316L vs. 904L Stainless Steel: What’s the Difference?

PMI gun testing 904L steel

The global maritime industry has been shifting towards greater environmental protection efforts in recent decades. The International Maritime Organization’s Marine Environment Protection Committee (IMO MEPC) works to address issues that affect and threaten our oceans, including air and water pollution, disaster preparedness, and ship recycling.

However, this organization also studies an environmental issue hardly known to the public: ballast water management.

Ballast water tanks keep a ship balanced as she transports cargo, but it must be fully filtered and treated before being released at her port of call. The IMO guidelines require that ships only discharge ballast water, that is:

  • < 10 viable organisms/m3 that are ≥ to 50 micrometers in minimum dimension
  • < 10 viable organisms between 10 micrometers and 50 micrometers in minimum dimension per ml
  • < 1 colony-forming unit (CFU) per 100 milliliters of Toxicogenic Vibrio Cholerae
  • < 250 CFU per 100 ml of Escherichia coli
  • < 100 CFU per 100 ml of Intestinal Enterococci
Organism SizeSize UnitNumber of OrganismsStatusAmountAmount UnitType
Over 50μm10viable organisms1m3Any
10 to 50μm10viable organisms1mLAny
--1Colony-forming units100mLToxicogenic Vibrio Cholerae
--250Colony-forming units100mLEscherichia coli
--100Colony-forming units100mLIntestinal Enterococci

These guidelines have solidified the importance of ballast water management systems (BWMS), making them a vital part of every ship. It is also more important than ever to maintain (and in some cases, upgrade) these systems – mainly because, as Filtersafe Head of Marine Mark Riggio recently said, “[the] global maritime industry [is] shifting its focus to operational compliance.”

The Best Alloy for Ballast Water Management Systems

Seawater filtration systems have a unique challenge to overcome. They must effectively filter debris and microorganisms while also withstanding the high chloride levels in the water, which constantly threaten to corrode the filter and its filtration screen. To achieve this goal, manufacturers often design BWMS with a filter that has a filtration screen made from a durable stainless steel alloy.

For many years, the industry-standard alloy of choice for filter screens has been 316L stainless steel. But due to the rising need for operational compliance, some of its shortcomings in seawater environments are coming to light. This has led some manufacturers to opt for the higher-grade 904L stainless steel when producing filtrations screens for seawater applications. Both these alloys can be applied in seawater filtration, but what are the differences between these two types of steel? And ultimately, as many shipowners are left to wonder, which is the right alloy for their BWMS, and their bottom line?

What is 316L Steel?

316L stainless steel is an austenitic alloy commonly known as “marine grade stainless steel” because it can be used for nearly 90% of marine applications – including filtration. In addition to metals like iron and nickel, 316L contains 16-18% chromium and 2-3% of molybdenum. These elements are important because they increase the alloy’s corrosion resistance; the chromium interacts with oxygen in the seawater to create a protective layer of chromium oxide, and molybdenum improves the metal’s ability to resist pitting corrosion. Additionally, 316L has lower levels of carbon (hence the “L” in its name), which gives it greater protection against corrosion.

What is 904L steel?

While 316L has long been the primary alloy ballast water filtration manufacturers use for their filter screens, it is not the only alloy available on the market. In fact, Filtersafe has been examining the value of 316L for many years now. As global leaders in the seawater filtration industry, our engineers sought to improve filtration standards by improving our materials. Our studies concluded that 316L stainless steel was not durable enough to meet our requirements – and so we opted instead to manufacture filter screens from 904L stainless steel In fact, Filtersafe – the global leader in seawater filtration – has been examining the value of 316L for many years now.

Like 316L, 904L stainless steel is a low-carbon austenitic stainless steel. However, its chemical composition includes greater numbers of chromium (19-23%) and molybdenum (4-5%), which gives the alloy greater corrosion resistance than 316L. Some people are familiar with 904L stainless steel as a metal commonly used to produce Rolex watches, which speaks to its high quality and durability. However, the same benefits that make it a must-use for Rolex also make it hugely beneficial for seawater applications.

316L vs. 904L: Durability

Seawater is highly unpredictable. It can have dramatic variances in temperature, chloride level, and the presence of microorganisms or debris. Therefore, a BWMS filter must be durable enough to withstand these changes.

