Definition:
Pump is a mechanical device which is used for transporting and increasing the pressure of liquids. A liquid flows only when there is a pressure difference. So we can say that the function of pump is to generate the pressure difference.
The Head (m) of a pump can be expressed as follows-
H = (p2-p1) / (ρ g) + v2 / (2g)
Where,
H= Total head developed in meter (m)
p2= Outlet Pressure (N/m2)
p1= Inlet Pressure (N/m2)
ρ= Density of Liquid (kg/m3)
g= Acceleration of gravity, 9.81 (m/s2)
v= Outlet Velocity (m/s)
Pump Efficiency (%) can be expressed as follows-
ɳ = P2 / P1
Where,
ɳ = Pump Efficiency (%)
P1= Power Input
P2= Power Output
Classification of Pumps:
Pumps can be classified as follows-
Working Principle of a Centrifugal Pump:
Impeller is the rotating part of a centrifugal pump which converts the driver energy (Electric motor) into kinetic energy. The volute or diffuser is the stationary part of a centrifugal pump which converts the kinetic energy into pressure energy.
Working Principle of a Reciprocating Pump:
A reciprocating pump works on the principle of a reversing piston motion within a cylinder by sucking liquid on the ingoing stroke and delivering it under pressure on the outgoing stroke.
Pump Characteristic Curve:
Pump characteristic curve shows the relation between Head and Flow Rate developed by the pump. Pump characteristic curve shows that as the Head of pump decreases, the Flow Rate developed by the pump increases. Pump characteristic curve has been shown below-
Shut-Off Head is the head developed by the pump when there is no flow. Actually it’s the maximum head developed by the pump.
Pump Runout is the maximum flow rate developed by a pump without any damages to the pump. Actually it’s the maximum flow rate in minimum head by which pump can be operated without any damages.
But it’s not a wise decision to operate the pump by Pump Runout. There is a point between Shut-Off Head and Pump Runout called the Operating Point. By maintaining this Operating Point pump is usually operated for the smooth operation.
Net Positive Suction Head (NPSH):
For avoiding cavitation, the pressure at the pump inlet must exceed vapor pressure of the liquid at the corresponding temperature by a certain value called the net positive suction head or NPSH.
Net Positive Suction Head (NPSH) can be expressed as follows-
NPSH ≥ (Hts - Pv)
Where,
Hts= Total Suction Head
Pv= Vapor Pressure of the liquid at the corresponding temperature
Standard Operating Procedure (SOP) of Centrifugal Pump:
One should follow the following sequences at the start up of centrifugal pump
Ø Inspection Work before start up:
1. Pipe cleanliness
2. Alignment
3. Condition of valve
4. Pump bearing
5. Lube oil system
6. Priming
Ø Pre-start up check points:
1. Lubrication system must be checked
2. Cooling water must be lined up
3. Prime mover power should be checked
4. Rotate the shaft of pump by hand
5. Level of liquid at suction must be checked
6. Discharge valve should be closed
Ø Start up procedures:
1. Suction valve must be in open condition
2. Priming should be done properly
3. Suction line should be full of liquid
4. Switch On the power to the pump
5. Open the discharge valve when the pump reaches at the normal speed
Ø Post Start up Checkpoints:
1. Process Parameters (Pressure, Temperature, Flow Rate etc.) should be checked
2. Vibration at all points should be checked
3. Abnormal sound
4. Seal leakage
5. Motor ampere
Ø Check Points During Normal Operation:
1. Bearing Temperature
2. Pressure
3. Flow Rate
4. Seal Leakage
5. Oil level at Bearing Points
6. Lube Temperature and Pressure
7. Motor Ampere
Ø Shut Down Works:
1. Close the discharge valve
2. Switch off the power to the pump
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