EN
High salinity seawater is one of the toughest environments for a marine heat exchanger to operate in. In locations like the Arabian Gulf, Red Sea and sections of the Mediterranean, the water can have salinity greater than 40ppt, as well as high water temperature, high levels of dissolved oxygen, a high level of bio activity, all of which combine to increase the rate of corrosion, increase scale deposition and to cause significant levels of fouling on the Plate Heat Exchanger (PHE). Vessel operators trading in these tough waters can’t rely on traditional designs. SME Group who have over twenty years experience in marine engineering and can provide solutions to meet this requirement using a combination of their products, ranging from ICCP, MGPS, SED & PHE to the high- salinity tailored design of our PHE.
1. Advanced Material Selection for Extreme Salinity
Material selection is the basis for any PHE constructed for high-salinity service. The 316L stainless steel may be adequate under typical seawater conditions, but will readily pit and crevice corrode under high-salinity conditions. SME advises titanium is the ideal material choice for PHE plates for high salinity applications. In fact, the passive oxide film on titanium is so exceptionally stable even in high-temperature chlorides, it enjoys a theoretically infinite service life in seawater-and the assurance that catastrophic tube perforation cannot occur.
Where titanium would be too expensive for some applications, we propose the use of super-austenitic stainless steels like 254 SMO or nickel-based alloys like Alloy C-276. Although this will give you an improved resistance to localized chloride induced corrosion, care needs to be taken on both temperature and velocity in use. Our engineers work with each of our customers on each individual vessel on the profile of their salinity, temperature ranges and operating velocities to make the most cost effective selection. In our Nantong based 5,000 sqm ISO 9001 certified manufacturing plant we adhere to the most rigorous material specifications for the construction of the PHE to meet the harsh conditions on vessels operating in the world’s harshest environments.
2. Scaling Prevention and Thermal Management
More salt in the fluid means a greater proportion of dissolved calcium carbonate, magnesium hydroxide and other scale-forming agents. As the seawater is heated up on either side of the PHE, these substances settle out on the plates forming a layer of scale which inhibits heat transfer and creates increased pressure loss. If scaling is allowed to accumulate unchecked, then this layer of scale will actually cause a blockage.
SME addresses scale by several design approaches. The plate corrugation patterns are designed for high fluid turbulence, enhancing the shear stress on the heat transfer surface to minimize the probability of scale adherence. The plates also have oversized flow channels in the hottest regions of the plate pack, reducing localized superheating to prevent precipitation. SME will also give guidelines to cooling water inlet temperatures based on trading zone calcium carbonate precipitation saturation temperature.
In environments of particularly harsh seawater conditions, SME is able to specify self-cleaning PHE’s, or propose appropriate chemical cleaning procedures that are safe for use on our plate materials. Together with an MGPS (Marine Growth Prevention System) to manage biological growth, that can intensify scaling phenomena, the operator has the most efficient way of retaining thermal performance in harsh marine environments.
3. Reduced Flow Velocity and Erosion Control
These high flow velocities will in part reduce fouling, but on the other hand, they will increase the erosion–corrosion rate. In high-salinity waters where the chlorides attack vigorously, this can be particularly problematic. SME designs this to strike a happy medium between getting a decent heat-transfer rate, and not pushing it so hard as to wear out the plates too quickly. The parameters that can be balanced between high velocity and erosion–corrosion are far more accommodating for the titanium plates than they would be for stainless steels.
We use CFD modeling of the flow patterns across the plate pack to see if any regions within the plate pack could experience high flow velocities at specific points. SME then use our experience of plate geometry, port sizing, and gasket configuration to optimize flow distribution such that it reduces erosion whilst ensuring the velocity is still high enough to control biofouling. This reduces the number of plates in vessels trading in high salinity regions where fouling can be a problem.
4. Integrated Corrosion Protection for System-Wide Reliability
Even top grade plate materials may be undermined by attack initiated elsewhere on the vessel by electrochemistry caused by high salinity water in contact with pipeline, fittings and connections. Stray currents originating from cathodic protection systems, electrical faults, or even from generated potentials from drive shafts may enter pipe work containing seawater and cause rapid corrosion attack on heat exchanger parts, irrespective of plate material.
Risks that are tackled by SMEs’ integrated protection approach. Precisely targeted hull protection is delivered by ICCP (Impressed Current Cathodic Protection) system, ensuring that no stray currents from the hull can reach the PHE and influence them. Static charge building on shafting due to rotation can lead to voltage levels which can cause seawater cooling circuit corrosion. SED (Shaft Earthing Device) neutralizes shaft static electricity and thereby prevent these potential corrosive current flowing into seawater via the shaft to shaft generator and finally into sea water via outer casing of the shaft generator. Combined together with an MGPS system (which prevents biological fouling on surface that can cause differential aeration cell on metallic surfaces), the combined system effectively counter acts the threat of both corrosive and biological agents in high salinity conditions.
The technical support team at SME work with you to analyze your entire vessel, including pipe materials, bonding and the existing protection, designing an all-inclusive solution to maximize the lifetime of your PHEs. With in excess of 500 new and existing projects annually and over 100 electrical and mechanical engineers, at SME Group we are perfectly placed to develop your bespoke PHE solution for even the most extreme saline environment and the peace of mind you need. With us, your solution extends beyond your worries.