Both 316L and 904L stainless steel contain chromium, which reacts with oxygen in the air to form a thin, protective layer of chromium oxide on the surface of the alloy. However, it is important to note that 904L contains more chromium than 316L. This means it is likely to provide greater protection over a longer period of time.

types of corrosion that afflict stainless steel in seawater applications

904L vs. 316L: Corrosion Resistance

Perhaps the most important feature of any alloy used in marine applications is its ability to handle constant exposure to corrosive substances. Seawater filters are prone to several different types of corrosion attacks, including from chloride, microbiologically induced corrosion, and crevice corrosion. The filter is particularly susceptible to these three corrosion types during the 1-3 week period between ballasting, when the filter is immersed in stagnant water, which can allow the development of a biofilm inside the filter and on the screen, and lead to corrosion. Therefore, a ship owner needs a filter with as much corrosion resistance as possible.

Shipowners and manufacturers can measure corrosion resistance by looking at an alloy’s PREN value. This formula looks at the amount of chromium (Cr), molybdenum (Mo), and nitrogen (N) in an alloy to determine just how well it will hold up against corrosion. In this case, 904L stainless steel is the clear victor over 316L; with an average PREN value of 36.7, it is much stronger than 316L (which only has a PREN of 26.1). Therefore, 904L is more likely to withstand the corrosive power of seawater.

904 vs. 316: Steel Hardness

To further understand the differences between 904L and 316L stainless steel, we must examine the hardness of both metals. The best way to do this is to look at the Rockwell Scale, which measures the indentation hardness of a material. Metals that score highly on the Rockwell scale are harder, which implies that they will be strong and withstand any bumps or bruises.

Both 904L and 316L stainless steel have a Rockwell hardness value below 95, which is typical for most stainless steel. This means that they will be able to withstand most forces, which is important for BWMS. However, it is far more important that seawater filter screens withstand the internal dangers like chloride and other corrosion attacks. 904L offers greater protection overall, which sets it above 316L.

904 vs. 316: Cost

904L stainless steel and 316L stainless steel are both readily available for purchase. Many industries and companies use 904L stainless steel, and the alloy has some brand recognition as it is famously used by watch manufacturers like Rolex and OMEGA. 316L stainless steel is commonly used for exhaust manifolds, heat exchangers, jet engine parts, and much more. As a result, BWMS manufacturers can introduce either alloy into their supply chains without suffering a delay in supply or a product bottleneck.

But which alloys offer the most “bang for your buck”?

If you look solely at the numbers, 316L may seem like the more attractive choice. Because of its widespread use across so many industries, this metal is both easy to purchase and available for a lower price. 904L tends to cost about 1.3 times more than 316L, which can make some manufacturers balk at the idea of producing all their filter screens from this material.

However, it is important to remember that filter screens made from 904L stainless steel will last longer and require fewer repairs than filters made from less durable materials. The enhanced corrosion resistance capabilities of 904L stainless steel make it less likely to wear down and break from corrosion, and therefore it will require fewer repairs over its lifespan. This is ideal for shipowners who do not want to incur additional costs to keep their ship in compliance with industry environmental and operating standards.

What is the Difference Between 316 and 904L Stainless Steel?

When ship owners are deciding between filters for their BWMS, they must consider a variety of factors before making their choice: cost, availability, value, etc. On the surface, the differences between 316L and 904L stainless steel may seem negligible, but in actuality, they have huge impacts on the long-term value of the filter.

To the Filtersafe team, the choice is clear: 904L stainless steel is the best choice for BWMS filters. 904L stainless steel offers greater durability, which makes it more effective and less costly over time (despite a higher initial cost). This alloy will help boost standards across the seawater filtration industry – and, in turn, improve environmental factors in oceans across the globe.

To learn more, download our white paper on Overcoming Corrosion of Stainless Steel in Seawater Applications for more extensive research on the common alloys used in seawater.

10 Most Invasive Marine Species

Ships worldwide transport over 10 billion tonnes of ballast water every year and by doing so inadvertently bring small plants and animals from one port to another, when they load and unload the water. Running the risk of ecological disaster via invasive species.

This is is why the International Maritime Organization decided vessels need to install ballast water management systems – to clean and treat the water before it is loaded into the ballast tanks. This way when the ship reaches its next port and needs to dump the water in its ballast tanks overboard, it won’t introduce any invasive species into a new ecosystem.

