Australian Pressure Equipment Knowledge

SIGNS OR INDICATIONS IN IN-SERVICE
INSPECTION OF PE

THERMAL FATIGUE CRACKS IN DESUPERHEATER

APEK GN 012.3

Case 3
Rev 0
Issued: 17/04/2014

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