Compressor Washing Systems
To achieve the best output and efficiency for your installed gas turbine, it is necessary to do a periodic compressor wash, to remove dirt and oil that will probably have collected. Even in relatively clean environments, dirt and oil can collect on the compressor blades. (For the non-believers, I’d suggest a test they can do easily at home. remove the vent cove from your home heating duct and rub your hand along the inner surface of the duct. You will quickly become a believer in compressor water wash). Most gas turbines installed since the mid-1980’s have a compressor water wash manifold installed on the compressor casings, and an associated water wash skid.
Off Line Water Wash
Most units have both an on-line water wash manifold and an off-line water wash manifold. The off-line water wash system is the most effective in removing dirt and oil from the compressor blades. This involves shutting down the gas turbine, and putting it on crank cycle (roughly 20% speed). Then, injecting water (50 to 90 GPM) thru nozzles installed in the inlet bell mouth. Typically the water wash cycle will last 20 to 30 minutes, but, to make sure the compressor is clean, you should monitor the water discharge, and let the system run until the discharge is clear. Water wash skids typically have a heater; to heat the water before the water wash cycle begins. Water used for off-line water wash should be “potable” (ok to drink)—no need for boiler condensate or treated water. The water temperature should be such that the temperature differential between the water and the compressor wheel space area is less than 200F. Typical time interval to cool down the compressor wheel space is 12 to 18 hours. For customers who need to minimize the time interval for a water wash cycle, can force cool down the gas turbine by putting the unit on crank cycle during the unit cool down to speed up the cool down process. Normally the hot wash water is sufficient to remove the dirt, but many operators also add a detergent to the wash water.
On Line Water Wash
Customers, who need to run continuously for long periods, also use an on-line water wash system. This system uses a separate manifold and nozzle system to inject water into the compressor. This is done while the unit is at normal load. Typical water flows are between 6 and 10 GPM depending on size of the gas turbine. Typically this water is not heated. Also boiler condensate or treated water should be used so as to avoid injecting any metallic contaminants that may be present in potable water. The on-line water wash process is not as effective as the off-line process, but it usually does remove most of the dirt from the first 6 stages of the compressor. This is the most important part of the compressor to keep clean, to recover airflow, output, and efficiency. Unfortunately, most of the dirt removed will then collect on the later stages of the compressor, thus, the need for periodic off-line water washes. Many customers schedule their on-line water washes coincident with the hottest part of the day, as the evaporation of the on-line wash water will cool the inlet somewhat, and give a little more power during the on-line water wash cycle. Use of the on-line water wash system can significantly reduce the need to shut down the unit for off-line water washes, by recovering most of the performance loss due to compressor fouling.
It is important to mention that the customer should never let the gas turbine output decline more than 5% due to a dirty compressor, before an off-line water wash.
Figure 1 - shows actual degradation over tine, including average degradation. Compressor fouling and online water washes are seen as the small “saw tooth’s” on the curve. Off line water washes show greater performance recovery. The large step changes are a result of compressor “hand scouring” during major maintenance outages. Over the life of the turbine there is some non-recoverable degradation due to casing roundness, blade finishes, seals, and other mechanisms.
Turbine Water wash
Units that use Crude fuel, Residual fuel, or any other type of heavier liquid fuel, usually have a turbine water wash system installed. The ash deposits (that result from adding an inhibitor for liquid fuels that have a significant Vanadium content) that get deposited on combustion hardware, buckets and nozzles will have a negative impact on gas turbine output and efficiency. The turbine water wash system is designed to remove most of the ash deposits. Typical water flow rates vary between 50 GPM and 90 GPM depending on gas turbine size. The wash water should be heated to about 200F, to maximize the wash effectiveness. Similar to the compressor wash process the turbine wheel space should be cooled down such that the temperature differential between the wash water and the wheel space is less than 200F. The wash cycle should continue until the drain water is clear. Immediately after the turbine wash cycle is completed, the unit should be fired to at least “full speed no load”. The ash deposits are usually porous, and if any ash deposits remain after the turbine wash, firing the unit will cause any residual retained water to flash, thus, removing additional ash deposits.
Any customers needing additional help with their compressor or turbine wash systems can contact the Keck Group International—keckgroupint.com for additional help.