The Top Alloys For Use In Seawater Filtration Applications

a comparison of the top seawater worthy stainless steel alloys

Filtration of seawater is a necessity across a diverse range of industries – such as intake for a desalination plant, ballast filtration for an oil tanker, or to maintain pressure and the production rate. When running a complex operation there are plenty of unexpected expenses to deal with on a regular basis, and the last thing anyone wants is a preventable failure of their filtration equipment. This is especially pertinent when dealing with seawater because it is filled with corrosive elements that can lead to material failure like corrosion or strength degradation. Therefore it’s important to have a quality, durable filtration system.

desalination pipeAs part of the search for the best system, an often underappreciated but critical component to its success is dependent on which alloy the screen filter, the heart of the system, is made of. The filter screen is the physical barrier that prevents particles (organic and inorganic alike) that are larger that its pore size from passing through with the water. Corrosion to the screen can damage its ability to be cleaned, clog its open area and inhibit water flow, or degrade the screen’s structural integrity, letting particles larger than its pore size to get through and compromise the integrity of the system. Various seawater filtration industries struggle with the same decision as to which is the best

Seawater filters have unique and specific requirements: they need to be non-corrosive, as well as effective at removing debris of various sizes, which can harm down-stream processes. This is extra important in industries that rely on very fine filtration (100-5 microns), such as in desalination and ballast water filtration. In these cases, the screens must also be highly effective at removing phytoplankton and zooplankton from the water. Any seawater processing application requires a durable pre-filtration protection system so that time and money isn’t wasted over the years to repair poorly protected systems. With this in mind, let us look at the most common alloys used in seawater filtration and determine which alloy best suits your system.

316L

316L stainless steel, also known as “marine grade stainless steel,” is the second-most common stainless steel used in manufacturing today (after 304 stainless steel). This alloy is made up of iron, chromium (about 16%), nickel (10%) and molybdenum (2%), as well as trace amounts of quantities of silicon, phosphorus, and sulfur. Although it is a popular choice, the latest advice from the International Stainless Steel Forum is that AISI 316 and its derivatives “are no longer recommended for permanent contact with seawater.” We’ll see some of the reasons behind this decision in the Cons section.

Pros

316L stainless steel is a popular choice for seawater filtration. This is in part due to the alloy’s widespread use throughout many industries, making it widely available, as well as the benefits described below.

Some Corrosion-Resistance

Like all of the alloys discussed here, 316L stainless steel contains chromium. This element is an important part of the chemical makeup; it creates a thin film over the alloy when exposed to oxygen, which helps prevent the steel from becoming corroded over time.

The chromium in the alloy does provide a degree of protection that prolongs the steel’s – and by extension, the filtration system’s – lifespan. However, it is important to note that 316L does not provide complete protection from corrosion. While the alloy can offer reasonable protection in the short term, its particularly susceptible to pitting corrosion caused by inconsistent salinity levels in the seawater.

Tolerant to Higher Temperatures

Temperature fluctuations are most acutely felt in the shipping industry, as seawater can vary wildly in temperature between locations, seasons or even times of the day.  316L can tolerate water temperatures up to Celsius (68 degrees Fahrenheit), which means it is able to successfully stand up to most seawater.

Low Cost

One of the biggest benefits of 316L stainless steel is its relatively low cost and wide availability. Manufacturers can purchase 316L scrap relatively inexpensively, which makes it a highly accessible material for filtration system engineering.

Cons

Despite 316L’s popularity, there are some industry experts who believe that this stainless-steel alloy is inappropriate for use specifically in ballast water treatment systems. These professionals cite issues with the welding process as a major drawback – and the consequences of these issues certainly warrant a second look in other seawater applications as well.

High Ferrite Content

One of the biggest drawbacks in using 316L stainless steel for seawater filtration can occur during the welding process. 316L must be welded under very strict conditions, or else it might develop a high ferrite content in the weld seam.  Ferrite can diminish an alloy’s ability to resist corrosion. Even a small amount (around 2%) of ferrite content in a weld seam can result in a less effective finished product.

