigeration temperature which could occur is below 32°F. This temperature must be calculated based on the specific composition of the LPG. Mixtures containing large amounts of propane or butane will have lower refrigeration temperatures than those with mostly C5+ hydrocarbons. If the auto-refrigeration temperature is above 32°F it is not necessary to specify Charpy impact tests. If it is below 32°F, specify a minimum average energy of 15 ft-lbs at 32°F or lower. Since it would be unlikely that the line would ever actually reach the lowest auto-refrigeration temperature while under substantial pressure, it is not always necessary to specify fracture toughness testing at that temperature. Consult CRTC Materials and Equipment Engineering for recommended testing temperatures for specific lines.
Gas and Multi-Phase Lines (ANSI B31.8)
For gas lines, the Code requirements for fracture toughness are more stringent than for liquid lines. The reason for the increased requirements is that in addition to brittle fracture concerns, the stored energy of the compressed gas in a large diam-eter or high pressure gas line can be great enough to propagate a ductile fracture. If a crack is initiated by an external force (backhoe, earthquake, etc.) the gas in the pipeline will start to decompress and release this stored energy. Whether or not the crack will propagate depends on the speed of the decompression wave inside the pipe relative to the fracture velocity in the steel, as shown in Figures 300-5 and 300-6. If the crack velocity exceeds the speed of the decompression wave, the pipe will “unzip” over a long distance. One way to prevent ductile fracture propagation is to slow down the crack. Since the crack velocity in the steel is related to the steel’s fracture toughness, specifying a high enough minimum Charpy impact energy will accomplish this. Another method is to install crack arrestors, which are discussed in Section 448.
The fracture toughness requirements in the Code are mandatory for all lines 16 inch NPS and larger which are designed to operate with a hoop stress over 40% of the specified minimum yield strength (SMYS) of the pipe, and for lines smaller than 16 inch NPS which are designed to operate with a hoop stress over 72% of SMYS (the Code permits maximum design stresses up to 80% of SMYS for some lines). Two acceptance criteria must be met:
• The average shear area for the Charpy impact specimens must be at least 35%
for each individual heat, and the average of all heats must be at least 50%, at
the lowest expected operating temperature of the line or 32°F, whichever is lower.
• The average absorbed energy for the Charpy impact specimens from all heats must meet or exceed the energy value calculated using one of several equations developed from pipeline research programs to predict the energy required for ductile fracture arrest. These equations and an example calculation are given in Figure 300-7.
Fig. 300-5 Ductile Fracture in Gas Pipelines
Fig. 300-6 Example of Ductile Fracture Analysis for Export Gas Line
Company practice has generally been to specify Charpy impact testing for all gas pipelines, with a minimum energy requirement of 20 ft-lbs at 32°F or the lowest expected operating temperature of the line, whichever is lower. This level of frac-ture toughness is adequate to prevent brittle fracfrac-ture, and will also exceed the ductile fracture arrest energy required for many small to medium diameter lines with typical operating pressures. This practice is included in the requirements in Figure 300-3, in addition to the Code requirements.
For lines up to 4 inch OD (3.5 inch NPS) which operate above 32°F and are designed using API 5L Grade B or Grade X-42 pipe, fracture toughness testing is not required unless the hoop stress exceeds 72% of SMYS, or the maximum allow-able operating pressure exceeds ANSI Class 1500 limits (3705 psi at up to 100°F).
These lines do not have a significant risk of brittle fracture, and the calculated energy requirement for ductile fracture arrest is low (less than 10 ft-lbs). For critical service, which includes all lines with operating temperatures below 32°F, fracture toughness testing should be specified with a minimum energy requirement of 20 ft-lbs at 32°F or the lowest expected operating temperature (whichever is lower) according to past Company practice. Note that API 5L SR5 does not cover testing of pipe 4 inch OD and smaller because it specifies transverse specimens which cannot be taken from small pipe without flattening. All of the requirements of API Fig. 300-7 Example of Ductile Fracture Arrest Calculations
1. Gas Pipeline: 24" outside diameter
1480 psi maximum allowable working pressure 2. Select Pipe Grade and Wall Thickness:
API 5L X-60 wall thickness:
hoop stress =
(SMYS = 60,000 psi)
0.438" required for hoop stress ≤ 72% of SMYS
= 40,548 psi (68% of SMYS)
3. Calculate Ductile Fracture Arrest Energy using equations from ANSI B31.8 Section 841.11
a. Battelle Columbus Laboratories (BCL) (AGA) CVN = 0.0108σ2R1/3t1/3 = 31 ft-lbs
b. American Iron and Steel Institute (AISI) CVN = 0.0345σ3/2R1/2 = 31 ft-lbs
CVN = full-size Charpy V-notch absorbed energy, ft-lb σ = hoop stress, ksi
R = pipe radius, in.
t = wall thickness, in.
PD ---2t
5L SR5 should be followed, except that the specimen orientation should be changed to longitudinal.
For lines up to 14 inch OD which are designed using API 5L Grade X-52 or lower strength pipe with maximum allowable operating pressures up to ANSI Class 600 limits (1480 psi at up to 100°F), a minimum energy requirement of 20 ft-lbs at 32°F or the lowest expected operating temperature (whichever is lower) will be adequate to prevent both brittle and ductile fracture. Specifying this require-ment is recommended in accordance with past Company practice, even though it is not mandatory according to the Code. However, if the hoop stress exceeds 72% of SMYS, then the Code requirements for shear area become mandatory and the ductile fracture arrest energy requirement may exceed 20 ft-lbs. Also, if higher strength pipe is used to reduce wall thickness requirements or the maximum allow-able operating pressure exceeds 1480 psi, the ductile fracture arrest energy require-ments may exceed 20 ft-lbs.
