Effective filtering

Ingesting submicron particulate and salt can cause performance degradation and possible major engine damage, such as hot corrosion. The mix of salt-laden air and high dust load means filtration needs to offer water repellency and high efficiency, while the reliability and availability requirements demand filter robustness and maximal filter life.

Hydrophobicity and efficiency

During heavy storms, filters are subject to a lot of water. Both filter efficiency and pressure drop can be impacted if filter drainage is not properly designed or the media hydrophobicity is not sufficient.

Filter standard efficiency tests, although valid references, are performed under dry conditions. Poor drainage capabilities will allow water to build up on the media, causing increases in pressure drop, which can force dissolved contaminants through the filter. As a filter’s dust load increases, the filter’s sensitivity to rain and humidity increases, and can force premature filter change-out. In the presence of salt, these issues are an even bigger concern due to the risk of corrosion.

What are the important design considerations for water handling?

• Louvres and rain hoods will remove some water from the airflow before reaching the filters.
• Coalescing pre-filters turn smaller droplets into larger droplets that can then fall out of the airflow before reaching the next stage filter. Good pre-filters drain large droplets at the front of the filter, keep a low pressure drop when wet, and coalesce small droplets into larger droplets at the back.
• Hydrophobic final filters are then needed to prevent any penetration of water and should have enough drainage to keep a low pressure drop.

Although there are industry tests for media hydrophobicity, such as the EN 20811 / ISO 811 standard, there is no industry standard for filter hydrophobicity. Many turbine manufacturers and suppliers have developed their own custom standards for filter hydrophobicity, but performance is always limited by site dust and conditions.

Signs that an upgrade is required

During an outage, if salt crystals or brown water carry-over can be seen after the final filter, then the final filter hydrophobicity is probably insufficient for the system water handling requirements. This will cause fouling or corrosion, and the consequences are reduced power output and increased heat rate.

Air inlet design features for water resistance

Gas turbines are susceptible to fouling and damage if water droplets and water soluble contaminants (i.e. salts) are ingested by an engine. Most gas turbine air intake systems have a stage to remove large rain droplets from the airstream, typically greater than 5 – 10 µm. Smaller droplets need to be removed by the final filter stages of the intake system. Camfil has numerous filters which utilise hydrophobic filter media and have design features that prevent water droplets and dissolved contaminants from entering the engine.

The risk of water bypass decreases when there are filter stages protecting the final filter, and for this reason, Camfil recommends a multistage static filtration system for wet and humid areas. Inlet filter systems which have horizontal filter cartridges, or a ‘cross flow’ configuration, however, have an extra challenge in that water can accumulate in the media pleats on top of the filters. In this case, water and contaminants could penetrate through standard media. If a cartridge filtration system already exists at the site, Camfil offers a robust design and hydrophobic media options that will prevent water penetration and keep the engine clean.

The consequences of a misapplied GT filter system for LNG trains are significant, ranging from reduced LNG output because of degraded turbine performance to no production because of a catastrophic engine failure – such as hot end corrosion. For new facilities, these consequences can be managed by selecting a multistage barrier filter system, with the features of high efficiency (E10 or greater), low pressure drop, and hydrophobic media and water drainage. For an existing system, replacement filters should have the same performance features.

An example where these recommendations have been made is an LNG production facility located in Trinidad and Tobago, where 26 units have been running for over four years without any signs of power degradation.

Read the article online at: https://www.lngindustry.com/liquefaction/13072017/effective-filtering/