Corrosion Resistance Chlorination Offshore Platforms Enhanced Tube Surfaces Applications Trends
Corrosion Resistance
The excellent corrosion resistance of copper alloys has long been recognized by the engineering community. This is due to the formation of a protective oxide film, which when combined with the inherent antifouling characteristics and high thermal conductivity results in their wide use. Typical applications include the large tonnage multistage flash evaporators for desalting plants, power utility surface condensers and petrochemical plant heat exchangers.
Clean, debris-free water, with a pH range of 7.2 to 8.5 and with adequate oxygen content, allows a protective film to form properly on the inside tube surface. Such film formation can be accelerated by ferrous sulfate additions, particularly in seawater, which greatly reduces the severe corrosion rate and effectively prevents both uniform and localized sulfide attack with concurrent improved resistance to impingement attack.
On-line tube cleaning effectively controls film formation while maintaining acceptable heat transfer efficiency. Water velocities must be sufficient to provide aerated water continuously and to prevent the settling of deposits, but not high enough to strip away the protective film. Generally, this design velocity is in the range of 4 to 8 ft/s, depending on the alloy. Maximum velocities in clean water service for the widely used condenser tube alloys range from 6 to 15 ft/s.
Excessive turbulence, industrial waste, sewage, or otherwise polluted waters are additional factors that influence the corrosion behavior of copper alloys and can influence maximum service velocities. For a seawater environment, minimum velocities in the order of 4 to 5 ft/s (1.22 to 1.52 m/s) are recommended.
Chlorination
Chlorine injection has been widely used for the control of algae and bi-organisms in cooling water systems augmenting copper's inherent resistance to biofouling. It is normally accomplished by charging chlorine gas or sodium hypochlorite into the intake piping controlled to give a residual of 0.5 ppm chlorine.
Offshore Platforms
Based on the successful service history of Alloy C70600 as condenser and heat exchanger tubes in coastal power stations and chemical plants, the alloy is being increasingly specified for seawater piping on offshore platforms, especially those designed for long-term service in deep water locations. Design emphasis favors material's reliability and life cycle savings, which are provided by this alloy's properties.
Typical copper-nickel piping applications include fire protection systems, sanitary plumbing services, and process cooling lines, as well as seawater piping for flare booms, generator coolers, and distillation plants. The alloy is also successfully used for seawater feed lines to mud and cement pumps and for drill water supply.
As with copper fire sprinkler systems, the superior corrosion resistance of the copper-nickel alloy permits pipe downsizing according to the high Hazen-Williams friction factor, thus making it an economically favorable material choice.
Enhanced Tube Surfaces
Significantly improved efficiency of air conditioning heat exchangers has been achieved via internal or external (or both) ridging or fins on the condenser tube surface. This improvement is due to improved film coefficients on both sides, and thereby overall heat transfer. The increase in performance can lead to a reduction in size and weight of existing units and can aid the design of new units with constrained size and weight limitations.
Application Trends
Changes in the pattern of heat exchanger tube selection for condensers installed in new electric generating steam power plants have occurred in the recent past. These changes have been toward the use of fewer types of materials and thinner wall gages, as well as a general upgrading of materials, with a shift away from the older traditional alloys.
Alloy C70600 (90-10 Copper-Nickel) has become the most frequently specified material for general use in new construction, largely supplanting Alloys C443900-C44500 (Admiralty) and Alloy C68700 (Aluminum Brass).