Safety and quality issues –
studs and nuts
Presentation API 6A Winter meeting Feb. 2011
1 - 2010-10-19
Background
• ASTM A320 require studs of Grades L7 and L43 to be equipped with nuts
confirming to Grade 4 or Grade 7 of ASTM A194, i.e. high strength nuts to be
equipped with high strength studs
• ISO 10423/API 6A allows for use of low strength nuts of type ASTM A194 2HM
with high strength studs like ASTM A320 Grade L7/L43
• The background for ISO 10423/API 6A allowing for use of low strength nuts (2HM)
to high strength studs (L7) is unknown (introduced in API 6A 1985, 2H was used
before 1985).
• Structural integrity of bolted connections with use of low strengths nuts raises is
an issue that needs to be addressed
• A technical note from Statoil on studs and nuts has been issued distributed to API
6A, API 17D, ASTM, ASME and ISO for comment.
• SC17 decided 18. January 2011 to monitor SC6 and present the 2HM issue at the
SC6 February 2011 meeting
2 - 2010-10-19
Example of 2HM nut failure during shell test
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Possible failure modes
Stud tension failure
Stud stripping
Nut stripping
Not acceptable failure mode in
design codes
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Assumed failure mode in design
codes
Why thread stripping is not acceptable?
• Tensile failure of the stud is easily detected.
• The initiation of stripping failure, though, is difficult
to identify because the stripping develops
gradually, some tension remains in the stud, and
there is little or no visible damage.
• Since replacement of damaged fasteners is
essential for sound joints, tensile failure of the
stud is desirable during overtightening during
make-up or overloading in service.
• Strongest stud to fail before weakest nut; checked
by nut proof loading.
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ASTM A320/A194 requirements
•
•
•
Hardness requirements, HBmin
− High strength assembly (L43/7):
Stud L43: 250
Nut 7:
− Low strength assembly (L7M/7M:
Stud L7M: 200
Nut 7M: 159
248
Proof load test load
− High strength assembly (L43/7):
1,4 x tensile capacity of L43 stud
− Low strength assembly (L7M/7M):
1,5 x tensile capacity of L7M stud
No requirement for proof load of nuts when d > 1 ½ in.
6 - 2010-10-19
API 6A requirements
•
Stud design capacity is based upon limit load
•
The nut limit load shall be higher than the stud limit load (only studs are
checked)
•
Stud allowable loads:
− 83 % of limit load for hydrostatic test
•
Proof load test load for diameter up to and including 1 ½ in.
− High strength assembly (L43/7):
1,4 x tensile capacity of L43 stud
− Low strength assembly (L7M/7M):
1,5 x tensile capacity of L7M stud
− “High” strength assembly (L43/2HM): 1,5 x tensile capacity of L7M stud
•
No requirement for proof load of nuts when diameter is greater than 1 ½ in.
•
API 6A proof load test of nuts for “high” strength assemblies with 2HM
nuts is ≈20% less than required by ASTM A320/A194.
7 - 2010-10-19
Calculation of 2HM nut capacities
•
API 6A and ASME VIII Div. 2/3 limit load
− Worst case dimensional tolerances nut/stud
− Coefficient of thread friction equal to zero
− Minimum specified yield strength 60 ksi (HB 159 ≈ Su = 80 ksi)
•
Non-linear 2D FEA performed for 2 ¼” and 1 ¼” stud/nut assemblies
•
Nut capacity formulas established based upon formulas in VDI 2230 Part 1 and
non-linear FEA
•
FEA results validated with 1 ¼” L7/7 tests and 2 ¼” 2HM nut testing
•
Calculation model also validated with 1 3/8” and 1 ½” 2HM nut testing
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Non-linear FEA analysis, 1 ¼”
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Non-linear FEA analysis of 2 ¼”
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Results of calculations
• Limit load calculations
− 2HM nuts have 5 % to 25 % less structural capacity than L43 studs for ½” to
4” size
− Example. 2 ¼” nut is utilised to100 % structural capacity when the stud is
utilised to 83 % of structural capacity. I.e. for pressure shell test where ISO
10423/API 6A allows 83 % utilisation of the stud, the nut has 100 %
utilisation. NOT ACCEPTABLE and this is a HSE ISSUE.
• Proof load testing
− Minimum HB for Grade 2HM/7M nuts should be increased from 159 to
minimum 200, which is the minimum hardness for ASTM A320 Grade L7M
studs to fulfil proof load testing
− Minimum HB of 248 for Grade 4/7 applied to L7/L43 is ok for proof load
testing
11 - 2010-10-19
Conclusions
• API 6A/ISO 10423
− Nuts is assumed to have higher structural capacities than studs, however,
calculations shows that 2HM nuts has less 5% to 25% less structural
capacity than L7/L43 studs, hence, derating have to be considered
− Nut proof load test of nuts for “high” strength assemblies (L7/L43 studs) with
2HM nuts is ≈20% less than required by ASTM A320/A194
• ASTM A194
− Minimum hardness of nuts should be the same as for studs, i.e. the 2HM/7M
nut minimum HB hardness should be 200 when used with L7M studs
• General
− Studs shall be equipped with heavy hex nuts with a grade of steel or
minimum hardness similar to that of the studs.
− Low strength nuts like 2HM shall not be used in combination with high
strength studs like L7/L43 – STRUCTURAL INTEGRITY AND SAFETY
CONCERN
12 - 2010-10-19
API 6A – Quality issues of closure bolting
• No traceability between heat treatment batch and material certificate
• No NDT (MPI/DP/UT/RT)
• No dimensional control
• Low sampling rate, e.g. hardness testing of Grade 7 nuts: 1 of 800
• No proof load testing for nuts when d> 1 ½”
• The studs (nuts) are the governing structural component for all API BX type
flanged connections and not the flange
• Not consistent quality requirements of fasteners compared with forged
flanges/components as fasteners are the governing (critical) structural component
in the API BX type flanged connection
13 - 2010-10-19
Thank you
Presentation title:
Safety and quality issues – studs and nuts
Presenters name: Finn Kirkemo
Presenters title: Technical Advisor
E-mail address: [email protected], tel: +47 901 27 901
www.statoil.com
14 - 2010-10-19