Centrifugal Pump – Complete Guide
Simple English explanation of working principle, major components, energy conversion and important interview concepts for plant & field operator interviews.
1. What is a Centrifugal Pump?
A centrifugal pump is a machine used to transfer liquid from one place to another.
It takes mechanical energy from a motor and converts it into hydraulic energy of the liquid.
The main principle of operation is centrifugal action.
2. Working Principle of a Centrifugal Pump
A centrifugal pump mainly works through the following steps.
Before starting the pump, the pump casing and suction pipe must be filled with liquid.
Why?
A normal centrifugal pump cannot properly pump air. If air is present inside the pump, the pump may not develop sufficient pressure to lift and deliver the liquid.
Priming = Removing air and filling the pump with liquid before starting.
The electric motor rotates the pump shaft. The shaft is connected to the impeller.
The impeller rotates at high speed and transfers energy to the liquid.
The liquid enters the pump through the suction pipe.
It reaches the centre of the impeller, which is called the impeller eye.
When the impeller rotates, it pushes the liquid from the centre toward the outer circumference.
This outward movement is the important centrifugal action.
The rotating impeller gives kinetic energy to the liquid.
Therefore, the liquid leaving the impeller has high velocity.
The high-speed liquid enters the pump casing. The casing, especially a volute or diffuser, slows down the liquid gradually.
When the velocity decreases, a major part of the kinetic energy is converted into pressure energy.
This increased pressure allows the pump to send liquid through the delivery pipe.
3. How Does Suction Happen?
This is a very important concept for plant and operator interviews.
When the impeller throws liquid outward, the pressure near the impeller eye decreases.
The pressure at the liquid surface in the suction tank is higher. Therefore, liquid moves toward the lower-pressure region.
In this way, the pump continuously receives liquid.
Impeller removes liquid from the eye → pressure at the eye falls → higher pressure pushes more liquid into the pump.
4. Main Parts of a Centrifugal Pump
1. Impeller
The impeller is the rotating part of the pump. Its main job is to give energy to the liquid.
It contains curved vanes and is available in different designs.
- Has front and back shrouds.
- Mainly used for clean liquids.
- Provides high efficiency.
- Has a back plate but no front shroud.
- Can handle some suspended solids.
- Does not have complete shrouds.
- Suitable for sewage, slurry and liquids containing solids.
- Lower tendency for clogging.
2. Casing
The casing surrounds the impeller and collects the liquid leaving the impeller.
Main functions:
- Collects liquid from the impeller.
- Reduces liquid velocity.
- Converts kinetic energy into pressure energy.
- Directs liquid toward the delivery pipe.
A spiral-shaped casing designed to gradually collect the liquid and help convert velocity into pressure.
Uses stationary guide vanes to gradually reduce velocity and increase pressure.
3. Shaft
The shaft connects the motor to the impeller.
It transfers rotational power from the motor to the impeller.
4. Bearings
Bearings support the rotating shaft.
- Support the shaft.
- Reduce friction.
- Maintain proper shaft alignment.
- Help control vibration.
5. Shaft Sleeve
A shaft sleeve protects the shaft, especially near the sealing area.
- Protects against wear.
- Protects against corrosion.
- Reduces damage near the sealing zone.
- Can be replaced when worn.
6. Mechanical Seal / Gland Packing
Where the shaft comes out of the casing, there is a possibility of liquid leakage.
A mechanical seal or gland packing helps prevent this leakage.
Seal = Prevents leakage around the rotating shaft.
7. Suction Pipe
The suction pipe carries liquid from the tank or source toward the pump.
8. Foot Valve
A foot valve may be installed at the bottom of the suction pipe where required.
It acts as a check valve and prevents liquid from flowing backward.
This helps maintain the liquid column and supports priming.
9. Strainer
A strainer is installed on the suction side to prevent large solid particles from entering the pump.
It helps protect the pump from blockage and damage.
10. Delivery Pipe
The delivery pipe carries the pumped liquid from the pump to the required destination.
A delivery valve can be used to control the flow.
5. Complete Flow of a Centrifugal Pump
For an interview, remember this complete sequence:
Energy Conversion
Important Interview Question
“A centrifugal pump works on the principle of centrifugal action. The motor rotates the impeller, which gives kinetic energy to the liquid and moves it from the impeller eye toward the outer periphery. The casing or diffuser then converts a major part of this kinetic energy into pressure energy, allowing the liquid to flow through the delivery pipe.”
