Gas Turbine Air Inlet Maintenance Checklist: Keeping a Plant Running

With the decline in coal-fired power generation over the last two decades in favor of renewable power, combustion turbines in both simple-cycle and combined-cycle configurations are serving as an important bridge technology for grid stability. A critical aspect of gas turbine operation is optimized inlet air flow. Poor intake design/maintenance or off-design operation can result in inefficient air compressor and turbine performance. Other issues may cause corrosion or fouling throughout the unit. Important air intake components include the ductwork, air filters (usual but not universal), silencer, and for many units, an inlet air cooler. This article outlines inspection/maintenance techniques to ensure that equipment is in proper condition.

By Scott Schreeg, Director, Sales, SVI Industrial

Upfront Protection

A too-frequent problem in the power industry (and no doubt other industries) for seemingly forever has been equipment protection from the elements during delivery and storage at the site. Too often, carbon steel ductwork, piping, and other components are placed unprotected in a laydown area, upon which significant rusting occurs. Not only does the corrosion reduce the life expectancy of the material, but, after the equipment is installed, the corrosion products may then transport to the operating equipment to cause further problems.

Painted carbon steel is common for gas turbine inlet ducts, but pre-painted components may suffer mechanical damage that can lead to exposed metal and localized corrosion. Specifications for shipment should include strict packaging requirements to keep equipment clean and dry prior to installation. Regardless, during unit commissioning and periodically thereafter, visual inspection of the intake duct and internals is important for finding paint failures, corrosion, and other problems.

Some equipment, such as silencers, may be fabricated from corrosion-resistant materials such as 304 stainless steel. Although the cost is higher than for carbon steel, the added protection can be economically beneficial over the life of the unit. A similar example is evident in steam generators, where use of low-chromium alloy steel in certain locations versus plain carbon steel can greatly reduce flow-accelerated corrosion.

Air Filters, Inlet Coolers, and Silencers

Combustion turbine air compressors, combustors, and turbine blades obviously operate under highly stressed conditions. Without air filters, air-borne particulates can readily enter the turbine and potentially influence air flow to the combustors or induce erosion of compressor and turbine blades. Particulate accumulation may require periodic off-line turbine backwashing, which may slowly degrade the material.

Units in arid locations such as the southwestern U.S. may be particularly prone to particulate ingress. Therefore, most units are equipped with inlet air filters. A common example is hydrophobic filter media, which allows moisture to collect and drain. However, just like filters for motor vehicles or other engines, turbine air filters will accumulate material to the point that changeout is required. Replacements may be scheduled on a timed basis, but it is more reliable to measure the differential pressure across the filters, and replace the filters at a design maximum setpoint. Personnel can change out filters during outage and inspection work. Furthermore, for those gas turbines that do not have air filters, sometimes redesigning air intakes to improve performance is an option.

Specific to CT intake acoustic perfor­mance, inlet duct design to control and manage break-out and inlet silencer noise in both near and extended dis­tances from the face of the filter house is an important factor. Intake noise is of much higher frequency than noise from the CT exhaust. Accordingly, the silencer baffles for the intake are typi­cally much thinner (to dampen the high frequency noise) than the exhaust si­lencer baffles whose greater mass is more effective on low-frequency noise. A proper intake system design is need­ed to meet acoustic guarantees for site-permitted criteria or for improved acoustic performance depending on the project goals.

A huge influence on combustion tur­bine capacity and efficiency is inlet air temperature; as temperature ris­es, capacity noticeably decreases. Accordingly, many combustion tur­bines, and especially those of com­bined cycle units, are equipped with inlet air coolers to reduce the inlet temperature in warm weather. The common design has water flowing down and along media through which the air passes. The media greatly en­hances air-water contact.

In many regards, the inlet air cooler is a reverse process of the cooling tow­ers at industrial plants, commercial buildings, and many other facilities. Instead of the circulating water being cooled for return to heat exchangers, the air is cooled for improved air com­pressor efficiency. However, like tradi­tional cooling towers, cooling comes from a fractional but steady evapo­ration of the recirculating water. This process requires periodic blowdown of some of the basin water with fresh makeup replenishment.

Vital to downstream equipment pro­tection is minimizing water carryover to the compressor and onwards. Water droplets can induce compressor blade erosion, while some of the entrained solids may deposit on turbine blades and cause significant corrosion, which, of course, is exacerbated by the high temperatures in the turbine.

Air cooler inspection is an integral part of intake evaluations. Misaligned media or mist eliminator sections should be repaired or replaced during the outage. Also critical is examination for scaling or microbiological fouling of the media. Not only can these phenomena restrict air flow, but they may cause ‘channel­ing’ in the media which in turn may increase moisture carryover to the tur­bine. In that regard, proper chemistry control of the cooling water is a must for any inlet cooler. Traditional scale control cooling tower treatment pro­grams are not recommended, but reli­able alternatives are available, including methods to control microbiological fouling. Further details may be found in Reference 1.

Summary

The inlet air intake and its internal equipment are an integral part of a gas turbine system. Regular in­spections of these components are necessary to ensure reliable turbine operation, and to discover potential problems before they become severe. By implementing inspection services and modifying or redesigning intake structures, unit efficiency and reli­ability can be improved.

Reference:

1. ”Water Supply Requirements for Gas Turbine Inlet Air Evaporative Coolers”; Report GEK 107158A, GE Power Systems, Gas Turbine, January 2002.