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Negative Suction Head

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MIL-HDBK-1164
Table 11 (Continued)
Inspection
Action
Bearings
Check bearing temperature and lubrication. Where
petroleum-based lubricants are used, follow the
manufacturer's manual and do not over lubricate.
Suction and discharge
Check these readings and compare with "normal" valves.
readings
Make sure valves are set as required. Check shaft packing.
Check discharge rate. Check driving equipment.
(1)
Positive Suction Head. When the intake (suction) side of the pump is under
pressure, use the following priming sequence:
a. Open all suction valves to allow water to enter the suction pipe and pump
casing.
b. Open all vents located on the highest point of the pump casing to allow trapped
air to be released.
Note: The pump is properly primed when water flows from all open vents in a
steady stream.
(2)
Negative Suction Head. Two priming methods are available for a negative
suction head condition--that is, when the pump lifts water to the intake (suction
lift).
a. Vacuum Pump or Ejection Method. When steam, high-pressure water, or
compressed air is available, prime the pump by attaching an ejector to the
highest point in the pump casing for evacuating the air from the suction piping
and casing. A vacuum may be substituted for the above equipment. Start the
ejector or vacuum pump to exhaust the air from the pump casing and suction
pipe. When water discharges from the ejector or vacuum pump, start the
centrifugal pump, but continue priming until the centrifugal pump has reached
operating speed.
b. Priming a Pump with a Foot Valve. A foot valve is used at the lowest point on
the suction pipe. The foot valve retains water in the suction pipe and pump
casing after the pump has been initially primed. To prime, open the suction
valve, if one is installed. Open vent valves at the highest points on the pump
casing. Fill the pump and suction line from an independent water supply. Allow
to fill until a steady steam flows from the vent valves.
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