Any industrial plant that uses gas turbine technology faces similar challenges when it comes to ensuring clean air is supplied to the engine. Fouling of the compressor blades, filter service life, system pressure drop, filter failure, and coping with water and salt are all issues that must be considered when specifying an air intake system, whether on or offshore.
Considering all that they face, offshore air intakes must be designed to operate in some of the harshest environments on the planet. In these circumstances, filter cost is almost a non-issue due to the invaluable job they perform protecting the gas turbine and ensuring optimum performance.
In these applications, gas turbines are used for compression and generation, operating on baseload. The most important task of the air filtration system is to protect the turbine blades from fouling to avoid the associated degradation in performance and the need to perform offline washes. Employing efficiency particulate air (EPA) final filters is becoming increasingly popular for this reason, as the high-efficiency filtration virtually eliminates fouling completely.
However, this is not without its own difficulties. Due to the typically small size of offshore air intakes, each filter element must contend with an extremely high airflow rate. As pressure drop increases with filtration efficiency, EPA filters can cause the differential pressure to reach unworkable levels, acting as a barrier to the airflow. Considering that, as a rule, a 50Pa increase in pressure drop means a 0.1% fall in turbine output, it is clearly important to choose a filter with as low a pressure drop as possible.
Furthermore, the high flow rate means that the filters - especially in the final stage - must have a very high burst resistance and be robust enough to cope with the high level of air passing through. A broken filter can cause massive damage to the compressor blades if ingested by a gas turbine.
The location of offshore platforms poses another air filtration challenge. Shipping a new set of filters to the middle of the ocean is an expensive business, and this is further compounded by the downtime required to change the filters.
Gas turbines in these environments typically operate for 8,000 hours a year, so any period where they are unable to operate must be kept to an absolute minimum. This is where the importance of a filter's service life comes to the fore. Essentially, this is dependent on how much contaminant a filter can retain before the pressure drop reaches an unacceptable level. Once this is too high, the airflow is choked and the performance of the turbine is negatively impacted. An overloaded filter is also more likely to burst, so changing on time and selecting a high dust-holding capacity is crucial.
Last but by no means least, air intakes in offshore applications must cope with high levels of water and salt, in addition to dust particulate. In these harsh conditions, conventional louvres are unable to prevent the damage that these contaminants cause, shortening the life of downstream filters and inhibiting turbine performance.
To combat these issues, Mann+Hummel Vokes Air has developed an air intake system comprising three filter stages. Each stage is designed to follow the next, progressively cleaning the air without compromising turbine performance.
First up is the ScandVane droplet separator, which uses vertical vane profiles to separate water droplets from the airstream. This is achieved through the combination of a multidirectional change of the airstream and integrated hooks that continuously drain water away from the downstream airflow. Collected water can either be drained to an inlet tray or drainpipe at the base of the housing.
With the larger water droplets removed, Macrogen GT Duo provides defence against the finer droplets that have bypassed the first phase. However, thanks to a unique hydrophobic media, Macrogen GT Duo also removes particulate from the airstream too, meaning that separate water coalescer and particle pre-filter stages are no longer required, saving valuable space and dramatically reducing system pressure drop. Available as either a stand-alone stage or integrated into the same housing as the ScandVane droplet separators, Macrogen GT Duo completely removes the last of the water in the airstream, as well as coarser particles such as dust and salt.
The three-stage system is completed by either the Compatex TMPC or compact TMPE filters. Fully sealed for coastal and offshore applications, Compatex TMPC provides defence against fine dust from F7 to F9 filter classes, while compact TMPE is rated from E10 to E12 for EPA levels of protection.
Both products feature rigid frames, extended filter areas of up to 30m² and robust mini-pleated filter mats. This delivers a low pressure drop, burst pressure in excess of 5,000Pa and a long service life, making this range of filters the industry leader.
Gas turbines sited on land still face the same challenges as their offshore counterparts but on a different magnitude. Fouling, pressure drop and burst resistance are still as important, but being onshore provides a useful asset when developing solutions to counteract these - space.
Air intakes are usually much larger on land and also do not have the same weight constraints, meaning that on average, the air intake should have a flow rate of 3,400-4,250m³/hour per filter element, allowing much greater flexibility. Water repellence is still a consideration onshore, although obviously not as critical, and salt usually only poses a problem for gas turbines situated in coastal regions.
One additional problem facing on and offshore turbine operators is oil mist from the rotating machinery. Gas turbines and other rotating equipment consume large amounts of oil to lubricate and protect their internal parts and bearing surfaces. When this oil is exposed to high temperatures and high pressure, large quantities of fine oil mist are created, which is often exhausted through the oil tank and into the atmosphere, leading to pollution, health and safety hazards, and significant oil wastage.
To combat this, Mann+Hummel Vokes Air has developed the ScandMist FBT oil-mist eliminator. In the ScandMist FBT vessel, the oil-mist fumes from the tank are intercepted by a static filter drum. Clean air is drawn out by an inverter-driven centrifugal fan. Liquid lube oil condensed by the filter is then drained and sent back to the lube oil tank for reuse. Filter pressure drop is monitored by an electronic differential micro-manometer that allows remote reading and alarm alerts in the control room. This allows ScandMist FBT to optimise power consumption while maintaining constant under-pressure in the oil tank.
Many gas turbine manufacturers and end users agree that air filtration has been overlooked in the past. With the increasing demand for greater efficiency, air intake filtration is now recognised as a crucial part of the system as a whole.
Air filters provide defence against myriad contaminants, reducing downtime and protecting the expensive inner elements of a turbine. Configuring the intake with the right combination of filters can yield huge differential pressure savings, boosting turbine output and reducing costs. Lastly, air filtration systems can improve health and safety while also protecting the wider environment to minimise the impact of power generation on the world around us.
