The Benefits of Upgrading a Gas Turbine to EPA Filtration
Gas turbine compressor fouling, corrosion and erosion are all linked to air quality. Compressor fouling can be caused by natural plant matter or solid particulate found in the air. It can also be caused by man-made air pollution hazards such as carbon smoke or hydrocarbon fume, which creates a sticky substance when deposited on the turbine blades.
Modern gas turbine rotating parts are complex in design and structure and have a critical profile for maximum working efficiency. The high-pressure blades/nozzles sometimes have small air holes to deliver cooling air as the working temperatures are close to, or above, the limit of the material. Compressor blades are made of a very sophisticated alloy of metals to provide strength and durability, and these are coated with a protective layer for extra durability. This makes effective filtration a major factor to the long-term life of the gas turbine.
So, how important is gas turbine filtration and what are the impacts of poor air intake filtration. In this article we explore the impacts and explain the route cause.
Particles, which have sufficient mass to irreversibly wear the internal rotating components, are typically identified as being greater than 10 µm in diameter. Their hardness, velocity and concentration in the air stream can cause erosion in a time-related manner. Such particles can be removed by inertial filters or pre-filters with consummate ease. A particle 10 µm in diameter is considered a very large particle but for context the human eye can only see particles >40 µm in diameter. Erosion causes irreversible damage to a gas turbine but can easily be avoided with adequate air filtration.
Those pollutants which are less than 5 µm diameter do not have sufficient mass to cause wear, but they can impinge onto the surface of the rotating and static components and in a short period of time change the blade profile away from its ideal shape. This is commonly referred to as fouling of the gas turbine and remains a very common problem. However, the impact of fouling can be very significant and although water washing the engine using detergents and copious quantities of fresh water can restore efficiency, there is still a slow and stable deterioration of engine efficiency and power output. It is also worth noting that the same small particles that result in compressor fouling can also plug the cooling air holes located in the blades which will increase the operational temperature of components and reduce engine operational design life.
Corrosion of the low pressure (LP) and hot pressure (HP) parts of a gas turbine can occur if airborne salts pass through the filtration system. It is a chemical process which is not dependant on the particulate size but on the presence of moisture and an electrolytic reaction between salts and metals of different types. Airborne salt and water ingestion causes low temperature corrosion whilst the combination of salt (NaCl) with air/fuel borne sulphur results in high temperature sulphidation/oxidation or ‘hot gas’ corrosion within the turbine section of the gas turbine.
Hot Gas Corrosion
Hot Gas Corrosion is of particular concern especially in coastal and offshore locations where NaCl is prevalent both as a dry particle and in solution in water. When mixed with sour (sulphurous) fuel it will cause accelerated degradation of key hot section components of the gas turbine. This is a common cause of engine failure in an offshore environment, often well before the design life of a gas turbine.
Erosion and Low Temperature Corrosion combined effect
A combination of these effects can lead to blade failure. Poor filtration erosion can result in removal of the blade protective coating, this will leave the blade susceptible to low temperature corrosion from a combination of salt and water. Should pitting corrosion develop near the root of the blade this could eventually result in catastrophic failure through detachment of the blade due to the excessive loads at the weakened blade root.
The impact of poor air filtration on gas turbine performance is entirely linked to air quality, and therefore the individual location of a gas turbine installation. It is the surrounding environment that will determine the severity of the air quality challenge and the potential impact of poor filtration.
Operators cannot simply rely on the gas turbine filters provided by the OEM to be the right match. To protect the rotating machinery from the impact of fouling, erosion, corrosion and hot gas corrosion, gas turbine manufacturers (OEM’s) issue mandatory air quality requirements to filtration suppliers. In general, the level of these requirements is not particularly stringent but also takes into consideration that regular water wash and maintenance of the gas turbine will also be required. For original equipment supply this enables the OEM to remain commercially viable in a competitive market whilst balancing the performance, health and life of the turbomachinery.
An Operator should consider the commercial and environmental benefits of improved filtration, particularly the value of upgrading to EPA filtration. AAF can work with you to quantify these benefits and provide details of the total cost of ownership so you can offset any increase in filter prices against the commercial benefits that will be achieved. The greatest dilemma in this scenario is even in the harshest of air quality challenges the particles that cause the most damage remain invisible to the naked eye and so it is easy to understand why the seriousness of the problem is not clear. Optimized air filtration can protect any gas turbine regardless of the location and the surrounding environment and AAF will work with you to find the right solution for an individual gas turbine. With a range of dedicated solutions AAF can support you with market-leading products that will eliminate the risks of gas turbine damage and maintain gas turbine efficiency and power output.