Superheater Stress and integrity analysis manufacturing and engineering ASME or GB
The strength of boiler tube
The strength of a boiler tube depends on the level of stress as well as on temperature when the tube metal temperatures are in the creep range. Because an increase in either stress or temperature can reduce the time to rupture, attention must be given to both factors during investigation of a failure by a stress-rupture mechanism, which can be encountered in a superheater.
This can occur particularly by two mechanisms: short-term overheating and high temperature creep. In the first one, a single incident, or a small number of incidents, exposes the tube steel to an excessively high temperature (hundreds of degrees above normal) to the point where deformation or yielding occurs.
Overheating results from abnormal conditions such as loss of coolant flow, internal oxide layer or flame incidence. The second one is either called long term or extended overheating failures, which results from a relatively continuous extended period of slight overheating, stress, or the accumulation from several periods of excessive overheating.
Creep deformation results in little or no reduction in wall thickness, but produces measurable creep elongation or increases in diameter in ferritic steel tubes.
Corrosion is another expected mechanism that can lead superheater tubes to failure. Localized or generalized loss of thickness occurs because of corrosion by the products of combustion (external) or from steam, especially when some contaminated water, coming from the drum, flows through the superheater (internal) after some process abnormality.
This causes increased stress in a tube operating at a constant internal pressure. In addition, corrosion is a source for overheating problems, by the formation of an oxide layer, which is a barrier to heat transfer. As its thickness increases, metal temperatures must also increase to maintain a constant outlet temperature.
Typically, tube metal temperatures increase from 0.6 to 1.1 °C for each 30 µm of internal oxide formed. Allowing for these changing conditions of metal temperature and stress over time is key to reliable creep life prediction of alloy superheater tubes.
1. Superheating raises overall efficiency as well as avoids too much condensation in last stages of turbine which avoids blade erosion. 2. The heat of combustion gases from furnace is utilised for removal of moisture from steam and to superheat the steam. 3. Super-heaters ususlly have several tube circuits in parallel with one or more return bends,connected between headers. 4. Heat from the hot gases to the vapour in the surperheater is transferred at high temperatures. 5. Therefore promary section of superheater is arranged in counterflow and secondary section in parallel flow to reduce the temp.stressing of the tube wall.