Condenser tube alloys

For each condenser-tube alloy, there is a recommended maximum as well as minimum velocity in sea-water service. Silt or other fine solids in the cooling water is a complicating factor. It may be helpful to raise the minimum velocity to the point where the silt is always maintained in suspension. 

Condenser Tube Alloys Including Copper-Nickel Alloys... 

Corrosion of overhead equipment in crude distillation units is associated with HjS and with acidity from hydrolysis of chloride salts. It is controlled by neutralization to pH 6-6.5 with ammonia or an amine, plus inhibition. Similar problems may occur in other distillation units, except that hydrochloric acid may not be present. The corrosive nature of the cooling water may require use of condenser tube alloys such as titanium or copper alloys. 

TABLE 8.20 Comparison of Test Results Obtained on Condenser Tube Alloys by Two Independent Laboratories (Testing Conditions Velocity of Jet 4.58 m-s k Air Added, 3% by Volume, Duration, 28 days)... 

Phosphorized deoxidized arsenical copper (alloy 142 (23)) is used for heat exchangers and condenser tubes. Copper-arsenical leaded Muntz metal (alloy 366), Admiralty brass (alloy 443), naval brass (alloy 465), and aluminum brass (alloy 687), all find use in condensers, evaporators, ferrules, and heat exchanger and distillation tubes. The composition of these alloys is Hsted in Table 5. 

Admiralty Brass and Naval Brass are 30 and 40% zinc alloys, respectively, to which a 1% tin addition has been added. Resistance to dezincification of Cu—Zn alloys is increased by tin additions. Therefore, these alloys are important for thein corrosion resistance in condenser tube appHcations. In these, as weU as the other higher zinc compositions, it is common to use other alloying additives to enhance corrosion resistance. In particular, a small amount (0.02—0.10 wt %) of arsenic (C443), antimony (C444), or phosphoms (C445) is added to control dezincification. When any of these elements are used, the alloy is referred as being "inhibited." For good stress corrosion resistance, it is recommended that these alloys be used in the fiiUy annealed condition or in the cold worked plus stress reHef annealed condition. 

Iron is added in small (usually 0.5—1.0 wt %) amounts to increase strength. More importantly, iron additions also enhance corrosion resistance, especially when precautions are taken to retain the iron in solution. Precipitation of the iron—nickel-rich phase does not result in strengthening and can cause degradation of corrosion resistance (47). A small (up to 1.0 wt %) amount of manganese is usually added to both react with sulfur and deoxidi2e the melt. These copper alloys are most commonly applied where corrosion resistance is paramount, as in condenser tube or heat exchangers. 

Copper alloys Seawater Condenser tubing. Piping. Pump impellers. 

Other gases which are occasionally present usually arise from pollution. Ammonia, which in various forms may be present in waste waters, attacks copper and copper alloys its presence in estuarine waters is one of the main causes of condenser-tube corrosion. 

In early times 70/30 brass condenser tubes failed by dezincification and Admiralty brass (70Cu-29Zn-lSn) was brought into use. This proved little better, but some time later the addition of arsenic was found to inhibit dezincification. Failures of Admiralty brass by impingement attack became a serious problem, particularly as cooling water speeds increased with the development of the steam turbine. The introduction of alloys resistant to this type of attack was a great step forward and immediately reduced the incidences of failure. 

Parker, J. G. and Roscow J. A., Method for the Assessment of the Quality of Surface Films Formed on the Cooling Water Side of Copper-Based Alloy Condenser Tubes , Br. Corros. J., 16, 2, 107-110(1981)... 

B 169 Aluminum Bronze Plate, Sheet, Strip, and Rolled Bar B 171 Copper-Alloy Condenser Tube Plates... 

Copper-Base Alloys. There is a wide range of copper-base alloys that have given good service in sea water. Admiralty brass, 70 Cu-29 Zn-1 Sn, plus an inhibitor such as arsenic, has found wide use as condenser tubes in marine-based plants using sea water for cooling. While it is not so resistant as the cupro-nickels, it often seems to be preferred because of the lower initial cost. 

The high-tin bronzes—e.g., 90 Cu-10 Sn—are known to have excellent lifetime in sea water as condenser tubes. Undoubtedly, their cost has restricted the wider use of these resistant alloys. 

