Improved Gas-Turbine Performance: Maintaining Inlet Fogging Systems
Jane Alexander | November 4, 2014
These turbine-output boosters work well with little maintenance, but require proper installation and regular monitoring.
By Jane Alexander, Managing Editor
Since their introduction more than two decades ago, gas-turbine inlet-air-fogging systems have been used with turbines across industry, including those found in power-generation applications. Inlet fogging boosts turbine output by cooling the air, especially on hot afternoons when demand is often at peak. While these systems usually require little maintenance and operate trouble-free for many years, they must be installed and maintained properly to produce the desired result. Fogging equipment expert Thomas Mee offers the following advice for keeping these systems—and in turn, the turbines with which they are associated—in top shape.
The most important factor for proper operation of a fog system, Mee says, is to use nozzles that make very small water droplets. Large droplets not only take more time to evaporate, they are less able to follow airflow around obstructions such as inlet-silencer panels and duct-support structures. Also, larger droplets are more likely to collect on duct obstructions or settle to the duct floor. If fog droplets impact on duct-support struts or trash screens, water will accumulate. This accumulated water can be stripped off by high-velocity airflow in the form of very large secondary droplets with a diameter of one millimeter (1000 microns) or more. Droplets that impact on silencer panels or duct walls can also create flowing and pooling water on inlet duct walls or on the floor.
Small droplets ensure that water evaporates quickly without causing excessive accumulation in the inlet ducts. Furthermore, because there is usually not sufficient time for all droplets to fully evaporate, any that don’t evaporate must be small enough to not cause compressor blade erosion.
Mee notes the importance of installing nozzles in the proper locations in the inlet duct. In fog systems used primarily for evaporative cooling, nozzles should be located in the filter house to allow maximum evaporation time. Systems used primarily for overspray or wet compression should have them located close to the compressor inlet. True fog droplets do not cause compressor erosion, but if large secondary droplets exist in the airflow, or if water is allowed to accumulate near the compressor inlet where it can be suctioned into the compressor as large droplets, blade erosion can occur.
Key needs: pure water, clean ducts
Fogging systems require that demineralized water be used to prevent mineral build-up on compressor blades or hot-gas-path corrosion that could be caused by the dissolved minerals present in untreated water. Mee recommends using demineralized water with less than one part per million of total dissolved minerals—conductivity of 2 mS/cm or less.
Prior to restarting a fog system that has been off-line for a long period of time, inlet duct surfaces and silencer panels should be thoroughly cleaned. This includes recoating duct surfaces that have rust or a compromised coating. If this process is skipped, dirt or other material that has accumulated on duct surfaces and silencer panels can be carried to the compressor, leading to compressor fouling. If compressor fouling occurs, it may be because the compressor is suctioning dirty water from the duct walls and floor.
While fog nozzles typically require little maintenance, they should be visually checked at least once per year. Any nozzles that are plugged or have deformed spray plumes—usually none or less than one percent—should be replaced. Most fog nozzles have an integral filter, which can be replaced in the field. A plugged or damaged nozzle can also be returned to the manufacturer for cleaning and reworking.
Excessive nozzle plugging can occur if carbon steel or galvanized pipefittings or components are installed in the flow path of demineralized water. Any such fittings should be replaced with stainless steel or plastic fittings. Bacterial fouling of fog-nozzle filters can also occur if water is allowed to stand in fog-system pipes for long periods. Ideally, automatic drain valves are in place to drain fog-system pipes every evening. Systems without drain valves should be drained at the end of the fogging season to prevent growth of anaerobic bacteria in feed and nozzle lines.
It’s also important to drain all water from any system that could be subject to freezing during the off-season. High-pressure filters should be used to collect any pump-seal material or other particles before they get to the nozzle filters. Also, a pressure sensor located downstream of the high-pressure filters should be used to ensure that the fog system shuts down if the filters become clogged. This will also prevent operation of the system with low-pressure water, which would result in larger droplets being formed. Upgrading to high-pressure filters is a good idea for any fog-system without them.
Correcting poor manifold design
A fog nozzle that produces large droplets, or an improperly designed fog-nozzle manifold, can result in excessive water in the inlet ducts. Due to minimal mixing of air in the inlet duct, a poor nozzle manifold design can result in over-fogging of some of the airflow and no fogging in the rest of it. According to Mee, this can greatly increase evaporation time for the bulk of the fog spray. It also increases droplet fallout and can decrease power boost because not all of the water is being used to cool the inlet air. Additionally, droplets that don’t evaporate in the inlet airflow are carried to the compressor, but evaporating water in the compressor produces much less power boost than it does when it evaporates in the inlet air.
