A gas turbine consumes vast quantities of air, and selecting the right filtration solution is a critical part of ensuring its ongoing performance and reliability as well as preventing unnecessarily high maintenance overheads.

Evaluation of local environmental conditions and operational goals will help determine which solution is optimum for a specific installation. Remember that contaminants can be solids, liquids or gases and each presents its own challenges, which the evaluation of dry particle efficiency alone from any of these standards is not going to tell you.

If a filter becomes blocked it can result in sudden pressure spikes, unplanned turbine outages and associated lost production. Hydrocarbon mists or moisture from mist or fog may produce droplets that are small enough to enter the filter media but large enough to become stuck within it.

When these liquids combine with dust and other solid particles already captured, they form a muddy or sticky substance that can rapidly block a filter or, worse, can be drawn through the filter and deposited in the turbine.

Salt is a particular challenge because of how easily it changes from solid to liquid in the presence of moisture and changes in humidity. If it enters the internals of the turbine it may combine with sulphur within the fuel and cause rapid hot end corrosion.

Designing a system to handle moisture, reduce the risk of sudden blockages and prevent moisture bypass can be just as important to the filter design as its particulate efficiency.

To best protect a turbine and a plant’s profitability, filter design needs to take all of these factors and more into account to produce a filter that will perform in the real world.

FILTER RATINGS

Historical standards for filter efficiency ratings have been based on the needs of the heating, ventilation, and air conditioning (HVAC) industry and have not truly reflected performance in the real world when installed in the harsh environments associated with gas turbines.

Although still very much aimed at the HVAC market, the new ISO 16890 standard relating to filter efficiency has some parts which better relate to gas turbine performance when compared with the current EN779 or
ASHRAE 52.2 test standards.

The main issue with referring to standard ratings for filter efficiency when selecting a filter for a gas turbine application is that the rating does not reflect how well the filter will perform in the real world or what specific benefit it will bring to the gas turbine.

To this end, there really is no replacing real-world experience of filter performance within specific environmental conditions. The changes within ISO 16890, however, do take us a step forward in understanding how filter selection affects gas turbine performance.

The new ISO standard was launched at the end of 2016 and the older EN 779 will be withdrawn in Europe in mid-2018. EN779 tested a filter in both clean and artificially loaded condition to adjudge its initial and through-life efficiency. The filter was artificially loaded with a standard test dust called ‘ASHRAE’, which was originally developed to test vacuum cleaners and so was not representative of the real world for gas turbine filters.

The filter’s efficiency was measured and reported at one particular particle size, that of 0.4 microns. The appropriate rating of F7, F8, F9 etc was then given depending on how the test results compared to a rating table within that standard.

The problem with this is that the test did not simulate real world conditions or provide an indication of the impact the selection of a particular rating would have on gas turbine performance. Efficiency at 0.4 microns is somewhat meaningless to the turbine operator.

In fact, filters that are artificially loaded with this type of test dust in the lab tend to experience a significant increase in their efficiency.

It has been shown that this does not occur in the majority of cases in the real world, leading to a general overestimation of the likely filter performance when reported to the EN779 standard.

The new ISO 16890 standard, however, performs the measurement and presents the results somewhat differently. It is only the initial efficiency of the filter that is now measured, with an artificial loading stage being optional and only used to determine the likely life of the filter, not trying to estimate its through-life
efficiency.

With regards to the initial efficiency, no longer is this evaluated at a single arbitrary and relatively meaningless point of 0.4 microns particle diameter.

Instead it uses a particulate matter (PM) reporting system as defined by the World Health Organization for airborne contaminants.

In addition, the measurements are normalized to representative real-world airborne particulate distributions for both rural and urban environments in an attempt to make them even more representative – although, once again, this new standard has been developed to fit well with HVAC users. PM numbers for the local external environment are often readily available and can now be used with the filter performance from this standard to determine the conditions after filtration inside a building.

While these PM particle size ranges have not been purposely defined to relate to gas turbines, it is by coincidence that their specific ranges, if interpreted correctly (as will be shown later), can now be useful to evaluate the likely impact of filter performance on turbine performance.

