External defects ControlExtrnal
From the graph above for carbon steel, it can be seen that there is a rapid drop in strength above 430oC.
Long term overheating is a condition where the metal temperature exceeds the design limit for a long period. The mechanical strength is reduced as a function of the increase in temperature.
Deposits on the external surface and thin gas film layer aid in reducing the metal temperature. Deposits on the inside increase tube metal temperatures.
Temperature drop across the thin film gas layer
Bulging of many different forms tend to precede bursting.
If the metal temperature exceeds a certain value dependant on the material rapid excessive oxidation can occur
This oxide layer can often form with faults, and can be exfoliated due to thermal stressing or vibration. The result is a thinning of the tube due to this cyclic thermal oxidation and spalling
A failed tube suffering from this will have the appearance of tree bark.
Plastic deformation due to metal overheating may occur. Microvoids form eventually leading to failure. Can be distinguished by a thick ragged edged fish mouth with small ruptures and fissures leading off.
Uncommon. Damage begins when iron carbide particles (present in plain carbon or low alloy steels) decomposes into graphite nodules after prolonged overheating ( metal temperatures > 427oC ).
If the nodules are evenly distributed then this not cause a problem. However, some tomes the nodules can chain together and failure occurs along the length of the chain ( as in ripping a postage stamp along the perforations)
Normally found adjacent to welds and determination as cause of failure requires examination under a microscope to observe nodules.
Short term overheating
Metal temperatures of at least 454oC and often exceed 730oC; failure may be very rapid. Not normally associated with a water chemistry problem rather than maloperation or poor design.
In very rapid overheating little bulging occurs and the tube diameters are unchanged in way of the fish mouthed failure ( normally thick walled edge)
Under less arduous conditions some bulging occurs and the failure may have a finely chiselled edge
Multiple ruptures are uncommon.
care must be taken not to confuse a thick walled short term overheating failure with the many other possibilities such as creep failure, hydrogen embrittlement and tube defects.
One of the most common causes of erosion within a boiler is sootblowing erosion . Especially those tubes adjacent to a misdirected blower.
Should the blower stream contain water then the erosion is much more severe. Ash picked up by the steam also acts as an abrasive. This is why proper warming through and drainage is essential
Other causes may be failure of an adjacent tube or to a much lesser extent by particles entrained in the combustion products
Internal water chemical causes
For a listing of the failures caused by water chemistry see relevant document 'Corrosion and failures in boiler tubes due to water chemistry'
Oil Ash Corrosion
High temperature liquid phase corrosion phenomenon where metal temperatures are in
the range 593'C to 816'C. hence normally restricted to superheater and reheater sections.
It can effect both the tubes and their supports.
May arise after a change of fule with the formation of aggressive slags.
Oil Ash corrosion occurs when molten slag containing vanadium compoundsform on the tube wall according to the following sequence
Catalytic oxidation of the metal surface by Vn2O5 occurs. The tube outer surfaces are thinned, stress increases in the inner layers and failure by creep rupture occurs
Corrosion of superheater by slag with a fusion temperature of 593 to 704'C requires all utility boilers to have a steam temperature not exceeding 538 to 551'C
Scale formation in the tubes leading to high metal temperatures can lead to this type of corrosion.
Elimiation may require the chemical analysis of both the fuel and the slag to determine the corrosive constituents. The fusion temperature of the ash can be determined. Fuel additives may be used. Magnessium compounds have been used successfully to mitigate problems by forming a complex with Vn2O5 and Na2O with a very high fusion temperature.
Low excess air retard the oxide formation
Water wall fire side corrosion.
may occur where incomplete combustion occurs. Volatile sulphur compounds are released
which can form sodium and potassium pyrosulfates
These chemically active compounds can flux the magnetite layer. This is more commonly found in coal fired boilers
On break out of an uptake fire the priority is to boundary cool to contain the fire and give cooling effect.
An uptake fire generally starts when the load on the boiler is reduced. This is due to the quantity of excess air being very low at high loads.
Should a fire break out then the possibility of speeding up and reducing the excess air should be considered.
The amount of feed heating should be reduced to lower the inlet feed temperature and aid with cooling parts.
Where the possibility exists of damage to the superheater, then after first relieving pressure, it should be flooded.
Where the excess air on older boilers is high even at high loads a different plan of attack must be used.
The flames should be extinguished and the air shut off. The amount of feed heating should be reduced.
The safeties should be lifted to keep a high steam flow and hence high feed flow requirements. ( the boiler is now being fired by the uptake fire )
Lifting the safeties give the added advantage of reducing the boiler pressure and hence corresponding saturation temperature of the water aiding the cooling effect
Tackling the fire
If a direct attack should be made on hot non-pressurised parts then the nozzle should be set to solid jet and aimed at the seat of the fire.
This should not be carried out on hot pressurised parts due to the risk of a steam explosion.
Dry powder is a suitable extinguishing medium.
