Technical Presentations at the October 2005 Meeting
After selecting materials for a seawater cooling system, the engineer has the problem of purchasing what he wants. The author described the use of Standards and their deficiencies for many commonly materials. This necessitates the writing of additional specification requirements. Even when materials are specified, it is still necessary to employ adequate QA/QC control to prevent inferior materials being supplied. Design is also very important. The author presented examples where poor design had led to failures. Finally, start up is very important for many materials in seawater systems. A good start up can ensure the satisfactory long term performance of many materials in seawater. This talk was taken from the final chapter of the author’s forthcoming book on materials selection for seawater cooling systems.
Maintenance and preservation of seawater and compensated fuel ballast tanks and voids resulting from the environmental degradation of coatings present a unique and very costly problem for the United States Naval Fleet. Current methods of inspection require human entry into tanks only after a series of measurements have been taken to ensure safety. The Navy is addressing this problem by utilizing advanced coatings technology, along with the introduction of condition based inspection techniques. These inspection techniques include use of an in-situ corrosion sensor system and optical inspection methods which require no tank entrance, minimal physical inspection by human inspectors and no gas free requirement. The in-situ corrosion sensor provides long-term data concerning global tank condition which can be accessed remotely to provide input for computer analysis and tank condition trending of each tank. Additionally, with the advent of remotely operated vehicles and cameras with a high level of accuracy and functionality, it has now also become economically feasible to implement these instruments for quantitative coatings assessment. The optical systems provide computerized, repeatable image scenarios, which are analyzed to provide coatings damage percentages and detailed optical documentation for maintenance scenarios.
Two new British Standards give guidance on the corrosion likelihood in water distribution and storage systems and in water recirculating systems.
· BS EN 12502: 2004 Guidance on the assessment of corrosion likelihood in water distribution and storage systems.
part 1 : general
parts 2-5: influencing
factors for copper and copper alloys, hot-dipped
galvanised steel, stainless steels and low alloyed steels and cast
BS EN 14868 :2005 Guidance on the
assessment of corrosion likelihood in closed water recirculation systems
In water distribution systems, several
types of corrosion are possible with all the metals of construction. Here,
sufficient oxygen is always present to drive anodic metal dissolution.
Then corrosion likelihood is determined by factors related to the metal,
and system design, installation, commissioning and operation. Although in
the vast majority of cases, copper performs perfectly satisfactorily in
plumbing systems, in adverse circumstances, it can suffer from erosion
corrosion, MIC, deposit corrosion, different forms of pitting and general
corrosion leading to blue water. Examples of these are shown in more
detail from actual cases investigated by CESL.
In closed recirculating systems, e.g. central heating systems, corrosion likelihood is determined above all by the amount of oxygen ingress. Corrosion damages are often the result of corrosion products formed in the system, e.g. magnetite - even at relatively low O2 levels. Air ingress should be prevented, if at all possible. The use of corrosion inhibitors may be necessary or desirable. www.cesl.biz