A Case Study: Air Inlet Filtration

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.

Typical compressor corrosion

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.

Air Inlet Filtration Simulation

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 m² 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.

Clearcurrent ASSURE - Advanced gas turbine HEPA filter house

Case Study Example

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 m².

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

Mitigating Filtration House Faults

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.