Tube failures occur due to the �Overheating� and the �Plastic Flow� conditions associated with restricted flow of water in side of the particular tube facing failure.
Such failures are generally of two types:
(i) Overheating in �Short Duration�
(ii) Overheating over a �Long Period�
Careful examination of the failed tube section reveals whether the failure is on account of rapid acceleration in the tube wall temperature or it is on account of a long term gradual build up/ accumulation of the cause of failure.
(a) Short Duration Over-Heating: -
When conditions causing a rapid metal temperature elevation at local spot occur, a violent rupture results. The �Plastic � Flow� phenomena of �Carbon Steel� materials at temperatures between: 700 0C � 800 0C is the cause of such occurrences.
Photograph 1
Interruption/ Restriction, in the water circulation by some blockage in tube leads to such failures, as the tube metal gets exposed to direct flames in such cases.
(b) Long Duration Over-Heating: -
�Long Term� conditions finally lead to a tube leak; wrinkled or bulged external surfaces are observed in such cases. Such appearance is caused by long term �Creep Failure� causing by repetitive scale formation, which leads to �Overheating�, thus �Swelling� the surface, forming a Bulge or Blister, visually observable minor fissures, etc. on external surfaces. Photograph 2 given hereunder illustrates the same.
Photograph 2
(c) Some Suggestions for Avoiding Such Failures: -
It is recommended that the quality of the �Boiler Feed Water� for boilers operating at 21 Kg/ cm2 (g) should be as per the annexure �A� attached herewith.
It is also recommended that the �Blow Down Time� & duration should be observed as per the annexure �B� attached herewith.
Blow Down must also be conducted on regular basis (When Boiler is in Low Steaming Stage) from side wall, rear & front wall headers; so that sludge accumulation in these headers may be avoided; which otherwise would rise in furnace tubes creating conditions for circulation restrictions/ blockage, thus overheating at local spots.
At times, foreign matters get left behind during the erection. Such happenings may be avoided by carefully examining tubes/ headers after fitting the same by a suitable method, such as, ball test/ by passing water & observing flow at the other end (where possible). Anyhow, these precautions should be taken before the firing of the Boiler.
ANNEXURE �A�
Sr. | Feed Water |
|
|
1 | Hardness, Max. (as Ca CO3) mg/L | : | 10 |
2 | pH at 25 0C | : | 8.8 to 9.2 |
3 | Oxygen, Max. mg/L | : | 0.1 |
4 | Total Iron mg/L | : | 0.100 |
5 | Copper, Max. mg/L | : | |
6 | Total Dissolved Solids, Max. mg/L | : | 200 |
Sr. | Boiler Water |
|
|
1 | pH at 25 0C | : | 9.8 to 10.8 |
2 | Phosphate Residual (as PO4) | : | 20 to 40 (If Added) |
3 | Total Dissolved Solids, Max. (T.D.S.) mg/L | : | 3500 |
4 | Specified Electrical Conductivity at 25 0C, Max. Microsiemens/ cm | : | 7000 |
5 | Total Alkalinity mg/L | : | 700 |
