Salt is one of the most troublesome contaminants for gas turbine operators. In the offshore/coastal environment or indeed anywhere close to bodies of saltwater, corrosion of turbines can be swift and severe if airborne salts are not adequately understood and properly filtered. Salt removal is one of the areas that needs to be understood and defined.
The enormous amount of air a gas turbine consumes means even the smallest percentage of salt can have serious consequences. Over time, advanced filtration systems have moved from offering 95% salt removal efficiency to today, greater than 99.9%. This 5% difference seems small but is very significant in terms of the reliability and performance of the turbine.
Salt is perilous to gas turbines for two main reasons;
- Firstly, the sodium within it can mix with the sulphur from the fuel to create an effect known as sulphidation (also known as sulphidization or hot corrosion). A chemical reaction between the sodium and the sulphur creates molten sodium sulphate (Na2SO4) which attacks and corrodes the base metal of the turbine blades. Adding in the heat from combustion (which acts as a catalyst), this corrosion effect is accelerated, very quickly eating away at the clean, smooth surfaces of the turbine blades, potentially leading to catastrophic failure of components. If sour fuels are used, the high level of sulphur in them will further increase the rate of sulphidation.
- Secondly, chlorine from the salt can act as a pitting corrosion initiator in the colder compressor end of the turbine.
To rub further salt into the wounds, the hygroscopic/sticky nature (has an affinity for water and absorbs moisture from the surrounding air) of salt when in moisture-rich environments means it easily adheres to the compressor and turbine blades; increasing the rate at which contaminants build up on the surfaces and so more quickly impacting the aerodynamic performance of the turbine and its overall thermal efficiency. Below 40% relative humidity (RH), salt appears in dry form. Above 75% RH, it is in a liquid form. In between these states, salt exists in a particularly problematic, damaging wet and sticky form.A bit of history on measurement
How much salt is in the air was defined by the National Gas Turbine Establishment (NGTE) 30 knot aerosol standard back in the 1970s? Based on the amount of salt present in a series of air samples collected by the UK Royal Navy in the North Atlantic, this figure was set at 3.6 ppm (parts per million). Unfortunately, 3.6 ppm is very high and not a realistic appraisal of the true offshore environment. This was recognised by the NGTE at the time and was only ever meant to be an interim measure until more detailed test information became available. Less salt in the air is of course a good thing, however, it is also key that GT operators have a realistic view of how much salt is in the air in order that they may correctly specify filtration systems that limit the amount reaching the inside of the machine. This anomaly prompted Parker Hannifin, to carry out an ASME paper research project back in 2004, resulting in a new ppm figure for ambient air salt concentration and size distribution in the marine environment. This revised figure, now known as the McGuigan Marine Boundary Layer (MMBL), provides a conservative, realistic figure for the average salt levels in the lower marine boundary layer (MBL) throughout the world. Still a realistic measure today, it puts the salt level at 0.1067 ppm.
Each percentage point counts
The more contaminants that are allowed through to the turbine, the quicker its aerodynamic efficiency will reduce and the greater the risk of damage to the machine. Ultimately this reduction in performance will result in reduced power output, lower system availability and reliability, and higher maintenance costs.
Unlike contaminants that cause compressor fouling (and can be cleaned away by water washing), one of the major problems with salt corrosion is that its effects are often not perceived in turbine performance data until something actually breaks. Unscheduled maintenance can be significant but, in many mechanical drive applications, the cost of lost productivity can also be huge.
If a turbine consumes 350,000 kg of air per hour (typical 30MW GT) and there is 0.1607 ppm of salt by weight in the air to start with (1ppm(w) = 1mg/kg), this equates to an unfiltered gas turbine exposure of nearly 300 kg of salt per year for an 8000hr operating year!. This means each 0.1% improvement in filtration efficiency protects the turbine from an extra 0.3 kg of salt exposure per year.
Designing a filter for salt removal
Salt can exist in a solid, a liquid or a sticky in-between state. Changes in relative humidity affect the state of solidity of the salt. As a liquid salt droplet is transformed through drying to a solid particle, it will also contract to about 25% of its original size. To handle salt effectively, a filtration solution needs to allow for input concentration, aerosol size distribution and aerosol physical state; whether droplet, dry particle or any in-between/sticky state.
A filter system also needs to provide this protection without getting easily blocked itself. The right solution for a given application therefore also has to consider the impact of increased pressure drop from multiple filtration stages with the Gas Turbine salt protection needs and must be designed to handle and remove both salt phases in order to properly protect the turbine. Dust filters will capture solid dry salt particles. Liquid removal stages and hydrophobic filters prevent salt from attacking the turbine internals in liquid form.
One of the vital elements in handling salt corrosion in offshore and coastal environments is to effectively handle the moisture in the air. Filter media with high dry particulate efficiency ratings may not necessarily be effective at handling liquid droplets. Coalescers can manage free moisture and salt in liquid form (brine) by agglomerating droplets to make them larger and heavier so they will fall out of the airstream. Although traditionally associated with large maintenance overheads, coalescers are now available that will run for extended periods without sudden pressure spikes and in configurations that can be easily cleaned with a water or air hose.
Dry salt particles can be dealt with by high efficiency (HEPA/EPA) filters but care needs to be taken with media selection to ensure these filters are not quickly blocked by moisture or sticky salt particles. Tests in real-world applications have shown that thicker glass fibre media is less prone to blockage in such environments than ePTFE membranes, which are 10 times thinner. Glass fibre media with effective hydrophobic coatings have been shown to prevent virtually all liquid salt and/or sticky salt particles from entering the turbine.
