Understanding the Air Quality Challenge
Gas turbines are essential to the production of oil and gas, running both mechanical drive and power generation applications on offshore platforms. However, they are repeatedly exposed to the most arduous atmospheric conditions.
The harshest environment
Air in the offshore environment contains numerous airborne particles that have the potential to permanently damage gas turbines (GTs). These include water droplets, sea salt aerosols, salt in solution and sub-micron particulate, as well as industrial airborne particulate from burnt and unburnt hydrocarbons, drilling activities, mud burn and grit blast. Compressor blades, inlet variable guide vanes and pneumatic parts are particularly exposed to contamination and corrosion offshore. This can lead to damage, seizure and ultimately the failure of these critical components.
Within the turbine section of the GT, cooling passages on the expensive turbine blades must remain free of contamination. If they are not protected, blockage can result in overheating, fatigue and cracking. Additionally, when operating with sour fuel, components within the turbine section are exposed to accelerated hot-end corrosion. This process results from the combustion of sour fuel gas, rich in hydrogen sulphide (H2S), reacting with salt (NaCl) from the intake air and ultimately leads to shortened life of critical and expensive parts.
Traditional offshore filtration
In Europe, products are tested and clarified in laboratories in accordance with EN779 and EN1822 use. EN filtration efficiency grades range from G1 to U17; the higher the number, the higher the level of filtration. In the GT market, filtration typically ranges from G3 to E12. Filters in the G3-M5 range are commonly used as pre-filters.
Typically, onshore modern GT filtration systems have a minimum of an F9 classification, although systems are regularly commissioned with levels of filtration up to EPA E12. However, in the offshore environment about 85% of offshore GTs are protected by small, high velocity filtration systems that fall into the pre-filtration classification range of G3-M5. This type of filter only provide protection against coarse particles and fails to capture sub-micron particles offshore. This can lead to lost production revenue, unplanned gas turbine shutdowns, reduced component and engine life, premature engine failure, and low turbine compression efficiency and high CO₂ emissions.
These systems were widely adopted from the late 1970s. Selection was based on research by the National Gas Turbine Establishment on an ocean-going marine vessel. It involved taking samples of air just above sea level as an indicator of offshore air quality. Testing at that time considered that 95% of offshore particles were above 5 micron, and that filter bags would be effective in arresting them. Recent research revealed that 98% of particles on offshore platforms are 1 micron or below (Table 2).
The traditional low efficiency filter bags in these small housings offer minimal filtration and particle arrestance for particles less than 1 micron. They are designed to allow water, moisture or fog to coalesce as it passes through the filter bags. This creates larger droplets that are captured by a downstream vane. However, water and salt in solution passes through the bag filters and can collect on the floor downstream of the bags and upstream of the final vanes. The water will then evaporate over time resulting in salt crystallisation growth and salts will be picked up by the airflow and to be carried into the GT.
Table 1: A comparison between the aerosols from the National Gas Turbine Establishment (NGTE) and AAF platform measurement, highlighting the difference in particle sizes at platform level.
Table 2: Air cleanliness at 0.3 micron. AAF’s N-hance EPA E12 filtration technology is 1900 times cleaner compared to traditional high velocity technology. Note, the air sample used was taken on a North Sea platform.
EPA E12 filtration
Air filters with EPA E12 classification capture 99.95% of particles at the Most Penetrating Particle Size – the most difficult particle size to capture at a predefined airflow. This is in stark contrast to traditional low efficiency filter bags that only capture 5% of these particles, which is reason why operators need to water wash so frequently and have to deal with unplanned shutdowns and reduced engine life offshore.
When they first became available offshore, EPA E12 technology required a large equipment envelope but this upgrade is now available as a retrofit within an existing high velocity filtration system, without the need to replace the smaller housing with a larger filter house. This unique system is known as N-hance Performance Filtration and you’ll find more detail on this solution on the AAF website. The upgrade is designed to deal with large amounts of moisture and salt. This type of filtration eliminates frequent water washing, increases production efficiency and lowers CO₂ emissions, while providing longer operational life for the gas turbine.
A gas turbine performing well offshore brings greater prospects of overall efficiency, profitability and sustainability. In addition to improving a business’ financial position, the latter is now increasingly necessary as a result of the growing recognition that resisting the pathway to decarbonisation is both futile as well as bad for business. N-hance could present a new frontier when it comes to overall offshore efficiency gains.