When the centrifugal pump is not primed ( filled with liquid ) before starting, the air pocket inside the pump case may give rise to formation of vortex and discontinuity of flow those lead to make noise, vibration and damages to the impeller.
Priming is done by pouring liquid in to pump casing ( when pump is not running ) through a funnel and air will be displaced through an air vent provided on pump case.
The formation of air pocket inside casing can be avoided by following methods:
1 – Installation of pump below the suction level
2 – Self – Priming device
- Vacumm pump can run con the same pump shaft or may be a diaphragm- or eductor-type unit, and electrically, mechanically or pneumatically driven. those suck air from suction pipe and throw it into the atmosphere.
- Properly design to form air-liquid mixture (the pump has an enlarged upper casing with a baffle – vách ngăn in it). The air which is discharged is replaced by water drawn up the suction pipe, until eventually the suction pipe fills literally and the air bubble in the eye of the impeller is blown out of the discharge pipe.
- Priming separation camber to be provided – The air escapes through the pump discharge nozzle whilst the fluid drops back down and is once more entrained by the impeller. The suction line is thus continuously evacuated.
3- Eccentric reducer Suction Pump System
In case of long horizontal pipe runs, air pockets are avoided by installing the eccentric reducer with the flat side up.
When the source of supply is above the pump, then the eccentric reducers must be placed with the flat side at the bottom.
AIR LEAK IN SUCTION LINE
As fluid recirculates in the pump and forces air out of the discharge chamber, it’s trying to create an area of low pressure. However, if there’s a leak in the suction line, air continues to be drawn into the pump, never allowing it to release enough to create that area of low pressure.
DEBRIS IN THE IMPELLER
If the eye of then impeller is plugged with debris, it removes the hydraulic capability of the impeller to create an area of low pressure.
PUMP IS AIR BOUND
A pump can become air bound if it experiences any of these conditions:
NO AIR RELEASE LINE
If there is no air release line, air cannot be vented to the atmosphere, and instead collects on the discharge side.
DISCHARGE LINE IS PRESSURIZED
Valve on the air release line is closed, and the valve on the discharge line is closed, again, giving no place for the air to go and get out of the pump.
EXCESSIVE IMPELLER CLEARANCE
If there is excessive clearance between the impeller and the wear plate, the pump has a difficult time creating a low pressure area. This is typically caused by wear, but could also be due to improper reassembly.
PLUGGED RECIRCULATION PORT
During the priming process, as discussed above, fluid is recirculated through the volute casing. If the recirculation port becomes plugged, the eye of the impeller is unable to create an area of low pressure in which to pull liquid up the suction line.