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Trouble-Shooting Suggestions: Centrifugal Fire Pumps
A centrifugal pump is designed for a specific duty and ordinarily will perform this duty satisfactorily over a long period of time. On occasion, however, trouble will show up in one form or another, and even an experienced operator may have difficulty in locating and correcting it. The suggestions listed below should be helpful.
IF PUMP WILL NOT PRIME OR LOSES PRIME
The following may prevent the pump from being primed or may cause it to lose its prime:
Air Leaks — Any faulty intake or discharge connection or fittings, such as the suction hose, gaskets, booster tank piping, discharge valves or gaskets, drain valves, pump packing or mechanical seals, pump gaskets, or gauge fittings may make it difficult to prime the pump. All suction hose gaskets should be wiped clean of sand, pebbles and any other foreign matter before hose is attached to the pump. Hose couplings should be tightened snugly enough to make airtight connections. Packing, if the pump is so equipped, should always be kept adjusted to prevent excessive leakage. (See operating instructions for proper packing adjustment procedure.)
The following procedure may be used to detect air leaks:
Connect suction hose to the pump with a cap on the opposite end of the hose.
Close all pump openings.
Open the priming valve and operate the primer until a vacuum of from 20 to 22 inches of mercury is shown on the vacuum gauge. (If impossible to draw this much vacuum, primer may need attention.)
CAUTION: Electric motor-driven primers should not be run longer than 45 seconds. Allow time for motor to cool between runs.
Close the priming valve and shut off the primer. If leaks are present, the vacuum will promptly decrease as shown by the vacuum gauge. If the vacuum drops from 22 inches of mercury to less than 12 inches of mercury in five minutes, check for leaks. Air leaks may frequently be detected by ear if the engine is stopped. Applying water or oil to the suspected points may aid in detecting leaks. They may also be found by applying water pressure to the pump and suction hose by means of an auxiliary pump or hydrant. Pressure should be approximately 50 PSI and should not exceed that recommended for the suction hose or pump.
Suction Screens — Clogged intake screens may make it difficult to prime the pump as well as cause the pump to lose its prime after it is started. When operating from draft, be sure to keep the end of the suction hose off the lake or river bottom.
Engine Speed Too Low — Follow the manufacturer’s recommendation for priming. Speeds higher than those recommended do not accelerate priming.
Lack of Oil in Primer Reservoir — Where a mechanically driven or electrically driven rotary primer is in use, an adequate supply of lubricant should always be kept in the reservoir to lubricate and “seal” the priming pump.
Improper Clearance in Primer — Due to wear in the priming pump, clearances may become so large as to adversely affect priming. In this case, the proper clearance should be restored by the method outlined by the manufacturer in the priming pump operating instructions.
Defective Priming Valve — A broken or deteriorated priming valve spring may cause the valve to leak and the pump to lose its prime. Worn parts in the priming valve may also allow air leakage.
High Point in Intake Line — High points in the intake line (caused, for example, by running suction hose over a bridge railing) create air pockets that may cause loss of prime. In some cases, it may be impossible to rearrange the suction hose to eliminate the high point. In this case, prime may be obtained by closing the discharge valve immediately when the pressure drops and then repriming. This procedure usually eliminates the air pocket that was drawn into the impellers when the pump was started after first priming; however, it may have to be repeated in some situations.
Lift Too High — Lifts of more than 22 feet should not be attempted, even at low altitudes, unless there are practically no air leaks and the equipment is in new condition. In this case, higher lifts may be obtained.
End of Suction Hose Not Under Water — The end of the hose may be submerged deeply enough for priming, but when a large volume of water is pumped, a whirlpool is sometimes formed, which, may uncover the end of the hose.
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