(1) Failure to start : Gland packing may be too tight. Check whether spindle is freely rotating. Motor is not getting electric supply. Check voltage at the motor terminals and the mains. Connections may not be tight (if starters used check relevant connections like star delta starters).
Check for loose connections.
(2) Failure to deliver water: Check if pump is properly primed and delivery valves is open. Check suction valve if provided. Check if water level in the well is above the foot valve. Check speed, it may be too low.
Foot valve or suction line may be leaking. Foot valve strainer or impeller may have become clogged up. If installation is new, the pump installed may not be the right one. Note the suction and delivery gauge readings and check against the Maker’s characteristic curves and installation data. If the pump is erected after overhaul one of the pipe joints may have been improperly connected or wrongly connected.
(3) Rate of pumping is low : Check all points in item (b) above. The valves must be fully open. Foot valve and impellers may have become clogged and neck rings badly worn out probably due to working for a long time. Pump to be overhauled. Check for leaks on the delivery line.
(4) Pump stops delivering after running for sometime: Leaks in the suction side especially joints. Check if sufficient water is in the well. Check motor control circuits and protection circuits.
(5) Pump vibrates badly : Check alignment, bed bolts not tightened properly, impellers not properly balanced. Check for worn out bearings or bent shaft. Bearings may have excessive clearances; shaft couplings are not properly assembled. Impeller may have been choked.
(6) Pump is noisy in operation : Check all points under item (e) above. The impeller may be touching the casing. Air-bubbles may be passing through the pump. Cavitation effect due to bad design.
(7) Pump motor getting overloaded and becoming hot : Voltage at the motor terminals may be too low. Pump delivering water at reduced delivery head. Single phasing in case of 3-phase motor. Check voltages on all three phases. In case of 3-phase motor, used after overhauling, coil connections may not have been proper. Incorrect size of impeller. Excessively tight stuffing box, seized spindle or insufficient seepage.
(8) Excessive corrosion : This may be due to presence of certain chemicals in the water. Stainless steel may be used for certain parts of the pump.
(9) Excessive scoring of impeller parts. : Due to passage of sand or gritty matter. Due to cavitation efforts, especially if at the tips.
(10) Cracked pipe flanges: Due to excessive strain on the flanges caused by improper method of supporting the pipes, improper alignment of joint surfaces, and use of excessive force to bring the flange holes to align when bolting. Due to water – hammer action. Due to excessive vibration.
Automatic operation of a pump
A pumping installation can be easily wired for automatic control by installing a float switch on the High Level storage tank. This ensures that the pump is started when the water level falls down to, say, half the tank and automatically stopped when the tank becomes full. The only thing to be ensured is that the pump is kept fully primed always.
Therefore, the suction system and the foot valve should be well maintained so that water is retained by the valve for long periods and the pipe joints are air tight so that no air may leak in. One condition which should be always ensured is that water level in the well should not fall below the foot valve.
This can be taken care by providing a low-level float switch in the well, arranged to cut off the power supply to the motor, if the water level falls unduly in the well. Quite often pumps are unattended for long periods. Where pumps are not working continuously and are likely to be off for long periods, it is best to ensure priming of the pump before the pump is started.
Construction of Centrifugal Pump
(i) Bed plate : This is usually made of cast iron or welded steel of rigid construction. It is bolted to the foundation.
(ii) Pump casing (volute) : This is usually made of close-grained cast iron. It is split in the vertical plane at the centre of the casing and the two halves held together by a number of bolts and one or two locating pins and a gasket to make it leak proof. For easy removal of the impeller an end cover is usually provided.
(iii) Impeller : This is usually of close-grained cast iron or cast steel, hydraulically and dynamically balanced to avoid end-thrust and vibration. To eliminate possibility of rusting, impellers are frequently made of gun-metal. Two types of impellers are widely used. They are closed type and open type as shown in Fig.1.
(iv) Spindle : This is made of steel. Where corrosive liquids are to be handled, stainless steel spindles may be fitted. The portion of the spindle which works inside the casing is usually fitted with a renewable gun-metal sleeve so that it may be replaced when it gets worn out and the spindle may have a long life.
(v) Stuffing box and gland packing : This serves two purposes: on the suction side it prevents leakage of air and on the delivery side leakage of water under pressure. The packing material consists of rings of soft, cotton, woven yarn, impregnated with graphite and tallow. The gland bolts should only be tightened lightly, just enough to prevent leakage. Water is admitted into the stuffing box either through an internal hole or by an external connection. It is very important that a centrifugal pump should not be allowed to run dry, as it may result in seizure of the spindle.
Modern pumps are often fitted with mechanical leak-proof seals. Basically it consists of a mirror-finished, hard, flat surface against which a spring loaded ring of a softer material like carbon, rubber, leather, or plastic is used. This combination permits free rotation without allowing any leakage. The seal is kept cool by the liquid pumped.
(vi) Bearings : These may be of the ball, roller or sleeve type. Grease lubricators are provided on the bearing housing.
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