Tim Nicholas

During operation, gas turbines suck in huge volumes of ambient air, which typically contains moisture, particles such as dust, grit, and pollen, and industrial contaminants such as hydrocarbons. Without effective filtration, these pollutants can find their way into the machine and cause fouling, erosion, and corrosion.

The fouling of compressor blades, by even the smallest airborne particles, rapidly impacts the aerodynamic efficiency of the turbine. Given that every element of an advanced gas turbine is precisely optimized to deliver thermal efficiencies above 60% in a combined cycle, even a minute loss in compressor performance can have a significant economic impact. At the tail end is a loss of megawatts.

While cleaning can restore most of the performance, consistent fouling means there is a gradual non-recoverable degradation in turbine efficiency. Fouling can also potentially restrict or block airflow through the cooling channels of the turbine buckets in the hot section, resulting in accelerated wear and eventually complete failure. In a worst-case scenario, particulates and other contaminants can corrode compressor blades and turbine bucket internals, causing a loss in performance and instrument downtime.

Today, gas turbines feature a two- or three-stage system that delivers high-efficiency particulate (HEPA) quality air that is used to remove at least 99.5% of the airborne materials larger than 0.1 microns or higher. Further, filter houses are constructed to protect the air inlet filtration system. A poorly constructed filter house can allow contaminants to bypass the filters and enter the turbine.

Simulating with typical PM10 air quality data, the removal of a single filtration unit from the filter house, which may have up to 700 filter elements, can leave an open-area gap of 0.35 m

 in an otherwise HEPA-grade system. Under standard conditions, approximately 1.5 kg of material passed through this gap in the filtration system over 8,000 hours, approximately a year of gas turbine operation. The entirety of the filter house running with HEPA filters in standard conditions without any bypass in comparison would only allow approximately 100 g of material through over the same period.

A recent example of how this affects the gas turbine is a multi-unit advanced gas turbine power plant in Asia outfitted with a filtration system comprising a pocket pre-filter and E10 vCell downstream HEPA filters. Downstream from the HEPA filters were gaps in the modular construction of the filter house and transitions that allowed water and air to be sucked in—a total open area for filtration of approximately 250 m

The operator noticed a drop in the machine's performance. Subsequent inspections revealed the turbines were fouled and even had pitted and corroded compressor blades. Further investigation of the filtration system, including water-spray tests, revealed that, although the filters were intact and installed correctly, construction errors had left gaps in the roof of the filter house. Since cleaning can't restore pitted and corroded blades, the operator had to replace some of the rotors of the compressor section.

Signs of construction errors, leaks behind HEPA filter stage

Filtration houses should be constructed with a filtration expert during the turbine commissioning phase of the filter house. Depending on the size of the turbine, an expert filter house inspection can take five hours to a full day. A final checkpoint by a filter house inspector before the turbines are on the grid is a simple risk-management step. Performing these checks and an inlet inspection before commissioning reduces the risk of damage to the gas turbines and compressors.

Tim Nicholas is the Powergen Marker Manager at Parker Hannifin.

Articles

Construction and inspection of an air filter house prevents gas turbine and compressor fouling and long-term damage.

A Case Study: Air Inlet Filtration

Environmental contaminants affect gas turbine performance. Sand, dust, salt, and other elements present in the huge volumes of air consumed by turbines can impact aerodynamic performance, corrode metal parts inside the machine, and lead to failure. The local environment can contain high volumes of sand, salt if near to the coast, excessive rainfall, elevated levels of moisture from mist, fog, or high humidity levels, or a combination of any of these with seasonal variations.


Inlet filtration houses are installed to protect the machines from contaminants. Their design needs to be specific to the environmental conditions of each installation. But how do we know how filters will perform against real-world conditions? To gain an in-depth understanding, mobile test rigs (MTRs) were deployed across the globe to review the performance of filters in four different environmental conditions: Coastal sites in the northeast of the United States (MTR4) and west coast of the Kingdom of Saudi Arabia (MTR3), as well as inland sites in the Midwest of the United States (MTR2) near its geographic center, and a desert plateau in the center of the Kingdom of Saudi Arabia (MTR1). These provided a range of conditions, namely coastal low dust (MTR4), coastal high dust (MTR3), inland low dust (MTR2), and inland high dust (MTR1). All equipment, flow rates, and filters were standardized.


