MIL-STD-419E
5.6.1 Procedure for system operating fluid flush. Flushing with system operating fluid or a fluid compatible with system fluid as approved by the acquiring activity shall be performed. For systems utilizing MIL-PRF-17331, MIL-PRF-17672, or PAO base fluids, flushing may be conducted with lower viscosity MIL-PRF-17672 or MIL- PRF-6083 fluids. In order to effectively remove particulate contamination from hydraulic piping, the flushing fluid should achieve turbulent flow. Reynolds number (Re) is calculated by the following equation:
Re = 3160*Q/(d*v) Where:
Q = flow rate (gpm)
d = waterway cross-section, pipe ID (inches)
v = fluid viscosity (centistokes c.s.)
Note: The value 3160 is a constant corresponding to units of gpm, inches, and c.s.
For pipe sizes greater than ½ inch, the fluid, flow rate, and fluid temperature shall be defined to achieve a Reynolds number of at least 4000. For ½ inch pipe, if a Reynolds number of 4000 cannot be achieved, then the pipe shall be agitated, the original established flushing time doubled, and a minimum Reynolds number of 2000 achieved. For pipe sizes less than ½ inch, when piping cannot be reasonably flushed in short enough sections to stay within the pressure limits of the piping (and system), the Reynolds number shall not drop below 2000. Upon completion of flushing with fluids other than the system operating fluid, the system shall be completely drained and blown free of flushing fluid, and the system refilled with operating fluid. Depending upon the complexity of the system and the amount of flushing oil left behind, a second or third drain-and-fill sequence may be required to provide minimal acceptable liquid (not solid) contamination.
5.6.1.1 Minimum flushing time. Flushing at turbulent flow shall continue until the volume of piping being cleaned, or the individual flushing loop as shown in figure 1, has been displaced a minimum of 20 times, as calculated in accordance with ISO 23309 below, or 20 minutes, whichever is greater.
T = 20V/qv where:
t = minutes
V = volume of piping being cleaned (gal)
qv = flow rate through the loop (gpm)
5.6.2 Flushing fluid samples. Unless otherwise specified by the contracting officer or ship supervisor (see 6.1), samples shall be taken from the bottom side of the pipe within 5 feet of the inlet of the flushing filter during full
flow. The size of each sample drawn shall not exceed 1 liter and shall include the fluid and contaminants trapped by the valve and adapter during the preceding 5-minute flushing period. (Open the valve to draw an amount greater
than the volume of branch pipe and valve, close the valve and wait 5 minutes and draw the sample.) The entrance to the sampling port shall be flush with the inner circumference of the pipe, and the valve connector (adapter) shall not
extend into the pipe beyond the inner surface of the pipe. For taking samples from the system during flushing,
diaphragm (packless) valves are recommended. Where the hydraulic system is equipped with sampling valves, samples may be taken from those valves, in lieu of the location specified above. The use of in-line fluid cleanliness monitoring, such as automatic particle counters, may be used to indicate the real time cleanliness level of the flushing fluid. When using in-line or continuous fluid cleanliness monitoring equipment, a periodic verification of equipment performance by an independent fluid sample analysis shall be performed. If this analysis indicates unsatisfactory equipment performance, then the flushing activity shall take the following corrective actions: (a) restore adequate equipment performance and (b) provide assurance of adequate system cleanliness, to include repeat flushing of any suspect flow loops.
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