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Most Turbine-Generator Lube Oil Systems have lube oil supply piping that is fully welded. There is no opportunity for anyone to turn a valve that can shut the lube oil flow off. 

For duplex heat exchangers and duplex filters, a selector valve, sometimes called a double circuit valve, can be turned from one flow path to the alternate flow path. These valves are designed so flow is not blocked during the transition. 

Flush Cock Valves are different.  These valves can block or divert lube oil flow, but it cannot be operated when the lube oil system is in service or pressurized because it will leak severely when it is opened.  

A flush cock valve is very advantageous when it is used at the junction of the main oil supply header and a branch to a bearing and there is a need to perform maintenance on that bearing.  The oil can be shut off for a few minutes, the plug is removed, rotated to the block position or to the flush position, the plug is reinstalled, and the oil system is turned back on.  The remainder of the oil system stays in service and this bearing can be maintained as required, though the shaft cannot be placed on turning gear.  Upon return to service, the reverse steps are made and the unit is ready to resume operation.

In order to rotate this plug, the retaining screws must be removed from the flange, and then reinserted in the threaded holes to jack the plug out of the fit.  Then the plug may be rotated freely and inserted in the body in one of four positions:  1.  Normal operation - oil flows straight through.  2.  Flush position - oil flows through the input port and out through the flush port.  3.  Blocked position - the closed side of the plug faces the inlet oil port. 4. A second blocked port -  the ports within the plug go from inlet to the closed side of the body.  Note that the groove that is milled into the flange of the plug shows exactly what the flow path is.

If there is any pressure on the oil line when the plug is lifted, oil will flow out of the valve, so if the oil system is pressurized at that time, a substantial oil flow will occur if the valve is opened.

In summary then, flush cock valves are used for those situations where it is absolutely critical that a valve that can shut off the lube oil flow cannot be operated when the lube oil system is pressurized and in service.