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|  MIL-P-17881D(SHIPS)
3.3.5.4 "O" ring seals shall be used between the shaft and the stuffing box shaft
sleeve. Other positive methods of sealing may be used subject to the approval of the
design review agency concerned.
3.3.5.5 Keys shall be used in the shaft to avoid relative rotation of rotor com-
ponents, such as impellers, sleeves, spacer pieces and couplings.
3.3.5.6 Impellers, shafts and associated hardware in the rotating assembly shall not
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contain radially oriented set screws.
3.3.6 Rotating assembly.
3.3.6.1 All rotors shall be dynamically balanced with all rotating parts connected
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thereto; for common shaft assemblies or those using a rigid coupling, this requires dynamic
balance with the rotating elements of the driving unit in place. However, rotating parts
may be balanced individually provided that when assembled the unbalance shall not exceed
the limits specified in MIL-STD-167. 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 maintenance
personnel.
3.3.6.2 The pump design shall be such and calculation shall be made to demonstrate
that the maximum operating speed of the pump will not be greater than 70 percent of the
first critical speed. If this proves to have an impractical effect on mechanical or -hy-
draulic design, the unit shall be designed for operation above 125 percent of the first
critical speed. The second critical speed shall be not less than 30 percent above the
maximum operating speed.
3.3.6.3 All pumps shall incorporate design features to achieve hydraulic balance to
obtain a thrust bearing life of 10,000 hours or greater.
3.3.6.4 Barrel-type pumps shall be so designed that the assembled pump rotor (exclusive
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of the coupling, including all non-rotating parts which can be installed only by disassembling
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rotating parts) can be removed, replaced and stored as a complete assembly. This assembly
shall be capable of passing through a hatch of the specified diameter (see 6.1).
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3.3.7 Stuffing boxes and mechanical shaft seals.
3.3.7.1 All pumps except those on nuclear ships shall be provided with mechanical
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shaft seals. The gland of each seal shall be designed to incorporate two or more rings of
packing for use in the event of a mechanical shaft seal failure. Seal internals shall be
constructed of 304 or 316 stainless steel. The mechanical shaft seal shall have a tungsten
carbide against carbon sealing surface.
3.3.7.2 The design shall be such that the pressure on the seal or stuffing boxes will
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not exceed pump suction pressure by more than 50 pounds per square inch (p.s.i.). Replace-
able pressure breakdown sleeves and bushings shall be separately fitted to the shaft and
casing fitted with packing. Leek-off connections shall be between the pressure breakdown
device and the stuffing box. The Leak-off shall be led to the suction side of the pump with
the point of entrance properly located to avoid flow disturbance to the first stage impeller.
Gage confectioner shall be supplied as specified (see 6.1) and located in such a manner as to
accurately determine the pressure on the stuffing boxes or seal. All stuffing boxes l xcept
those with mechanical seals shall be fitted with throat bushings. For packed pumps all
stuffing box glands shall be set up by nuts threaded on gland studs secured in casing.
Sufficient space shall be provided between bearings and stuffing boxes to permit easy renewal
of packing without disturbing bearings, and inspection end overhaul of. bearings. The gland
design shall insure that gland leakage cannot reach the bearings or be thrown over bearing
housings and driving unit. This shall be accomplished preferably by use of internally cored
glands and by the use of shaft flingers. Ample drip pockets, spray shields and drains shall
be provided. Glands shall be split in halves with the halves secured together in a manner
to form a solid ring for setting up.
3.3.7.3 Space shall be provided between bearin s and stuffing boxes to permit easy
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renewal of packing inspection end overhaul of mechanical seals without disturbing bearings,
end examination and overhaul of bearings.
3.3.7.4 Mechanical shaft seals shall be positioned on the shaft by means of stub or
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step sleeves. Mechanical shaft seals shall not be positioned by Use of set screws.
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