How offshore installations are different
For an offshore oil and gas installation, a multi-stage filtration system is required to handle the various, harsh environmental challenges faced. As space offshore is at a premium, it often makes sense to install a more compact, high velocity filter system with a multi-stage Vane Coalescer Vane (VCV) salt removal system. Stage one is an inertial vane separator that removes bulk water including rain, sea spray and coarse aerosols. This is followed by a coalescer that coalesces fine salt aerosols into larger droplets (>20 µm). This stage also captures fine dust and dry salt. The final inertial vane separator stage stops and removes the re-entrained larger salt droplets.
A traditional M6 (EN779) VCV inlet filtration system is a popular choice for offshore turbines and offers excellent wet salt and bulk water removal capability. However, at high air velocities (as the units are designed to be compact) these systems are typically limited to an F7 (EN779) dry particulate removal efficiency rating. If the environment is considered dusty then this configuration can have a limited filter life, produce high operating pressure losses and have reduced effectiveness against dry salts. As well as a need to replace filters on a regular basis, maintenance of the drain system on these units is also critical to operation, so overheads can be higher than desired. These VCV systems are therefore typically employed when salt is the main concern rather than a combination of salt and high dust levels.
To address issues that result from a combination of high concentrations of salt as well as high concentrations of dust, the latest high-velocity filtration systems are designed with extra filtration stages that provide very high small particulate removal efficiency with hydrophobic properties; meaning they prevent liquid penetration while still capturing fine (< 3µm) dust and dry salt particles. These F8 (EN779) to H13 (EN1822) rated high-velocity filters can also be configured with deeper glass fibre filtration cells (24” deep) which means that although turbine inlet air velocity is high, the air velocity at the media is similar to low and medium velocity systems. This increases filtration efficiency and reduces pressure loss across the system. The thicker filter media has the added benefit of being less prone to blockage than thinner, high-efficiency alternatives.
Testing salt efficiency removal
Ultimately, whatever the ppm of salt in the air inlet, the more that is removed the better the turbine is protected from deteriorating performance and catastrophic failure. However, salt leaching rates are not covered in standard efficiency tests (EN779, ASHRAE 52.2, EN1822, JIS Z8122). The only real test of the effectiveness of an inlet filtration system is the performance of the turbine over time in the variety of environmental conditions it faces. To simulate the real world, Parker (CLARCOR) created a hydrophobic salt test protocol to help determine salt removal efficiency and in order to correctly and realistically evaluate new filter system designs when they are subject to variations in salt concentration, dust concentration and relative humidity. Specifically designed to test for phenomena such as salt leaching, the test takes the filter through a total of nine wet/dry cycles in a ten-day testing protocol.
The test requires that the filter(s) are clamped and installed as they would be on site. This ensures that the seal and clamping system are not weak points in the unit where liquid and contaminants can bypass the filtering media itself. Salt is first introduced into the test as an aerosol. Repeated tests are then completed (10 days’ worth) when the filter is loaded with dust. This adds back pressure to the system while the dust coats the fibres within the media as it gets captured, simulating real-world particulate build up. This combination of salt aerosol and loaded filter is very important to analyze as captured dust often acts as an alternative flow path for wet salt transfer downstream, in effect acting as a shortcut for wet salt transmittal downstream, bypassing the media!
Testing a filter system in this way provides comprehensive data about performance over time including the amount of water and salt that passes through the filter during various test stages and highlights any pressure loss increase that occurs. There are two widely used filter tests with accepted ratings used in the filtration industry – the EN779 standard and the EN1822 standard. The EN779 test standard has a set of criteria that a filter must meet to achieve a certain rating. Depending on the filter, a rating of G1-G4, M5-M6 or F7-F9 will be given. The higher the rating, the more effective (typically) the filter. The EN1822 standard is used to test higher efficiency filters, in the EPA/HEPA range, and produces a ratings scale of E10-E12, and H13-H17. When the multi-stage hydrophobic high-velocity system was tested against this salt test protocol, it showed salt removal efficiency to be improved by a factor of 10,000 compared with traditional M6 (EN779) units, giving an E11 (EN1822) efficiency rating with similar pressure loss to a standard M6 (EN779) efficiency system. Such units have also been proven to reduce the frequency of offline turbine water washes by up to a factor of 6 (from around four weeks to six months) without creating sudden pressure spikes.
No single filtration solution is right for all installations
Understanding the nature and impact of salt is a vital consideration in designing a filtration system for use in offshore or coastal environments. Systems need to be tested and evaluated for their performance in handling wet, dry and sticky salt to protect turbines from serious damage without sudden pressure spikes. If filters are selected solely on efficiency rating, operators may be left with systems that are difficult to maintain and, although higher rated, may not protect assets as well as lower efficiency solutions. With a careful assessment of conditions and selection of filter configuration, modern filtration solutions have been shown to virtually negate corrosion of turbine blades over 20,000 fired hours in real-world conditions.
About Parker Gas Turbine Filtration Division
Parker Hannifin supplies a full range of inlet systems and filters engineered to meet your operating goals, including:
- Higher power output.
- Lower operating costs.
- Proven performance utilizing advanced filter technology.
- Extended gas turbine availability.
- Maximum protection against corrosion and fouling.
- Easy maintenance and change out.
We are the choice for advanced filtration for new units and replacement filters. Our inlet system designs include self-cleaning (pulse) and static inlet systems for all gas turbine OEMs. We supply a full range of filter types at all efficiency levels. The predictable and reliable performance of our air filters significantly reduces compressor contamination and the need for unplanned maintenance.
This article was contributed by Peter McGuigan, global LNG market manager, Parker Gas Turbine Filtration Division. It was originally published in Gas Turbine World, September 2017.
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