The following infographic shows the top ten invasive marine species that were transported by international shipping and the impacts they had.

Filtersafe is proud to be a leader in the movement to end the spread of invasive species and to protect our marine resources.

top 10 most invasive marine species
Click on the Top 10 Most Invasive Marine Species image to see in high quality.

Filtersafe’s Smartweave Screen: Water Filtration at a Whole New Level

filtersafe default image with logo

Most eCommerce clicks not only put new clothes or appliances in your virtual shopping cart, they also are putting your packages on cargo ships for delivery. The continued dependence on online shopping has meant more work for  international shipping vessels, which are constantly moving from one corner of the globe to another – and they’re discharging ballast water in every port they visit.

This can be dangerous for our oceans, as zooplankton, phytoplankton, and other microorganisms that hitch a ride in ballast tanks can damage the native ecosystems if they are deposited into a new body of water. Therefore, it is essential that ships filter their ballast water before releasing it back into the sea.

As a company founded by engineers, Filtersafe has been dedicated to finding innovative solutions to filtering sea water. The current seawater screens often suffer two main problems: blockage, which can result in pressure depressions and a decreased flow rate, and an inability to filter out smaller microorganisms.

To address these issues, the Filtersafe engineers created the Smartweave screen. This unique technology is a weave-wire screen specifically designed to tackle the challenge of seawater filtration. By using a combination of weave wire filtration screens, protective screens, and a reinforcement layer — as well as the automated cleaning power of our Everclear system — this innovative, highly effective screen filters out sediment and microorganisms, protecting native marine life in every port. Its combined efficacy and durability make the Smartweave screen a filtering element ships can rely on for years.

 

How It’s Made

Filtersafe understands the unique challenges facing ballast water filtration (one of the most challenging seawater applications). Seawater contains microscopic particles that can cause harm when transferred to other ecosystems and encourage the creation of corrosion within the ships’ interior. Filtering out elements this small requires a complex and multi-layered system, so the Smartweave designers created exactly that.

The Smartweave screen is made up of three stainless steel weave-wire screens and a fourth reinforcement layer to enhance the screen’s integrity. This includes one filtration control layer, two protective layers on either side, and one reinforcement layer. We then take this combination of screens and sinter them together, creating one mega-screen that is durable enough to filter without additional support.

These screens are available in a variety of sizes, from 500 microns down to 10. This allows ship owners to choose a Smartweave screen that best suits their vessels’ unique needs. The Smartweave seawater screen is also made from 904L stainless steel: an stainless steel that offers high corrosion resistance, higher quality, and a longer-lasting screen than the standard 316L stainless steel and other options in the market today.

The innovation and careful design work behind Smartweave have made it one of the top weave-wire screens for seawater filtration today. This screen offers enhanced strength and top performance, so shippers can trust that their ballast water is safe to discharge and is compliant with the IMO G8 requirements.

 

How Does It Work?

All the innovation in the world doesn’t matter if a product doesn’t work. The engineers at Filtersafe know how important seawater filtration is, and so they’ve spent years testing and perfecting the Smartweave screen. As a result, Smartweave is one of the most effective filtration systems available today, keeping zooplankton, phytoplankton, and sediment out of ballast water across the world.

The term “zooplankton” refers to small microorganism and the immature stages of larger species. It is vital to filter these from ballast water, as they can be detrimental to the native ecosystem at a port of call. According to tests from Filtersafe and other research groups, the Smartweave screen removes 99.95% of zooplankton from ballast water – more than any other filter available today.

Shippers also need to filter phytoplankton from their ballast water before releasing it into the sea. Phytoplankton, or microscopic marine algae, are an important food source in the ocean’s ecosystems – but only if they’re in the right location. While phytoplankton can be very small (as small as 10 microns), Smartweave is an effective filter for them, too. that the Smartweave screen removes 100% of phytoplankton larger than the micron of your selected screen.

Finally, a filter must be able remove sediment and other suspended solids from any ballast water. Once again, Smartweave delivers here:  A series of tests we conducted on our filter show that even in extreme conditions when the TSS levels reached 2500 ppm, we were able to decrease the decrease suspended solids from 2500 ppm to 100 ppm – a removal rate of 98%.

