Magnetite is a form of iron particulate that forms naturally on the inside of steam and water pipes. In power plants, particles can shed, damaging a plant's steam and water analysis system (SWAS) by plugging components and analyzers. And when the SWAS doesn't operate properly, the water chemistry for the whole plant is at risk.
The Monitor sat down with John Powalisz, Sentry Equipment's Director of Business Development, to discuss how plants can mitigate magnetite in the system to prevent issues.
The Monitor (TM): How do magnetite and other particles get into the process stream, and what do they do?
John Powalisz (JP): Magnetite usually comes off the protective layer within the piping system of a power plant. The plant's chemistry program puts a protective layer onto the piping, but when the plant cycles – typically during startup and shutdown – this layer is upset and gets sloughed off into the steam and water circuit of the plant. The sample system tends to capture that corrosion, which can plug up the components.
TM: What are mag traps, and why do plants need them?
JP: When excessive amounts of magnetite enter the system, operators need to physically remove the magnetite to protect the steam and water conditioning components and analyzers and, ultimately, the plant. Magnetic traps use high-strength magnets to isolate and remove corrosion products (such as magnetite particulates) from the sample stream. This helps protect sample conditioning and analytical instruments from downtime and expensive repairs.
TM: How do mag traps work?
JP: As an oxide of iron, magnetite is attracted to magnets. When the sample flows past the magnetic surface of the trap, the magnets attract and capture the magnetite. However, a mag trap only traps magnetic particulates. If there's non-magnetic particulate in the sample stream, it will flow uninterrupted through the trap to the downstream components, providing a representative sample while still protecting equipment.
Magnetic traps won't plug lines or prevent flow to downstream components. When Mag Traps become "loaded," meaning the magnet can't attract additional magnetite until the particulates are flushed out. The traps are equipped with a flush handle to quickly purge trapped magnetite from the line – only temporarily disrupting the sample flow. Then, it's ready to attract more magnetic particles and prevent them from plugging up the system.
TM: Where should a mag trap be located?
JP: The magnetic trap is typically located upstream of the pressure reduction device (Variable Pressure Reducing Valve or Pressure Control Valve), where most particulate lodges. However, when you measure iron or corrosion products, you don't want to remove them from the sample stream. In that case, you'd put your mag trap below the corrosion product sampler so you can see if this material might be having an impact on other measurements.
TM: What happens in a plant that doesn't use a mag trap?
JP: If a plant doesn't have a way to capture iron and magnetite, it could cause disastrous plant shutdowns and equipment failures. Even if the plant uses another solution – such as a filter or High-Pressure Blowdown Valves (HPBVs), which remove deposits in the sample lines and protect coolers and conditioning equipment – they'll encounter interruptions in service. That's because the HPBV isn't as effective as a mag trap. And filters require you to shut down the sample line at the primary source to replace the filter element or clean it – creating unnecessary downtime that interrupts power generation.
A mag trap is an optimal solution because it captures all the iron or magnetite in a stream, then allows you to quickly flush the magnetite to the drain, interrupting the sample stream only for a minute or less. That's not possible to do that with a traditional filter arrangement.
To learn more about how a magnetic trap works, watch this animation of the Sentry® MT-5 Series.
Sentry is the expert in creating customized solutions to protect a plant's SWAS from critical downtime and costly repairs.
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