Since 316L stainless steel is prone to developing excess ferrite during production, it can make some filtration experts leery of using this alloy in their systems. In fact, some argue that the potential for high ferrite content in 316L stainless steel can lead to greater incidence of pitting corrosion or crevice corrosion. This kind of damage can require significant repair or even a completely new filtration system – effectively counteracting the reduced costs of using the 316L alloy.

Duplex 2205

Duplex 2205 is a stainless steel characterized by its strength. With a makeup of 22% chromium, 3% molybdenum, and about 5% nickel, it is about on par with 904L in terms of corrosion resistance. However, duplex 2205 is (as the name suggests) a duplex stainless steel; this means that it contains both austenite and ferrite phases in its metallurgical structure, giving the alloy a greater overall durability.

Pros

Duplex 2205 is the most popular of the duplex stainless steels, and it is often used in pipework systems for offshore oil and gas. This alloy’s strength and chemical composition, as well as its durability against
seawater in offshore uses, makes it a logical choice on paper.

seawater filtration for oil and gas applications

Good Corrosion Resistance

Duplex 2205 has a PREN average of 35.9 – just shy of 904L stainless steels average of 36.7. This indicates that this alloy would be able to fair just as well as 904L when dealing with pitting corrosion, crevice corrosion, or any other type of destructive wear and tear within the filtration system. Theoretically, duplex 2205 would be a reasonable alloy for a seawater filter.

Cons

While duplex 2205 has had plenty of success within the oil and gas industry when used as a solid structure such as piping, it’s not used often in complex manufacturing procedures such as a weaved screen mesh used in seawater filtration. This is likely due to two main issues: There are challenges when working with it and its lower temperature threshold for corrosion.

Hard to Work With

One of Duplex 2205’s assets, it’s strength, is actually a double-edged sword. Since the alloy is so strong, it makes it difficult to work with, and requires special heavy machinery that is not commonplace. In addition, it makes it very challenging to mold the steel into the very tiny and delicate structures, such as ballast water filters or desalination filters that require filtration down to 10 microns.

Corrodes Faster than 904L

While Duplex 2205 has better corrosion resistance than 316L, it still corrodes at a lower temperature than 904L (50C vs 55C for 904L).

904L

Another popular alloy used in seawater filtration is 904L stainless steel. This alloy contains many of the same elements as 316L steel, but it is comprised of higher levels of chromium (about 19%), nickel (23%), and molybdenum (4%).

904L stainless steel is most famously used in luxury watches (Rolex, for example, swears by this alloy), but it is also a popular metal for seawater filtration. This is because the high amounts of nickel in this alloy
makes it particularly corrosion resistant, protecting your seawater filter from damage.

Pros

Choosing the right alloy is essential for a durable and effective seawater filtration system. When alloys have similar compositions, like 316L and 904L, the decision may seem insignificant; however, 904L stainless steel has some major benefits that can make a big difference for a seawater filter.

Highly Corrosion-Resistant

As we mentioned above, 904L is highly resistant to corrosion due to the high levels of nickel in its composition. This means that seawater filtration systems made with 904L tend to have a longer lifespan than their competitors.

In fact, 904L stainless steel has an average pitting resistance equivalent number (PREN) of 36.7 – more than 10 points higher than 316L’s PREN average (26.1). This is an important parameter to consider when
designing seawater filters, as it will help determine how effective the filter’s corrosion resistance and durability will be over its life.

Widespread Availability

Another benefit 904L has to its credit is its availability in the market. This alloy is readily available from most suppliers, which means that engineers and manufacturers can procure the metal and produce filtration systems much more easily than they might with other metals.

Cons

While 904L stainless steel offers both practical and logistical benefits, it is not a perfect alloy. Some engineers or manufacturers may opt for another metal due to the one major flaw that accompanies using this metal: the cost to use it.

Higher Cost

The costs of most alloys vary based on the amount of chromium and molybdenum in the metal. As we’ve mentioned previously, 904L stainless steel has higher levels of both these elements compared to 316L – which means that it tends to be about 1.3 times more expensive.

This price difference might drive some budget-conscious manufacturers to a lower-quality alloy. However, it is important to mention that the greater strength and durability of 904L stainless steel means that filters made from this alloy are less likely to need repairs during their lifespan. The cost savings you will earn from this lack of repairs more than offsets the cost of the alloy itself.

