Downtime can be mitigated with proper filtration
In the already challenging liquefied natural gas market, any operational problem becomes magnified and any advantage—however small—can make all the difference.
One way to drive incremental improvements at LNG processing facilities is through clean air, said Peter McGuigan, global LNG market manager for the Parker Gas Turbine Filtration Division. Gas turbines are widely used as mechanical drives when compressing refrigerants, which are used to help turn methane gas into liquefied natural gas (LNG). The easy transport of LNG via specialized tankers without the need for vast, cross-continental pipeline infrastructure has helped spur demand for LNG.
“Gas turbines are the primary vehicle to drive compressors within that liquefaction plant, and if the gas turbine is down, the whole plant is down so they can’t make any more liquified natural gas,” said McGuigan. “You’re talking in
the region of US$8-10 million a day in terms of downtime for some plants. So they want these gas turbines to run and run and run. And a key method of making sure that happens is to make sure the air is clean.”
McGuigan has written a whitepaper, “Avoiding Shutdown of LNG Processing Facilities,” about the subject. According to McGuigan, a single unscheduled turbine shutdown may result in the whole LNG train being taken offline with lengthy shutdown and start-up procedures leading to huge losses. In addition, this may also mean that all gas being processed needs to be flared instead of liquefied, with the extra associated cost and environmental impact.
Getting an edge
In the current challenging marketplace, those kinds of problems can be catastrophic.
“(Operators) are under pressure, so any incremental improvement in gas turbine efficiency, overall plant efficiency and reliability will give them an edge in the market,” McGuigan said.
Fundamentally, the gas turbines used to help produce LNG need to provide extended, reliable operation with maximum uptime and maximum predictability of output. For this to happen, a carefully designed inlet filtration system is required.
Land-based refrigerant compressor stations for the liquefaction of methane are always located near coastlines to facilitate onward tanker transport of the LNG. This means gas turbines will be exposed to dust, moisture, salt and many other airborne contaminants, all of which will put gas turbine operations in jeopardy if not addressed.
“Gas turbines ingest enormous amount of air, and the operators realize that any salts or hydrocarbons, any nasty st
uff getting into their gas turbines will significantly impact operations. And then they connect the dots in terms of lost potential, lost revenue,” McGuigan said. “And they’re very, very risk averse as a result.”
With the colossal volumes of air passing through a gas turbine air inlet, having the correct inlet filtration system in place is vital for ensuring ongoing reliable operations. To handle these various, contaminants, protect the turbines and avoid shutdown on the basis of either pressure loss (DP) increase or degradation in output, LNG filtration systems typically have multiple stages. Ensuring the right stages of filters are selected has a huge impact on the overall process availability, reliability and profitability. A filtration system that is correctly designed and engineered to meet the real-world conditions of the gas turbine installation can mean shutdowns are limited to scheduled maintenance periods only. Choosing the wrong system, however, can have major financial repercussions. For LNG applications, the turbine needs to be protected from the corrosion, erosion and fouling which would be caused by salt and particulate getting downstream, with a primary focus on keeping the turbine operating reliably and predictably over long periods of time.
Seawater, seasonal challenges
As onshore and offshore LNG installations are in close vicinity to seawater, the presence of a combination of sand, dust, salt and moisture all need to be factored into the turbine inlet house design. Where there are significant volumes of dust and sand, self-cleaning filters are often utilized. These use pulses of compressed air to periodically remove layers of dust from filters and reduce differential pressure across the system. If there are also high levels of moisture in the air, coalescers can be added up-front to help remove this prior to it reaching the main filters. If there are also high levels of dust, in a desert region for example, care also needs to be taken that the coalescers do not get blocked, which may cause pressure spikes or force the coalescer out of place, rendering its protection useless.
There are also challenges posed by seasonal anomalies, like heavy pollen in spring or the dust and dirt churned up during spring planting or fall harvest. Dust storms in regions such as North Africa and West Australia also tax filtration systems.
“In certain times of year, fog in the Middle East can be a big, big problem,” McGuigan said. “Fog droplets getting into high-efficiency filters is a real pain.”
And even the plant facilities themselves may cause problems, McGuigan said. Some plants are so big that there’s a constant hum of infrastructure work going on, which also is kicking up dust.
“The facilities themselves are commonly forgotten about when people are doing environmental assessments,” McGuigan said. “They might be considering purely the fog, purely the seasonal pollen and dust storms and that kind of stuff. Whereas there’s challenges right on site from manmade activity.”
The inlet filtration system for an LNG process turbine can have three, four or even five unique stages, providing an ability to change filters online without the need to shut down the turbine. The first few (prefilter) stages are designed to remove larger particles and 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 change out need to be incorporated. The final filtration stage should use high efficiency hydrophobic media, typically rated F9 (EN779) to E12 (EN1822) to achieve optimum results. Options to use an extended 24-in. deep final filter (compared with traditional 12- in. to 17-in. vCell filter depths), provides for extended filter service life if required. Another area for consideration in correct filter selection is the type of high efficiency media used. Levels of moisture are obviously going to be high in offshore and coastal environments, and small moisture droplets can quickly block thin ePTFE membranes. Sudden blockages equate to sudden and unpredictable pressure spikes (the ‘hockey stick’ effect), which can result in complete turbine (and therefore plant) shutdown.
Micro fiber glass media, however, offers the same efficiency but is around 10 times thicker, making it more resistant to blockage and more predictable in its performance, with slow, gradual, pressure increases as contaminant is captured. To avoid unplanned maintenance activities, 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 three to four years. If a filtration system requires more regular maintenance, a review of its design and the choice of filter grades used is recommended.
Filter system designers are continually developing and improving technology to enhance performance in challenging offshore and onshore installations. When considering the result of an unexpected shutdown, investment in the right filtration solution offers operators lightening quick return on investment.
McGuigan said operators tend to understand the importance of filtration, but aren’t always able to keep abreast of developments and new technologies.
“They absolutely get right from the outset, every single one of these LNG operators get the quality of air entering the gas turbine is one of the things that, if they don’t adequately control, they’re in big trouble,” McGuigan said. “But a lot of them aren’t aware of the latest technologies. So that’s my job is to educate and tell them what’s new and what extra incremental benefits new technologies can bring to their operations.
“What I’m feeling and my genuine belief is that LNG operators want more than just the filter, they want the expertise, the recommendations, the new product development, all the kind of stuff that goes with a customer-operations focused, leading-edge company.”