Australian Pressure Equipment Knowledge

ACCEPTANCE OF NON-CONFORMING WELD ROOT OVERLAP,
REINFORCEMENT, SPATTER IN CS PE

APEK GN 234
Rev 1
Issued: 25/10/2017

1 Scope

Shows and assesses 4 significant signs or indications (Fig 1) detected during in-service inspection of PE. Gives basis for acceptance of these flaws and non-conforming weld root overlap.

2  Purpose

Feedback for all, including trainees; show that some DNCs can be accepted; & support AS/NZS 3788

3  PE involved

New vertical air-oil separator: Carbon steel, Class 3 pressure vessel 9mm thick, 700mm OD. Imported, ASME VIII-1, ŋ=70% ie no RT or UT, P = 2MPa, Hydrotest = 1.3P, T = 60C, Hazard Level = C

4  Signs
    (Indications)

Root reinforcement: 2.5mm continuous; root overlap: 2mm – continuous on both sides; minor spatter from circumferential weld; and weld oxide scale.

5  Sign location

Inside vessel on vertical longitudinal weld.

6  Detection

By IS Inspector during commissioning inspection in Australia by VT in 2014.

7  Assessment
    (by inspector, advisor
    & owner user)

.1 The inspector sought competent advice on flaw identification etc, and which Standard applied.
.2 Check with 100% UT showed no other flaws.  Various Standards were reviewed.
.3 Overlap did not conform (DNC) with AS 4037, but was acceptable in ASME VIII-1 UW35a). Overlap is equivalent to a joint joggled or with backing strip - both allowed for circumferential joints. Overlap (and spatter and oxide scale) might not be acceptable for severe corrosion service or where hygiene is critical as with food or medical PE.
.4 For this Service conditions: Corrosion negligible with oil – very low risk off corrosion failure. Low stress (effective safety factor = 3.5/ (ŋ=70%) =5,0; low stress range, medium no. of cycles and “crack” parallel to stress. Hence very low risk of fatigue failure. Spatter and thin oxide scale acceptable.
.5 Weld root reinforcement of 2.5 mm complies with AS 4037, is within 0.1mm with ASME VIII-1. It might not be acceptable for flow-accelerated corrosion or complete drainage.

8  Occurrence

During manufacture 2013 overseas. Root overlap is rare - some inspectors did not recognise.

9  Failure Modes
    (damage mechanism)

Corrosion and fatigue possible – very low probability

10 Probable cause(s)

Poor fitting backing bar (eg copper) for small diameter vessel

11 Outcome

Put into service without removal or repair because:
a) Conformance with ASME
b) For this application – corrosion and fatigue failure are very unlikely
c) Owner-user happy as PE was urgently needed at minimum cost

12 Fix

No correction was required.

13 Prevention

In manufacture:   better shaped backing bar, edge bending or clamping; better QA or inspection.
In service: after 10 to 15 years, spot UT of the vertical weld lower half might be desirable

14 Lesson(s)

.1 If a flaw is judged DNC, consider if PE might be fit for service and if an assessment is worthwhile.
.2 Different laws, Standards or service may have different acceptance criteria eg see 8.3 & .5.
.3 Acceptance of DNCs requires proper assessment, justification, documentation and sign-off.

15 References

AS/NZS 3788  PE In-service inspection
AS 4037  PE Examination and testing
ASME BPV Code Sect. VIII  Pressure vessels Div 1