Magnetite is a form of iron particulate that naturally forms on the inside of steam and water pipes. In many aging power plants during plant startup and cycling, particles of the coating can shed – known as a crud burst or iron throw. These magnetite particles can cause issues and damage within the plant’s steam and water analysis system (SWAS) by jamming and plugging sample conditioning components and analyzers. And when the SWAS does not operate properly, the water chemistry for the whole plant is at risk.
Ways To Remove Magnetite
In cases where magnetite is in excess, cycle chemistry is not enough. Whether it’s the plant age or a shift in how it’s being operated, the plant will need to take steps to physically remove the magnetite to protect the steam and water conditioning components and analyzers, and subsequently, the plant.
Depending on the severity of the magnetite issue and the equipment you are trying to protect, multiple solutions exist, including strainers, filters and traps. If you are looking to protect the analyzers and the sample conditioning system, then at a minimum you would need a solution that operates at elevated temperatures and under high pressures. Because of the more extreme conditions of high-pressure applications, the solutions tend to be higher priced. If protecting only the analyzers is the goal, typically a low-pressure and low-temperature solution would be sufficient. Typically in these applications, low pressure is less than ~ 100psig, and low temperature is less than 120°F (49°C), with high pressures being up to 5000 psig.
As mentioned above, magnetite physically can be removed from the system in three different ways:
A generally low-cost option, strainers are appropriate for both high and low flow, and are designed to capture larger particulates of magnetite. While there are varying designs for different sizes of particulates, strainers typically are used where the particulate to be captured is visible – about 40 micron and up. As the strainer traps the particulate, it begins to restrict flow and build up a pressure drop across the straining element, indicating it is time for maintenance. If left unattended, strainers eventually will become clogged, choking off flow to the downstream devices.
Additionally, a strainer traps all particulate, which may be undesirable if trying to obtain a representative sample within the system.
Strainers are available over a wide range of sizes, pressures, temperatures and performance. When specifying a strainer, the larger the surface area of the straining element, the more time between cleanings.
Filters work in a similar way as strainers, but typically are designed to capture finer particles within the system. While not a requirement, many filters, like in your coffeemaker, have a disposable element to be changed. If the filter does not have a disposable element, it will have one that must be cleaned. Regardless of filter media, proper maintenance is required to avoid plugging, which then chokes off flow to the downstream conditioning and analytics system. Filters are available in either high- or low-pressure versions, and like a strainer, surface area of the filtering element should be maximized to eliminate the need for frequent cleaning and maintenance.
Magnetic traps are another method of preventing magnetite particles from disrupting sample conditioning and analysis, and offer both advantages and disadvantages over filters and strainers. Rather than a physical filter or strainer media, a magnetic trap uses a magnetic surface to trap particulates. Magnetite is an iron oxide mineral and is attracted to magnets, so as the sample flows past the magnetic surface, the magnets attract and capture the magnetite. An important distinction of a magnetic trap is that it only traps magnetic particulate and does not target a specific particulate size. Additionally, if there is particulate in the sample stream that is not magnetic, it will flow uninterrupted through the trap, to the downstream components, providing a more representative sample while protecting equipment.
Unlike filters and strainers, magnetic traps theoretically will never plug or prevent flow to downstream components. When not properly cleaned and maintained, they simply become “loaded,” which means the magnet cannot attract additional magnetite until the particulates are flushed out. The downside to magnetic traps is that unless they are designed with the appropriate magnets, they typically have temperature limitations. For example, a standard neodymium magnet permanently degrades at 175°F (79°C). Fortunately, magnetic traps are commercially available for use in both low- and high- pressure applications, protecting both the sample conditioning equipment and the analyzers.
For more information on how a magnetic trap works, see an animation of the Sentry® MT-5 Series magnetic trap, which can protect a plant’s SWAS from critical downtime and costly repairs.