What gas turbine filters face offshore

Below are excerpts from the paper "Gas turbines air filtration systems for offshore applications" by Dominique Orhon of Total Exploration and Proudction, Rainer Kurz of Solar Turbines, Stephen Hiner of Clarcor Industrial Air and Jim Benson of Camfill Farr Power Systems at the Turbomachinery Symposium in 2015.

All Gas Turbine (GT) inlet filters are designed to deal with the common types of contaminants such as; sand, dust and soot. Operation in the marine environment brings its own additional contaminant challenges that must be addressed by the GT filtration system to maintain a reliable and available machine, the main ones being discussed below.

Salt Aerosol 

Salt aerosol is the mixture of very small particles or droplets of salt with the surrounding air. This is formed when waves trap air as bubble, which rise through the sea to its surface where they burst expelling small droplets into the atmosphere.

The amount and make-up of these aerosols is a function of the wind and sea states. Strong winds lead to more and larger droplets. The resulting form of the salt within the aerosol when it eventually reaches the gas turbine inlet is directly related to the ambient relative humidity. At relative humidity’s (RH) below ~40%, salt is crystal and can be considered as a dry particulate and can therefore be filtered just like any other dust type particle with traditional filtration methods. Complications arise when the relative humidity changes as salt crystals are hygroscopic in nature, i.e. they have an affinity for water and like to absorb moisture from the surrounding air.

At a critical relative humidity, the salt crystal will continue to absorb moisture (and swell accordingly) until it reaches super saturation, at which time it deliquesces. Deliquescence is the change undergone by certain substances, which become damp and finally liquefy when exposed to the air, owing to the very low vapour pressure of their saturated solution. Salt is such a substance.

At this critical RH the salt crystal becomes a saline droplet, which requires different filtration methods to capture and retain it. At a relative humidity between 40% and 70%, the salt crystal is neither completely ‘wet’ nor completely ‘dry’ and can be considered as dynamic or ‘sticky’. The RH at the surface of the sea approaches 100% and has a vertical distribution with height which is dependent upon wind velocity. Offshore operators with meteorological capabilities have recorded frequently and regularly varying relative humidity, often below the 70% RH level where salt begins to change from wet droplet to dry crystalline.

Consequently, a gas turbine inlet air filter system design for coastal, marine and offshore installations must be able to handle salt in its wet, dry and dynamic phases. When salt reaches a gas turbine, it can foul and corrode the compressor section, but more importantly, the sodium in the salt combines with the sulphur in the fuel (if present) to cause highly accelerated corrosion in the hot section of the GT (Stalder etal, 2001).

Bulk Water

Bulk water is the mixture of large quantities of water with relatively large droplet sizes within the air entering the GT filtration system. This can be generated by heavy seas in the form of waves and spray, spray from the motion of vessels or produced by the weather, such as driven rain from heavy storms, monsoons etc. It can even come from man-made sources, such as on board fire deluge systems etc.

Sea spray

Large quantities of water entering the filtration system can cause the filter pressure loss to rapidly increase, due to over loading and filter plugging or blockage due to the expansion of captured dust via the soaking up of moisture. It can also cause captured dirt and salts to be washed off and through the filters further downstream of the system. If the water is able to pass the filters, then the large size of the droplets have the real potential to do damage to the rotting parts of the gas turbine.

Mist and Fog 

Mist and fog is another form of aerosol, but this time is in the form of fine pure water droplets in the airstream, typically with a droplet size of less than 2 microns in diameter (Figure 7). On its own, this type of contaminant is small enough and chemically inert to be harmless to a GT, but it can have affects upon the filtration system itself, causing the swelling of captured contaminants leading to high pressure losses, or even pressure loss “spikes” during fog periods. This affect can be greatly exaggerated if the filters also experience contamination from hydrocarbons, such as from lube oil vent outlets etc. and can also be more pronounced with higher efficiency filters.

Hydrocarbon vapours 

As with bulk water, the wetting of the filters can lead to contaminant being washed through and further downstream. Other Contaminants The air at the gas turbine intake can also be contaminated by products of normal platform operation, such as hydrocarbon vapors, dust from well completions, drilling mud and others.