For lines up to 14 inch OD which fall outside the specific limits given above, and for all lines 16 inch OD and larger, the minimum energy and shear area requirements must be determined in accordance with the Code. There are four equa-tions given for calculating the minimum energy required for ductile fracture arrest (refer to Figure 300-7). Since all four equations generally give results which are easily achievable with modern line pipe steels, we recommend using whichever equation gives the highest value for the particular line in question. The specified energy requirement should not be less than 20 ft-lbs, even if the calculated values are lower. If the calculated values are below 10 ft-lbs, consult CRTC Materials and Equipment Engineering regarding whether or not fracture toughness testing should be waived (unless it is mandatory per the Code).
Note that, as stated in the Code, the equations for calculating the minimum energy for ductile fracture arrest are based on pipelines transporting essentially pure methane. Gas mixtures containing substantial amounts of heavier gasses such as propane and butane will have different decompression behavior, and may require higher Charpy energy to insure ductile fracture arrest. An arbitrary safety factor of 1.5 has sometimes been applied to the calculated energy requirements to account for this “rich gas” effect. CRTC Materials and Equipment Engineering can perform an analysis of the decompression behavior of a gas mixture using a mainframe computer program called EQUIPHASE to accurately determine the required Charpy energy.
Company specifications require testing of the weld and heat affected zone of seam welded pipe (ERW or SAW) in addition to the base metal whenever fracture tough-ness tests are specified. The Code requirements for ductile fracture arrest energy are not mandatory for the weld seam, based on the assumption that the weld seam in each joint will be rotated with respect to the next joint. Therefore, a fracture would destroy at most one joint of pipe before it arrests in the next joint. However, it has been Company practice to apply the same requirements to the weld and heat affected zone as for the base metal, and this has generally been achieved without much difficulty. Contact CRTC Materials and Equipment Engineering regarding relaxation of this requirement if necessary.
Use of drop-weight tear testing (DWTT) in accordance with API 5L Supplementary Requirement SR6 should also be considered for high pressure gas lines 20 inches in diameter or larger and Grade X-52 or higher. The Code permits this test as an alter-native to specifying a minimum shear area for the Charpy impact specimens.
However, Charpy impact testing is still required to verify that the ductile fracture arrest criteria are met.
CO2 Lines
CO2 lines which operate at super-critical pressures (where the CO2 is a dense phase more like a liquid than a gas) are also a special case. Extremely high pressures combined with auto-refrigeration concerns can result in fracture toughness require-ments which are significantly greater than for typical natural gas pipelines. Crack arrestors have been used for CO2 pipelines, as discussed in Section 448. Consult CRTC and CPTC specialists for advice on design of high pressure CO2 pipelines.
314 Corrosion
Internal Corrosion
Carbon steel pipelines are typically designed with a zero corrosion allowance.
Adding a corrosion allowance should be an economic decision. Corrosion in pipe-lines usually takes the form of pitting for which a corrosion allowance offers little benefit. Corrosion can usually be controlled more economically with either inhibi-tors or corrosion resistant linings.
In the special instances where corrosion allowances are desired the following rules of thumb may be used:
• The corrosion allowance depends on the product or medium in the line.
• As small as possible corrosion allowance is usually selected because it will add to the weight and cost of the line.
• For refined products the rule is zero to 1/32 inch (0.8mm).
• For crude lines with significant water the typical allowance is 1/16 to 1/8 inch (1.60 to 3.20 mm).
• In gas lines that contain water, and CO2 or H2S an allowance of 1/8 inch is reasonable.
• In special cases a higher allowance may be warranted.
• Pipelines carrying gas meeting transmission pipeline specifications should not require a corrosion allowance.
In systems where corrosion cannot be controlled or carbon steel is inadequate, several options can be considered:
• Internally lined pipe (e.g., cement-lined, plastic lined, or epoxy coated) is used in water services. See Section 350 for more information.
• Nonmetallic pipe (e.g., fiber reinforced plastic (FRP) or plastic) is sometimes used for water or chemicals. See Section 370 of this manual and Section 1100 of the Piping Manual for more information.
• Corrosion-resistant alloys (chromium and duplex stainless steels, and nickel alloys) are available as line pipe as indicated in Section 311. There are also weldable low chromium alloy steel grades (0.5% to 2% Cr) with enhanced CO2 corrosion resistance available from some of the Japanese manufacturers.
• Clad or bi-metallic pipe with a conventional steel backing and an alloy liner is available. API specification 5LD applies to these materials. These materials are very costly but very effective in mitigating corrosion. Long lead times are necessary for procurement. See Section 311.
External Corrosion
Codes B31.4 and B31.8 require external corrosion control of buried and underwater pipelines by a combination of external coating (see Section 250 of the Coatings Manual and Sections 340 and 444 of this manual) and cathodic protection (see Section 460 of this manual and Section 500 of the Corrosion Prevention Manual).
It is not necessary to provide a corrosion allowance for pitting.