It’s a Cold, Harsh World Out There for Gas Turbine Filters
Pictured above: Inside filter house, with filters removed to reduce pressure loss (Texas, February 2021)
Gas turbines are installed in some of the harshest conditions on the planet, so filtration house design needs to allow for varied contaminants and weather unique to location. The events in Texas last winter demonstrated what happens when gas turbine filters and filtration systems are not equipped to prepare for extreme weather conditions.
The impact of ice can be catastrophic. Icing caused filtration systems in Texas to freeze suddenly and brought power grids to a standstill. If ice is allowed to block gas turbine air inlet filters, it results in high pressure losses, engine runback and, eventually, complete engine shutdown leading to brownouts and power outages.
Icing comes in different forms (rime and glaze), and operators need to choose the right gas turbine filter design to withstand extreme conditions. A filter pulse cleaning system used to remove sand and dust may not be effective when dealing with sudden ice formation and build-up.
By using military test standards and developing new testing strategies, Parker is beginning to understand how ice behaves, and how filters can be designed to handle it more effectively. Results of our extensive ice testing:
- Compared to the impact of rime ice, the ability of air to create natural flow paths through the ice rapidly decreases with the formation of glaze ice, along with the effectiveness of the filter pulse cleaning system, making performance highly dependent upon the type of ice that forms.
- Greatest hazard to gas turbine filters is at colder temperatures when impermeable glaze ice is prevalent.
- Even with observably large quantities of rime ice present in the system, Parker filters were still able to function.
- Even as temperatures continue to drop, when moisture droplets cease to be available to form ice, previously formed ice starts to “disappear into thin air” through the process of sublimation – and filter pressure loss recovers quickly.
- Results of testing different filter types against various concentrations and types of ice reveal not only is the selection of filtration media important, but construction of the filter element itself, its support structure, the mesh within the filter, and even the shape of the filter can all affect its performance.
Filter shape and outer mesh
Round filters enable ice to form in an inherently strong, circular structure, negating the effect of filter pulse cleaning systems. Testing revealed the structure of the outer mesh of the filter, if specified correctly, will allow a filter to continue to “breathe” when covered in ice.
Ice that forms on the outer mesh produces a honeycomb effect which leaves space for air to flow into the filter media, and collects further airborne moisture, preventing it from reaching the filter media and icing over.
This means filters without an outer mesh, or meshes with the wrong characteristics in their construction, are unlikely to work well in icy conditions. Testing revealed that mesh pattern, size, shape, gauge and even surface finish can all affect filter performance. Smooth powder coating on the mesh, for example, appears to allow excessive accumulated ice to be more readily removed by the filter’s pulse cleaning system; it provides an extra level of protection against rapid inlet pressure loss increase during icing conditions.
Glaze ice is a heavy coating of glassy ice which can occur when relatively large droplets of rain or drizzle freezes on an object, or when a filter is already wet before the temperature drops.
Due to their shape and size, only a small part of impinging water droplets initially freeze on impact with the cold surface of the filter – and the rest spreads out and mingles with other droplets to form a solid sheet of clear ice which severely restricts or prevents the passage of airflow. Its amorphous, dense structure helps glaze ice cling to surfaces, making it very difficult to remove.
Rime ice, on the other hand, is formed by the rapid freezing of tiny water drops, such as mist and fog, as they impinge upon an exposed cold object. Generally milky-white and crystalline, rime is a lighter ice that forms because the whole droplet freezes on impact.
The water droplet retains its circular shape, forming pores between the crystals allowing air to pass, which creates a much weaker structure than glaze ice. Rime ice forms in a narrower band of temperatures than glaze ice, at temperatures between -3° C (26.6° F) and -8° C (17.6° F).
Tests on different types of ice revealed the gas turbine filter system was able to handle a surprisingly high volume of rime ice and still function. However, a far smaller amount of glaze ice can lead to a rapid increase in pressure drop across the filter. It was observed during testing that the lower the temperature, the faster the increase in pressure drop over the filter with the increased formation of glaze ice, which is far less permeable than rime ice, which remains porous.
The type of ice forming affects gas turbine filter performance. With increased formation of glaze ice, the ability of inlet air to create natural flow paths through the gas turbine filter decreases along with the effectiveness of the filter pulse cleaning system.
With glaze icing, the time required to take action to reduce its effect on pressure loss is also decreased. The system must react quickly to reduce the impact of an icing event. With rime ice, even with observably large quantities present in the system, Parker filters are still able to function.
Testing carried out by Parker on the impact of ice is part of its continuous efforts to improve filter designs. New filter shapes and the improvements to the construction of the mesh will help make gas turbine filters more resilient.
About the authors:
Luke Scott is Lead Development Engineer, Gas Turbine Filtration Division, Parker Hannifin Ltd., and can be contacted at: firstname.lastname@example.org
Steve Hiner is Chief Engineer, Gas Turbine Filtration Division, Parker Hannifin Ltd., and can be contacted at: email@example.com
Gas turbine air filter models tested:
Parker Hannifin Clearcurrent ASSURE Filters were tested for performance with ice, as well as new next gen prototypes.