Erosion corrosion, tube

Occasionally, tubes affected by erosion-corrosion or erosion processes may be confined to specific regions of the equipment. In this case, only the metallurgy of affected tubes needs to be altered. For example,... 

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. 

Corrosion and erosion of the tubes immersed in the bed are at a low level, although there is evidence that the addition of limestone or dolomite causes some sulphide penetration. The chief operating problem is corrosion by chlorine. 

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. 

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 main objective of the test program is to burn processed MSW in a fluidized-bed boiler environment to investigate combustion characteristics, heat transfer, corrosion and erosion, and control parameters. The tests, are being conducted in a CPC-owned 0.7 sq m fluidized-bed combustor which has been reworked to incorporate water tubes for heat extraction in both the bed region and exhaust. Additional tubes will be air cooled to simulate boiler temperatures and to observe erosion and corrosion phenomena, detailographic examination will be made of these materials. 

Problems that may be manifested in the operation of evaporators and reboilers are numerous (1) corrosion and erosion, (2) flow maldistribution, (3) fouling, (4) flow instability, (5) tube vibration, and (6) flooding, among others. The final design must take into account some or all of these problems in addition to the thermal and mechanical design. 

Erosion-corrosion is a combination of corrosion and mechanical wear effects. Many metals and alloys depend on surface films or corrosion products for corrosion resistance. If these surface layers are removed, active and rapid corrosion occurs. Erosion-corrosion is often encountered under conditions of high velocity, turbulence, impinganent, and solids in suspension. Valves, pumps, agitators, and heat exchanger tubes often fail because of erosion-corrosion. 

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. 

When the flame from a small leak impinges on another tube it causes wastage by corrosion and erosion. Currie et al. [6.14] ve typical data on wastage rates in austenitic steels. A leak of order 1 g/s impinging directly on to an adjacent tube causes wastage at a rate of about 0.01 mm/s, so secondary tube failure in a period of a few minutes can be expected. 

Part A gives general guidelines for the design of large commercial fluidized bed reactors with respect to the following aspects (1) solids properties and their effect on the quality of fluidization (2) bubble size control through small solid particle size or baffles (3) particle recovery by means of cyclones (4) heat transfer tubes (5) solids circulation systems (6) instrumentation, corrosion and erosion, mathematical models, pilot plants and scale-up techniques. 

A variety of problems may occur in condensers and heat exchangers using seawater as a coolant. These include impingement corrosion, sand erosion and pitting. External corrosion may also occur if gases, such as carbon dioxide, are dissolved in the condensate. The following are the causes of major tube failures ... 

Oil refineries. Titanium tubing has been extensively specified where seawater and brackish water cooling is employed in refinery steam and overhead condensers. Seawater cooling of petroleum fractions at temperatures 110-120°C is not known to cause corrosion or erosion problems of... 

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. 

Corrosion, erosion, and power consumption versus tube veloc-ity. 

Corrosion was caused by carbonic acid. A film of condensed moisture and dissolved carbon dioxide formed the acid. The erosion was caused by high-velocity movement of air across the tubes. Attack occurred intermittently. Deepest metal loss was 33% of the 0.040 in. (0.10 cm) wall thickness. 

Figure 11.3 Sand dunelike erosion-corrosion patterns on the inlet end of a steel heat exchanger tube. (Magnification 7x.)...

Favored locations for erosion-corrosion are areas exposed to high-flow velocities or turbulence. Tees, bends, elbows (Fig. 11.5), pumps, valves (Fig. 11.6), and inlet and outlet tube ends of heat exchangers (Fig. 11.7) can be affected. Turbulence may be created downstream of crevices, ledges (Fig. 11.8), abrupt cross-section changes, deposits, corrosion products, and other obstructions that change laminar flow to turbulent flow. 

Erosion-corrosion problems on the outside of tubes are frequently associated with impingement of wet, high-velocity gases such as steam. This typically involves peripheral tubes at the shell inlet nozzle (Fig. 11.9). Baffle and tube interfaces may also be affected. 

Figure 11.5 Erosion-corrosion at elbow of a brass tube. Note also the borseshoe-shaped depressions and comet tails aligned with flow direction in the straight section.

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