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Life on Board Marine vessels can be one of the toughest environments imaginable. The high salinity, presence of oxygen and numerous marine activities in the seawater constantly challenge heat exchanger equipment. Corrosion, pitting, crevice corrosion and SCC are a constant challenge that can reduce the lifespan from decades to months depending on the materials selected. Choosing appropriate materials for their Plate Heat Exchangers (PHEs) is one of the key and often difficult decisions a shipowner, technical superintendent or a newbuilding specifier faces, with serious financial implications that can affect the cost of operations, maintenance and equipment availability. SME Group has been engineering, designing, manufacturing and commissioning marine equipment for over 20 years, offering a comprehensive product line of ICCP, MGPS, SED, and PHE solutions, and provides you with this easy guide to making smart material decisions.
1. Titanium: The Gold Standard for Seawater Service
Titanium has no equals as a coolant in seawater service. The chloride-induced pitting and crevice corrosion resistance of titanium are a consequence of a tenacious, self-healing oxide film that quickly regenerates upon exposure. Titanium PHE plate elements can withstand years of constant contact with seawater even in the most adverse high temperatures and velocities at which stainless steel would be seriously affected.
Cruise Ships, War Ships, Deep Sea Traders where in failure is not an option – Titanium is the ultimate selection for Main Engine Cooling, Charge Air Coolers & Freshwater Generators. SMEs can provide titanium plates with advanced surface finishes to minimize fouling and maximize ease of cleaning. Titanium, whilst initially more expensive, has a life expectancy in seawater, in practical terms infinite, making it the ultimate long term economic proposition. Utilization of our MGPS to control biofouling in the seawater intakes further enhances the value of our titanium PHEs leading to drastically reduced spare parts requirements and the prevention of plate perforations and the associated consequences.
2. Stainless Steels: Balancing Cost and Performance
If using freshwater, distilled water, or an in-line closed-loop cooling process, chloride levels in your water are likely low and the usage of stainless steel may provide you with an affordable solution. Stainless steels such as 304 and 316L are popular choices. Grade 316L adds a little extra corrosion resistance with its added molybdenum to help prevent pitting in mildly acidic conditions.
Selection, however, requires careful thought. Even 316L stainless steel is prone to pitting in chloride-containing waters due to external contamination, or by the creation of differential aeration cells through varying levels of oxygen in the fluid. For slightly more aggressive fluids such as the emergency cooling water that may sometimes be taken from the sea for cooling circuits, SME advises duplex grades of stainless steel such as the 2205, which can give the combined strength of ferritic steels with the corrosive resistance of the austenitic grades. This bridges the gap between 316L and titanium. SME will analyze the water chemistry to recommend the most suitable grade of stainless steel for the application, so that cost savings do not result in premature equipment failure. We can manufacture plate heat exchangers in our 5,000 sqm Nantong workshop to your precise material requirements for any vessel.
3. Gasket Materials: The Critical Interface
While plates may comprise the bulk of the heat transfer surface, the weak point (the junction between plates and their seal, called a gasket) ultimately dictates a plate heat exchanger's life service. Gaskets in Marine PHEs must contend with seawater, high temperatures, heat cycling and chemical cleaning solutions. Lower temperatures in seawater use will allow Nitrile (NBR) to perform, however, it is compromised above 80C. EPDM, well suited for water resistance and moderate temperature resistance is compromised if there is the presence of oil. Marine use requiring resistance to both high temperatures and chemical attack suggests the use of Viton (FKM) or specific fluorocarbons.
Our clip-on and snap-in gaskets have been engineered to make replacement and maintenance quick and efficient, a key consideration on ships with tight port windows. SME also provides condition monitoring advice to the ship’s crew so that gasket failure can be predicted and prevented. It’s our policy to provide long-lasting gasket solutions along with practical designs so that the gasket connection does not become your system’s vulnerability.
4. The Synergy of Active Corrosion Protection Systems
Material Selection Is not the only issue Even the best plate material and gasket material can be susceptible to attack due to electrochemical reactions somewhere else in the vessel. Stray electrical currents (from welding or due to a cathodic protection system or an electrical fault) can cause local attack on heat exchanger plates and connections by stimulating corrosion and thereby 'jumping over' the material’s resistance.
That’s the point where SME’s complementary corrosion management solution excels. Our ICCP (Impressed Current Cathodic Protection) solutions protect the hull and relevant pipework, mitigating galvanically induced currents from flowing towards heat exchangers. Our SED (Shaft Earthing Devices) cancel the stray shaft currents which flow along seawater piping and attack the cooler plates. If your vessel is equipped with correctly implemented and maintained SME ICCP and SED, using higher grade materials may no longer be required to secure sufficient corrosion resistance for marine heat exchangers. Conversely, without these measures even high grade titanium plates may experience excessive accelerated corrosion from stray current induced corrosion. SME’s experienced support engineers will collaborate with you to evaluate all corrosion influencing factors on your vessel such as hull condition, electrical bonding and any implemented measures to select a corrosion management solution, i.e. Material and complementary measures package, which is specifically tailored to your needs. Backed by our ISO 9001, 14001 and 45001 certifications and more than 100 engineers and technicians, we do more than just provide heat exchangers, we provide total corrosion management to maximize lifespan and minimize costs over the vessel’s life. We go beyond with you.