Australian Pressure Equipment Knowledge

SIGNS OR INDICATIONS IN IN-SERVICE
INSPECTION OF PE

CREEP AND THERMAL FATIGUE CRACKS IN CARBON STEEL BOILER TUBE

APEK GN 012.1

Case 1
Rev 0
Issued: 02/01/2010

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.