📊 Performance Parameters & Head Calculations
Engineering analysis evaluates centrifugal pumps using specific structural "heads" measured in meters of fluid column.
| Head Type | Engineering Definition | Mathematical Definition |
|---|---|---|
| Static Head (Hs) | Total vertical height the fluid must be raised. |
Hs
=
hs
+
hd
(where hs = suction lift,
hd = delivery lift)
|
|
Manometric Head (Hm) |
The actual head developed by the pump to overcome all system losses. |
Hm
=
Vane Head
−
Internal Losses
OR
Hm
=
Hs
+
hfs
+
hfd
+
Vd2
2g
(where hf = friction losses,
Vd = delivery velocity)
|
Pump Efficiencies
• Manometric Efficiency (ηmano): Ratio of the actual manometric head to the ideal head generated by the impeller:
(where Vw2 is the whirl velocity at outlet, and u2 is the tangential blade velocity at outlet)
• Overall Efficiency (ηo): Total performance tracking actual fluid output power vs. input shaft power:
Positive Displacement Pump
Complete Easy-English Guide for Plant & Field Operator Interviews
1. What is a Positive Displacement Pump?
A Positive Displacement (PD) Pump is a mechanical device used to move fluid from one place to another by trapping a fixed volume of fluid and then forcing (displacing) that fluid toward the discharge side.
A PD pump first traps a definite amount of liquid and then pushes that liquid toward the discharge.
Unlike a centrifugal pump, which mainly increases the velocity of the fluid using a rotating impeller, a PD pump works by changing the available volume around the trapped fluid.
2. Centrifugal Pump vs Positive Displacement Pump
| Centrifugal Pump | Positive Displacement Pump |
|---|---|
| Uses a rotating impeller to increase fluid velocity. | Traps a definite volume of fluid and mechanically displaces it. |
| Velocity / kinetic energy is important in the pumping process. | Trapped volume and displacement are the basic principle. |
| Casing converts a major part of velocity into pressure. | The pump mechanically forces the trapped fluid toward discharge. |
Centrifugal Pump = Velocity Based
Positive Displacement Pump = Volume Displacement Based
⚙️ 3. Working Principle
The working cycle of a positive displacement pump can be understood in two main phases.
Suction / Expansion Phase
The internal mechanism of the pump moves in such a way that it creates an expanding cavity on the inlet side.
This mechanism may be a:
- Piston
- Plunger
- Gear
- Vane
- Screw
As the cavity becomes larger, the pressure inside it decreases. The higher pressure on the source side then pushes the liquid into the pump.
Discharge / Compression Phase
The pump mechanism then moves so that the cavity becomes smaller.
The liquid becomes trapped inside the pumping chamber and, as the available volume decreases, the liquid is forced toward the discharge port.
Expansion → Suction → Liquid Trapped → Compression → Discharge
4. Why is it Called "Positive Displacement"?
It is called a Positive Displacement Pump because the pump traps and displaces a definite volume of fluid during each pumping cycle.
If a pump displaces approximately 1 litre per cycle, it attempts to move that trapped volume toward the discharge during each cycle. Actual flow can vary because of leakage, slip, pressure, viscosity and operating conditions.
⚠️ 5. Important Operating Characteristic
For a given speed and operating condition, a PD pump generally provides a relatively predictable displacement per cycle.
However, actual flow may change because of factors such as slip, leakage, viscosity and discharge pressure.
A positive displacement pump must not be allowed to operate against a completely closed discharge line unless suitable pressure-relief protection is provided.
The pump continues trying to displace fluid. If the discharge path is blocked, pressure can rise rapidly and may damage equipment.
🔧 6. Types of Positive Displacement Pumps
Positive displacement pumps are mainly divided into:
7. Reciprocating Pumps
"Reciprocating" simply means back-and-forth movement.
A simple example is a syringe.
Reciprocating pumps generally produce a pulsating flow.
Piston Pump • Plunger Pump • Diaphragm Pump
🔹 Piston Pump
A piston pump uses a piston that moves backward and forward inside a cylinder.
The piston has a sealing arrangement that moves with the piston.
🔹 Plunger Pump
A plunger pump uses a smooth, rigid rod called a plunger.
The plunger moves through a stationary packing/sealing arrangement.
Plunger pumps are commonly used for high-pressure applications.
Piston = Piston moves inside the cylinder with its sealing arrangement.
Plunger = Smooth rod moves through stationary packing.
🔹 Diaphragm Pump
A diaphragm pump uses a flexible membrane called a diaphragm.
The diaphragm moves backward and forward to create suction and discharge.
Because the pumped fluid can remain separated from the driving mechanism, diaphragm pumps are useful for difficult fluids.
Common applications include:
- Corrosive chemicals
- Toxic fluids
- Abrasive liquids
- Chemical dosing
8. Rotary Pumps
Rotary positive displacement pumps use rotating elements to trap pockets of fluid and move them continuously from the inlet toward the outlet.
