Filter out unnecessary downtime

Gas turbine (GT) combustion air intake filtration is a crucial aspect of ensuring the efficient and reliable operation of LNG production facilities. GTs play a pivotal role in the process of converting natural gas into its supercooled liquid state. They either provide mechanical energy as directly coupled refrigerant compressor (GTC) drivers or provide electrical energy as the front end of turbo generators (GTGs) for motor-driven refrigerant compressors. Similar GTGs are also commonly used for general site power needs. GTs are powered by internal combustion processes, requiring enormous volumes of clean and properly filtered atmospheric air for combustion.

Importance of air intake filtration

GTs are highly-sensitive internal combustion engines that ingest and compress atmospheric air, mix, and then burn it with fuel and release it. The energy of the released hot gas is converted into mechanical shaft power in the power turbine. Airborne particulates, such as dust, pollen, and industrial pollutants, can adhere to and clog the GT’s highly optimised internal components, leading to reduced efficiency, increased fuel consumption, significant maintenance issues, and – especially important for LNG production demands – potentially very expensive plant downtime. Moreover, in the salt-laden coastal and offshore environments in which LNG production facilities are located, corrosive particles and liquid salts which are always present in the ambient air, will, if ingested, eventually destroy the GT cold and hot gas path internals. Corrosion within GTs does not normally show up on measurements within the control room and can remain undetected until mechanical failure occurs.

In LNG production facilities, any unplanned disruption to GT operations will result in production slowdowns or even complete plant shutdowns, leading to significant financial losses. The implications of unscheduled interruptions are even more acutely felt when operating floating LNG (FLNG) vessels offshore. Limited personnel and spare parts on board will likely mean specialised crew and equipment need to be mobilised at great time and expense. To mitigate these risks both on land and offshore, high-quality multi-stage LNG air intake filtration systems are employed.

Filtration requirements

The inlet filtration system for an LNG process turbine can have three, four, or even five unique functional stages, providing an ability to change filters online without the need to shut down the turbine. The first few (prefilter) stages are designed to coalesce liquids and remove larger particles and thereby extend the life of the later (high efficiency) stages. As prefilters can typically be changed out without taking the turbine offline, designs that facilitate quick filter changeout need to be incorporated. The final filtration stage should use high-efficiency (EPA – Efficient Particulate Arrestance) hydrophobic media, typically rated E10 to E12 (International standard EN1822) to achieve optimum particulate removal results. Options to use an extended 24-in. (600 mm) deep final filter (compared with traditional 12 – 17 in. filter depths) provides for extended filter dust holding and service life if required to meet operating goals. A lower efficiency ‘guard’ filter may also be employed as a ‘final-final’ stage, allowing changeout of all the main filter stages online, including the EPA stage.

Another area for consideration in correct filter selection is the type of high efficiency pleated media (‘cloth’) used. Levels of moisture are obviously going to be very high in offshore and coastal environments, and small moisture droplets can quickly block the pores of thinner 2D membrane (expanded PTFE) cloth, typically around 0.05 mm thick. Sudden blockages equate to sudden and unpredictable pressure spikes (the ‘hockey stick’ effect as the loaded filter quickly limits airflow passage downstream and the pressure loss [delta P] quickly escalates), which can result in turbine trip and shutdown. 3D microfibre glass media (around 10 times thicker [0.5 mm]) is the preferred filter cloth employed in advanced filtration systems today for most LNG applications. Microfibre glass offers excellent particulate removal efficiency, but the vastly increased depth provides better resistance to blockages and much more predictability in its performance, with slow, gradual pressure increases as contaminants and liquids are captured.

To avoid unplanned maintenance, filters should be designed for long life. Prefilters should require changing no more than around once per year. Second stage filters once every two years, and third or fourth stage filters around just once every 3 – 4 years to achieve the 32 000 hrs+ continuous operation frequently requested by LNG operators. If a filtration system requires more regular maintenance, a review of its design and the choice of staged filter grades is recommended. It is not uncommon to adjust product selection as the reality of site requirements becomes clear during actual operations. The ‘best’ fit filter solution may therefore change with operating profile and/or season and differ from that originally supplied. A well designed filterhouse should allow direct interchangeability of various products in this regard and, equally importantly, should be future proofed to allow for the introduction of new products as filtration innovation continues at pace and new products are released to market.

One job of the rotating equipment and maintenance engineers is to align filter replacement intervals with a strategy for spare parts planning within the pre-defined GT maintenance and inspection windows (BSI, HGP, major inspection). These inspection windows are normally defined by the GT OEM based on fired hours and/or the number of starts. Defining a maintenance programme that reduces the owner’s costs and keeps the gas turbine available when needed requires careful thought, as well as early and frequent engagement with filter system professionals.

Filter system designers are continually developing and improving technology to enhance performance in challenging offshore and onshore LNG installations. Filter selection needs to be reviewed in terms of the risk/cost of reduced plant production efficiency created by contaminants bypassing the filters and moving downstream into the GT, an equation highly dependent on the duty cycle and reliability needs of the processes. There are very few more critical applications than GTs when used for LNG production.

The filtration requirements for gas turbine air intake systems in LNG production facilities are stringent and involve several key factors:

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