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An equation will greet all Marine Engineers and Fleet Managers on the equipment they specify, and here it is: When it comes to your seawater cooled exchanger selection, the primary factor you have to consider has to be the material. The truth of the matter is, seawater is nasty. It's very corrosive, has high chlorides and dissolved oxygen, and is teaming with critters that just can't get enough of eating anything improperly specified materials. SME completes over 500 retrofits and repairs annually and it is consistently found that the selection of a material directly affects the lifespan and final cost. Here we will focus on three of the most commonly addressed materials-Titanium, 316L SS and SMO 254 super austenitic SS-addressing them from the aspects of Corrosive Resistance, Mechanical Properties, Value and Fitness for Service.
Corrosion Resistance in Marine Environments
Localized corrosion, in the form of pitting and crevice corrosion is the main cause of heat exchanger plates in seawater use.
The default model is 316L Stainless Steel. The level of resistance to chloride attack of the material is in moderate range because of the amount of molybdenum in it (around 2-3%). But in low flow/ stagnant sea water application the risk is on crevice corrosion under the bio fouled deposits and under the gasket. Most of time we replace the 316L plate on vessels which is operating at tropics & on saline/ brackish water, to find that the plate got attacked at ~12-18 months by pitting.
SMO 254 is a super-austenitic grade as well as it includes much more nickel. It has a PREN (Pitting Resistance Equivalent Number) greater than 40 thus making it significantly more pitting immune to chlorides compared to the regular 316L material. It functions in warm salt water and is resistant to brief chlorination or a bit stagnant environments. The longevity of SMO 254 without large corrosions could differ and has actually currently been found in 3 to 5 years on various SME projects for shipbuilding in seaside settings.
Titanium (Grade 1 or 2) has essentially unlimited reliability when used in seawater. Passive oxide film is very stable even at a high temperature (up to 120℃) and in high-velocity flow. Unlike stainless steels, titanium is resistant to problems such as crevice corrosion, erosion-corrosion and stress corrosion fracture. When price is not a factor and downtime is non-negotiable, then titanium rules the roost.
Mechanical Properties and Thermal Conductivity
Aside from corrosion, the heat exchanger plates must resist pressure differentials, vibrations and thermal cycling.
The yield strength of 316L is approximately 170 – 220 MPa. It is malleable and easy to form, but it is less strong and will therefore result in thicker plates for high-pressure systems, resulting in less thermal efficiency.
A maximum yield strength in the order of 300 – 350 MPa is possible with SMO 254, which results in thinner and more efficient plates than was previously possible, yet without compromise to the structural integrity. The high strength to weight ratio often surpasses the low thermal conductivity, making it an ideal design for small structures.
Grade 2 titanium has a yield of between 250 and 400MPa and is also an excellent tough fatigue material. By contrast the thermal conductivity of this materials is less than 50% that of their stainless steel counterparts, with thin gauges/effective flows necessary to attain equal amounts of heat transfer. It carries a considerable temperature drop in the plates, the design staff in our tech team recommend optimum corrugation to offset this as we carry out our retrofits.
Weldability and Repair Considerations
In reality, plates get damage from mechanical damage, gasket failure and erosion. Replacement or repair on the site must be simple to choose.
316L can be easily welded with a TIG system and is easy to repair. There are many examples of 316L on most of the shipyard workshops and stock can be purchased of 316L spare plates. But welding results in a decrease in corrosion resistance in the HAZ, sometimes necessitating post weld pickling.
It is important to develop a keen sense of welding with high nickel filler rods to apply SMO 254, and be cautious of the heat input during weld work. Corrosion benefit may be lost if improper welding allows for the precipitation of intermetallic phases. SME's certified technicians are trained in the super austenitic welding procedures, however many general repair yards do not have this capability.
Prevention of embrittlement requires full shielding during titanium welding, either with argon or with helium. It is not something for unqualified people to do. But, since titanium is very resistant to erosion, less repairs overall. SME states that it is best to replace the plates with a factory weld when damage has taken place, and not to patch in the field.
Cost and Lifecycle Value
The initial economical cost can often be the determinant factor in purchasing a product, but when it comes to life cycle analysis, that is not the case.
The cheapest on the front-end is 316L. But with its short life in salt water (typically 2-3 years), replacement, gasket replacement, and disruptions can increase the total cost by three times of the vessel's operating decade.
SMO 254 costs 2 or 3 times the cost of 316L, but will give 4 or 5 years use in typical sea water conditions. It is usually achieved in about a year and a half, creating a cost effective choice for medium needs in performance.
The 1st cost is highest cost, the original titanium plates have the 1st cost which can be 5-8times high to 316L. Because plates are to run 15/20 years, so replacement must very little; if ship needs long working life or high mission, this has the lowest life-cycle cost to those kinds of ship then. Here we have supplied Titanium to 1000+ships already and they do not meet failure any more in last 10+yrs.
Conclusion: Let SME Guide Your Decision
We are committed to achieving results! At SME there is not one single solution. We don’t do that sort of thing here. Our team of over one hundred engineers & professional technicians provide on average over five hundred completed projects every year. Each year for every shipyard in China and the service providers of the world; every one of your projects carries a twelve-month guarantee. And if not: “it’s about even getting involved if that meant providing the correct material to get you going.” For that cost effective and time driven enablement for 316L, for reliable performance from SMO 254, and for that assured delivery from your titanium- we are promising you the prompt technical assistance, the on-site global on installation services and of course precisely matched cuts and fabrications at our 5000 sq meter plant in Nantong. It requires an in depth evaluation of the service operation and ship life-cycle in addition to the client’s budget to reach a proposal with a guarantee that prevents failure and minimizes cost for the optimum.