A pulse filter option for addressing operating difficulties in salt conditions

Sea Salt Aerosol and Sea Salt Spray are primarily associated with operation of platform off shore generators. Fine Salt Crystals however, can be suspended in atmospheric air along the coast line and in some cases be carried further inland.

In a coastal environment air entering an inlet filtration system could contain high concentrations of dry salt crystals. Most range in size of 1 to 2 microns. There are two primary inlet filter housing designs for Gas Turbines. Static Barrier filter systems and Pulse Filter type systems. Both can be used with pre-filtration and can do an adequate job retaining high concentrations of salt crystals. In static systems these crystals are held within interstices of the filter media. With pulse systems the majority need to be captured at the filter surface.

Salt crystals are typically cubiform in shape and hygroscopic in nature. Critical humidity of pure salt is 80% relative humidity. At this humidity, the salt crystal will continue to absorb moisture until it reaches “super saturation”, at which time it deliquesces. The resultant saline droplet is five times larger than the original dry crystal cubiform.

Therefore, at relative humidity below 70%, the salt is always dry and cubiform. At relative humidity between 70% and 78%, the salt crystal has not changed its shape, but is supersaturated. At 80%, the critical relative humidity, the salt crystal becomes a saline droplet 5 times its original size. The relative humidity between 75% and 80% is referred to as the dynamic range for salt.

Along coastal environments, when the relative humidity gradually increases above critical relative humidity of salt. There may be no free moisture entering the filter system. However, potential catastrophic conditions exist. The collected or embedded dry salt crystals, being deliquescent, have absorbed moisture in the vapor phase from the incoming airstream. The salt crystals are now in their dynamic phase and can hold no more moisture. The relative humidity exceeds the critical phase causing the salt crystals to deliquesce, forming saline droplets. If the dry crystals are not removed on an ongoing basis or the high efficiency barrier filter media is not waterproof, the saline droplets will leach through the media and into the turbine inlet.

When saline droplets reach the second stage of the axial compressor, the heat of compression is sufficient to evaporate the moisture. At this point dry crystals enter into the hot gas section of the turbine. The filtration system has failed.

The ultimate solution to prevent system failure would be a completely waterproof media in all stages of the filter system. In other words, collect and retain both liquid and solid particulates in multi stages and inhibit them from passing through to the turbine.

Unfortunately, no filter is 100% impenetrable to either liquids or solids. As both the water vapor and solid particulates load onto the Barrier or Pulse filter, they create an ever increasing restriction. At some point that restriction is greater than the resistance the turbine can effectively operate within. Filters must be changed, interrupting energy production and costing the power generator additional capital expense.

There is a great controversy in the industry on which style of filter housing prohibits or limits the amount of saline droplets through to the turbine. Since such a great variation exists in atmospheric conditions, and controlled environments cannot duplicate reality, it is not clear if any single solution or design exists to cost effectively eliminate the problem.

There are several manufacturers who have developed exotic solutions to address the symptoms generated by deliquesced salt crystals. They are very expensive, fragile in design and may not provide a 100% successful result. However, if you operate a pulse style filter housing there are basic design parameters that can address the issue.

• High efficiency surface capture of sub micron particles
  • The smaller the salt particle the more readily it absorbs moisture and the more quickly it can deliquesce into a liquid.
  • Capturing the particle on the surface of the filter will help prohibit the migration of the particle or liquid through the filter.
  • Capturing the salt particle on the surface would also offer the chance to remove it during pulse cleaning.
• Water resistance of primary filter media
  • Providing water resistance as a property in a filter increases the time it takes for a particle to migrate through a filter medium.
  • Use of 100% synthetic is recommended. Blended paper media contains a percentage of cellulose fibers.  Although there are chemical binders that coat the fibers, they are not sufficient to eliminate absorption of moisture into a cellulose fiber.
• Increased frequency of pulse clean particle removal
  • Pulse cleaning of filters removes surface buildup of dust particles to assist in the recovery or control of pressure drop.
  • Removal of salt particles on a more frequent basis reduces the concentration of solids that can be deliquesced over time, further reducing the severity of salt dust or droplet penetration through a filter media.