Under certain conditions an extremely destructive fire, commonly known as a hydrogen or 'rusting' fire, may occur Under high temperatures water will tend to disassociate to hydrogen and oxygen. The percentage amount increases with increased temperature These will recombust again liberating heat In a fire there is a danger that the use of superheated steam as an extinguishing agent (say sootblowers on an air heater fire) could in fact feed the fire and accelerate the growth. For example the displacement which occurs about 707oC
Heat + Hot 3Fe + 12H2O Ћ 3FeO3 + 12 H2
see Theory section for a more complete explanation
Tackling this type of fire is very hazardous and consists mainly of boundary cooling and shutting off water and air supplies as effectively as possible.Under no circumstances should steam smothering be considered.
A typical scenario for this fire is a badly cleaned uptake igniting leading to tube failure
L=Length of cylinder
t= Material thickness
Fp= Force acting on cylinder due to pressure
Fh= Resolved horizontal component of force
Equal forces act on all surfaces. If a vertical section is cut then the forces may be considered to be resisted by the longitudinal seam for the horizontal direction.
i.e. horizontal forces to left=Horizontal forces to right
= resisting force in seam
Pressure x projected area = stress x C.S.A of joint
By using projected area the horizontal component of the pressure force is automatically resolved
p x dia x L = stress x 2 x t x L
(p x dia x L)/ 2 x t= Stress ( longitudinal joint)
Horizontal forces to left = Horizontal forces to right
Pressure x end plate area = Resisting force in circumferential joint
P x (pi x d2)/4 = stress x csa (circumferential joint)
= stress x pi x d x t
(p x d)/ 4 x t = stress ( longitudinal)
Hence, circumferential stress is twice that of the longitudinal stress and hence seams in the longitudinal axis must be twice as strong.
Hot gasses acting on the thick section tube plate set up a temperature gradient leading to creep, plastic flow to relief thermal stress and high tensile stress on the surface at cool down. In addition grain growth leads to the metal becoming brittle
A more severe form may lead to distortion of the entire drum in two possible directions. The thick section tube plate is exposed to the heat of the furnace and is subject to overheating. Thermal distortion takes place leading to stressing. This stressing is relieved by creep . When the drum cools a set distortion is in place
The distortion may occur in three ways, in a radial or axial direction as shown below
Advantages of Water tube boilers over smoke tube (Tank)
Advantages over tank
Ligament (ligature) Cracking Mechanics
Generally associated with failure of refractory plug located beneath steam drum. The ligature is the space between the tube plate holes. Classification rules typically allow isolated ligature cracks to be gouged and re-welded. For continuous cracks repairs are not normally allowed and a new drum/tube plate may be required
If a boiler was open ended to atmosphere then boiler panting would not occur. However it is not, instead combustion products must flow over a whole range of items all of which contribute to a pressure drop indicated as P drop. For example, screen tubes, generating tubes, superheater tubes, economisers etc. All of these items cause a pressure drop which varies according to the combustion variations.
Mechanism of panting
The system shown above is considered to be in steady state. The windbox pressure is at a slightly higher pressure than the furnace pressure which is at a higher pressure than atmospheric.
If there was a sudden disturbance to the plant, for example, poor combustion caused by say low atomising steam pressure then combustion of the fuel would be less efficient. The pressure in the furnace will drop, the P drop increases and the mass/volume of the furnace gases increases. The actual volume of the gas has however reduced.
The furnace pressure drop will then cause increased air flow from the windbox ( after some period allowing for inertia). The density of the air remains high and Pdrop remains high.
This in rush of air into the furnace aids the combustion process of the flame and also burns up any fuel products not completely combusted. This has the effect has the effect of reducing the density of the furnace atmosphere, increasing its volume, reducing Pdrop and increasing furnace pressure.
The flow of air from the windbox reduces as the pressure differential reduces. The poor combustion of previous is re-established and the whole process is repeated.
The cycle time will depend on the aggravating process i.e. in this case the poor combustion caused by the low atomising steam pressure., the volumes of the respective chambers as well as the size of the inlet for windbox air flow and also the amount of restriction caused by the elements forming the P drop.
This example only describes one possible scenario, in reality there may be many different sources all acting together or independently to cause the panting.
Probably the most common cause of panting is an uptake fire, others may be such as slagging of the tube stacks or even build up of the furnace floor on front fired boilers.
The effects of panting are too cause a low frequency ( governed by volume/ P drop criteria ) oscillation of furnace spaces repeated to a lesser extent in the windbox and flue gas spaces.
For membrane boilers which are by design air tight the effect can be to cause heavy mechanical loading on all points especially on the drum connections, placing unwanted tensile stressing on welds. Other no less important effects are poor combustion leading to inefficient operation and choking of the tube stacks.
Remedies and remedial action
Modern combustion control equipment by their design inherently act to prevent panting. When the drop in furnace pressure is detected by the air flow transmitter it is sent to the P+I controller as a reduced air flow measured value.The P+I controller acts to increase the air flow hence going some way to negate the cycling problem caused by the inertia of the air.
Should a boiler start panting during its life, the condition of the internal surfaces should be inspected and deposits removed.