254 SMO

Like the two alloys we’ve already mentioned, 254 SMO is an austenitic stainless steel. However, this alloy was originally developed for use in chloride-heavy environments – such as seawater cooling pipes, heat exchangers, pulp and paper plants and more. The metal contains a similar amount of chromium as 904L, but it has a greater amount of molybdenum (6% vs. 4% in 904L).

With this composition in mind (not to mention the alloy’s intended use), it is no surprise that 254 SMO is an alloy that generates lots of interest among seawater filtration engineers. There are unquestionable benefits that come with using this alloy – but there are also a few drawbacks that can complicate its use.

Pros

The greatest benefit of using 254 SMO for seawater filtration is its corrosion resistance. The chemical composition of this alloy makes it highly resistant to both microbiologically induced corrosion and chemical corrosion – making it incredibly durable against seawater even after prolonged exposure.

Excellent Corrosion Resistance

254 SMO has a PREN average of 43.8, which means it has the greatest protection against corrosion of all the alloys studied here. It also has a low carbon content (around .02%), which means that there is little risk of ferrite development during production. These two facts mean that 254 SMO is one of the best alloys for protecting your seawater filtration system from the corrosive effects of seawater.

Cons

The excellent protective qualities of 254 SMO might make it seem like the gold standard in seawater filtration materials. However, at least in their ballast water treatment systems, few developers use it – why? 254 SMO is often passed over due to two flaws in its practicality.

High Cost

As we mentioned earlier, alloy costs vary depending on the amount of chromium and molybdenum in the steel. 254 SMO contains higher levels of these elements than both 316L and 904L stainless steel – which means that it tends to cost much more than its competitor alloys.

254 SMO can cost as much as three times more than 316L, and many manufacturers are unwilling to spend more to produce their filtration systems. This cost is above the threshold deemed “acceptable” by most industry professionals (unlike 904L stainless steel, which has a slightly higher cost but offsets the cost of maintenance).

Limited Availability

One of the hinderances that limits 254 SMO from being suggested for more seawater filters is that this alloy is not produced regularly due to its higher costs.

Which Alloy is Best?

radar diagrams for common stainless steel alloys

Due to the nature of seawater filtration and its association with heavy industry, engineers of all applications endeavor to build filtration systems standing up to the ravages of seawater — all without costing you too much money. At the same time, the cost of using a lower-quality alloy will corrode more easily and lead to expensive problems – specifically more frequent maintenance and reduced filtration capabilities as corrosion overtakes the screen. These outcomes will, in the long run, increase operating costs, as equipment owners will have to pay for more frequent servicing, and where applicable such as in regulated industries like ballast water management systems (BWMS), potential fines for non-compliance of malfunctioning systems.

The solution? Invest in a filtration system that’s made from a durable, yet cost-efficient material. And when it comes to choosing the best alloys for seawater filtration without consideration of cost – SMO 254 is definitely the favored alloy. When dealing with a high end application where even the smallest downtime can be very costly such as in the oil & gas or desalination sector, the initial investment in using a Filter with an SMO 254 screen pays off in the long run. However, when a cost-effective solution is required, there is one option that stands head and shoulders above the competition: 904L stainless steel.

904L stainless steel is the ideal marriage of strength, corrosion resistance, and accessibility – all at a reasonable cost to manufacturers. Seawater filters made from this alloy will stand up to corrosion and keep your filtration system running effectively for many years, saving you thousands in fees and maintenance costs.

For a deeper investigation into the alloys discussed above for use in seawater applications please download our peer-reviewed white paper on the topic – Overcoming Corrosion of Stainless Steel Screens in Seawater Applications.

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.

How Does Filtration Purify Water: Filtration processes across the spectrum

Membrane Filtration Spectrum Infographic

Clean water is essential for all living things. Humans, animals, and plants all need clean water to live — which is why water filtration is a massive industry today. Whether you’re watering crops on a large farm, pumping ballast water at a foreign port, or simply getting a glass of water from your refrigerator, you are likely to be using water that’s gone through some form of filtration.

