LCC Quick Notes, Turbine Oil Flush, a Good Preventative Maintenance Exercise.
Turbine oil flushes have become rare. Although one of the best preventative maintenance procedures that can be implemented, they are often victims of a shortage of maintenance time and thought of as too messy. An oil flush cannot be performed at a computer keyboard, and therefore loses popularity. With careful planning and the right equipment on site a successful flush can save the turbine from major contamination issues, removing internal piping, hydraulic relay bores, and internal oil passages of contaminants otherwise ready to foul trip systems, wipe bearings, and seize control devices.
(1) Why Flush?
A popular argument against performing oil flushes is based on over confidence of filtration systems. While it is true that good filters can provide very fine filtration of the feed oil streams, this is not a guarantee of internal contamination prevention. The internal passages and pipes downstream of the filters are carbon steel, and subject to wall corrosion and debris release over time. Also, years of replacing front pedestal covers in routine inspections introduce excess sealant release. The resultant particles find there way into the piping and lodge in the runs. Oil flushes were once very common and considered essential for safe and proper turbine operation. Under the heavy pressure to reduce turbine outage work tasks the oil flush has been a routine victim. This can be tolerated... for a time, but if lube oil based turbine control systems are not flushed at least once every five years contamination problems can be predicted.
(2) What to Flush and What Not To Flush
While it is great to flush contamination OUT of the oil system, it is very bad to flush contamination INTO control relays and hydraulic solenoids. For this reason a carefully designed and executed Flush Plan is highly advised. Plan components, and the reasoning behind them, are presented in this Quick Note.
■ TARGET AREAS... For turbines operating controls or autostop systems on lubricating oil the key flush areas are servomotor pilot chambers and all feeding piping and passage ways on governor valve operators and latching solenoids and all feeding piping and passage ways on stop valve operators. Each of these areas needs to be subjected to a high volume, cycling temperature flush under drain return particle collection monitoring. Cyclic temperature is needed to help de-scale pipe and drilled passage way foreign material coatings and corrosion. The same temperature induced contamination effect experienced post-outage when the warm up of oil for service breaks loose contaminates and creates hydraulic system headaches is used to advantage in the flush procedure.
■ SYSTEMATIC APPROACH... Pilot control relays should be removed or replaced with parts drilled for flush drains. Each critical hydraulic cavity should have a low flow drain tap feeding a collection sack of cheesecloth to detect debris removal and judge flush duration. Solenoid selection (3 or 4-way) valves should be replaced by manifolds which divert flow into and back out of normally dead-headed pilot chambers. Sensitive E/H converters should be isolated. During the flush oil temperatures should be cycled as much as cooler manipulation permits, usually with manual cooling water valves. Flows should be as high as filtration systems will permit prior to bypass.
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