In the US the comparable standard, ASHRAE 52.2, will continue as it is for the time being. This was originally based on EN 779 but has changed in recent years to also give filter ratings based on three particle size ratings (0.3–1 µm, 1–3 µm and 3–10 µm) and, as such, is more in line with the new ISO standard.

However, ASHRAE 52.2 does not normalize the results to a typical atmospheric dust distribution and still uses the results from a similar artificial dust loading step to EN779, which uses the same ASHRAE vacuum cleaner dust as part of the filter’s overall MERV rating.

The new ISO standard therefore gives a clearer picture that is more representative of likely real-world performance.

It should be noted that all of these standards are only guidelines and not legal obligations for turbine users, who are free to choose any or none of these standards when selecting filters.

IMPACT ON GAS TURBINES

Different particle sizes impact on gas turbine performance in different ways.

Particles of contaminants larger than five or ten microns (µm), such as sand and mineral dust, are hard and abrasive. If sufficient quantities of these are allowed to flow into the compressor and hot gas path parts of the turbine, they will chip away at the metal inside, causing erosion.

The change in the metal structure impacts aerodynamic performance of the turbine and, over time, as sections of metal become thinner, increased local stresses can result in parts breaking loose, creating serious or even catastrophic damage to the turbine.

In desert regions, particularly those subject to frequent sand and dust storm events, dust loading can be exceptionally high and erosion of turbine parts is a primary concern.

PM NUMBERS FOR ISO 16890 ARE DEFINED AS FOLLOWS:

Small particles less than one or two microns can attach to gas turbine blades and change their smooth, precisely engineered aerodynamic shape that has been optimized for performance. Operators may see a reduction in power output of the turbine by as much as 10 per cent and an increase in fuel consumption, both caused by compressor blade fouling.

The rate at which fouling occurs will depend upon which contaminants are present in the air flow and how ‘sticky’ they are.

Operators can remove fouling materials and recover most of the lost turbine performance through on-line or, for greater effect, off-line washing.

Selecting a filter to protect from these contaminants helps to reduce the frequency of off-line washes with its associated lost production, as well as the rate of reduction of power output between washes.

In the hot section of the turbine, hundreds of tiny precision holes are used in the blades to allow cooling air to flow. This cooling air protects the turbine blades from excessive temperatures that may distort or even melt them. Particles between about 1 and 3 microns tend to plug these vital cooling air passages and cause the parts to exceed temperature limits, reducing component life and increasing the risk of failure.

Knowing this, the PM ranges specified within ISO 16890 can be used to relate to GT performance as follows:

• PM1.0 will tend to indicate the degree to which a gas turbine compressor will become fouled;

• PM2.5 will tend to indicate the degree to which the blades in the turbine section may experience cooling hole plugging which can lead to catastrophic failure;

• PM10 will tend to indicate the degree to which the blades throughout the machine may experience erosion of their profiles, which will lead to reduced life and can also lead to catastrophic failure.

This then shows how filter performance measured to ISO 16890 can be a benefit to operators as it offers a stronger understanding of how the filter will perform in situ and what affect it is likely to have on the turbine.

DIFFERENT CHALLENGES

To be absolutely clear, the new PM rating within ISO 16890 does not give a single number that completely defines how a filter will perform in particular environmental conditions, and filters cannot simply be compared using their rating alone. Each turbine installation presents different challenges – whether from hygroscopic salt from nearby seawater, seasonal dust storms, fog events, snow and ice, or a multitude of other factors that may affect turbine performance and reliability.

To fully evaluate an installation and factor in specific site goals for maintenance intervals and process criticality, a holistic approach is required in the selection of an appropriate filtration solution.

Manufacturers such as Parker use their experience and advanced test procedures to consider the full range of environmental factors and worst possible conditions a filter may face in the real world.

The new ISO 16890 standard is a step in the right direction for gas turbine operators to glean a clear picture of how a turbine will perform in the often harsh environments associated with gas turbine installations.

However, it is still by no means a panacea on which to solely base filter selection. Indeed, different filters and filter media materials with the same PM number may well give completely different results once installed.

It is, of course, turbine performance that ultimately matters and this is the only reason filters are installed. Arguably, the only true judgement of how a filter is performing is still the resulting real-world performance of the turbine after a filter is installed.

Steve Hiner is chief engineer at Parker Gas Turbine Filtration, formerly CLARCOR Industrial Air