As the name suggests, filtration “filters out” harmful substances from our water, making it debris-free and usable in commercial systems. But how exactly does this process work? The answer depends on the type of filtration system you use.

Why Water Filtration Matters

Before we can discuss how filtration purifies our water, it is first important to understand why we filter water at all. According to the United States Geological Survey (USGS), water is a “universal solvent” — in other words, it can easily dissolve a great number of other substances.

While water solvency makes it an effective cleaning tool, it also makes it easy for contaminants to combine with the water itself. Debris, bacteria, and microorganisms can make themselves at home, and this can cause real harm to plants and animals who are exposed to contaminated water.

Membrane Filtration Spectrum Infographic
This membrane filtration spectrum chart shows the process of separation for various materials at increasingly smaller sizes.

Filters are necessary to protect your system from any debris or other contaminants in your water. If particles manage to get into a system, the equipment can suffer from buildup, clogging, and a host of other issues that can shorten the lifespan of the machines. In fact, these risks are so great that many organizations also implement a prefiltration stage. Systems that use ultrafiltration, for example, often use prefiltration to keep out larger particles and protect their primary filter from clogs or other damage.

For people working in industries like agriculture, water treatment, desalination, and much more, having clean water is vital to keeping your customers healthy and your business thriving. Therefore, these industries must rely on filtration to keep their water in safe and healthy conditions.

How Filtration Helps

Simply put, filtration removes the impurities from water, nearly eliminating the risk from any debris or particles that might have been present. This results in filtered water that is cleaner and purer than in its original state, making it safe for use in ballast tanks, industrial systems, and much more.

Filtered water can help prevent contamination of other things. For example, farmers use filtered water to prevent chemicals or bacteria from changing the pH levels in their soil. pH balance is a key component in growing various crops, and therefore farmers cannot have unexpected contaminants changing their soil composition.

Similarly, filtration is necessary for cargo ships that plan to dump ballast water at their next port. The ocean is a wide and varied ecosystem, and microorganisms from one area could drastically alter the ecosystem in another place. Filtering ballast water before dumping it at a port will prevent animals or bacteria from entering a new part of the ocean and drastically altering its biodiversity.

Ultimately, any industry that uses water to run their business can benefit from filtering their water before use. Investing in filtration will result in cleaner and safer water for everyone.

How Filtration Works

Water filtration has been a part of society since 500 B.C.E. when the Greek scientist Hippocrates developed a cloth filter for purifying water. Since that time, humans have developed and tested countless filtration methods for their water, from boiling it under the hot sun to using charcoal to chlorine to kill microorganisms.

But how does filtration work today? Some of the earliest filtration methods (like charcoal filters) are still used in some capacity. However, modern technology has opened the door to a wide range of different filtration methods. The most common filtration systems are:

  • Particle Filtration (two types are discussed below)
  • Membrane Filtration
  • Reverse Osmosis

Particle Filters

Screen Filters

One of the most common types of water filtration system is a screen-based filter. The vast majority of all commercial and industrial filtration systems first start with a screen to filter out as much suspended material as possible, before heading to treatment or use.UF and RO pre filtration

Screen filters use a mesh screen (usually made from polyester or stainless steel) to trap dirt, sediment, and other debris that might be in the water.

Screen filters were one of the earliest filtration tools developed by mankind, and they remain a highly effective method for purifying water even today. However, it is important to note that screen filters have one major drawback: they’re a flat, 2-D system. The standard screen filter doesn’t have any depth, and that means that the screen can become clogged very easily. Many get around this issue by regularly cleaning the filter – either by removing and cleaning it routinely or by using a self-cleaning system that removes debris automatically and keeps the screen fully functional throughout its life cycle.

There are different levels of screen filters that use varying sizes of mesh, which allows you to determine how many particles your filter captures. Additionally, some manufacturers, including Filtersafe, have created multi-layered screens, which capture a greater number of contaminants as the water moves through them. These tools make removing sediment, sand, and microorganisms a quick and painless process, thereby making it easier to have clean and pure water for your business.

Candle Filters

Candle filters are another approach for removing fine particles.

