|
|
Click below to view:-
|
This information is based on the most common type of horizontal, single stage pump, operating with flooded suction and fitted electric drive motor.
Ensure power supply is isolated.
Check pump rotates freely by hand. ie. there are no obstructions in the pump or drive motor.
Open any isolating valves and ensure pump is flooded. ie. release any air-locks in the pump or pipework.
Start the pump briefly to check for correct direction of rotation. The correct rotation should be indicated by an arrow on the pump or drive motor - if not, refer to operating manual.
Note: Some pumps have screw-on impellers which may be loosened if the pump is run in reverse direction.
On start-up, it is advisable to partially close a valve in the pump discharge line to limit the initial flow surge. Also monitor the power taken by the drive motor.
Once running, the discharge valve should be progressively opened until the required flow rate and pressure is obtained, ensuring that the power taken is within the maximum rating of the drive motor.
The absorbed power is dependent on the pump performance and the properties of the fluid being pumped. ie. specific gravity, viscosity and entrained solids etc.
If the required flow rate, pressure or power limits can not be achieved, refer to the original pump selection data or our article 'The Correct Selection of a Centrifugal Pump'.
Remember that most centrifugal pumps must not be run 'dry' of process liquid because of probable damage to the shaft seal or bearings on a magnetic drive unit.
It is important to avoid unnecessary restriction of flow to the pump suction because this may cause cavitation.
Therefore, any control valve or other flow regulating device, should be installed in the pump discharge line.
Refering to a centrifugal pump performance graph, it will be seen that the highest pressure is achieved at, or near, zero flowrate - commonly known as 'closed valve' condition.
At this point, the pump is absorbing the least amount of power but most of that power will pass into the small volume of liquid recirculating within the pump.
Continuous operation at closed valve can cause over-heating and may damage the pump.
As the control valve is opened, flow rate increases and the power absorbed also increases. If the flow rate exceeds the maximum rating for that particular pump, the pump may cavitate, become unstable or over-load the drive motor.
The aim is to operate the pump somewhere near its optimum flow rate, which corresponds to the point of maximum efficiency.
An important characteristic of a centrifugal pump is that the flow rate achieved is dependant on the pressure acting at both the suction and discharge connections.
Therefore, performance is expressed in terms of 'Differential Pressure' generated between the suction and discharge connections.