The cupro-nickels have been widely accepted as the best available alloy for condenser tubes. For handling hot sea water, there has been some favorable experience with cupro-nickel for pumps and heat exchangers. 

The impetus for further developments was the recognition of the economic significance of corrosion phenomenon during the 19th century that led the British Association for the Advancement of Science to sponsor corrosion testing projects such as the corrosion of cast and wrought iron in river and seawater atmospheres in 1837. Early academic interest in corrosion phenomenon (up to the First World War) was followed by industrial interest due to the occurrence of equipment failures. An example of this is the corrosion-related failure of condenser tubes as reported by the Institute of Metals and the British Non-ferrous Metals Research Association in 1911. This initiative led to the development of new corrosion-resistant alloys, and the corrosion related failure of condenser tubes in the Second World War was an insignificant problem. 

High velocities of aqueous solutions impinging on copper and brass tubes produce impingement on the metal or alloy. The aluminum brass and cupronickel alloys have great resistance to flow-induced attack up to a well-defined maximum for the flow rate, beyond which the film on the metal surface will be disrupted. Admiralty brass and aluminum brass have lower values for maximum velocity of flow than cupronickels. Admiralty brass and aluminum brass are preferred to cupronickels for use in media containing sulfide species. Coatings have been developed for cupronickel and aluminum brass condenser tubes for land-based and marine systems. 

Some typical applications of the alloys are in propeller shafts, propellers, pump impeller blades, casings, condenser tubes and heat exchanger tubes. The corrosion rate in flowing seawater is <0.025 mm/yr, but can pit under stagnant water. Alloy 400 is immune to chloride SCC. 

Behavior of copper alloys condenser tubes under velocity conditions. 24... 

The erosion resistance of copper alloy condenser tubes can be related directly to velocity. The velocity limits for common copper base alloys are shown in Figure 1.18. The velocity limit is roughly proportional to the strength of the alloy (i.e., copper is the weakest material and displays the poorest erosion resistance) copper-30% nickel is one of the strongest alloys and displays the best erosion resistance. 

Figure 1.18 Behavior of copper alloy condenser tubes under velocity conditions.13...

Muntz metal. An alloy containing approximately 60% copper and 40% zinc a low percentage of lead is sometimes added for free-cutting. It is classified as a brass and used primarily for condenser tube plates and other electrical applications. It is formed by hot-working and is not amenable to coldworking. 

Copper-nickels contain up to 30% nickel and have moderate strength and better corrosion resistance than other copper alloys. They are used for condenser tubing, tube sheets, salt water piping and ferrules. 

Aluminum alloys can also be attacked by microorganisms. For example, there have been MIC problems with aluminum fuel tanks and transfer lines. In this case, microorganisms grow in the water layer under the fuel to produce volcano-shaped tubercles, frequently evolving gas. Pitting occurs under the tubercles. MIC attack on copper alloys is usually insignificant. However, corrosion of copper condenser tubes by microbially produced ammonia has been reported. In addition, sulfuric acid has been produced by microbial activity by corrosion of underground copper pipes. 

Cupronickels contain from 15 to 20 per cent of nickel, the remainder being copper. They can be cold-worked for example they can be cold-rolled from I inch down to 0-05 inch without annealing being necessary. They have been extensively used for bullet jackets. The 2 5 Ni, 75 Cu alloy used in coinage has already been mentioned. A 30 Ni, 70 Cu alloy is used for condenser tubes. Another useful alloy, sometimes known as constantan, has 40 Ni and 60 Cu. Owing to its high electrical resistance and low resistance temperature coefficient it is used for standard electrical resistances. 

It is possible for the passivation (oxide) layer on the surface of a metal to be continuously removed or not allowed to develop, by erosion from particulate matter or gas bubbles. Not only is the surface eroded but the removal of the protective oxide layer allows corrosion to take place. The problem is accentuated by the presence of an obstruction or debris, on the metal surface that diverts and accelerates the flow near the surface along a defined path. Sato et al [1977] report experimental data on erosion-corrosion resistance of condenser tubes fabricated from various cupro nickel alloys. They suggest that high iron bearing cupro nickels are superior in respect of erosion corrosion by clean sea water. 

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