Dear sir,I am thankful to you for sharing very good technical knowledge of power plant.In our power plant,for last 06 months,we have been facing problem of pitting and tube failure in convective superheater tubes,which is causing huge loss.Pls suggest for remedial action in this regard.
CAUSE OF BOILER TUBE FAILURE -बॉयलर ट्यूब पंक्चर होने के मुख्य कारण
BOILER MAINTENANCE
1
CAUSE OF BOILER TUBE FAILURE
Today we discussed about the boiler tube failure and resin behind it. Typical resin of boiler tube failure given below:-
CAUSTIC ATTACK :–
when the tube is under caustic attack at that spot where the caustic attack concentration is more tube wall loss the inside diameter surface of the tube and stress and strain concentration increase on tube wall.
Causes:-
During the caustic attack there is a excessive deposition on the ID tube surface. This leads to restrict the cooling water flow in contact with the tube which causes the local under deposits boiling and concentration of boiler water chemical. Higher ph water is also responsible for caustic attack it breaks the protective magnetite due to corrosive attack.
Oxygen petting symptoms:-
When the feed water contains the excessive amount of oxygen then these water goes into the boiler and reacts with the tube wall and to form iron oxide. Then the internal surface area of the tue get corrosion and form the cavities on the inside tube surface this type of corrosion called is petting corrosion. This type of pitting corrosion symptoms show on the economizer inlet pipe line and boiler non drain-able points. Pitting corrosion caused by excessive amount of oxygen presence in feed water and improper passivation. When corrosive ash is present moisture on tube surface from condensation or water washing can react with element in the ash to form acids that leads to a much more aggressive attack on metal surfaces.
When super heater tube have not cleared of condensation during boiler start up , obstruction steam flow. Tube metal temperature reach combustion gas temperature 750 – 900 digree Celsius which leads to tube failure. This conditions may be occurred when the plant get BLACK OUT no power supply for the running auxiliary of plant so the no feed water supply available for the boiler available water get vented through the drain and vents due to higher pressure so the fluids supply reduced to the superheater and superheater tube temperature goes upto the dangerous limit and finally tube get rapture.
LONG TERM OVERHEATING
In long term over heating tube get rapture over a long interval of time due to restriction in flow, scale deposits on tube . So the tube metal temperature increase and continue to operator for a longer period and tube get rapture finally . Super heater and reheat super heater tubes commonly failed after many years of service as a result if creep. This type of failure is characterized by minimum swelling and a longitudinal split that is narrow than fish mouth opening. This type of problem most commonly from waterside such as deposits, scale or restricted flow. In this case of either super heater or water wall tubes eventually failure is by creep rupture.
SHORT TERM OVERHEATING
Short terms overheating occurred when the insufficient fluid present in the super heater tube. Then the tube metal temperature increase up to a dangerous limit and cause the ductile rupture of the tube . This is characterized by an opening called as fish mouth.
EROSION :-
this type of failure take place due to impingement on tube external surface .in this condition tube experience metal loss from the OD of the tube. Damage will be oriented on the impact side of the tube. The erosion medium may be abrasive particles of combustion product or steam. Erosion of the tube surface mostly due to impingement of ash or during soot blowing.
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