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Stuffing boxes and mechanical shaft seals. |
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|  MIL-P-18682D(SH)
3.3.7 Rotating assembly.
3.3.7.1 Rotors shall be dynamically balanced with all rotating parts
connected thereto; for common shaft assemblies, or those using rigid coupling,
this requires dynamic balance with the rotating element of the driving unit in
place. However, rotating parts may be balanced individually provided that,
when assembled, the imbalance shall not exceed the limits specified in
MIL-STD-167-1. .
3.3.7.2 Where balancing is required as a maintenance procedure to maintain
proper vibration or noise performance of the pump and driver unit in service, the
pump shall be designed such that balancing may readily be performed by overhaul
activity personnel.
3.3.7.3 The pump design shall be such and calculations shall be made to
demonstrate that the maximum operating speed of the pump will be not greater
than 70 percent of the first critical speed.
3.3.7.4 Impellers, shafts, and other parts of the rotating assembly shall
not contain radially oriented setscrews except setscrews solely used as balancing
weights in the balancing ring design required by 3.3.6.5 are acceptable.
3.3.8 Stuffing boxes and mechanical shaft seals.
3.3.8.1 Pumps shall be provided with mechanical shaft seals. The stuffing
box for surface ship pumps shall accommodate a minimum of two rings of packing
for use in the event of a mechanical seal failure. The mechanical shaft seal
shall have a tungsten carbide against a carbon sealing surface. Mechanical seal
housing material shall be as specified in table I.
3.3.8.2 Sufficient space shall be provided between bearings and stuffing
boxes to permit easy inspection of bearings and mechanical seal leakage.
3.3.8.3 Pumps for submarine application shall be provided with pressure
breakdown device. The pressure breakdown device shall be located in way of a
shaft sleeve and not in way of the impeller. The device shall limit the leakage
to 5 gal/rein in the event of failure of the mechanical seal, under conditions of
maximum suction pressure specified. The stuffing box for submarine pumps shall
accommodate a minimum of four rings of packing for use in the event of a mechan-
ical seal failure.
3.3.8.3.1 Pumps shall be configured so that normal gland leakage, whether
from mechanical seal or when operating under emergency packing, shall be collected
and piped to waste. The collection area shall be tapped for drain connections so
that the leakage can be drained away from the pump casing foundation, bearing
housings, or driving units. With the pump running or shut down, the collection
area shall collect and drain all gland leakage, up to the greater of either
(a) one quart per minute or, (b) three times the minimum allowable leakage rate
recommended by the contractor and approved by the design review agency for normal
operation under packing.
3.3.8.4 Mechanical shaft seals shall be positioned on the shaft by means
of stub or step sleeves. Mechanical shaft seals shall not be positioned by use
of setscrews alone.
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