 

Constantly Improving

Today, Filtersafe treats 25% of the world’s ballast water – and all because of unique, innovative, and effective products like the Smartweave screen. For some teams, the Smartweave screen’s impressive filtration rates would be enough to call the filter a success. But Filtersafe is dedicated to making the ballast water filter even better.

The team is constantly innovating and perfecting our ballast water treatment systems. We’ve outfitted our Smartweave screens with our Everclear cleaning sequence, which automatically returns your filter to its original operating parameters. This ensures peak performance and an optimum flow rate – and provides the user with peace of mind.

Each Smartweave screen is used in conjunction with Filtersafe’s patented nozzles – the Nozzlex system. This uses suction technology to clean the screen thoroughly and completely with each use(without damaging it). These features help maintain the Smartweave’s exceptional performance, so captains can focus on their cargo and getting to their next location and not just about meeting USCG or IMO water treatment standards.

*Efficacy is dependent on the size and distribution of the TSS.

Sizing Up BWTS Filter Options To Reduce Operational Compliance Risk

filtersafe default image with logo

In a recent panel discussion for Riviera’s Ballast Water Webinar Week, Dr. Guillaume Drillet, Regional Manager at SGS, shared his views on the main reason for ballast water treatment system (BWTS) failures during compliance testing. Speaking on the webinar titled “BWMS commissioning testing: making it work in practice”, he said that organisms over 50 microns in size are responsible for clogging BWTS, subsequently leading to test failure.

His comments underline the critical importance of BWTS filters – the component responsible for preventing organisms from entering the tanks. Without a strong and robust filter, the effectiveness of the entire BWTS could be compromised, leaving manufacturers open to criticism from ship owners and operators, who must repeat unsatisfactory tests at a later date.

 

Compliance challenges

Selecting the right filter brings significant rewards to the shipping industry, from driving compliance, to preserving marine biodiversity and increasing operational efficiency.

Each BWTS and vessel is unique and faces its own set of challenges and specific stresses. These are impacted by the BWTS used and the conditions it must operate in. For example, there are challenging testing conditions in shipyards with poor water quality, shallow harbors with a high silt load, and difficult initial loading conditions. If a filter is unable to withstand these conditions and clogs, water flow through the BWTS will be limited or even prohibited.

 

Regulation requirements

Under IMO G8 requirements, filters must prove their ability to perform effectively in water with total suspended solids (TSS) up to 50 mg/liter. However, the threshold to pass this test is not reflective of water quality standards in some key maritime locations. For example, TSS in the Ports of Shanghai and Hamburg are twenty times higher than IMO G8 requirements for BWTS type-approval testing. This means high-quality filters that exceed IMO performance standards are critical.

Fortunately, filter performance in more challenging marine environments can be tested through the Control Union Shanghai filter test (Procedure CUW-HBR-P-2), which uses proxy mud up to and beyond 1,000 mg/L to test filter performance.


Size is part of the solution

In order to prevent organisms over a certain size from impacting BWTS and compliance testing, filters need to be engineered with a design that has effective mesh sizes.

Filtersafe use mesh sizes from 10 microns upwards, at flow rates from 50-5000m³/hr in a single unit. Using these fine mesh sizes ensures the system prevents organisms from entering or impacting the system by settling into the ballast tanks or clogging the system.

A filter capable of withstanding the highest TSS conditions found in the world’s oceans will provide confidence to shipowners that the BWTS will pass compliance tests in any port around the world. In its Shanghai Test, which simulates an excessively high particle load, Filtersafe filters did not clog, even when the test reached 2,500 TSS.

With compliance testing coming ever more to the fore, BWTS manufacturers, shipowners, and operators need to be confident that systems are capable of performing effectively throughout the lifecycle of the vessel across all marine environments, including areas where water quality challenges are acute. It is important to remember – mesh size matters.

Filtersafe’s NozzleX: An Innovation that will Save Your Filter Screen

filtersafe default image with logo

At Filtersafe, we understand that water filtration is essential to helping many industries run smoothly. However, we also understand that today’s filtration technologies can always be improved — which is why we strive to be a leader in the world of water filtration. Our engineers have decades of experience in the industry, and they’re dedicated to creating the top technologies for automatic filtration.