Improperly designed nozzle manifolds can also result in temperature and airflow distortions at the compressor inlet. A common error is to leave large gaps at the edges of the nozzle manifolds—between duct walls, floor and ceiling and the nozzle lines. If too much untreated air is allowed to flow past the nozzle lines, uneven cooling can result, which can lead to compressor blade flutter and compressor maintenance issues over time.
Computational fluid dynamic (CFD) modeling can be used to determine ideal nozzle spacing arrangements for even cooling at different loads, ambient temperatures and relative humidity levels. In some cases it can be beneficial to locate a higher concentration of fog nozzles in areas of a duct cross-section that have higher airflow and fewer in areas with lower airflow.
Retrofitting poorly designed nozzle manifolds is inexpensive and can be accomplished in a day or two of outage. It’s recommended for systems that don’t produce the expected power boost or that have excessive amounts of water in the inlet ducts.
Upgrading duct drains
Properly designed drainage systems are vital for the removal of water that accumulates on walls or duct floors where it could be suctioned into the compressor. Simple water diverters and gutters can be used to direct water to drainage points. But high-velocity airflow over the floor near the compressor can cause water to pile up even if there is an open drain point nearby. If this occurs, a false floor can be installed above the drain point to allow water to flow to the drain without being suctioned into the compressor.
It’s recommended to install a viewing window and adequate lighting near the compressor inlet so the area can be monitored both for water accumulation and/or to validate the design of nozzle manifolds and duct drains.
Sub-micron water filters on the fog pump skid are normally replaced annually if the full-flow pressure drop is more than 5 psi. Excessive filter plugging can occur if carbon steel or galvanized fittings are used in the demineralized water piping system.
If the raw water source is surface water, such as from a river, lake or pond, the water-treatment system should be designed to remove colloidal solids. These are particles of sand or clay typically less than one micron in diameter, and can be present in water that appears clear to the naked eye. A properly designed fog pump skid has sub-micron filters to remove colloidal particles because the particles can damage the fogging nozzles. However, high levels of particles will rapidly plug the skid filters. Frequent plugging of the pump skid filters can often be resolved by installing high-capacity sub-micron filters with automatic backwash capability.
High-pressure ceramic plunger pumps require oil changes and seal replacement. Pump manufacturers often recommend changing oil every 500 hours, but many operators have extended oil changes to 4000 hours or longer without problems. Because demineralized water is a poor lubricant, pump seals in ceramic plunger pumps can have a short life span—typically around 500 hours with highly purified water.
Water dripping from a high-pressure pump indicates that seals have reached the end of their useful lifespan and require replacement. Fog systems that operate for more than 500 hours per year should use seal-flushed pumps to extend pump-seal life. Seal-flush pumps, which feature a secondary water flush for low-pressure seals, have been demonstrated to operate more than 6000 hours between seal replacements. Those who want more time between seal replacement should consider replacing existing pumps with seal-flush pumps.
Certain pump designs don’t require seal replacement or lubricating oil. These use the water as lubricant and are purported to require no maintenance for extended periods. However, they cost more than ceramic plunger pumps and require a factory rebuild approximately every 8,000 hours. Also, proper installation is critical for these pumps because they are susceptible to total failure if air is entrained in the water supplied to the pumps. Air-purge systems are available to ensure against this type of pump failure.
Protecting the compressor
Several gas-turbine OEMs have identified poorly designed fog systems as a source of compressor blade erosion. As noted, droplet size is the single most important factor in avoiding blade erosion, and research and experience have shown that inlet fogging droplets of 20 microns or less do not cause erosion. The tactics described above that include retrofitting better nozzles and manifolds can also protect the compressor from damage.
If a fog system suddenly shuts down due to system failure or operator error, it is possible for the sudden introduction of much warmer air to cause a compressor surge. This can be avoided by installing a pressure vessel on the high-pressure water lines that allows the fog spray to be gradually reduced over a few seconds.
Also note that while some older versions of the fog-system software that controls the amount of fog injected into the inlet do not account for changes in inlet air mass flow when the turbine is operated at part load. However, software modifications can often be made inexpensively that will take input from IGV (inlet guide vane) position so fog flow decreases in proportion to inlet airflow. MT
Thomas Mee, III, is CEO of Mee Industries, Irwindale, CA. The company has installed nearly 1000 fogging systems on gas turbines worldwide. It also provides fogging for building and data-center humidification, special effects, dust control and other applications. For more information, visit meefog.com.