🔹 Gear Pump
A gear pump uses rotating gears to trap and carry liquid from the inlet to the discharge side.
External Gear Pump
Uses two external gears that mesh with each other. Fluid is carried in the spaces between the gear teeth and the pump casing.
Internal Gear Pump
Uses a gear-within-a-gear arrangement.
It is particularly useful for high-viscosity fluids such as heavy oils, polymers and asphalt, depending on pump design.
Gear Pump = Gears trap liquid and carry it from inlet to outlet.
🔹 Screw Pump
A screw pump uses one or more helical screws to move liquid along the axis of the pump.
The rotating screws create spaces that carry the fluid continuously toward the outlet.
- Smooth flow
- Low pulsation
- Relatively quiet operation
- Can be suitable for some shear-sensitive fluids
Screw Pump = Rotating screws move liquid forward.
🔹 Lobe Pump
A lobe pump uses rotating lobes to trap pockets of liquid and move them from inlet to outlet.
The lobes are designed so that they do not directly contact each other during normal operation.
Lobe pumps are widely used in:
- Food industry
- Pharmaceutical industry
- Sanitary applications
Depending on the design, they can handle products containing relatively large soft solids.
Lobe Pump = Rotating lobes carry liquid gently.
🔹 Vane Pump
A vane pump contains a rotating slotted rotor.
Rectangular vanes slide in and out of the rotor slots.
As the rotor rotates, the vanes maintain contact with the surrounding casing or ring and move the liquid from inlet to outlet.
Vane Pump = Sliding vanes trap and move liquid.
🔹 Peristaltic Pump
A peristaltic pump works by compressing a flexible tube with rotating rollers.
Think of toothpaste inside a flexible tube. When the tube is squeezed, the toothpaste moves forward.
In the same basic way, rollers compress the flexible tube and push the liquid forward.
The pumped liquid remains inside the flexible tube and does not contact the pump's rotating mechanism.
Useful applications:
- Precise chemical dosing
- Laboratory applications
- Some medical applications
- Applications where contamination must be minimized
Peristaltic Pump = Roller squeezes tube → Liquid moves forward.
⭐ Complete PD Pump Flow
📊 Performance Parameters & Characteristic Differences
Positive displacement pumps operate fundamentally differently from centrifugal pumps under varying plant conditions.
| Engineering Parameter | Positive Displacement Pump |
Centrifugal Pump (For Comparison) |
|---|---|---|
| Flow vs. Pressure Relationship | Relatively constant flow for a given speed and displacement, although actual flow can change because of slip, leakage and operating conditions. | Flow generally decreases as system pressure/head increases. |
| Viscosity Handling | Many PD pumps can handle high-viscosity fluids effectively. For some designs, higher viscosity can reduce internal slip and improve volumetric efficiency. | Pump performance can be affected significantly by increasing viscosity, with increased losses and reduced flow/head capability. |
| Self-Priming Capability | Many PD pumps are capable of self-priming, depending on pump type, design and installation. | A conventional centrifugal pump generally cannot pump air effectively and commonly requires priming before startup. |
A PD pump physically traps and displaces fluid, so its flow is mainly related to the pump's displacement and speed.
A centrifugal pump depends more strongly on the relationship between its pump curve and the system resistance.
Volumetric Efficiency (ηv)
In a real positive displacement pump, the seals and internal clearances are not perfectly leak-free.
Therefore, a small amount of fluid may move backward from the high-pressure discharge side toward the low-pressure suction side through internal clearances.
This internal leakage is commonly called slip.
Qactual = Actual flow delivered by the pump
Qtheoretical = Theoretical flow based on pump displacement and speed
If the pump should theoretically deliver a certain amount of liquid but some liquid leaks internally back toward the suction side, the actual delivered flow becomes lower.
Therefore:
Actual Flow < Theoretical Flow
🚨 Critical Safety Rule: Over-Pressurization Risk
Never operate a positive displacement pump against a completely closed discharge line unless the system is specifically designed with suitable pressure-relief protection.
A PD pump continues attempting to displace fluid even when the discharge path is restricted.
If the discharge is blocked, pressure can rise rapidly and may damage the pump, piping or other equipment.
PD pump systems commonly require a suitable pressure relief valve or other engineered protection to prevent excessive discharge pressure.
In an actual plant, always follow the equipment design, approved SOP, interlocks and site safety procedures.
If the interviewer asks: "What happens if the discharge valve of a PD pump is closed?"
Answer:
"A positive displacement pump continues to displace fluid, so closing the discharge can cause pressure to rise rapidly. Therefore, suitable pressure-relief protection is required."
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