4 SALT: THE TURBINE KILLER Salt is particularly perilous to the GTs on naval vessels because there is so much of it stirred up from the sea and continuously present in the ambient air. While the filter system housing and internals will typically be manufactured from 304 or 316 grade stainless steel, sodium from sea salt (NaCl), if allowed to get downstream of the filtration system, will still combine with sulphur in the fuel to create sodium sulphate (Na2SO4). This chemical reacts with the base metal of the turbine blades in the high temperatures of the hot gas path causing rapid corrosion and component failures. This very common effect is known as hot corrosion or sulphidation. Also chlorine in the salt very commonly acts as a pitting corrosion initiator in colder parts of the turbine, potentially leading to catastrophic damage. Salt can be difficult to handle because of its hygroscopic nature, meaning it has an affinity for water. It absorbs moisture readily and can move easily from solid to liquid form with changes in ambient relative humidity. A filtration system, therefore, needs to efficiently defend the turbine from both liquid and solid phase contaminants. When salt is wet, it easily sticks to turbine blades and attracts other contaminants, quickly affecting the aerodynamic performance of the turbine. Unlike contaminants that cause fouling, one of the major problems with salt, is that the corrosion it causes is often not perceived in monitored turbine performance data until something physically breaks. Naval vessels, by their very nature, will always be exposed to a great deal of salt laden sea spray. To handle this salt effectively, a filtration solution needs to allow for input concentration, aerosol size distribution and aerosol physical state, whether droplet or particle. The right solution must also consider the impacts of increased pressure drop introduced by adding multiple filtration stages with salt protection. Salt can also contribute to erosion and crucially for salt compounds, are a substantial factor in the corrosion of turbine internals such as compressor blades, combustors, bearings or even elements of the turbine section. Corrosion and wear will inevitably reduce the service life and increase the requirements for maintenance. While this is costly, for military endeavours a more significant concern is that servicing and repairs have direct implications for service readiness - any downtime is a major issue in terms of naval responsiveness and repairing or replacing worn components can be a lengthy procedure, especially if the vessel is at sea. The same considerations apply to less severe contamination too. A typical response to compressor stage contamination is an offline wash cycle which will restore most, if not all, the lost performance. However, it also necessitates shutting down the turbine with an inevitable, if short-term, loss of availability. The marine environment in general then is particularly unforgiving when considering precision equipment like an advanced gas turbine and with bad weather, rain, fog and sea spray on top, these issues are only exacerbated. Thus, a critical consideration for naval turbine reliability is to prevent corrosive materials like salts and other airborne contaminants from entering the machine. This places huge emphasis on effective air filtration. HMS Queen Elizabeth, Equipment: Parker Neptune Image 2 – Parker Neptune high velocity intake system
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