Australian Pressure Equipment Knowledge

SIGNS OR INDICATIONS IN IN-SERVICE
INSPECTION OF PE - CASES

1 Figure

a) External view showing through cracks

b) External side view showing creep bulging

c) Inside view – showing many crack initiation sites

2  PE involved

Modified CO water tube boiler in oil refinery; steam 2 MPa and 220⁰C; steady service

3  Sign(s)

Two major circumferential cracks up to 60 mm long through tube wall; bulging; leakage; many internal initiating circumferential cracks. Light scale on both surfaces.

4  Location of signs

In bottom of horizontal boiler roof tubes, DN 75, 4 mm thick, low carbon steel seamless tube.

5  When & where occurred

1985, boiler age 25 years. In Australia.

6  Detection & Investigation

VT and water loss; metallurgical examination

7  Failure mode(s)

    (damage mechanisms)

.1Thermal fatigue - “downshock” (cracking)   .2 Creep (bulging)   .3 Leakage

8  Most probable causes

Frequent, intermittent steam blanketing (bubbles) on inside surface due to reduced water circulation. Resulted from modified protective refractory for vertical tubes opposite burners; and possible internal scale.

Unstable bubbles caused down shock due to colder water cyclically contacting hotter steel causing cyclic high local longitudinal thermal tensile stress.

Steam blanketing caused high tube temperature and local creep bulging.

9  Outcome

Boiler shut down for days, repairs made, protection modified, firing modified. Extra cost and some reputations impaired; good inspection acknowledged. Cost $200,000 approx.

10 Fix

Immediate/emergency:       Cut out and replace tube and others as convenient
Long term:  VT all tubes; replace as above; hydrotest; modify firing and refractory set up.

11 Prevention

Better boiler thermal design, refractory location and firing system. Thicker tubes would not help. Periodic VT for bulging and VT for cracks (not UTT).

12 Frequency

Very rare globally for 2 such adjacent failures.

13 Lesson(s)

Design and operate to ensure steady fluid and heat flow, with no local overheating.

14 References

Brett, S. J. Cracking Experience in Cr-Mo-V Steam Pipework Systems, OPE/AINDT 2009, Queensland, August 2009.

1 Figure

2 PE involved

Cold water pressure piping supply system P = 1MPa, T = 20^oC

3 Sign(s)

Circumferential crack with slight rust stain at stress concentration

4 When & where occurred

2009; hose coupling age 18 years; after 1000 full pressure cycles. In laundry in DN 15 in polythene/nylon hose coupling at stress raiser (thread) under tightening up tensile stress on side facing window and occasional sun for 5% of total life

5 Detection & Investigation

Leakage on floor by VT – then crack by VT

6 Failure mode(s)

(damage mechanisms)

Actinic degradation and cracking and fatigue

7 Most probable causes

Long term exposure to sun, continuous tensile stress due to tightening, no blocking of pigment in plastic

8 Outcome

9 Possible Prevention

Don’t expose to sun; add pigment to coupling

10 Reference

Compare with diesel hose failure in HMAS Westralia (4 deaths)

1 Figure

2 PE involved

Main utility water tube boiler stainless steel desuperheater (atemperator) nozzle.
Steam 500^oC and 16MPa

3 Sign(s)

a) Shallow circumferential surface cracks in bowl base;
b) 1 radial crack in offset opening in pipe restriction; and
c) hard deposit build up inside bowl lip.

4 When & Where occurred

Circa 2005. Boiler and desuperheaterage 20 years . In desuperheater bowl and pipe restriction

5 Detection

Bowl cracks PT; other signs – VT; (based on earlier serious failure in other unit); thermal stress.

6 Failure mode(s)

(damage mechanisms)

Thermal fatigue cracking; deposit

7 Most probable causes

(DT = 50^oC) this high temperature difference induces local thermal stress.

8 Outcome

Deposit – material in water.

Effective inspection, shutdown and cost but no other damage. A similar unit fractured earlier and severely damaged desuperheater, piping and downstream components.

9 Most Possible Prevention

Cracks increase water temperature; reduce thickness; radius hole edges; moderate initial water injection rate.
Deposit – if unacceptable, improve water quality.

10 References

1 Figure

2 PE involved

3 Sign(s)

4 When & Where occurred

5 Detection

6 Failure mode(s)

(damage mechanisms)

7 Most probable causes

8 Outcome

9 Most Possible Prevention

1 Figure

2 PE involved

Boeing 747-200 passenger aircraft at P = 60 kPa;  T = -50 to 30°C

3 Sign(s)

Initially – axial gouges in hull with probable shallow flat (dent).
Later – possibly brown stains after patch repair.

4  Location of Signs

In bottom and rear outside of pressure hull (fuselage) at x above.

5 When & Where occurred

In 3 major stages: Initial damage:- “tail strike” on right landing or at rapid rise take-off, in 1982 when aircraft 2 years old. Temporary repair with patch added. Final explosion May 2002 on reaching nearly 35000 ft over South China Sea.  Age:  23 years.

6 Detection & Investigation

Initially – VT. Final accident investigation showed brown stains to be nicotine:- based from early cigarette smoke in cabin leaking through crack in fuselage and patch.

7 Failure mode(s)

(damage mechanisms)

Fatigue with about 10,000 pressure cycles.    Pressure hull sheets mechanically gouged, scratched and dented along the outside and slight deformation; fatigue crack growth; and finally ductile overload rupture.

8 Most probable causes

9 Outcome

10 Most Possible Prevention

Improved landing/take-off; proper and timely repairs (follow rules and check done); take heed of unusual signs, (stain or leakage); good communication and management. 

11 Comment

This disastrous failure shows the need to:

.1  reassess “temporary repairs” (not possible to see condition of pressure hull sheets under patches).

.2  consider properly all signs – brown stain was possibly noted early and well before final explosion but not followed up. If noted, contamination from external service may have been assumed.

.3  replace temporary repairs before becoming unsafe

.4  not ignore unusual signs – may be a warning – investigate if consequences are very high and find most probable cause.

.5  Note that pressure vessels with axial gouges or cracks in dents are highly hazardous as pressure opens up the gouge any fatigue will extend crack as above. A derailed LPG rail tank car with similar defects exploded in USA in 1970s killing 12 people, and causing multi-million $ damage. This was due to morning sun heating LPG, raising its pressure and so reverse the dent and rupture the shell. See Case A15.

.6  Consider knock-on effects - an additional outcome in 2009 was $10 million fine for a US airline for not inspecting on time.

12 References

Air Crash Investigation. Channel 7. Sydney 23/12/2009

http://www.youtube.com/watch?v=DAQO2JBXnwg&list=PLtKoPr63_39us6R-9SceIDJ4FOxAIwuiS
http://www.youtube.com/watch?v=sARVZfltER8???????
http://en.wikipedia.org/wiki/list_of_Mayday_episodes. “Scratching the surface”.