The study ran for 6,000 fired and instrument hours, covering all four seasons. Performance data and weather conditions were recorded to identify any abnormalities. Each MTR can run multiple stages of filtration, including a coalescer stage in the weather hoods, pulsing cartridge filters, and downstream vCells stage. Each rig is built to run two independent, identical wind tunnels side by side with multiple filters in each stage, enabling direct performance comparisons. At the high-efficiency cartridge stage, it is possible to run six filter sets per wind tunnel, all capable of being pulse cleaned.


As well as comparing known filters in the industry, having two wind tunnels in each MTR provides the capability to develop and try new filter types without putting a gas turbine at operational risk if it was to be tested at a larger scale. Weather conditions that can affect filtration performance are measured, including rain, mist, fog, snow, ice, dust and sand, and wind speed. Each is measured and rated, including droplet size, particle size, relative humidity, and temperature.


Coalescers are usually installed near large bodies of water, such as oceans, lakes, and rivers. However, one of the larger reactions seen in the MTRs on the pulse filters came from MTR2, an inland low dust test site. Normally such an environment would be classified as benign. But results confirmed that coalescers are beneficial in areas that experience high levels of humidity and fog even where no large body of water exists. As opposed to rain or snow, moisture is formed of tiny water droplets that can become trapped in the matrix of the filter media and cause blockages (Figure 1).


At the MTR3 coastal high-dust installation, the impact of high winds created minor differential pressure increases across the inlet and coalescer stage (Figure 2). Such observations help in the selection and design of filtration systems and GT placement for new power plants.

Figure 1: Differential pressure spikes during mist and fog events without coalescers. Small moisture droplets present in mist and fog can restrict air flow through fine filtration media.

Figure 2: Observed minor pressure increases during high winds at the coastal high dust installation


The importance of being able to test multiple high efficiency products at the same time can be seen from a trial completed in MTR2 (inland high dust). The test rig had two different filtration product types running, one in each wind tunnel with identical test parameters. After six months of operation, a distinct shift in their effectiveness became evident (Figure 3). As current industry testing standards utilize accelerated test methods using a known type of test dust, they produce radically different results. Thus, such standards may not provide a true comparison of products.

Figure 3: Differential pressure measurement of two products in a high dust environment. For the first six months, performance was similar. After six months, the differential pressure across the industry standard filter rose more rapidly compared with Parker clearcurrent ASSURE filters.

The effect of snow, ice and moisture can be seen at the MTR4 (coastal low dust site) (Figure 4). The results showed sharply different reactions to moisture over time that produced differential pressure spikes.

Figure 4: Differential pressure measurement of two products in a moisture-rich environment (MTR4).

The industry standard filter experienced a greater reaction to moisture over time, producing differential pressure spikes. Such events could lead to de-loading of the gas turbine.

Parker clearcurrent ASSURE Filters protecting latest generation gas turbines.


In what appears to be low-moisture risk (inland low dust) regions, there was a consistent reaction to moisture, showing the benefits of installing coalescers. Without coalescers, the inland low dust site’s reaction to moisture is more pronounced than coastal sites in similar conditions with coalescers. The impact of winds on the coalescer stage was also a notable learning from the data set. The measured wind and associated increase in differential pressure was significant enough that OEMs should consider this when installing new gas turbines.


Ultimately, a filtration solution needs to protect high-performance gas turbines from different contaminants, seasonal variants, and distinctive environmental conditions. While lab testing provides some guidance, it does not replace testing in real-world conditions. ■

Tim Nicholas is PowerGen Market Manager, Gas Turbine Filtration Division, Parker Hannifin, whose clearcurrent ASSURE filters provide protection and performance for advanced gas turbines.

Jim Lenox is Senior Product Manager, Gas Turbine Filtration Division, Parker Hannifin. For more information visit parker.com