Centrifugal Pump Vacuum Test
Vacuum test is carried out for normally for bilge pump and general service pump, if centrifugal type. These pumps normally have to take suction from bilges and tank top which is below the pump installation. Any air leakage from the pump, air-pump, valves, pipe lines, gland packing, etc. can be investigated and rectified by the test. Since centrifugal pumps are not self priming, an air-pump is attached to them, which sucks the air from suction line to lift up the water and fill it up in the pump body.
Procedure of Vacuum Test
- Open sea suction valve
- Start the pump and raise to normal speed
- Open sea discharge valve
- Close sea suction valve
- Open suction valve to an empty compartment until the vacuum has fallen away and then close the valve. The pump vacuum should rise rapidly to 64.5 mbar or more, if the air pump is in satisfactory condition and the pump is free from air leakage.
- Stop the pump. The vacuum should be retained if the pump is air tight.
If the pump is not air tight, stop the pump, readjust the pump gland and fill the recess with oil. Make a further test, and if no difference in performance occurs the gland is not at fault.
If the pump casing joints are quite air tight and the suction valve properly shut, the fault is then attributable to a faulty air-pump (self priming pump), a leaky non return valve, a choked basket strainer or a detached float, which can be easily checked by removing the float-gear distance piece. Remove the non return valve cover plug and grind in the valve with fine abrasive paste.
With the pump running, and the air-pump suction pipe disconnected, note the pull excreted by the air-pump upon a hand placed over the suction branch of the air-pump. This will indicate if the air-pump is pulling up the required vacuum. If the air-pump is found to be in order, open the pump suction valve and the discharge valve, and, with all other individual bilge or tank suctions closed on the line, ascertain the maximum vacuum which can be obtained in the main line suction when the pump is operating. The pump should be watched for overheating in the water end under a prolonged run when not pumping, and if necessary, a fresh supply of water admitted by opening the sea suction valve for a few minutes.
If the vacuum fails to rise, then the defect is due to excessive air leaks in the suction piping through bad joints, badly packed valve spindles or valves left open or leaky. if the vacuum rises satisfactorily, open the suction valve of the tank or bilge to be pumped and watched vacuum gauge. If the vacuum falls away suddenly and fails to recover, look for a leak in the tail pipe above the water level in the tank. If the vacuum remains higher than that required to overcome the static lift, and the pump fails to deliver, there is a choke in the suction pipe, which can be readily ascertained. A choked impeller will show little or no increase in vacuum after the pump is primed and a low motor current value.
It should be remembered that, although the air-pump will continue to operate pulling water from the suction system via the float valve, it is not desirable to operate the unit under these conditions. Should the water be constantly discharged by the air-pump, it is probable that the float valve is dirty or requires to be lightly ground in.
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