Also known as backwashing tubular filters, candle filters are not automatic like many screen filters are, instead of using gravity to produce clean water. Named for its candle-like shape, this filtration system uses a filter cartridge (which is usually made from ceramic or a fine sieve) to filter particles out of the water as it runs through the filter. This system can be an effective way to eliminate sediment, bacteria, and other particles from your water, though most industrial users prefer automatic filtration systems.

Membrane filtration: Micro-, Ultra-, and Nanofiltration

When filtration needs to be done to a much finer degree than particle filters can manage (below 10 microns), membrane filters are the solution (though screen filters are the first step for filtering water, before they pass through membranes). This means that the system uses hydrostatic pressure to force water through a membrane; as the water passes through, any particles or contaminants in the water become trapped, resulting in pure filtered water.

There are three main types of membranous filtration: microfiltration, ultrafiltration, and nanofiltration. While these three types use the same process to filter water, they differ in the size of particles they can trap. The key difference is the size of the pores in each membrane:

  • Microfiltration membranes have a pore size of around 0.1 microns, which means it can catch particles, but not dissolved substances. Microfiltration (MF) membranes developed specifically to solve complex process challenges such as microbial removal, protein fractionation, and pretreatment to other membrane processes. Microfiltration membranes have the most open pore sizes of all polymeric membranes. With a pore size range of 0.1 to 10μm, microfiltration membranes are capable of separating large suspended solids such as colloids, particulates, fat, and bacteria, while allowing sugars, proteins, salts, and low molecular weight molecules pass through the membrane.
  • Ultrafiltration membranes have a pore size of around 0.01 micron, which means it can catch smaller particles and smaller contaminants such as viruses. However, ultrafiltration cannot trap dissolved substances.
  • Nanofiltration membranes have a pore size of 0.001 microns. As one of the smallest pore sizes available (only reverse osmosis membranes are smaller), this filtration type can trap virtually all organic matter, multi-valent salts, and other particles in the water. In addition, nanofiltration membranes are capable of rejecting multivalent salts and larger molecules, while selectively rejecting varying amounts of monovalent salts.

 

Reverse Osmosis (RO) Filters

The term “osmosis” refers to the phenomenon of a solvent (like water) passing through a semipermeable membrane into a solute or dissolvable substance. When this happens, the water will dissolve and “pick up” some of the solute molecules, resulting in a water-and-solute mixture that is equally concentrated on either side of the membrane.

Reverse osmosis, as the name suggests, reverses this phenomenon – removing particles from a water source through the use of a membrane. This type of filer is most commonly used for desalination in conjunction with a media filtration or ultrafiltration system as a prefilter.

In reverse osmosis, the semipermeable membrane (which is most commonly made of cellulose acetate) does not allow the water to dissolve the solute; instead, it acts as a barrier that keeps material such as minerals or microorganisms from passing through the filtration system. Through this method, you can get purified water that is debris free.

Compared to traditional filtration technologies that rely on a screen or filter to remove particles, reverse osmosis (RO) is a pressure-driven separation process that employs a semipermeable membrane and the principles of crossflow filtration.

Reverse osmosis water treatment provides the finest level of filtration. The RO membrane acts as a barrier to all salts and inorganic molecules, as well as organic molecules with a molecular weight greater than approximately 100 Da (Daltons). It is therefore a highly effective process for removing contaminants such as:

  • Endotoxins/pyrogens.
  • Insecticides/pesticides.
  • Herbicides.
  • Antibiotics.
  • Nitrates.
  • Sugars.
  • Soluble salts.
  • Metal ions.

Get the Best Filtration System for Your Needs

There are a variety of water filtration systems available today, and they each serve a unique purpose. Whether you are purifying water for a water treatment plant, to nourish your crops, or for any other industrial purposes, it is critical to have the right tools at your disposal to do your work effectively – and that means finding the right filtration system for your needs.

And no matter which filtration technology you use for your system, you will need a prefiltration screen to protect and increase your system’s lifespan. A simple prefiltration system will prevent excess wear and tear on your machinery and save you in maintenance costs! Filtersafe has over 30 years of experience in advising and supplying the best system for your needs, helping you protect your systems and reduce costs and downtime. Contact us today to learn how our team can help your facility get cleaner, better water.