One such technology is the NozzleX suction scanner – an innovative tool that enables the cleaning of organic and inorganic material from your filter’s screen, without wearing down the nozzle or wearing out the screen. With a typical automatic screen filter, the nozzles sit on a bar called a raiser. The raiser moves the nozzle heads back and forth along the length of the screen, while also rotating the nozzles in a corkscrew motion. The nozzles scan the surface, and by using negative pressure, dislodge debris one square inch at a time. While this does help the screen filter water effectively in the short term, the force from the nozzle can damage the screen and shorten its lifespan.

NozzleX, by contrast, uses low head pressure (as low as 23psi) to clear off all the buildup on the screen without damaging it. First, the raisers move the nozzles in such a way that 100% of the screen is scanned and cleaned. Next, in contrast to other filters, NozzleX nozzles safely come into contact with the screen, utilizing gentle pressure to pull materials off of the screen without impacting the screen’s integrity. This results in improved performance and a longer lifespan for your ballast water filter or other filtration technologies.

 

How it Works

The NozzleX uses passive suction to remove organic matter like debris and sediment from the filter screen. Through an innovative combination of consistent force and passive pressure, whereby the nozzle both actively removes material from the filter while allowing the naturally occurring changes in pressure to carry it away, this nozzle makes cleaning your filtration system an easy and automatic process.

This patented system moves around the filter screen automatically, using as little as 1/32nd the force of typical cleaning nozzles to offer a thorough cleaning with minimal wear. Then, the nozzle disposes of reject water through the system’s flushing chamber, guaranteeing that buildup won’t remain on your screen.

NozzleX is part of Filtersafe’s Everclear cleaning system, which includes the Smartweave screen filter. Working together, these two technologies provide exceptional filtration power while minimizing space. This ensures that operators can use their filtration system as long as possible.

 

Benefits of NozzleX

In the world of water filtration, NozzleX is truly a revolution. This unique cleaning tool offers several significant benefits to an automated filtration system, which can make a real difference in the quality and longevity of a system. Here are just a few of the unique benefits you can get from using NozzleX:
  

    • Continual System Operation: Firstly, NozzleX runs as part of the automated filtration system. There’s no need to halt system operation and clean out your filtration screens; instead, NozzleX will clean your screens throughout the filtration process.
    • Minimal Surface Area: In addition to its consistent cleaning functionality, the NozzleX is exceptionally compact. The nozzle takes up a mere 1% of the screen area, which allows your filtration system to continue running even while it’s being cleaned, and without significantly hindering the flow rate. This will result in greater filtration capacity and greater overall efficiency for your system.
    • Improved Performance: Dirty or clogged filters can have a serious impact on a filtration system’s efficacy. In fact, Filtersafe testing has shown that some systems lose performance in as little as six hours. In that time, a system’s flow rate can drop from about 3,000 m3/hr to less than 1,900 — but NozzleX maintains a consistent flow rate throughout its operations. With NozzleX, you’ll be able to get the optimum performance from your filtration system all day long.
    • Zero Screen Wear: Finally, one of the greatest benefits NozzleX provides is the way it minimizes screen wear. Other nozzle-based cleaning systems use significant force to remove debris from their filters, which can lead to punctures and other damage that cuts the screen’s lifespan short. NozzleX, in contrast, equalizes system pressure to use far less force — around 1.6 Bar (23psi) of head pressure. This means your filter screens suffer next to zero wear, allowing them to operate much longer than other screens.

     

    Why It’s Important

    The importance of a properly functioning nozzle is simple – a water filtration system only works when it’s clean. Whether you are filtering ballast water before filling your tanks, protecting your oil and gas exploration equipment safe from organic oceanic materials, or prefiltering water for desalination, each filtration system should have the same thing in common  a reliable nozzle to clean your screen.

    So, why not let NozzleX do the job for you?

    NozzleX ensures that a filtration system stays clean throughout operations, thereby making sure that the system remains effective and efficient. As part of Filtersafe’s Everclear system, NozzleX will provide users with a completely clean filter screen — and that will help guarantee superior performance from your filtration system.

     

    To learn more about the NozzleX cleaning technology, contact Filtersafe today. Our team will be happy to answer questions and help you find the system that best suits your industry and your organization’s unique filtration needs.