Graeme Turnbull explains how a new understanding of offshore air has prompted a gas turbine filtration re-think
The evolution of research into offshore air particle size illustrates the reality that low-efficiency filter bags are no longer suitable in this harsh environment – despite their ongoing popularity. So what are the alternative solutions available to the oil & gas industry’s engineers?
From rotating equipment engineers, to mechanical engineers, reliability engineers and C-suite executives, all disciplines concerned with the performance of gas turbines have long focused on the importance of maintaining the engine. However, a critical example that often escapes attention within the process is the fundamental importance of air filtration systems, which have the ability to exponentially enhance gas turbine performance and availability while promoting better compressor cleanliness and long-term part integrity.
Research delivers radical new thinking with high-efficiency particulate air filter technology
Currently, around 85% of offshore gas turbines are protected by small high-velocity filtration systems that utilise low-efficiency filter bags, which only provide adequate protection against large coarse particles, and fail to capture the majority of sub-micron particles offshore. It is worth noting that this scenario traces its roots back to the 1970s when the National Gas Turbine Establishment (NGTE) located at Pyestock, England, published technical paper 59/1975 for ocean going vessels, later to become known in the offshore industry as the ‘30 knot aerosol’. The research was taken less than 10m above sea level on a Royal Navy frigate. The measuring equipment was also appropriate for the time and concluded 95% of particles are >5 microns, with the majority bigger than 13 microns. In the world of filtration these are very large particles and why smaller filtration systems with low-efficiency bags were highly appropriate.
However, fast forward to 2017 and recent measurements at platform height (which is often >30m) with modern measuring equipment tell a different story. It is in complete contrast to the NGTE research with 75% of particles in the North Sea smaller than 0.3 microns and in the Middle East that percentage increases to 97%. This is why low-efficiency filter bags are not suitable for this environment. Indeed, in the onshore environment these bags are now only used as pre-filters to protect intermediate and final filters.
The legacy of this situation has resulted in lost production revenue, unplanned gas turbine shutdowns and costly downtime, reduced component and engine life, premature engine failure, and low turbine compression efficiency as well as high CO2 emissions. All of these impacts are significantly magnified and unwanted given today’s current market dynamics and the low price of oil. What’s more, small high-velocity systems are designed to allow water, moisture or fog to coalesce as it passes through the filter bags. This process creates larger droplets that are designed to be captured by a downstream vane after the filter bags. However, vanes are not 100% efficient and some of the water and salt in solution will pass through them. In addition, water will often collect on the floor downstream of the bags and upstream of the final vanes. The salt-laden water will evaporate over time, which will allow salt crystals to enter the airstream and hence a proportion of these dry salt crystals will pass through the vanes into the gas turbine.
By contrast, high-efficiency particulate air filter (EPA) E12 technology captures 99.95% of 0.3 micron particles, compared with low-efficiency filter bags, which only capture 5% of particles at this size. This significantly protects and enhances the performance of expensive gas turbine components. As offshore operators have become aware of the benefits of EPA E12 air intake filtration, there has been a push to upgrade existing high velocity units installed offshore. However, traditional EPA E12 filtration technologies - with much larger equipment envelopes - have necessitated that the air intake filter system is replaced in its entirety. This increases foundation loads and incurs significant costs and downtime. However, there is another route, using a new EPA E12 system that provides all the associated benefits of EPA E12 air filtration, but can be quickly and seamlessly installed within the existing high velocity air intake filtration system.
BP Clair moves to new EPA E12 high velocity filtration: A case study
BP’s Clair platform in the North Sea, demonstrates the optimum role air filtration systems can play in unlocking considerable financial, operational and environmental benefits. The platform operates three Titan 130 gas turbines (GTs 1, 2 & 3) employed in power generation application to provide power to the asset. Each gas turbine was experiencing compressor blade fouling, corrosion and erosion, as well as turbine section hot gas path corrosion. Operationally, this resulted in poor engine reliability, reduced availability and premature engine overhaul and/or replacement. All of which severely impeded the long-term strategic planning for the platform for both production and maintenance.
Eventually, the poor filtration provided by the high-velocity bag system resulted in a catastrophic failure of GT2 after 12,000 operating hours, which equated to only a third of the engine design life. The root cause of the failure being inlet guide vane seizure and in turn compressor section imbalance and ultimately blade liberation. This resulted in irreparable damage and a new replacement engine was required, incurring unplanned long-term shutdown and significant unbudgeted costs.
BP was aware that AAF International was in the final stages of developing a new EPA E12 high velocity filtration solution. Critically this new design could be installed within an existing high velocity housing with no penalty in differential pressure (dP), therefore negating the need for a larger housing replacement. As a consequence of this failure on GT2, BP was expediting the GT OEM for fast-track delivery of a replacement engine and approached AAF to determine if this new technology (N-hance Performance Filtration) could be urgently deployed in a field trial as a technology collaboration initiative. The N-hance filters and conversion parts were delivered to BP within five weeks and commissioned along with the new GT2 engine on BP Clair in February 2017.
The pilot delivered excellent results. There was a significant increase in engine availability due to the elimination of frequent monthly water washing, which was no longer required. There was also a decrease in CO2 emissions improving sustainability, as well as retained power output (compressor efficiency) and heat rate. Critically, BP has also eliminated the risk of potential GT failure due to corrosion at just a third of design life.
Shifting purchasing decisions to high-efficiency particulate air filter technology amid stark market realities
Modern thinking has turned the dial on what is now regarded as an optimum air filtration system on offshore assets such as BP Clair. The decision to harness EPA E12 gas turbine filtration in existing small high velocity systems will build further resilience in the market, increasingly energy security through a reduction in downtime, increasing production efficiency, reducing CO₂ emissions and lowering operating expenditure, all while significantly reducing the risk of catastrophic failure.
Graeme Turnbull is with AAF International