Erosion-corrosion boilers

Leaky valves are also a cause of erosion. Most turbine erosion-corrosion problems come from damage that takes place when the unit is not running. A shght steam leak into the turbine will let the steam condense inside the turbine, and salt from the boiler water will settle on the inside surfaces and cause pitting, even of the stainless blading. There must be two valves with a drain between them, i.e., a block valve on the header and an open drain in the line before it reaches the closed trip-throttle valve. 

In a turbine that is running, erosion-corrosion is pretty much confined to units that are operating on saturated steam with inadequate boiler-water treatment. This type of erosion takes place behind the nozzle ring and around the diaphragms where they fit in the casing. 

Special devices have been used to study erosion-corrosion by boiler water moving at high velocity, and an example is the method used by Wagner, Decker and Marsh . 

Oxygenation treatment also reduces the risk of erosion-corrosion problems and limits iron transport to other parts of the boiler system where fouling could take place. 

Morpholine is still the standard by which other amines are compared for pH control, and AMP has commonly been employed to control carbon steel boiler tube erosion-corrosion problems in European gas-cooled reactor stations. 

Excessive us of high-pressure steam soot blowers is a common source of tube erosion-corrosion. Other boiler cleaning methods less threating to boiler tubes are available such as mechanical rapping, shot cleaning, and compressed air soot blowing. 

The flame imprinted characteristics of pulverized coal ash relevant to boiler slagging, corrosion and erosion have been discussed previously (1,2). Silicate minerals constitute between 60 and 90 per cent of ash in most coals and boiler deposits are largely made up from the silicious impurity constituents. This work sets out first to examine the mode of occurrence of the silicate mineral species in coal followed by a characterization assessment of the flame vitrified and sodium enriched silicate ash particles. The ash sintering studies are limited to investigations of the role of sodium in initiating and sustaining the bond forming reactions to the formation of boiler deposits. 

F. Raask, "Flame Imprinted Characteristics of Ash Relevant to Boiler Slagging Corrosion and Erosion," presented at the 1981 Joint Power Generation Conference, ASME Paper No. 81-JPGC-Fu-l. 

Under certain corrosive conditions, many metals form covering layers. If these are sufficiently dense, they act as protective films against the corrosive removal of the material. An example of this is the protective layer of iron oxide formed in unalloyed or low-alloy boiler tubes. Corrosion with erosion is understood as the combined action of mechanical surface removal and corrosion. With some soft and loose layers, the shear forces obtained with pure flowing liqnids at medium flow velocities are sufficient to damage the protective layer without the involvement of abrasive solid particles. Where drop impingement or cavitation is involved, the mechanical removal of material is understandable. 

Boiler tube corrosion and turbine blade erosion... 

Final control elements In rare instances the final control elements can be duplicated, in cases when the erosive/corrosive or sticking characteristics of the fluid could cause unacceptable downtime or in cases of critical controls (viz, boiler drum level control with control valves in medium-sized power plants). The major cases are as follows ... 

Applications involving the reduction-to-practice of pulsed thermography techniques include an Electric Power Research Institute (EPRI) project to detect the extent of erosion/corrosion damage in power generation boiler tubes. The material in this section is reprinted with permission from EPRI. 

An example of this type of attack is erosion-corrosion of brass tubes in boiler reheaters. The attack takes place where the direction of flow changes. Overheating and local boiling takes place with a disruptive effect on protective films, particularly at the exit and the entry, where turbulence is the greatest. 

The erosion-corrosion problem in boiler tubes is addressed by ... 

A good summary of the behavior of steels in high temperature steam is available (45). Calculated scale thickness for 10 years of exposure of ferritic steels in 593°C and 13.8 MPa (2000 psi) superheated steam is about 0.64 mm for 5 Cr—0.5 Mo steels, and 1 mm for 2.25 Cr—1 Mo steels. Steam pressure does not seem to have much influence. The steels form duplex layer scales of a uniform thickness. Scales on austenitic steels in the same test also form two layers but were irregular. Generally, the higher the alloy content, the thinner the oxide scale. Excessively thick oxide scale can exfoHate and be prone to under-the-scale concentration of corrodents and corrosion. ExfoHated scale can cause soHd particle erosion of the downstream equipment and clogging. Thick scale on boiler tubes impairs heat transfer and causes an increase in metal temperature. 

Steam/CR system corrosion debris pickup, tranport, and re-deposition. Wide ranging FW line blockages and impingement/erosion Enhances risk of pre-boiler corrosion... 

Poor steam purity Contaminated steam leads to corrosion, erosion, sticky valves, and boiler operation problems. 

Many types of WT boiler are of a one-drum design, but where mud drums are fitted, these should also be inspected. As its name suggests, much of the mud, sludge, dislodged scale particles, and other general debris in the boiler ends up in the bottom or mud drum. This material should be removed and the drum inspected for underdeposit corrosion, wall thinning, erosion, and other problems. 

Ash deposition in biomass combustion systems has been the focus of numerous research efforts.559,659 The basic mechanism for deposit formation in biomass combustion systems starts with the vaporization of alkali metals, usually chlorides, in the combustor. Fly ash particles, which are predominantly silica, impact and stick to boiler tube surfaces. As the flue cools the alkali metal vapors and aerosols quench on the tube surfaces. When the ash chemistry approaches equilibrium on the surface and the deposit becomes molten, the likelihood increases that additional fly ash particles will stick, and deposits grow rapidly. Ash deposits can also accelerate the corrosion or erosion of the heat transfer surfaces. This greatly increases the maintenance requirements of the power plant often causing unscheduled plant interruptions and shutdown. 

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